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What if I have a too-small boiler installed?

You've seen my posts elsewhere on this forum: I'm shopping for a new gas FHW furnace with indirect domestic for my 2200 square foot Massachusetts colonial. I want a good, efficient, direct vent furnace. My gas use suggests that I don't need more than 28kBTU/hour on the coldest winter days, so my current 95kBTU/hour output boiler is three times what I need. Almost everything I've read here and elsewhere agrees with this.

But, plumber after plumber wants to install something as big or bigger than what I now have. They're worried that they'll put something in too small, I'll complain loudly, and they'll get burned. It's a reasonable concern, even if misplaced. I doubt they get many call-backs for too-large boilers.

So here's my question: what if I prevail upon my plumber, and then find that the boiler I demanded is too small? Either I run out of hot water during the morning's second shower, or the house starts cooling on cold windy days, or both. What are my options for fixing it?

I ask because I want to know how dire a fix I'd be in. If I'd need an electric booster that only kicks in a couple of times a year, then that's not to bad. If I'd need a swap for the next-size-up plug-compatible boiler, then that's pretty bad. But if I'd have to rip everything out and start from scratch, then that would really be nasty.



Asked by dtgriscom
Posted Jul 20, 2014 12:29 PM ET


5 Answers

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First of all, we need to clear up a confusing point: Your title refers to a "boiler," while the first two sentences of your question refer to a "furnace." Since you are the author of a previous Q&A thread that discusses hydronic systems in detail, I'm guessing that you meant to write "boiler" when you mistakenly wrote "furnace."

(A furnace is an appliance that makes hot air for a system that uses ducts for distribution.)

The way to make sure that you don't have a boiler that is too small is to perform a Manual J calculation to determine your home's design heat loss. If you don't want to perform the Manual J yourself, and if you are having trouble finding a heating contractor with the competence to perform a Manual J calculation for you, you need to hire an engineer or a home energy rater to perform the Manual J for you.

For more information on this issue, see:

Saving Energy With Manual J and Manual D

How to Perform a Heat-Loss Calculation — Part 1

How to Perform a Heat-Loss Calculation — Part 2

Answered by user-756436
Posted Jul 21, 2014 5:54 AM ET
Edited Jul 21, 2014 5:58 AM ET.


Daniel , the odds of your new boiler whose size was chosen after performing an in depth heat loss analysis being too small is almost nil . There is fudge built into the programs and or numbers used for calculating the load . The only issue you may have is not reaching desired indoor set point during a period that gets lower than what you designed for but that would suggest a perfectly sized heating system .
There are a few qualified contractors roaming the planet , mind you very few , one in your area should be Dennis Foley . Give him a call .
In another thread you stated your NG consumption (actual) for the past winter was 30K per hour . Is this accurate , are you questioning that a BTU is a BTU ? If you want to pad yourself against future cooler than normal temps and give yourself a little room don't go too big . Remember the fudge , a 22 mph wind cannot hit all the walls at the same time for instance , yet the programs recognize this phenomenon as fact .
I gave my number on the other thread, you should use it . I will make a one time offer to help you gratis in case the hesitation is of a monetary concern .

Answered by Rich Mc
Posted Jul 21, 2014 6:15 AM ET


First issue, Martin: boiler. Boiler, boiler, boiler. (Sorry.) And thanks for the heat loss pointers.

And Richard, thanks for all the help, and thank you for the kind offer. It isn't the money; I'm just swimming in a sea of experts, trying to figure out which ones to follow. What I'm hearing here makes abundant sense, but then again I talk to the locals, with installation experience out the wazoo, who insist otherwise. I'm 90+% sure of the way to go, but I still have a niggling concern that I'll get burned.

Which brings me back to the original question here: what if a year down the line I find that my boiler doesn't have enough "oomph"? Or, if I'm selling my house, and the buyer's home inspector laughs at the boiler size, how much would it cost me or the buyer to step up the boiler size? Are we talking a whole new installation, or is it just swapping the boiler for a higher-capacity model?

