Replacement boiler sizing help

Firstly, I have to say THANKS to all the regulars on this forum. I've been reading for days and days, trying to educate myself as much as possible. Today I registered to ask some q's. It's going to be a long 1st post - trying to get it all in here...

Intro:

1. I'm in Milwaukee, WI. Design temp: -4 deg F
2. 1400 sq ft 1.5 story bungalow. Built 1924.
3. Wall composition: Brick exterior, air space, sheathing, stud framing, lathe, plaster. I'd say about 1/3 of the exterior walls are insulated - only what we can reach from inside the attic.
4. LOTS of windows: 29 single pane double-hungs w/high quality storms. I've rebuilt and re-weatherstripped all sashes. We're keeping them for charm - the uppers have orig leaded panes.
5. Cast iron radiators on 1st and 2nd floors.

Existing boiler: natural gas Burnham "Series 2" 205
From the data plate:

1. Mfg Date: 1983
2. DOE Htg Cap: 107,000 BTU/hr
3. Net IBR: 93,000 BTU/hr
4. AGA Input: 130,000 BTU/hr

Burnham says their current 205 is 80% AFUE ... I doubt my 26 year-old is still that efficient, if it ever was.

I'm guessing the house was originally steam. The piping is gigantic, and has reducers to mate with our existing boiler.

I DIY'd a heat loss calulation using the Build it Solar online calculator. I *think* what these calculators want is 1) surface area having unconditioned space on the other side, and 2) the R-value of these surface assemblies. My combined 1st & 2nd floor heat loss came to about 113,000 BT/hr. Since I don't know what's inside some of my walls & ceilings, I estimated low on R-value for those assemblies.

Like many home owners, I'm interested in the federal and state tax incentives for the purchase of high-efficiency heating and DHW-heating systems. Though they're both old, my existing boiler and hot water heater are working fine. But, if I replace them now I get $1500 back. If I wait for them to die there may not be such incentives.

I do not expect new appliances to ever "pay for themselves" in reduced heating costs. But new appliances *should* make the house more sell-able than 20+ year-old equipment, when we need to sell it (within the next 5 years).

Additionally - and this is what started this dream/project - our chimney is disintegrating and leaks slightly into our attic. I've got it temporary patched together, but it's a time bomb. Our current boiler and water heater vent into the chimney. Instead of paying for a chimney rebuild, I'd rather replace the boiler and water heater with direct-vent appliances and just remove the chimney.

I recently got a quote for a Buderus GB142/45 + indirect DHW system. It was more expensive than I was expecting, though not un-affordable. After reading a lot it seems the Buderus is a very high quality boiler, and the quote was fair.

However, I question the /45 model sizing. The dealer measured the dimensions of our radiators - which, from reading this site, is not the correct sizing method. Did not do a heat loss calc. Our current 80% (at best) efficient boiler is a 130k BTU/hr input boiler. Does moving to a 92%+ efficient 160k BTU/hr boiler make any sense? I would have thought we've move to a lower input boiler... Maybe the extra because of the indirect DHW?

Also, based on lots of reading here, I'm starting to question the aluminium mod/cons. It seems like too much is left to the installer's ability, and I'm extremely risk-averse! I'm waiting to hear back from TT on who installs them in the MKE area. I'm certainly more comfortable with stainless. But, honestly, I'm thinking a nice cast iron boiler might just be in my future instead of a mod/con. I'm looking closely at the Burnham Revolution and Weil-Mclain GV.

I'm going to call around for quotes on the Revolution and GV. I'll ask for rates on a pro heat-loss calc as well.

Anything I should be considering that I'm not?

Again - THANKS for such an awesome resource!

 

If your current boiler is heating the house with no problems, then your heat loss would be 107,000 BTU or less. Having the DHW set as priority would not require a larger boiler because the boiler will be either heating the house or the hot water, not both.

Your house sounds very similar to mine. The system may have had gravity system, not steam. A mod/con may not be the best choice for what you have. You won't get the highest efficiency from the boiler unless it is condensing and that happens when the water temps are below something like 125F. Given your location and your cast iron radiators, this probably won't happen very often.

I wound up choosing the Burnham ES2 over their mod/con based on that and the higher maintenance cost. I'm not sure about side venting for the ES2. There is some discussion of this in another thread.

 

Wow Long post.

I say go Triangletube Solo 110, or even the Excelllence with the built in indirect. You need to weight your domestic hotwater needs though to ensure you are not going to run out of hot water.
Heatloss calcs are hard, complex and it's usually a three day course with an exam at the end of it. It's hard to get online calcs right, as it's not always as simple as they make it out to be.

