Need help with Boiler Selection and Sizing

Users who are viewing this thread


New Member
Reaction score
Marlborough, MA
Hey Guys, I live in Eastern Massachusetts and my home is around 2900 sq ft. I bought this home last year during fall (Built 1988) and my last winter gas bills were skyrocketing. I knew that I had to replace the boiler at some point when I purchased the home but looking at the last year winter bills I'm now seriously contemplating of replacing my existing boiler with a more efficient one(95% AFUE). My current boiler is a Burnham 136K BTU boiler with 5 zones. I have 40-gallon hot water tank powered by Gas and Cooking Range powered by Gas too.

Baseboard sizing below,

Zone 1 - 44' | Zone 2 - 39' | Zone 3 - 33' | Zone 4 - 29' | Zone 5 - 34'

Energy Consumption:

Dec - 332 Therms | Jan - 344 Therms | Feb - 309 Therms | Mar - 219 Therms

My summer usage for water heating and cooking averages around 50 therms/month.

I had Mass save perform an energy assessment and added more insulation in my attic. I'm hoping this should help in some kind of energy saving this year. As my boiler is nearing the end of life, I would like to replace it before the winter and avoid any kind of emergency replacement during the winter. I'm in the process of sourcing quotes from multiple contractors. I'm trying to educate myself with everything that I find on the internet regarding boilers to make a decision and I still consider myself a novice in this field. Appreciate any kind of comments which would help me make a better decision on my boiler replacement.

Couple of questions,
1. Is it a good idea to replace my current boiler with a combi boiler/water heater? The reason I'm asking this is that my current vent would be too big for just the water heater to vent out once the boiler is replaced. I need to add a liner through the chimney to support the water heater venting and looks like the cost to add the liner would close to replacing the water heater with a power venting heater.
2. If combi units are a bad idea, should I go with indirect water heater or electric/gas with power venting?
3. What brands of boilers should I consider? Based on my analysis, I was thinking of going with Bosch Greenstar. Is it a good or bad idea?
4. How should I size my boiler? Many contractors were sizing the boiler based on the length of my baseboards. Is it good enough?

Please feel free to chime in on anything that I need to look for while assessing the different brands and models.


In the trades
Reaction score
At your energy use that boiler is something like 3x oversized for your likely 35-45K-BTU/hr heat load. A tight 2x6/R19 house with R30 in the attic, clear glass double panes and no foundation insulation would typically come in between 12-15 BTU/hr per square foot @ 0F. Your actual 99% design temp is in low positive digits (yes, I know it sometimes hits -10F in Marlborough).

Sizing it by the total radiation is a BAD idea- that only puts an upper bound on the size, not the ideal. At 170F average water temp (180F out, 160F return) the ~150' baseboard can only emit about 75-80,000 BTU/hr, so the Burnham (is that a P206?), so the absolute biggest boiler that should EVER have been hung on that radiation would be an 80-85,000 BTU/hr (input) condensing boiler, or a 90-100,000 BTU/hr (input) cast iron.

Rather than monthly totals, look at the EXACT meter reading dates from November through February or early March, and total therms between those times, and run a BTU/heating degree-day analysis on it using data from a nearby weather station, which amounts to measuring the heat load using the boiler as the measuring instrument. Don't bother subtracting out your summertime hot-water /cooking use from that, since that can introduce more error than it corrects for, and even if perfect isn't going to matter much. (Solar gains can be a bigger factor, and hot water fuel use corrects for the solar gains.)

For all boilers, but especially for low mass CONDENSING boilers, run this analysis on the zone radiation to figure out what boiler is the best fit (hint- it's not going to be a wall hung combi boiler). Cutting to the chase on that...

Your median zone is 33', which at water temperatures needed to achieve 95% combustion efficiency emits about 200 BTU/hr per running foot, or 6600 BTU/hr. If the zone can't emit the minimum fire output of the boiler it's going to cycle on/off during continuous calls from just a single zone. The smallest zone is 29' which would only put out 5800BTU/hr at 95% efficiency water temps. The smallest wall-hung combis run about twice that much, so forget about it.

A decent but inexpensive modulating condensing boiler that would work here without thinking too much about it would be HTP's UFT-080W (which also sold by Westinghouse as the WBRUNG-080W). That model has a high fire input of 80,000 BTU/hr and has a 10:1 turn down ratio, so at min-fire it would be taking 8000 BTU/hr source fuel input, and at 95% efficiency would be delivering 0.95 x 8000= 7600 BTU/hr into the zone. While that would be 1000-2000 BTU/hr of excess, there is just enough thermal mass in the boiler and plumbing to prevent it from true efficiency-robbing/boiler-destructive short-cycling. (I can run that math for you too, if you like.) That model has been out there for a handful of years now, with very few reports of problems, and being a low pumping head fire-tube boiler it's an easy retrofit for a cast iron, unlike many water-tube boilers which would require hydraulic separation. HTP's marketing even makes the claim that it can be pumped direct, without hydraulic separators, which is mostly true (but for 2 systems out of 100 , maybe not...). It's almost certainly going to work pumped-direct in your case.

But there are several others that would fill the bill, but none of the Bosch Greenstar boilers can handle zones as short 33' baseboard zone without significant cycling issues.

Run the heat load analysis on the wintertime fuel use and report back- there may be more appropriate choices, but that boiler covers about 95% of all houses comfortably, and HTP's headquarters are in MA- local support for their products tends to be pretty good, and if it craps out on you it's less than an hour's drive to chuck the thing through their front office window. :)

An indirect water heater operated as the "priority zone" works. (Here again, the UFT boilers have a separate pre-plumbed port and controls specifically for supporting an indirect water heater), and would be the most appropriate way to go if using a modulating condensing boiler. But depending on where your space heating load number come out you might be better off using a condensing water heater for both heat and hot water, especially if the water temp requirements for the baseboard to emit the heat are at domestic water temps or lower, which it might be. With 150' of baseboard at 200 BTU/hr per foot you're already at 30,000 BTU/hr of heat emitter even at 120F average water temp (125F out/115F back). If the design heat load is 35K or less this is worth analyzing more closely.

FWIW: The heat load at my house in Worcester is ~35K @ +5F (a 2400' 1.5 story 1920s antique bungalow with 1600' of insulated but not actively heated basement). With currently four people showering daily we're burning under 20 therms/month during the summer season.
Last edited:
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks