I finally finished upgrading my current system with a boiler bypass (via a 3-way mixing valve), Taco 4900 air separator, bladder expansion tank & a Tekmar 260 controller. The system runs much better now and the boiler actually reaches temperatures hot enough to prevent condensation without the hours of slowly creepy up from a cold start. The system is a 12-year-old Utica CI boiler into a converted gravity system with CI radiators. (Please note I am not currently using the DHW portion of the Tekmar.)

My question is what is the best option to set the Tekmar up to take into consideration the difference between the boiler temp vs. the system temp? The bypass valve is currently set for a 50-degree delta T between the return temp and boiler temp to ensure the boiler is hot enough to prevent condensation. The system temp (going out to the radiators) ends up with a delta T of 25 degrees (i.e. 175 boiler temp =150 degree system temp & 125 degree return temp). I currently have the Tekmar boiler sensor strapped on the boiler side and quickly programmed the following setting in the 260 to get it up and running: Indoor design 70, Outdoor Design -15 (Milwaukee), Thermal unit 5, Design water temp 180, Boiler max 190.

Should I continue with the Tekmar sensor on the boiler side and set the unit to take into consideration the lower supply temp or should I move the sensor after the mixing valve to monitor the actual system temp and make sure the boiler doesn’t exceed a certain temperature? As always, thanks for the input.

 

The 260 supply sensor should go on the distribution side.

The boiler aquastat should take care of the boiler high limit.

More later.

 

Yep, you're right. I should have done a little more research prior to posting my question. I moved the sensor to the system side and wired the burner/auto damper circuit back through the Honeywell aquastat this morning. Now I need to figure the optimum Tekmar settings and the aquastat high limit temperature.

 

What mixing valve are you using? (Just curious.)

Three suggestions for the 260 (I have one, too):

1) use (or do) your heat loss calculation, and estimate the radiator output in each room to figure out what water temp you need at -15F design, on average. It might be 180; it might not (probably less). That will help dial down the curve.

2) Go slow and steady on adjusting the 260. It took mine nearly 8 weeks (!) to settle on supply temps at a given outdoor temp.

3) Leave the differential set to Auto.

This control really rocks if you have a room or zone that approximates the average temperature of the house, and can employ the indoor sensor (076). There are a few caveats and something of a change in control philosophy, but I can elaborate if you're interested.

 

Thanks xiphias. I'm using a Tekmar 712 valve.

I calculated the heat loss and total EDR of each radiator over the last month as I researched my options. I only have two rooms that require a water temp greater than 160 using an outdoor design temp of -15. My formal living room needs approximately 170 degrees and our open sun room needs 180. The sun room opens into the dining room and is always cold compared to the rest of the house since it is over a unconditioned crawl and has full windows on three sides. The rest of the first floor should be fine with 160 and the 4 bedrooms upstairs should be good with 155 (the 2nd floor is always warmer since it's a single zone system).

I didn't purchase the indoor sensor. Do you think it's worthwhile? What are your thoughts on a system design water temp? I'll probably start around 165 - 170 with a maximum of 180. I assume I should set the boiler aquastat high limit greater than the Tekmar max since the boiler temp will be around 25 degrees higher than the system?

 

Try setting BOIL MAX to 170.

I have the same kind of cold-room-over-unconditioned situation. With outdoor reset you should be circulating water a lot more. I've found that really helps even out the cold room temp. Not perfect, but way way better.

I spent last year on just the ODR/260. About a month ago I added the indoor sensor. The performance difference is phenomenal.

Since you have a single zone, find a place that's a good average (not upstairs) and put an indoor sensor there. Turn up your existing thermostat to a couple degrees above what you set OCC for on the 260 so it generates a continuous call for heat. The control will go for constant circulation and constantly tweak the supply temp and differential to even out the space temp and maximize boiler performance.

There is a catch, sort of. If you don't set back at night, then no catch. You just effectively replaced your thermostat with the control and indoor sensor. If you do set back at night, the best way to set back is to have a timer or switch to operate the OCC/UNOCC function. You don't need a fancy $$$ tekmar timer. I'm using a cheapie ($30) battery-powered programmable thermostat mounted near the control. Set the timer/stat for, say 40F during the times you want OCC temp. Set it for 90F during the times you want UNOCC temp. Or you could use a simple switch (requires remembering to throw it, though).

