Short cycling on DHW side of Quietside DPW-120

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  #81  
Old 09-29-14, 08:40 PM
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I'll ask tech support about the sensor.

...as long as the DHW heat exchanger is large enough...
Did the "70 degree temp rise at 3 gpm" info give you anything useful regarding the size of the secondary/DHW heat exchanger?
 
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  #82  
Old 09-29-14, 09:06 PM
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I believe this means that at a boiler water temperature of 180f the domestic hot water will rise 70f . The heat btus must balance input from boiler to DHW or the boiler water temperature will continue to rise until the high limit shuts burner off. the manual sets out DHW min. flow 0.5. DHW GPM at 50f in 100f out at 0.7GPM : 50f in 110f out at 3.7 GPM : 50f in 120f out at 2.6GPM : 50f in 130f out at 2.3 GPM .
 

Last edited by saves; 09-29-14 at 09:27 PM. Reason: more information
  #83  
Old 09-30-14, 12:39 PM
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Same boiler, same problem, it's flow rate

I have the exact same boiler. Your problem is not enough flow rate, resulting in not enough heating load to keep the boiler firing. I have the same boiler with the exact same problem. Engineering flaw in my opinion. I noticed the newer boilers from quiet side will modulate down farther than ours. I will look at specs again, but the lowest BTU input for domestic hot water on my boiler is 47,800 btu.
Mine short cycles if the wife showers (she takes colder showers, not enough delta T)
Running the sink faucets, only 1.5 GPM total flow, not enough flow to keep the boiler firing constantly.
Summer time I have more short cycling problems (incoming water is too warm, not enough delta T).
Bad engineering, as in order to save money on water and heating, a person buys a high efficiency boiler. But with a 1.5 gpm shower head, they short cycle, which can't be good for heat exchangers. So you use a 2.5 gpm shower head (offsetting some of the savings). I will have to up my shower head flow to 3 gpm so when the wife takes showers it won't short cycle. My showers are hot enough I can get away with a 2 gpm shower head.
Do a lot of nasty math, and I'm sure you will find your problem.
Sorry about the book, but I'm hoping that somebody can come up with a more elegant solution than wasting water to get the boiler to operate properly.
 
  #84  
Old 09-30-14, 01:13 PM
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The math

Our boilers have 47,800 btu/hr minimum input @ 85% boiler efficiency. That would be 40,630 btu minimum for heating water. It cannot supply any lower than that, it just shuts off (short cycles). I don't know your specifics. But for easier math, arguments sake, lets say you want your outgoing hot water temp to be set at the exact temp you like in the shower (so when you shower all the water is coming from the hot water tap, nothing from the cold). It helps to know your shower temp and incoming water temp for this test. And GPM flow at the tap your testing.

BTU/HR = TEMP RISE X GPM X 60 MINUTES X 8.34BTU/IB
Let's put my wife's numbers in here. She showers at 90 degrees. Incoming water this time of year is 58 degrees. Shower 2.3 GPM.
BTU/HR = 32 X 2.3 X 60 X 8.34 = 36,829 btu/hr.

My boiler doesn't modulate that low, so it has to cycle. If I turn on the hot tap at the bathroom sink a bit while she showers, the unit doesn't cycle. Try this first, I wouldn't undo all of your plumbing just yet. My boiler has no storage tanks, and the only way to cure the cycling problem is to increase flow, or take hotter showers. It must be too hard/expensive for these companies to make a modulating gas valve that can go from 10,000 BTU to 200,000 BTU. Boilers are made and marketed to women that want to fill a whirlpool tub with really hot water really fast. They are not marketed to guys like me that would take a 1.5 GPM pressure washer type shower to save on water/natural gas.
 
  #85  
Old 09-30-14, 01:19 PM
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BTW I'm not an installer

Dont' take this as gospel. I'm not a professional installer. Any errors feel free to correct. I'm trying to help and get help myself.
Money wise, it has got to be cheaper to buy higher flow fixtures and burn up more water/gas than to short cycle our boiler, which can't be good for longevity.
 