Thanks for everything,

Answered by dtgriscom
Posted Jul 21, 2014 8:30 AM ET


What you're talking about is a small drop in house temperature and a sluggish response to thermostatic setback on extremely cold days. Most boiler's are set with domestic hot water priority so that during a simultaneous call for hot water and heat, the boiler sends heat only to the indirect tank for some amount of time (usually 20-30 minutes). The probability of running out of hot water is quite low - you would have to have an enormous hot water draw coincide with some extremely cold outdoor temperatures. Further, the probability of you under-sizing your boiler is quite small - just rounding to an available size will probably give you >25% safety margin.

Even though this has no application to your situation, finding oneself with an under-sized boiler it's generally easier to change the building shell properties to match the boiler size. I'm very skeptical that a building inspector would comment on the boiler size, especially in the context of a proper Manual J.

It seems crazy, but all of the contractors you're speaking with are wrong. For years I trusted installers to be right about this stuff, and very seldom had good results. For instance, many of my original customers have 55 gallon indirect tanks connected with over-sized boilers (>70kbtu/hr). That combination has a first hour DHW rating of at least 151 gallons, which is ridiculous. Installers of this combination had no reservations about putting 50 gallon standalone tanks with < half the first hour rating of that combination. They simply weren't reading the manuals of the products they installed. See p 43 here: http://s3.supplyhouse.com/manuals/1321036885438/68124_PROD_FILE.pdf

Answered by user-945061
Posted Jul 21, 2014 10:02 AM ET


There are very few boilers on the market that would be too small for any 2200' house in Massachusetts that has glass in the windows, doors that close, and at least some insulation. A really crappy 2200' 2x4 house with no wall insulation and R19 in the attic w/ clear storms over wood sash double hungs might come in as high a 50,000 BTU/hr @ 0F, but it won't be anything like 75K unless it has air leaks big enough for skunks & raccoons to crawl through (that can and should be fixed before they do!)

That is about the smallest of the line for many mod-con manufacturers, and is about the output of a code-min 82% AFUE 3-plate cast-iron beastie. If somehow it doesn't keep up, it's easy to add insulation, tighten the place up some, and maybe swap out the storms for low-E storms.

A lot of local MA HVAC-hacks got used to installing 3x oversized boilers to get some reasonable hot water performance out of embedded coils. Even the very smallest oil-fired boilers out there will be more than 2x oversized for your estimated 28K heat load. But a 28K output boiler heating hot water with an embedded coil would not support even a single mid-summer shower flow @ 2 gpm.

There are a few tiny 2-plate cast iron boilers with output of about 28K (eg: the derated version of the Burnham P202, which has now been discontinued, as have most 2-plate boilers), but there are plenty of mod-cons that will modulate well below than number. The smallest direct vented versions (important for eliminating backdrafting issues) are 3-platers like the Peerless PSC II-03 or Burnham ESC3, with output roughly 2x your estimated load. But a 50K like the Peerless PF-50 has a min-fire input of 16K (output ~15K), but a max-fire input of 50K, which even at non-condensing output temps will deliver over 40,000 BTU/hr. If your heat load is 30K @ a design temp of 0F and an interior temp of 70F, that means you're good down to about -30F with that boiler, a temp probably not seen more than once in most of MA since 1776. The Triangle Tube PT-60 has a comparable min-mod, but a higher high end. There are many others.

Don't be afraid to specify the boiler rather than simply asking for a dozen proposals from contractors, and getting a dozen oversized recommendations. Call the local distributor for contractor recommendations (especially for mod-cons)- they know better than anyone who is pestering tech support about stuff that's clearly in the manual, and making stupid-attack design errors & warranty calls, and who is taking them by the dozen without any issues.

Answered by user-1004076
Posted Jul 21, 2014 10:59 AM ET

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