With the indoor sensor, the 260 will provide an automatic boost to get you from the UNOCC to OCC temp. It doesn't have "adaptive recovery" or "optimal start" so you need to figure out by trial/error what time to start the recovery. But once you figure that out, the ODR will automatically shift the curve to give you nearly the exact same time length of recovery every morning. (We've had overnight outdoor temps from 45 down to 5 over the past few weeks and the recovery time differs by maybe a couple minutes every day.)

I see the 712 is a manual valve. Not sure how that might affect a setting for BOIL MIN. I think if you let the aquastat protect the boiler then you will have effectively partial reset with a lower limit defined by the lowest return temp you can safely give the boiler for prolonged periods without condensing.

We need radioconnection for a consult on the bypass with 260 thing. As he knows all too well, bypasses confuse me.

 

I added the indoor sensor last week and it's been interesting to watch how the controller monitors the system now. One quick clarification- you stated with a constant boiler demand the system should go into constant circulation? I've noticed if the Tekmar indoor system is at the specified room temperature set in the controller (ie 68 degrees) the system will completely shut down (no circulation), but still show boiler demand on the screen. Is that normal? I've also found unless the room temperature is below the set point on the Tekmar, the unit always uses the minimum boiler temp even if its 5 degrees outside. It then constantly short cycle since the CI radiators never warm up enough to raise the temp in the room with the sensor. It appears to operate a little more effciently with a minimum temp of 130 -135 compared to the 120 I started with. Overall, I think I'm finally on the way to a decent system with boiler protection.

 

Hmm. I think the way it should work (at least this is the way mine is working) is like this:

if water is circulating constantly, the only way the 260 can change the space temp is by changing the characteristics of the water entering the space heating loop. It can either raise or lower the supply temperature, and it can figure out how wide a differential it can use at a given water temperature to cause the space temp to go up or down. I think the PID logic would probably seek to run the lowest possible water temperature at the widest reasonable differential. Those two parameters probably have the most to say about overall system efficiency.

The only time I have seen the 260 stop the zone circs on my system is during the first 24 hours after I installed the indoor sensor. It started out by jacking up the supply temps a bit above what the ODR curve would have said, and the building overheated (went 4-5F above setpoint). It responded by turning off the zone circs until the building cooled off.

Since then, on days with lots of solar gain, it will not shut off the zone circs, but it does set the boil target to stupid or near-stupid low values (a BOIL TARGET of 48F was one that I recall...) so the water is still circulating.

Do you still have a thermostat on the system? How is the control wired to the boiler and circulator?

Not sure I understand what you're saying here. The sensor should tell the control that the space temp is below setpoint, and the supply temperature should go up.

Can you list the various programmable parameters here?

We still need radioconnection for a primer on manual bypass with the 260.

 

The system is currently set with the following parameters:

Occupied: 68
Unoccupied: Not Used
System Design:160
Room Design: 70
Boiler Max: 170
Boiler Min: 130
Auto Differential
Warm Weather Shut Down OCC: 70
Warm Weather Shut Down UNOCC: 60

The thermostat is wired to the boiler demand circuit, but with the added indoor sensor, it only functions as an on/off switch.

Event the Tekmar literature leads me to believe the circulator should always be energized (constant circulation) with boiler demand. With my unit, that is not the case. If the indoor sensor reads the current target room temp (68), the system shows demand, but does nut energize the pump or fire the burner. When the indoor sensor reads less than the desired room temp, everything operates within the desired parameters.

 

Eureka. Maybe. Probably. I think I know why it won't run the circulator or fire the burner when the room is at setpoint.

The control is probably computing a supply temperature to hold the room at setpoint that is below BOIL MIN. So its only alternatives are

1) supply BOIL MIN water (and the room overheats), or

2) turn off the circulator until the room cools off sufficiently so that a supply temperature at or above BOIL MIN is required.

You could test this by setting BOIL MIN to OFF and seeing what the BOIL TARGET temp does when the room is at setpoint. My guess is the target temp will be less than your current BOIL MIN, and the circulator will continue to run.