My first question would be, currently does your boiler provide enough heat during design conditions? Have you ever noticed your boiler running 24/7 during January / Febuary ?

If the answer is no, then your current boiler is at least near the right size but still oversized.

The Solo 110 has an input of 110, and depending on your heat emitters it should have enough HP to heat your house.
If it's cast iron emitters sized at 140, then you should be about 95% eff. which would be 104,500 BTU out.

I am not sold on aluminium blocks, if the water is not treated and monitored every year you could be in trouble. The condensation is said to build a protective coating on the block, therefore if the boiler is not properly cleaned then that coating may be at risk.

You house was likely a gravity system, not steam. The big old round boilers had no pumps and relied on convection to start the flow to the rads, hence why the pipework is never level.
The only benefit to this was some nice boiler protection. By effeicency forgetaboutit.

 

You sneaked in past me :-)

Remember, its return water temp that governs condensing.
I can run a setpoint of 160F and bring it back at 120F and condense the boiler all day long. I have designed systems to run at a 40 degree delta tee and the customer reported outstanding fuel savings vs a steel boiler.

This was in Southern ontario, where is does get cold. Not in the city.

 

I'm no expert, so I really don't know. I brought my choice to the forums at the time and they helped me make that choice.

It's damn near impossible to find someone to actually design a system appropriately. I had a few people come through to do estimates and they didn't even want to do a heat loss. The guy I wound up using is a good plumber, but he didn't seem to be that good on the design side. If I came across this forum a year ago, I may have just done it myself.

 

Thanks drooplug @ TOHeating!

Gravity system, huh. All the radiators have separate ins & outs - does that help narrow it down? Ie, would a gravity system have separate in/out? I though it just "trickled back down" once cold? Or maybe that's steam.

Either way, it's forced now!

To answer some of the questions:

Yes, our current boiler is heating the house with no problems. And we (just two of us) have plenty of hot water. I replace the d@mn relief valve every other year, but that's my own fault (it leaks after testing, or I bump it or something and it starts drip drip dripping). And we have it cleaned/serviced every other year - not as a rule, but that's just how it's worked out. We get a small amount of air entering the system. I bleed at the start of the heating season and usually somewhere in the middle (when the upper floor rads start to gurgle). The boiler feed regulator doesn't work (it's stuck open, thankfully), and I usually have to manually add a little water at the start of the season. This is most likely how we're getting air into the system.

When we bought the house, the 2nd floor radiators were EMPTY! The previous owner did almost zero maintenance on anything. Including the heating system. Most of the work we've done to the property so far is an attempt to un-do the previous owner's transgressions.

We've lived in the house for just under 5 years, and the boiler has never run 24/7. In the depths of winter, I'd say it cycles off for as little as 15 minutes before re-firing. I don't remember exactly, as I've never really paid attention before. But I think I'd notice if it were running continuously, because I can hear the circ when it kicks on.

I'm going to re-do my heat loss calculation. I now realize I forgot the exterior brick in my wall assembly r-value.

The installation manual for the ES2 does not show side venting. In fact, I think we'd have to add a steel liner to our existing chimney to use the ES2. But, the Burnham Revolution and Weil-Mclain GV series can side vent.

Re: the difficulty in finding someone to properly design the system ... Burnham's web site has a dealer/installer search that's really nice. There are dozens of installers within 15 miles of our location. I wish Triangle Tube had a similar search feature.

This has been a steep and fast learning experience so far. Given the expense of all this, I really want to get it right and get the best value for our situation.

 

I had one guy near me. I called him up and he refused to come out to me because it was too far. I guess the 30 minute drive was too much for him.

You should call Burnham and see if you could side vent the ES2. It may be possible.

 

LOL. Last summer I did a lot of landscaping, and had a about 4 cu yards of left over washed stone that I was trying to GIVE away. Like, for free. When people called about it and learned I was ~30 minutes away they spazzed.

The Buderus dealer who quoted me was awesome. Came out at night for the estimate, spent a lot of time explaining things, answering questions, etc. Then again, he's only 3 miles away...

Good idea re: calling Burnham on side venting the ES2. If we can use the ES2, it'd be great. It appears to offer the best combination of features for what I want (cast iron heat exchanger + modern controls).

The stainless TT Solo 110 is roughly 1/3 more expensive (per pexsupply.com pricing), but this would be more than offset by the federal tax credit. I'm going to call TT this afternoon to locate an installer.

Any particular questions I should ask dealers/installers to "weed out" the less-competent? Maybe: how many of these have you installed? How many per year? Can I get references from those customers? Etc.

Are those unreasonable demands?

 

The amount of radiation or water volume has nothing to do with the boiler size.