  #86  
Old 09-30-14, 02:37 PM
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Radiantfloored wants to use his tank and as I have stated, my solution is to disable the thermistor #339. rd543759 were you do not have a tank the sensor is used to control the DHW temperature and will result in burner short cycling unless the flow is high enough to keep boiler water temperature low enough to keep burner on. If you pay for the volume of water used and of course the extra fuel burned probably no savings to be had.
 
  #87  
Old 09-30-14, 03:06 PM
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I have another idea to keep burner on when there is no tank, which is to install another plate heat exchanger between the tap and the boiler. DHW cold would be piped to this new exchanger tap to the same side of heat exchanger. A pump installed between the new heat exchanger and the boiler cold in , the hot out from boiler piped to the new heat exchanger. A flow switch installed in pipe going to tap would turn new pump on and the original flow switch would turn boiler on. The sensor #339 would have to be moved to a new well in the hot water pipe to taps .This would allow boiler water to rise to high limit 180f. When DHW reaches the temperature as set by DHW boiler control the burner would shut down .
 
  #88  
Old 09-30-14, 04:39 PM
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OK,
So, lets assume 47,000 BTU.
since we are trying to charge a storage tank the delta t will be much lower.
if your aquastat in the tank is 120, and lets say your DHW setpoint is 140.
Makes nice easy math.
you going to need a flow rate of 4.7 GPM thru the DHW side.
Should be doable, not sure a 008 will do it.
I have yet to DL the install manual, some may have and they can look at the heat exchanger head loss at 4.7 GPM and see how many feet of head it consumes.

When you incoming water is close to 60 degrees, yeah it's gonna short cycle on lower flows.

You can play with the delta T and get some different flow rates, but you do have a limit to the DHW temperature. If you could raise the setpoint to 160 or 180 F it would perform better and heat your tank faster.
 
  #89  
Old 09-30-14, 09:09 PM
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rd543759, great to hear from you. I've been hoping to get some input from someone who could corroborate what I'm seeing as normal or faulty. Although I suppose it's starting to seem as though "normal" means "performing as (poorly) designed". Thank you for the insight and suggestions. And no need to apologize for post length. If yours was a book, several of mine amount to War and Peace.

Have you tried calling for DHW with DIP switch #5 ON or would that not matter since the minimum btu input for this unit is too high for moderate DHW use whether it's actively modulating or forced to min fire?

saves, thanks for the explanation. Also, I trust your suggested modification is well thought out and valid, but again I'll have to get a better handle on it before I can seriously consider it as an option.

TOHeating, this is along the lines of the test you initially proposed, yes? Thanks for the additional break down. My supplier initially suggested a 3 speed B&G NBF-25 as a replacement. I wonder if that would provide sufficient flow. Whoa...pricey.
 
  #90  
Old 09-30-14, 10:25 PM
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RD, what is your DHW set point at.
Do you have a mix (antiscald) valve installed ?

If your setpoint is high, and / or you have a mix valve installed your problem may be flow issues due to the mix valve mixing down the hot water with cold and sending it up to the tap. This of course reduces the flow thru the DHW circuit of the boiler.

Try setting the setpoint at say 110 F, see if it will still short cycle. The idea of course is to get as much water thru the heater as possible.
Keeping a lower DHW temp will force you to use more at the taps, plus it's cheaper to produce 110 F water then 140F water.
It looks like the min flow rate thru the DPW120 is .7GPM. They do rate the delta tee at a starting water temp of 50F. Since your much closer to 60F this does have some impact.

Radiant, as you know, the manual sucks. There is little info about the heat exchanger other than its a brazed plate. They are restrictive, but not horribly so unless your trying to flow the crap out of it.
Shop around for pumps.
Try to find a bronze or Stainless Steel (they seem to be cheaper than bronze theses days)
Taco 009,0014,0011
Grundfos UPS26-99 three speed.

Theses are going to be overkill, but they will move the water thru the heat exchanger. I wish I could just lend you one to see what the difference was.

Probably not a ton of help other than reinforcing what has already been said
 
  #91  
Old 10-01-14, 06:14 AM
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TOHeating would you look at the manual and parts list and make a comment on eliminating the sensor 339 as a solution when heating a storage tank thanks.
 