Don't leave it that way forever, because I'm still bypass-challenged and can't provide a useful opinion on whether the manual bypass valve will provide adequate boiler protection.

 

Xiphias--per your PM

I don't understand how his valve works, does it monitor the return boiler temp to provide protection? Only way to determine boiler protection is whether the return is above 135 degrees in a short period, and the boiler should be running at above that temperature before shutting off.

Pete

 

Pete,

The valve is a tekmar 712.

http://tekmarcontrols.com/literature/acrobat/d710.pdf

It's just a plain fixed manual 3-way valve. It would accept a motor to open/close, but that would require a different control than the 260.

The question is whether simply diverting a fixed portion of the boiler's output would provide sufficient boiler protection. If not, would the kind of ESBE valve that you have do the trick?

Thanks.

 

Is he using that valve for system or boiler
protection?? What does the valve operate on?

Pete

 

Boiler protection, but it would be good to see some pics of the piping. jborders, got any you can post?

The valve operates by a human hand turning it to a position and walking away. That's the source of my concern. I don't want to suggest permanently changing the 260 settings if that jeopardizes the boiler.

 

As far as I know, the Tekmar would have to be set to a boiler min. of 150 degrees, as shown in the manual to ensure the return temps are above 130 degrees. The bypass can work by shunting some of the return around the boiler to slow the return of cold water to the boiler, or it can be used to feed some of the heated supply back to the return to temper the return water to reduce condensation. I still think the Tekmar would have to be set to the 150 degree Boiler Min. in both cases..

Now, if he had a thermic bypass valve, or an automatic mixing valve controlled by a remote sensor to control the amount of bypass, then a lower setting might on the Boiler Min. might work?

I honestly don't know, just musing here.

Pete

 

Hi all, and especially xiphias.

I'm going to drag this thread up, because I am thinking about putting in a Tekmar 260 on my current boiler system. I read what xiphias was doing (with his Tekmar/thermostat logic) and thought it was brilliant.

But after sleeping on it I have a few questions.

Xiphias -- I am not sure that I follow your logic in post #10. I would have expected the Tekmar to run the pump because of the demand signal. I don't see why the calculated boiler temp being below BOILER MIN should matter.

This would be consistent with the data manual (page 5 of http://www.tekmarcontrols.com/litera...bat/d260.pdf):

I am also confused as to how the 260 uses the indoor sensor and the significance of ROOM OCC.

To me, the first 2 sentences say that the Tekmar simply adjusts the differential better when it has the indoor sensor. Ok. But the last sentence, about using ROOM OCC, make it seem that with the indoor sensor connected, the Tekmar takes over as the thermostat AND (my logic leap) ignores the boiler demand signal from the thermostat. (One can't have it both ways.)

But if the presence of the indoor sensor makes the Tekmar act as a thermostat and ignores boiler demand signal (contradicting the first manual quote), then I am confused as to how your thermostat@70 tekmar@68 example allows your pumps to circ all the time, unless your indoor sensor never hits 68.

I am inclined to think that ROOM OCC simply shifts the heating curve, and that the last sentence about adjusting the room temperature using ROOM OCC is simply about shifting the heating curve. This would be consistent with the manual (I won't quote that here)

In that case, I have no explanation for jborders5's behavior.

A final question: If your logic in #10 is correct, then I would hypothesize that as the heating season has wound down, you should have encountered the same circ-cutoff behavior that jborder5 has described. (Did you?) Assuming that you have a BOILER MIN enabled, I would imagine that with warmer weather, there would be a number of times where the 260 calculated a boiler temp below BOILER MIN - did your circ shut off?

I look forward to participating in this forum more as I start up my project this summer.

 

Hi and welcome. Must have missed this post.

BOIL MIN matters because it is used to reject the boil demand if the supply target is below BOIL MIN. In other words, there can be a boil demand, but the circ(s) won't run and the boiler won't fire until a supply target >BOIL MIN is computed.

For the past couple months, I have had BOIL MIN set to 80F (its lowest setting before OFF). Sometimes, a boiler demand is still present, but the circ(s) won't run and the boiler won't fire until the 260 calculates a supply temp >80F.