 

I assume this is a response to what I wrote in my initial post re: the contractor measuring my radiators? Yeah, I agree. I gather from reading here that the only accurate way to size a boiler is to calculate heat loss.

In other news, I found two local Triangle Tube dealers via TT's Milwaukee distributor. I have calls in to both of them. Finger are crossed.

 

Just got a quote for the TT Excellence, which looks like an awesome value. I'm totally sold on TT, and the bid came in roughly 25% under the Buderus (though I think the Buderus was oversized).

So - new but similar question re: DHW. The Excellence claims to supply 180 gph (3 gpm). A single low-flow shower head will eat about 2.5 gpm, right? We only have a single bath now, but have tossed around the idea of adding a 2nd. Is the Excellence going to keep up? The contractor says YES, no problem. But the math has me doubting it.

So should we go Solo + a larger indirect tank just to be safe? We certainly have the space for an additional tank.

 

I'm sorry but this is simply not true. Manual J is important as far as sizing the radiation required to heat a room. You use Manual to J to design the proper radiator/baseboard/in-floor radiant heat tubing when designing a new system.

When REPLACING a boiler, but keeping the existing piping and radiation infrastructure, you need to have a large enough boiler to heat the water volume and radiation. Many old homes have massively oversized radiators. If you simply do a Manual J on the home, quite often your boiler will not be able to maintain limit temperature in the system. If the boiler is not sized to match the pipe size(i.e. you must increase the load if you have 3" mains compared to a home with the same amount of radiation but only 2" mains. Just like it would take you longer to boil 1 cup of water on your stove vs 2 cups of water on your stove).

The best source for properly sizing boiler replacement is to get the following book: E.D.R. - Ratings for Every Darn Radiator (and convector) you'll probably ever see - BNP Media Store

The book is written by Dan Holohan who is the most famous steam/hot water expert in North America. This guy is unbelievable and if you are a contractor he is the type of guy who will even answer your individual questions personally.

Once again Manual J is great for determing the load on the home and in a forced air application is everything(assuming your ductwork is adequate), but with hot water or steam you must have the output to heat the radiation and just like boilers are usually oversized the radiation is also usually oversized. Unfortunately you have to make sure you can get that 160 to 200 degree water temp. If not some radiators will not get ANY HEAT WHATSOEVER. You'll still end up reducing the size of the boiler(often significantly) in most apps, but Manual J will cause problems if you don't account for water volume and sq ft of radiation.

 

This oughta open up some lively discussion!

I agree with you if you are talking about STEAM. The boiler MUST be matched to the EDR for proper operation. That's a given that noone will argue.

But you are saying hot water also?

So for example, an older home... one that has been all upgraded with insulation, new windows/doors, etc... areas of infiltration sealed, all that good stuff. When built, the radiation in the rooms was oversized by ohhh, maybe 2X to account for the higher heat loss... and what, you would go in, measure the radiation, and install a boiler with the BTU output to match that?

You only now need HALF the heat? Why do you even NEED 160-200 water anymore? You don't! You might get by with 130 now... so you don't need to heat the water as hot. The boiler doesn't have to keep up with the radiation...

Yes, you would need boiler protection... that's a given... or a perfect excuse to install a mod/con contraption...

Let the CIVIL discussion begin! and nobody go gettin' pissed off if someone doesn't agree with you.

 

Mr Heat I am sorry to inform you that your thought patten on boiler sizing needs readjusting. The only thing that matters in sizing a replacement boiler with an existing system is the heat loss. If the building has 1,000,000 btu's of cast iron radiation and the heat loss is 100,000 the boiler should be 100,000.
As far as heating the home, the more radiation you have the lower the waster temperature you can operate with.
The near boiler piping becomes more critical with cast iron boilers and p/s more important with very large water volume systems. It all comes down to flow in the system, boiler and mixing.
I recall a job that had 231k of cast iron radiation and the home was tightened up. The heat loss was 117k. the boiler installed was 120k. The house heats very well at 64% less fuel. Just installed an ES2 80K DOE output, the connected load is 198K. Works well also. Removed a 116k gross output boiler.
I have been ignoring the amount of connected load and sizing to heat loss for almost 20 years, sizing to the Gross output(DOE Output) and have never had a problem and have an average fuel savings of 25% - 40% and have seen as much as 74% fuel savings.
To reiterate, the boiler output only has to match the heat loss of the home.

