Boiler BTU output, calculating home heat loss and replacement sizing
I’m hoping you all at GBA can help me understand my 3500 sq. ft. house’s HVAC system (coastal Massachusetts). My house currently has 1983 Weil Mclain CGM-7 boiler with cast iron recessed baseboard radiators, which I supplement with mini-splits (three 1:1 mitsubishis, 12K and 18K) and a wood stove. As I understand it, the boiler has an input of 210K BTU and DoE heating capacity of 165K BTU (79.5% efficiency). At some point this unit will need to be replaced, so I have been looking into sizing a replacement.
I’ve been tracking my gas/electricity/firewood consumption for two years, and calculated heating loads using heating degree days for my location (after removing non-hvac loads for electricity and domestic hot water). For example, this past month we used 245 therms of natural gas, 1316KwH, and 1/4 cord of red oak (75% efficiency stove, 25Mbtu/cord). Subtracting out domestic use (600kwh and 25 therms), I calculate that we used 29MBTU for the month. With 1119 HDD, I calculated 1070 BTU/hr/ºF. At a 99% design temp of 9ºF outdoors and 68ºF indoors, I calculate that I would need 63K BTU/hr. I have done the same calculations for other months and get the same answer, within ~20%. This seems low, but my house is oriented SW with lots of glass so solar heat gain is substantial (we use cellular shades at night). Is it really possible that the boiler is oversized by a factor of 2.5x? If so, would I be ok going with a Weil Mclain Eco Tech 110?
The other question is efficiency. Based solely on boiler specs, I calculate that the payback period would be 30+ years going from a 78% to 95% efficiency unit at my current therm consumption (savings ~26% per year, or $600) and an install cost of ~$20K (ballpark). However, I know my old unit does not modulate output. Does this mean that when a single zone calls up heat (say 20′ of baseboard at 500btu/ft), my boiler will run at 210K BTU input to satisfy the heat needed, even if it’s only 10K BTU required? If so, wouldn’t the Ecotech 110 (10:1 turndown) use far less gas in the real world than my present setup, irrespective of stated efficiencies? Does anyone have any real world examples of replacement boilers in their homes, and how much their fuel consumption changed?
I’ve learned so much from this community over the years as a reader; you all are great.
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Replies
It's very much believable that the boiler is oversized by 2.5x. I'd also look at the feet of baseboard, the boiler can't put out more than the baseboards can put out.
If the house does have meaningful solar gain that will lead to a lower estimate for heating load. Also, when you calculated the contribution of the heat pumps did you factor in a COP?
DC, I appreciate the reply. The two primary zones (out of five total) are approx. 90' and 60' of cast iron, and each has a 30" x19" cast iron radiator as well. The other three zones (fin baseboards) are 40', 36' and 20'. These three ancillary zones don't get used much between the mini-splits and wood stove. Based on 600 btu this would be ~160,000 btu/hr output from baseboards.
For the splits, I used a COP of 3 since I only use them above ~20ºF, and it looks like their rated COPs at 17ºF are 2.86 (two MSZ-GE12NA) and 3.61 at 47ºF. I believe 2.8 is standard practice, but bumping to 3.0 should slightly bump my estimated heating requirements.
Since we are coming into February you have the opportunity to test how oversized it is. Saturday and Sunday will be pretty cold. If you can track how much the boiler runs you can tell how oversized it is. In theory the perfectly sized boiler will run all night on a design night. My guess is it will be well under 50 percent duty cycle.
HOw old is the house? If it is pre 1973 it may have been designed for single pane glass and little insulation.
Gus, that's a good idea. I will test and report back. Right now, the unit really only runs for an hour in the mornings after a temp setback to 55ºF overnight, then maybe 15 minutes per hour the rest of the time with splits running. Bedroom zone stays at 68ºF (we have a newborn). House is 1952, with a mid 1980s addition that doubled the square footage. We still have the original single panes, which are in great shape. Cellular blinds at night have helped a lot, as has adding attic insulation and polyiso to basement walls.
63MBH seems a little low compared to other structures I've looked at in the past but it is not inconceivable and it all depends on how your home is constructed. When was the house built? What are the approximate R-values for the roof, walls, etc? Is this a tightly constructed home? Is it standard stick frame or mass walls? Almost 40years on one boiler, that's pretty good. When you look at replacement systems you might look at the efficiency at part load as well since it may be more efficient at part load. Same for pumps. You may end up with slightly larger equipment that operates more efficiently at part load and then you'd have some wiggle room if the sizing is not exact.
Thanks for your reply. The original house is a 1950s stick construction (2x4) shed-dormered gambrel, ~1600sqft with original single pane windows with storms (every window has cellular shades). A 1980s addition added two staggered gambrel sections (2x6 stick), so 70' long in total. Long side faces the SW, with ~50% of walls comprised of windows/siding doors. See the attached drawing. Since moving in I've improved the envelope by extensively air sealing, bringing the 1950s attic up to ~R50, 1.5" polyiso on the basement walls, replacing four SW-oriented sliding doors with high SHGC Marvin units in our sun room which helped a lot.
Good point about the partial loads with a larger boiler. I will look into this. If a Weil Mclain ecotech 150 modulate down to 10:1, I wouldn't lose much compared to the ecotech 110 yet still have top end for the rare -10ºF night we get every few years.
An interesting data point that may lend credibility to my numbers is my experience with our wood stove (Jotul F400, 55K BTU max), which is located on the north end of the house on a 12' wide brick hearth. We have two flights of stairs in the house, to the warm air makes a convection loop to the S end of the house with all hallway doors open. With an outdoor temperature of 20-25ºF, that stove can keep the entire house at 60+ºF (75ºF in the room with the stove).
Just a comment.
With multiple heat sources, and setbacks, it is really really difficult to get a handle on actual energy usage.
If you make an improvement, how will you measure the success of it?
I am not questioning your heating strategy, just that it makes getting 'an answer' very difficult