80F also seems to be a good compromise between constant circulation and no sensible emitter output from my fin-tube baseboard. There's just not a need to go constant circ in the shoulder season -- sometimes the supply targets on warm days are even well below room temperature. And below ~85F, the fin tube really doesn't output much, if anything. But using 80F allows the control to see how system water temps are changing at these low target temps, and I suspect that's good for the PID logic in the control. Also consistent with the tekmar philosophy of maintaining flow past the sensor for that purpose.

After living with the indoor sensor for a few months, it seems there is not as much tweaking of the differential -- at least in my system -- as I thought. It still seems to be anywhere from 18-24F or thereabouts. What the indoor sensor does do, however, is provide input to the control that allows it to dial down the supply temp to the absolute minimum required to hold the setpoint. In my system (and likely in others), that spaces out the time between firings a lot.

You are correct that the indoor sensor makes the 260 act like a thermostat in that it is now controlling both the circulators (assuming you set your thermostats to some high value and enable some value for BOIL MIN) and the supply temperature to hold setpoint at the indoor sensor. For example, I could in theory simply remove the thermostat near the indoor sensor and tie the R/W wires together to generate a constant boiler demand for that zone.

With the indoor sensor, the ROOM OCC temperature does indeed become the target temperature the 260 is seeking to hold. If you set your room thermostat to 90F, it doesn't matter and it'll never get there. The ROOM OCC is the setpoint and the control will modulate water temperature to hold it.

So basically, by setting the room thermostat(s) to a degree or so higher than ROOM OCC, they serve as high limits and can turn off the boiler demand should the room temp increase due to solar gain, cooking, occupancy, etc. Or in my case, with two zones, I have the indoor sensor in the zone that is more thermally stable. The other zone has more radiation and also more solar/cooking/occupancy gains. So it will periodically bounce off its thermostat.

Having watched the performance of this arrangement over the past 5 months or so, I'm really happy with it. Works great.

 

Interesting.

I recall that the original poster (in a separate message) was worried about running his boiler too cold -- because of condensation in the boiler.

This seems like an important issue that I do not understand yet. I haven't found a good explanation of this yet -- doing a search for "condensation in a non-condensing boiler" yields a lot of Mod/Con posts here and on google, and I haven't had time to refine down and dig.

I was going to ask about condensation problems (not knowing exactly what I am talking about, but guessing the gist of it) but then I see the answer to that question over at another forum where you write that your boiler "...has a built-in control and circulator that does injection mixing (basically internal primary/secondary piping). So the boiler core runs hot to prevent condensation, and you can run any supply temperature down to about 55F."

I looked at your indoor-sensor graph on another forum. That looks interesting. You've got me on wikipedia looking at "heating degree days"

My intention is to go slow and start-to-finish document the project, asking a lot of questions along the way so that I don't kill myself or my boiler.

 

xiphias has a boiler that has built-in protection against cold return temps.

Typically, you'd want to keep return temperatures for an oil fired boiler at 116° or higher and NG fired boilers at 135° or higher to prevent condensation.

 

Slow and steady wins this race. You're on the right track.

"Condensation" as we refer to it here means flue gas condensation in the heat exchanger and/or the exhaust vent.

Oil- and gas-fired appliances have differerent condensation temps, which depend on a number of things like humidity, excess air, etc. But in general, for oil it's about 110F, and for gas about 135F. (As Who said, in practice keep return temps a few F above these.)

What this means in a standard cast-iron boiler is that you don't want the boiler to experience prolonged periods of return water temps below these values, or the flue gases will begin to condense in the heat exchanger. The condensate has a pH of around 3-4 (acid) and will eat away the heat exchanger, the exhaust vent, etc. and the boiler will eventually fail in an ugly, expensive, and potentially dangerous manner.

There are ways around this, which include:

1) designing the boiler for condensing conditions by using stainless steel or aluminum heat exchangers. Modcons take this to another level by using the latent heat of condensation to achieve a few more percent efficiency. (There is also a new modcon that is made of very thick cast iron and has an interesting internal burner and fan arrangement that is designed to condense. Mestek RAY.)