  #92  
Old 10-01-14, 06:25 AM
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Saves,
While eliminating that sensor (removing from the pipe) might effectively (and likely would) allow the heater to produce hotter water, it would likely only offset when the boiler shut down. Instead of short cycling, I suspect it would now hit the internal high limit and lock out on an error with a hard lockout. Then someone would need to reset the unit. Removing the sensor may also add another issue if the boiler looks at that sensor to see a rise in temp as the unit fires, and it might not modulate to low fire anymore.
I do not know how the software is written, so it's impossible for me to know exactly what the outcome would be, but those are the possible problems I would for see.

Secondly, removing that sensor on a permanent basis would void the manufactorers warranty, it would leave the homeowner liable in the event the high limit failed and did not shut the boiler down. If there was ever a fire due to the heater, the insurance company would have a large "out" to refuse the claim.

As a tech, I might pull the sensor off the pipe to see what the heater did, and use that information to try to solve the problem. I would never leave the house with it removed, nor would I instruct the home owner to even think about removing it. It really all comes down to liability, right know the product is tested with all the sensors in place and functional, and the manufacturer is liable for damages if the product fails.
 
  #93  
Old 10-01-14, 06:44 AM
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TO thanks for your incite , I would suggest as I did before that the company teck. be consulted on removing the sensor from its well. I would, like you if I were there remove it to see what effect it would have on cycling. The Viessmann that I removed the combe unit was set up to run an indirect so there were no control problems.
 
  #94  
Old 10-01-14, 07:01 AM
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Doing more testing tonight

I believe last year I did drop the temp to 98 or 110F. Ran shower tap wide open. Still had the same problem. I will try it again tonight to see if anything different happens. I do believe that no matter what a guy cobbles together, unless you can get a load of 40,000 BTU's or more on the boiler, it will short cycle to keep from overheating the heat exchanger.
I will look at dip switches again, I will set to minimum fire and see if I get anything different.
I'm not sure how to reset high limit temperature, I will check in the manual, but I don't want to get too hot. I'm positive that high limit is there for a reason. And eventually, even with a temp at a higher limit, without enough load the burner is simply going to short cycle at a higher outgoing hot water temp.

Thanks for all the replies.
Does anybody know if a different gas valve can be put into these boilers, for low cost? Let's say instead of a 40,000-120,000 BTU range … maybe a 14,000-100,000 BTU range. If a person could do this, would it simply mess up the microprocessor controls, electronics, etc?
 
  #95  
Old 10-01-14, 04:10 PM
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Gas valve alone is not going to help.
Actually the gas valve has nothing to do with firing rate.
The burners and heat exchanger determine the firing rate alone.

Many sized boilers can use the same sized gas valve.

And it's true, if you have under a 40,000 btu load there is nothing you can do in the way of piping or anything to keep the temperature from raising if your firing rate is higher than the load. Simple physics.
 
  #96  
Old 10-01-14, 07:11 PM
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If the boiler water is allowed to go above the 140f the transfer of btus into the 50f DHW is greater and hopefully the two loads will equal out before the boiler water reaches the high limit .Good Physics?
 
  #97  
Old 10-01-14, 07:18 PM
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Concept shows you're thinking...

but I don't believe the two lines will converge.

In other words, it's doubtful that the modest increase in driving force attained by the greater delta between the entering water and the boiler water will gain enough to overcome the excess BTU produced by the burner, even at low fire.
 
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Old 10-01-14, 07:30 PM
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Thanks , I had a stroke Sep.18 on my Harley at 50 mph and passed out hit a guard rail ,flew through the air missed two power poles and guide wires .Went dancing Sat night had a couple of beers . I have too drink more to stay healthy in my opinion.
 
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Old 10-01-14, 07:43 PM
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Thanks , I had a stroke Sep.18 on my Harley at 50 mph and passed out hit a guard rail ,flew through the air missed two power poles and guide wires .Went dancing Sat night had a couple of beers . I have too drink more to stay healthy in my opinion.
Oh my!!!!!!

At least your still with us................

I can relate to the crash, but not too the stroke part....

I crashed twice in 30 yrs...
 