 

first of all I don't completely disagree. You can certainly adjust your limits down if you no longer need the massive radiators. The problem is the radiation was probably oversized even before your changes in insulation. Regardless you still need to be able heat the water volume and that can be absolutely huge in old homes.

if 130 degree water temperature can get the job done then fine. You would still use the EDR calculation but the btu's per sq ft of radiation would be less. For example at 170 degrees you would get say 150btu's per foot. At 130 you might get 110 btu's per foot(this is just an example I have the book in my car).

The problem is lets say you have copper convectors or copper baseboard. How much heat can you get out of copper baseboard/convectors at 130 degree water temp? If you look at a chart from the baseboard/convector manufacturer? You are probably below the min water temp allowed right?

The biggest problem comes up when you have massive mains in the basement. To say that you shouldn't make adjustments for 3" or 4" mains is IMO crazy. I don't care what the manual J is you still have to be able to achieve your desired limit temps for your radiation regardless.

 

I'm not trying to pick a fight here at all. I love these sights and talking with other contractors. I'm not married to my beliefs and am persuadable.

Good talking with you guys!

Mr. Heat

 

Size to heat loss only and forget about water volume. It is all in the piping like boiler bypasses and p/s piping. To get water temperature up in the system you slow the water down in the boiler and that picks the water temperature up in the system quicker as the boiler Delta T is increased.
I hate to say it but if you are sizing off EDR you are oversizing your boilers.

 

I'll run it by Dan. I'm not opposed to this at all. The fact is using EDR I'm still (drastically in a lot of cases) greatly reducing the boiler size. My competition often just looks at the tag and maybe knocks a 250k down to a 210k and let r rip.

Your customers must be shocked when they see your quote and tell you that some other A hole quoted them one 2.5 times larger.

I just feel nervous because even when using EDR and say a Grunfos UPS-15 3 speed pump(tiny pump pretty comparable to a B&G NRF-22) it still often takes an hour and a half or so on the original start up to get the water temp to limit. I know I'm coming from 45 degree ground water, but still I'd be a apprehensive to get even more aggressive.

 

I have cast iron radiators and large piping in the basement. Just had my 160k boiler replaced with a 105k. Yes, it takes longer for the water to go from room temp to 130F. So if you want to go from a 65F house to a 70F house in 15 minutes, a boiler sized to heat loss will not do it.

My only problem is that my newly separated 2nd floor zone over shoots the set temp. Other than that, it maintains a comfortable temp. I don't see any problem getting more than enough heat out of my boiler this winter.

Ideally, the boiler has long run times to maintain the temperature if it is sized properly. So the water shouldn't be reaching room temp before another call for heat is made.

It's all math. If the house loses 50k btu/h, you need to have a boiler provide 50k btu/h to the home.

 

I know that... and it's all good! Some times though, differences of opinion evolve into flame wars. Just want to nip that in the bud is all.

Like droopy said, about the only drawback that I can think of when sizing to actual heat loss instead of water volume, etc, is one of responsiveness. It's kinda like having some extra horsepower under the hood when you pull out to pass that tractor trailer on an upgrade... and the 'shoulder seasons' are the times when that would be most apparent, as droopalong also said, once the water is warmed up and the weather is colder, it wouldn't/shouldn't have to come up from cold. (just funnin' ya drooplug with yer different nicknames!)

Proper near boiler piping is extremely important in situations like this, because you need to ensure that the FLOW in the system is not compromised in any way in order to keep the 'balance'. a good primary/secondary setup is probably the best bet, and circ choice is critical.

Droop, if your second floor t'stat has an 'anticipator' adjustment, check that to make sure it's set correctly, might help with the overshoot (move it to 'shorter' cycles)... also, make sure that you've got the flow check valves needed, etc, so there's no 'ghost flow'. (off topic, but wanted to add...)

 

Hi Mr. H and welcome.

I'll go with "heat loss rules" for sizing.

Also consider some of the common scenarios for heating up a large-volume system.

Say you have a system with 100 gallons of water. Assuming a worst-case scenario of needing to go from stone cold city/well 45F to 180F.

100 gallons * 8.33 pounds/gallon * 135F temp rise = 112500 BTU

(In actual practice, it would probably be near-room temperature so call it 65F to 180F, which would be about 95800 BTU)

But... once the system gets going, let's assume a standard 20F dT. The requirement is now only 16700 BTU.

So for the worst case, if you have a design heat loss of 60k btu/hr and a 60k btu/hr DOE output boiler, it will take about 2 hr to get the whole thing cooking. I don't think that's unreasonable.

And of course you're almost never at design, so the heat requirement will be met in much less than that.

You could even throw in the 15% IBR pickup factor, and the numbers don't change that much (15%... ).

IMHO, designing heating systems for the 0.6% to 1% condition is overkill. When they occur, those conditions last for a few hours, typically overnight. Unless the structure is really, really, really lossy, the space temperature won't change by more than a couple degrees. The mass of objects in the house, even less.