2) providing for "boiler protection" by using a boiler bypass, thermic bypass, injection mixing, or other means. The tekmar website has a technical essay on this, IIRC.

3) limiting the range of water temperatures in the system, e.g., "partial outdoor reset" so the return temps are always out of the danger zone.

 

4) Buffer tank...

 

For boiler protection? Please explain. My mind's eye is fixated on a buffer tank adding volume to the system, which takes even longer to heat up and thus potentially worse condensation problems. (Like the dude with the gravity conversion who had 100+ gallons of water and a boiler that never saw >130. Or something like that.)

Not disagreeing, just need explanation. You are in all likelihood correct as usual.

First thread resuscitation and now thread drift. Ain't the internet grand?

 

One way to think of flue gas condensation is to use an analogy to 'dew point' ... which is basically what it is.

When the weather conditions outdoors are right, the humidity in the air will condense on cooler objects.

Since fuels are hydrocarbons, when they burn, the hydrogen combines with the oxygen and a percentage of the flue gas is water, in the form of superheated steam. Since fuels contain carbon, and may contain sulfur, and air is mostly nitrogen, some of these elements combine to form carbonic, sulfuric, and nitric acids which would dissolve in the water.

When these hot gases encounter a cooler surface, they form an acidic 'dew' on that surface.

Try search on "flue gas condensation", you may get more specific hits that way.

 

xuphias, actually the buffer tank in conjunction with a thermic bypass...

Maybe 2B instead of 4.

 

Xiphias, if I understand what you are saying -- are you suggesting that he control his OCC/UNOCC by hooking directly to the the UNOCC pin 13 on the 260, and not simply by controlling this through boiler demand pins 1-2?

Why is this best way to control the OCC/UNOCC? Does the Tekmar PID logic take special account of the OCC/UNOCC pin signal in building its logic state? Is there a down-side to having my existing programmable thermostat (which is already upstairs and does have adaptive learning of its own in terms of when to start sending a boiler demand signal) control the night set back by controlling the boiler demand signal pins (rather than, potentially, controlling OCC/UNOCC pins)?

(Sorry for jumping around a bit! Also, thanks all for the helpful explanation on condensation. The dew point analogy works for me and I'll do a search on that in the future. I'll probably end up asking questions on those posts at some point too. For any future readers of this thread -- like me again 2 months from now -- I found a/the Tekmar essay that xiphias refers to at http://www.tekmarcontrols.com/litera...robat/e021.pdf and that helps me understand this as well.)

 

Not sure if I follow. But in any event, the control should be wired according to one of the tekmar application diagrams

http://tekmarcontrols.com/literature/acrobat/a260.pdf

which show the OCC/UNOCC opening/closing a connection between terminals 13/14.

OCC/UNOCC is just a simple switch that tells the control whether it should use the OCC temp setting or the UNOCC temp setting. Typically, OCC is the (higher) non-setback temp and UNOCC is the (lower) setback temp. The control shifts the heating curve downward during UNOCC, and when the temperature at the indoor sensor reaches UNOCC, the control holds that temperature until you return to OCC again, at which point the system jacks up the water temp to recover from setback. After recovering, the control reduces the water temp to hold the OCC setpoint.

You could use just about anything to switch between OCC and UNOCC, including:

1) a simple switch

2) a simple programmable thermostat

3) a genuine tekmar timer

I use a LUX500 (Ace Hardware version). Simple programmable stat. Just set the temperature values really high to close the switch, and really low to open the switch. The progammable times on the stat then determine when you tell the 260 to go into and come out of setback.

Don't really know about the Honeywell (or whatever) adaptive learning stats with the 260. I also use LUX500s for the two space heating zones. They are simple. If the temp is below setpoint, they call for heat. I guess it's likely that a fancy adaptive or cycles-per-hour-based stat would eventually figure out what the tekmar is doing to the water temperatures and why the temp increases (or not) when the stat calls for heat, but I don't know. I'm content to let the 260 run the show and have the room thermostats serve as simple high limits.

 

Maybe what I was asking is something stupid. What I was wondering was -- which is better? :

Option 1: Wire existing upstairs night/day thermostat to 13/14, and feed constant 24V signal to the boiler demand 1/2 pins.