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Old 10-01-14, 07:45 PM
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Water flowing at one gal/min will deliver 10000btu /hr. of heat at a 20f drop in temperature . Therefor 5 gal/min at a delta t of 100f will deliver 4166 btu per min. It looks like the burner would have 10 min. to deliver 41660 btu.
 
  #101  
Old 10-01-14, 07:58 PM
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I have too drink more to stay healthy in my opinion.
You'll get no argument from me!

Glad you survived, stay healthy, ride safe!

Not gonna tellya about the time I flipped and rolled a Triumph Spitfire and walked away...

Therefor 5 gal/min
Not gonna check the math, but when was the last time you drew hot water at 5 GPM? I'd say that's an EXTREME amount of hot water!
 
  #102  
Old 10-01-14, 08:07 PM
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I am thinking of the pump that is circulating the water from the bottom of the tank through the plate heat exchanger and back to the tank and not the draw to tap.
 
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Old 10-01-14, 08:35 PM
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Ohhh... I haven't looked at the curve for the 008, maybe that would be a good idea.

Didn't TOHeat recommend a larger pump?
 
  #104  
Old 10-01-14, 09:20 PM
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rd543759, I'd like to hear the results of your tests. I don't get a chance to do much during the week and haven't gotten around to trying the min fire test, amongst others. Assuming you have the same control unit (DSF-100) setting the upper limit of the DHW is easy. Just select the "tea kettle" DHW button on the right and then use the up/down arrows to adjust the temp (range 98-114F in 2 deg increments, then 120-140F in 10 deg increments). The space heating side is the "radiator" button (range from 122-176F in 2 deg increments).

saves, wow...that JUST happened. It's a miracle you are even able to use a computer at this point. Congratulations on surviving and here's to a speedy and complete recovery. Thanks again for the input.

TOHeating, my supplier offered a trial of the B&G NBF-25 to see if it solved there problem. Maybe I should take him up on it?
 
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Old 10-01-14, 09:25 PM
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Ohhh... I haven't looked at the curve for the 008, maybe that would be a good idea.

Didn't TOHeat recommend a larger pump?
Sry I am slow to the party... But post #35 I said we use 009 minimum... but sometime a smaller pump works depending in unit...

IMO again it seems like pump too small...

That was post #35 here we are on #102...Ugggggg
 
  #106  
Old 10-02-14, 07:32 AM
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A few test results

My dip switches are set correctly, I didn't mess with them.
Now, in theory, if we wanted a 98F shower, we could set the outgoing temp to 98F and run full hot tap for our shower, or we could set the outgoing higher all the way to 140F, and use more cold tap and get the same shower temp.
At the SAME shower temp, my boiler doesn't short cycle as much if I turn the outgoing water temp to 98F and use more hot water tap to get the desired shower temp.
I think the boiler can modulate temp easier with higher flow rate, less temp rise. I wonder if it even modulates as low as it can with the outgoing temp set to 120, 130, or 140. Maybe to get the boiler to fire as low as it can, we need to ask for lower outgoing temps?
I haven't tried 114F yet. I know that I was finally able to get my wife to take a full shower with no cycling when I set the water temp to 98F. Now this gauge isn't accurate, the hot water actually coming out of the boiler/hot water tap was well over 98F, but it did work. I will actually measure the temp of the water tonight. I took a shower, which I like hot, about 105F, and I had plenty of hot water. If the boiler actually put out 98 degree water, the shower would have felt cold to me, but I think the temp gauge we use is just a VERY rough estimate of outgoing temp.

Its too warm to use my radiant floor heat, but I don't remember having short cycling problems, my pump has 3 speeds, I use the middle speed. Will give updates on this as well when the weather gets cooler.
With radiant heating, it could have the same problems if you try to close or reduce flow through zones that you don't want to heat as much. Example: in order for my boiler to not short cycle while heating my house, I might have to open up my garage zone and keep the garage warmer than I need it, in order to have enough total GPM to keep the boiler from cycling since I have a low temp rise.
 
  #107  
Old 10-02-14, 07:53 AM
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Concerning Poster's original problem

I think it was said before, your storage tank brings water into the boiler that is too warm, and there isn't enough flow rate to put enough load on the boiler to keep it firing constantly. If there is an easy way to bypass the tank, try it and see what happens.
You could also set your outgoing boiler temp to 98F, see if it helps? It helped my situation, but I have no storage tanks. It is a very simple setup, cold city water comes into boiler, and hot goes out to a manifold that goes to the bathrooms/kitchen.
 