 

I would like to add let's not confuse boiler btu's and water temperature. As the water passes through the boiler and the boiler is inputing let's say 60,000 btu's. The water temperature exiting is a factor of flow. If the 60k boiler has 6 gpm the water temp will be 20º higher than the inlet temp. Let's use 120º return temp. So we return at 120º and exit at 140º. If said boiler has a flow of 3 gpm (boiler bypass installed) the exiting water temp will be 40º higher, 120º + 40 = 160º. On the other hand if the boiler has a delta T of 10 too much water flowing then the exiting tamp will only be 130º thus taking longer to heat up. So flow through the boiler and the system may be different dependent on the piping.

 

No anticipator. It's a cheap digital thermostat. I have the swing set to .25 degrees. Yet it seems to need at least 1 degree before it kicks on the heat or a/c.

I don't have any flow check valves. Just the zone valve. Each radiator for this zone has it's piping coming from and returning to the basement before it goes up to the next radiator.

I don't have a boiler bypass either and I really think I should. I'm debating whether I should tackle this now or in the spring. It's also all black pipe so I'd have to rent a threading machine to do it or just use copper. I'm trying not to spend too much on anything at the moment.

 

Do you have a main for the first floor and a main for the second floor or are the rads intermixed on a single main? Each zone has to be completly isolated from the other.

 

The zones are completely separate. The 2nd floor rads or on a loop in series. The first floor is a little more complex. Each side of the house has it's own supply and return pipe. The radiators are run in parallel.

 

Resurrecting my dead thread...

I had the TT Prestige Excellence 110 installed a few months ago. We got our first gas bill for the billing cycle that would have completely included the new boiler. I went through old bills to find a comparable billing cycle (based on average monthly temp). And we're actually using MORE gas (therms) with the TT 110 than we were with our old CI boiler. I realize this is probably a very crude way of determining this.

I called our contractor, who came out to check on the boiler. I wasn't home, but my wife says he measured the boiler's efficiency somehow and came up with 90%. Looked over our bills and didn't understand the additional usage. And concluded that it's impossible that it's using more gas. OK.

So I started looking at the ODR functions. With MUCH help from rbeck via PM, I determined that I could set my upper supply temp at 160F. I set that in parameter 4.

*BUT* It's like the ODR is getting ignored. I'm often in the basement (wood shop, etc.) and notice the display saying supply temp is 180F and above (this morning it was 182F). Even though target CH water temp is much lower (this morning 140-ish, per Info item 6).

I don't get it. The boiler shouldn't be cooking up above the ODR setpoint, right?

 

There is something in the settings or wiring that is throwing this off or you are looking at it when it is calling for hot water.
Is the boost feature turned on?

 

This evening I'm going to reiew the ODR wiring job. Thought, I assume it's correct since the boiler IS reporting outdoor temperature. Thought it's slightly off.

I saw something about boost in the manual this morning, but didn't have time to check my boiler before work. Per these forums, I see I need to use code 54 to even access that settings area.

How does the boost setting work? The manual says default = 00. But reading these forums, I think that value is a unit of time?

I also thought maybe it was just reporting the DHW temp, but DHW water temp has it's own "info" step (item 3). DHW shouldn't have been firing anyway - I checked this morning at 5am, and there would have been zero demand.

I have a cheap little IR thermometer - it would be easy to measure the copper CH supply pipe to confirm this. I should have thought of this earlier.

 

Boost will increase the high limit if the call for heat has not been satisfied within the specified amount of time.

What is your setback like on your thermostats?

My boiler was installed in Sept. and I was setting my thermostats back at night and during the day until the beginning of Jan. Since I've decided not to use any setbacks, I have seen a dramatic change in my natural gas usage. I am reluctant to share the information right now because I want to make sure as many anomalies are removed from the data as possible.

 

IR t'moms have their place I guess... but they are notoriously inaccurate most of the time. When measuring copper, you MUST paint a flat black spot on the pipe. It's the only way you will even get close to an accurate reading. Rusted old cast iron will give fairly accurate results... it's all got to do with 'emissivity' of the material... Scroll down a bit and there's a little about the subject HERE

 

Ok, so if boost defaults to "00" (according to the manual), then it will wait ZERO minutes before increasing the high limit? That could certainly account for the higher temps.

For the 1st billing period we did not use setback. I read here and was advised by our HVAC installer that setbacks were counterproductive with modcons.

But, we like it cooler when we sleep. So I've recently programmed a 3F drop at night.

I have some of that too!

 

No. If it is set to 0, it will disable the boost.