Option 2: Wire a LUX500 to 13/14 and, existing upstairs night/day thermostat to the boiler demand 1/2 pins.

Option 3: Wire existing upstairs night/day thermostat to 1/2, do nothing with 13/14.

(This might be a stupid question since I don't know enough about thermostats -- I'm assuming that my thermostat can provide 24V/0V as well as short/open -- I assume the former is how I use mine now, and the latter is how you suggest to control OCC/UNOCC pins.)

Assuming these options are all valid, then the advantage of option 1 in my particular setup is that I can control my OCC/UNOCC times without going to my basement (which is 3 stories and a side door away).

(In the last message I wondered if option 3 was worse than option 1 -- but I think you answered this in your last message as "Yes, it is worse." The reason why it is worse is that with option 3, at night, when it gets cold and upstairs thermostat generates the boiler demand signal, then Tekmar shoots for daytime temp and not nighttime temp.)

 

There are a bunch of ways to solve this problem. In general, though, your thermostat is not wired directly to the control. (I suppose it could, but haven't thought about it.) The thermostat is used to close a zone valve end switch or a switching relay that actuates a circulator. In the grand scheme, I think you don't want to simply have a constant boiler demand; there should be a means to control it on the basis of input (too hot, too cold, etc.)

But you're getting somewhat ahead of yourself, I think. The first thing you should do is figure out what range of water temps you'll need to satisfy your range of heat loss. i.e.,

1) start by doing a heat loss for your space. Free software at www.slantfin.com

2) figure out how many BTUs/hr your radiation puts out at various supply water temps

3) match the heat loss at various outdoor temps to the output values of the radiation at various supply water temps (e.g., using totally pretend numbers, at 30F outside your heat loss might be 20,000 BTU/hr. Your radiation can output 20,000 BTU/hr at a supply temperature of 110F).

Once you've done the heat loss, the whole exercise is simple to do in a spreadsheet like Excel. Basically, you're doing a heat input/output budget for a range of temperatures (say 5F increments from 60F down to your design temp which might be 0F depending on where you are). All of this is quite approximate (heat loss by Manual J method, which is what the slantfin-ware does, is often overstated by 20-30%), but it gets you in the ballgame.

Now you know the range of water temps you need, and this will dictate the extent to which you need to plan for boiler protection using some means described above. If your calcs show that you never need supply temps <140, you might be good to go. That would be quite unusual, I think. More likely you'll find that you could get by with a range of supply temps in the 80-150 range, in which case, you need to protect the boiler.

It's not quite as simple as slapping a control on there and pressing go.

 

I appreciate the advice. I've come to realize the likely need for boiler protection in the last few days, based on this thread, and have been reading about the various ways to do this in the Tekmar literature, and looking at various mixing controls for fun.

I actually did do a Slantfin heat loss analysis a few weeks ago. [BTW I do mean to do a proper introduction with photos and data in a month or so when I get to the point when I get some time to really wrap my head around the system. ]

Slantfin said 67k BTU/HR at indoor temp 75F and outdoor temp -5, which seemed right to me for Boston area. IIRC (I'm not at the property right now and don't recall exactly), my Bryant boiler is rated at 180k BTU/HR output. This oversizing is another issue -- but my impression and calculations from reading about short cycling losses is that there is a long payback for fixing this oversizing problem.

The last owner of my place put in a night/day thermostat, and I don't believe he added any boiler protection when he did that. So I may have some thermal shock or stress issues because of the night to day switch, especially in the shoulder seasons when the boiler doesn't need to kick on at night. (Once I get moved into the new place, I can really look at the piping picture. I can't take a look until some safety renovations are complete.)

I do have a question about what you wrote below, and this also gets to something that has been nagging me.

You say that I should "figure out how many BTUs/hr your radiation puts out at various supply water temps."

One question that I have is whether my huge cast-iron radiators are properly sized for their rooms (or inversely, based on what you write, what temperatures and flow rates I need to run in the system to make them properly sized). Is this also what you mean with your comment above?