  #108  
Old 10-02-14, 07:58 AM
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I just reread your original post/looked at picture. I know its messy, but if its plumbed with pex you can probably bypass it without too much headache. Do you have any pex tools or was this installed professionally. I have enough Pex tools that I can replumb stuff without a lot of cost/hassle.
If you have to call/pay a professional, I can see why you don't want to tear the plumbing all apart, especially if it might not even cure your problem.
 
  #109  
Old 10-02-14, 08:14 AM
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Just found a technical manual, that didn't come with my unit

I did an online search and found a technical manual that didn't come with my unit. I would have been very handy and explains everything we've been talking about. They have minimum flow rates at different incoming water temps, they talk about short cycling problems and possible solutions.
An expansion tank is built in, so you should be able to bypass your tank without damaging anything.
If you don't have this manual, do a google search and download DPW 120 installation technical manual.
 
  #110  
Old 10-02-14, 08:36 AM
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There is a valve between the cold inlet and the tank. Closing this valve effectively bypasses the tank and that has been tried with no joy..............
 
  #111  
Old 10-02-14, 09:01 AM
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Yep, I see that valve now in the pic. What I would try, is close that valve, set your temp to 98. Turn your tub on full hot, and see if your boiler still short cycles. The storage tank we both need is in Quietside's technical installation manual, but I can't find it for sale anywhere. I noticed their storage tank has more inlets, it must have a dip tube, which is probably the key. They claim it is sold to help eliminate short cycling.
My guess on how it works. Correct me if I'm wrong here. Let's say you have a small heat load, it takes hot out of the storage tank only, and cold water refills the tank. Once the water temp in the tank gets below a set temp, the pump in the tank turns on and recirculates this water through the boiler at a VERY HIGH RATE, keeping the boiler from short cycling. The boiler only runs when the tank demands it, not when the taps demand it. And when the tank finally demands heat, it has a large enough pump to keep from short cycling.

The original posters set up is close to this, except I think when he opens a tap the flow goes through the boiler as well, which fires it, whether its a large load or a small load. So he is drawing hot water from both his storage tank and his boiler.
 
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Old 10-02-14, 09:35 AM
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Got the day off, sorry for all the posts.
The diagram for the Quietside storage tank, and the brochure, shows 4 taps. The OP's storage tank setup has 2 taps. The quiet side tank only delivers hot water to the faucets from the tank, not from the boiler directly.

Is there a way to plumb up the OP's setup so the boiler water only goes to the tank, and not the faucets? I'm looking to do the same type of setup with a simple electric water heater for a storage tank, but I can't seem to sketch-up a way to make it work.
 
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Old 10-02-14, 10:38 AM
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SORRY. Just saw that you already tried Tank Hot going directly to Boiler input side. The specs on your pump show good flow rate. If you want to keep the tank system, last resort might be to set aqua stat to not run pump until 110 degrees, this would give you a bigger delta T. I know there is a limit to how low you can go before you get bacteria problems in a water storage tank that isn't hot enough.
 
  #114  
Old 10-02-14, 10:56 AM
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I think our boilers also use water from the cold input to mix with the heated water from the exchanger to get our set temperature. Maybe if your cold line coming from the storage tank is too warm, the boiler mixes the water, but it's still not cool enough, so the boiler shuts the burner off to keep from having outgoing water that's too hot. I believe it does this to even out the outgoing water temperature.
Maybe this is also a possible culprit? I checked the specs on your pump, and it looks like it should flow plenty. But I guess you won't know for sure unless you put a flowmeter on the line, to find out exactly.
 
  #115  
Old 10-02-14, 02:39 PM
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Is there a way to plumb up the OP's setup so the boiler water only goes to the tank, and not the faucets?
I've been trying to come up with a method, but unable so far.

This is the 'hydraulic separation' that I mentioned a while back, meaning that domestic water flow won't affect flow through the boiler and vice versa. Two separate flows.