If so, I have no idea how to do this right now. I imagine that I can measure the supply temperature at the boiler, and then measure the radiator temperature with my handy-dandy infrared thermometer. And then I can probably multiple this by the length and width of the radiator (or some such) (and somehow incorporate flow rate?) and come up with a BTU measurement for the radiators, and compare that with the size of the room? Any thoughts/tips? Am I being too simplistic?

It also seems to be that the convection properties of a room are details that are lost in the noise of all of these calculations. (Eg: the location of the radiators, the circulation of a room, perhaps aided by a fan both affect how quickly the room gets warm, and how quickly the boiler can be shut off.) At the end of the day, I suppose people chock it up to "close enough for a hand grenade" and just approximate.

 

To reply to myself on my own question (since the "edit" button seems to disappear a while after making a post), a bit of googling reveals this thread:
http://hearth.com/econtent/index.php...wthread/10220/
which has a link to a possibly useful calculator and an old book.

However, what I will no doubt find -- given the energy improvements (insulation, windows) that have been made to the property since these huge radiators were installed -- is that a lower water temperature is needed than whatever the boiler currently puts out, for a large range of outdoor temps.

(I may not need to go through the process of calculating target water temperatures since I should be able to rely on the control to do this for me? Perhaps this will let me turn down my aquastat minimum to 140 or something if it is not already there, but beyond that, I know that I could use boiler protection. What the calculation of desired water temp may tell me is how much energy savings I might get with a more complication boiler protection scheme like what I'll discuss below. Maybe I'll do it for fun anyway. )

The question then perhaps becomes -- how to boiler protect given what I can afford.

My open musing of the moment is: will any energy savings of a more complicated system justify the installation/run-time cost over a simple manual valve solution such as is used in the original posting?

For example, Tekmar seems keen at http://www.tekmarcontrols.com/litera...robat/e021.pdf on a variable speed injection system (though perhaps partly because it requires a more expensive control.) The advantage of that system is the ability to run my system loop at a lower temperature, and presumably the boiler will need to fire less often. (I'm concerned it may not be THAT much less often because the boiler aquastat has to keep temperatures up when system water returns to the boiler loop to make up water as a result of injection. Clearly, it takes less time to heat boiler return water when it is hotter, but I wonder if other factors (e.g., minimum boiler fire time) mitigate/eliminate potential energy savings by using the variable speed injector.)

I wonder how the (gas savings) - (energy cost of running 1 more system pump + occasional variable pump kick-on) - (amortized cost of more expensive control, 2 new pumps, any installation cost that I don't do myself)

compares with

(lower gas savings) - (any installation cost that I don't do myself) for the simpler job of putting in a manual valve ala the original poster in this thread for boiler protection control.

The thing is -- if I were in charge, I would call both scenarios "partial outdoor reset" since we're supposed to be resetting the supply temperature of the boiler. (Yes, people call the more complicated mixing schemes "full reset" because of the ability to set full system temperatures after a mixing device, but the BOILER MIN in my mind makes mixing systems a hack to emulate full reset for non-condensing boilers.)

The question I really have is how much time can I shave off firings with the more complex system -- I sort of doubt it is very significant (but feedback is welcome -- I have no experience in this area!) because of the need that boiler return water is hot enough to avoid thermal stress. I can see how the more complex system could give me greater temperature stability (comfort) in rooms. Greater comfort is good, but to the best of my knowledge the previous owner had no comfort complaints with the existing system. (I have yet to experience a winter in the property.)

As a final aside: I think for the first time I understand enough to speculate on the primary reason why a mod/con boiler has 95%+ AFUE (since AFUE includes cycle losses, etc) -- is it because there is no boiler min on these boilers? (I imagine there is other stuff as well, just as heat re-capture or some such.) Too bad the ROI is so high

Any and all thoughts are welcome!

 

Yes, sort of, but there is no boiler min _because_ of the capture of the latent heat in the exhaust stream... in a non-condensing boiler that heat that goes up the chimney comprises the 'lost' 15% or so.

 

milesjamie, I started going through your post above and quoting out pieces to reply, but it got too complicated.

First thing to understand is that it was not all that long ago (maybe 10 years) when modcons didn't exist in a widely consumable residential form. Yet system designs (e.g., radiant floors) that require low supply temperatures have existed for many many many decades, as has the desire to have a fuel efficient system. In a world of cast iron boilers, doing so means ensuring that the boiler is protected. There are a bunch of ways to do this, as you've read.

Reset, partial or full, technically refers to the target supply temperature, not the boiler temperature. It's called boiler reset because typically the boiler is controlled in some way. Again, it's only in the past few years that modcons have made it possible to fire a residential boiler to directly provide supply temperatures below the fuel condensation temperature. The reason modcons get 90s+% AFUE is that they are designed to fire to low temperatures for long periods of time at a low firing rate, and thus capture the latent heat of condensation released in the heat exchanger.

BOIL MIN is not a hack, it's a control convenience that allows an installer ensure that in the absence of a real boiler protection scheme, that the boiler doesn't suffer from condensation damage. If you want or need full reset, then you need to take appropriate design and installation steps. There are other reasons to use a minimum boiler temperature, such as to avoid reversing thermal exchange in a large concrete slab, or cutting down on system runtime if there is not functional heating output at lower temperatures due to the emitter type (fin-tube and fan-coil leap to mind).

If you want to retrofit a reset control on an older cast iron system, then a bit of homework is required, and as you note, some thinking about "how much do I want to spend relative to the potential payback" is certainly in order. Certainly a full repipe to primary/secondary with injection mixing, done by a pro, would run into some serious dollars. Worth it on an oversized, old boiler? IMHO, no. The next best thing would probably be a thermic bypass. That would still be some bucks, professionally installed. If you've got good plumbing skills, all of this stuff can be done DIY with proper thought and attention to design. This is a DIY site, after all. Plenty of us here to help!

If you are going to be in the residence for a long time (>7 years, it probably makes sense to look long and hard at a new modcon boiler. Particularly with cast iron radiators. You'd likely spend most or all of a typical year in the condensing range, and thus achieve some excellent efficiencies. The stories one hears about replacing a decades old oversized boiler with a new modcon typically involve fuel use reductions of 30-50%. It might be worth it to run the numbers and see what your ROI could be.

Design temp for Boston is 5-9F, not -5. Indoor temp is your pick but 70F would be more common.

 

Thanks for your thoughts. And sorry for writing so much -- can you tell that I'm interested in the project?

At the moment, I am leaning toward going a winter on this existing boiler, probably with an outdoor reset and manual or thermic bypass, to get a baseline for my own energy numbers.

I did look seriously into installing a mod/con. (For other readers, an AFUE payback table is here: http://www.aceee.org/consumerguide/heating.htm ) I got three quotes with the intention of doing it before realizing both that my existing boiler (at 79%) was more efficient than I had realized, and that I had to allocate renovation dollars to some non-HVAC safety issues first.

If I pessimistically back off that boiler efficiency number a bit for oversizing and assume an AFUE of 65% and optimistically bring that to 95% with a mod/con, then the yearly payback is $32 for every $100 I currently spend on heating gas.

I hesitate to use the previous owner's gas bill (which I have). I estimate his usage at at $2000 a year for heating-only fuel. But I'm not sure that will apply to me because of energy upgrades I have made, and because he had an odd lifestyle with travel and roommates. But assuming his number, that is $640 in gas savings. My lowest bid to install a mod/con (with simple payback) puts it at about 12 years for ROI.

I plan on staying here 7 years. I can make the argument to myself both ways on this -- lowering the carbon usage is good, and if I sell the place there might be a small difference in property value (7yo vs 27yo boiler), though around here location trumps all.

Right now, though, it comes down to a short-term question of $$, even with the possibility low-interest loans to fund it. (With my current financials I effectively would be borrowing money to pay back the low-interest loan.)

Also I take back my hack language as too strong. Though I was reacting to the need for a mixing system to approximate a heating device that can offer supply temps below 140 (and wasn't referring to the need for boiler min.)

If I do go forward with the manual or thermic bypass, a good part of me wants to try the diy method ("brain surgery! that sounds fun!"), and then hire in a trusted heating contractor I know at the end to check the work. But I have never done any plumbing before, and this might be a stupid idea. (I am comfortable with the idea of propane soldering, getting dirty, and going very slowly, but don't know enough to handle the situation if something really goes wrong.)

Thanks again,
-mj