The QS tank with the four ports achieves this goal.
 
  #116  
Old 10-02-14, 03:30 PM
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Trooper is there not hydraulic separation now when the tank is in play. The boiler water is kept separate from the DHW by the boilers DHW plate heat exchanger . The tank with its water at 120f is Hydraulic separate from the water in the boiler plate heat exchanger due to the extra resistance of flow of the 3/4 inch piping resistance to flow through the plate heat exchanger as compared to the ease of flow from the heated water of the tank .
 
  #117  
Old 10-02-14, 04:22 PM
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When domestic water is drawn there will certainly be MUCH more flow through the tank than through the boiler, but there will be SOME flow through the boiler. It's a parallel path so there has to be a 'split flow'.

When the pump is running with no domestic being drawn, all the flow has to be between the boiler and the tank, but when the pump is running AND domestic is being drawn, the flow will again be split.

So no, by definition, there is no complete hydraulic separation.

Separation means just that... separate flows and that can't be achieved when there is 'common' piping.
 
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Old 10-03-14, 07:54 AM
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Thanks trooper, I was wondering about that. Seem to explain it perfectly. To make this setup work great, we probably need the Quietside tank built to work with this, which I can't seem to find. I'm sure there's another company that makes a tank to work the same way.

What I've been doing. Set my temp to 98F. I can run my bathroom faucet tap wide open on hot, and the water temp will slowly rise, because I still don't have enough flow. But I need to run the sink for at least 90 seconds before it will cycle off. Maybe as much as 2 minutes. I still don't have enough flow, but when set at 98F it is surely firing lower than when I had the outgoing temp set at 120 or 130F. It would short cycle a lot quicker at those settings. At 98 the wife never short cycles the system in the shower. It runs nonstop. The ONE problem with this is that at high flow rates, it will actually deliver 98F, like when filling the tub. So the baby's water feels lukewarm or warm. It wouldn't be hot enough for me if I wanted to take a bath. With the new style bath faucets, I can't reduce flow to increase heat of the water, I need to increase my boiler temp to 102F.

I don't see myself paying for a pump, additional tank, etc. I will use my bandaid approach until my boiler craps out, and then I will spend the money on a HE tank type heater. With a tank I will be able to use lower flow rates at fixtures, hopefully saving enough water and overall heating to offset the lower efficiency of the tank type heater. I'm sure with enough math I person could figure out if it would save or cost money. 2.5 GPM showers at 85% efficiency, vs 2 GPM showers at 75% efficiency, etc
 
  #119  
Old 10-03-14, 08:09 AM
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These companies are hurting themselves by not explaining these little gremlins that you get with boilers. People install them, they don't act like traditional tanks, and people say they are "JUNK" and get a tank installed. I love the efficiency, it's just too temperamental for most people. I can change my habits from old, but 90% of people absolutely will not.
Quietside should have stated that low flow fixtures will cause short cycling. HE washing machines won't work well with the boiler. HE dishwashers won't work well with boiler. The storage/recirculating tank should be on the brochure as a highly recommended item for use with newer appliances, low flow fixtures. I found no such info when I purchased my boiler.

Another problem is without a tank, at low flows it short cycles, but at a high flow, it doesn't deliver enough heat to keep the shower hot while the washing machine is filling. These boilers should be advertised as LIMITED FLOW RANGE BOILERS, say 2-4 GPM. If you are between that range, you are golden. And also make up your mind… do you want cold water or hot water. You can only get warm water in the shower without short cycling. You can get it at your faucets but the boiler will kick on and off constantly.
 
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Old 10-03-14, 08:14 AM
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I think they hide this info because by the time a consumer looks at the cost of the boiler itself, then the tank system, the stainless steel pumps, plumbing, possible labor if hired install, it adds up to $3000 in materials alone.

Well if you're burning natural gas, how long will it take to recoup that, if you can get a standard water heater for $400, run lower flow fixtures (which is nice if you live in a city where you get charged for water) so you save on water/sewer costs.

I'm sure my payback period will be close to ten years, and with all of the short cycling, will it last that long? If it does, I will probably just break even, except with a HELL of a lot more headaches and head scratching, and wife complaints
 
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