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# Help figuring out what heat load calculation to believe

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Member Posts: 9
We're looking to get central air and heat installed. We are getting quotes for ducted central air/heat on the second floor plus mini-splits on the first floor.

We have a couple of different quotes to evaluate where the equipment size for the second floor is pretty different, and I'm struggling with who to believe. One is 60,000BTU, one is 20,000BTU. Both installers basically say "I did the heat load calculation and got this number".

For context. We're in Massachusetts, the 2nd floor is about 1100 sqft. 2 rooms (each ~200sqft) have lofted ceilings (~16ft peak, 8ft at the outside wall). The rest of the house has 8ft ceilings. 16 total double paned windows and 2 skylights, blown in cellulose throughout the walls and 8" of fiberglass in the lofted ceilings. House was built in the 30s.

Right now we have steam heat, and there are 6 different convectors/radiators I estimate put out closer to the 60k btu based on similar sizes from here (pg 13 is closest to ours) https://modine.worksmartsuite.com/PORTAL/io_modules/IOGETIMAGE.php?type=stream&filename=13-111.pdf

One quote suggests an 18,000BTU mitsubishi hyper heat ducted air handler (SUZ-KA18NAHZ and SVZ-KP18NA), the other suggests a 60k BTU high efficiency (97%) furnace and 3 ton bosch heatpump.

I'm not a heating pro, so I have no idea how to check the work. What could I ask that might help me know who did a better job with the math? Is it obvious to anyone here that one quote is way off?

• Member Posts: 1,595
edited April 2022
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Down load the slantfin heat load app. It is detailed but does a great job.
• Member Posts: 22,327
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Are the load numbers for the upper and main levels?
Could one ducted system supply both up and down? Or keep the steam and mini split the cooling?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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And what about the existing steam? I hope you will be keeping it -- you will be much happier if you do. Or buy some good quilts and warm wrappers...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,897
edited April 2022
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That 60kBtu heat loss is garbage, but high efficiency furnaces don't come much smaller so the furnace in a hybrid set up will be that size even if the true heat loss is 5kBtu. Very doubtful they actually did an honest heat loss. On the flip side, the Mitsubishi comes in larger sizes, so that contractor believes in their number, otherwise they could have upsized to the 24,000btu Mitsubishi. With that particular Bosch, you could likely never turn on the furnace. In fact, the 24kBtu has better dehumidification, so that's a consideration on the cooling side.

Now, https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler can be used to establish a ceiling for the heat loss. But since this method uses all fuel usage, it'll be for the full structure, not just the second floor. For example, say you get 40,000 for the whole house. Then obviously 60,000 is wrong, but perhaps 20,000 for the second floor is right.
• Member Posts: 9
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Sorry it was unclear - the load numbers (and my details) are just for the 2nd floor. The whole house is closer to 2800 sq ft. The 1st floor quotes of mini-splits each match a lot more closely, so I was focused on understanding the 60k vs 20k difference (also on that floor we intend it to be our heat, so I'm more worried).

I've also gotten more info from the 60k quote - he said the heat loss from his software was ~46,000btu, cooling was just over 2 tons. So he didn't want to go to the 40,000btu model of the furnace (the only smaller size). For both, he admitted he was conservative in the insulation R-value, but he'll send me the full report from the Wright Soft software.

@Hot_water_fan, thanks for that link, I'll do that. I looked at the Mitsubishi lineup and thought that the 24kBTU model wasn't hyper heat and fell off at 5F?

On the steam, our goal is to be as efficient as possible from a carbon standpoint without completely sacrificing comfort, so we're likely to convert the radiators on the first floor to hot water and run a condensing boiler. The steam system is comfortable, but even with the pipes well insulated its at best running at 82% efficiency.
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Keep in mind when you think of converting the radiators on the first floor from steam to water... it's not as simple as it sounds.

First, make sure they can be converted at all -- not all steam radiators can.

Second, be aware that unless they are two pipe radiators to begin with, you may have difficulty installing a return line.

Third, be aware that you will need mostly if not all new piping, even if they are two pipe now.

Last -- be very aware that you will get much less heat out of them than you do now with steam; if they are run at condensing temperatures, somewhat less than half. If they are run at 180, you'll get about two thirds -- but your efficiency will be the same as it was with steam, so you will have gained nothing.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 9
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@Hot_water_fan I followed the instructions on the website using a few different temperatures/designs (65 vs 60HDD, using a few options for a design temperature (5F vs 20F)). The range I got depending on my math puts the whole house between 46,000 and 68,000btu/hr of heat loss. (281.4 therms used, 1098 - Base 65, 950.3-base 60).

Given that, I'd expect to spec a total of about 95,200btu for the house (I used the larger number and oversized equipment at 1.4). We have more square footage on the first floor, so just dividing the house by sq feet (second floor, but higher ceilings) puts us at a 37,400 needed for the second floor, so I'm more inclined to believe the 46k heat load (where he admitted to being very conservative). I suppose I could push that installer on a 40k BTU furnace instead, but with a modern high efficiency/variable speed unit, whats the lowest output it could give? Would a 60,000BTU max furnace scale down to <20,000 BTU if it turns out we are very oversized?

For the conversion to hot water - the steam heat is actually pretty oversized already, as it feels designed single-paned windows and no insulation. So I'd rather have the radiators on longer at lower temp, and back-of the envelope with hot water at 160F I'll have enough heat.
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Also, while we are running single-pipe today, there is easy access to the inlets on each convector and we'd just replace a single radiator in the bathroom with a baseboard. It is all unfinished space below the radiators we're replacing, and they have threaded plugs on the high side so converting seems reasonable on the surface.
I know we'll have to pressure test them, so I get there is risk here. The only reason we're not converting the radiators on the second floor too is the need for new piping through finished spaces.
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Keep the steam,  Use the heat pump during the shoulder seasons. Once the outside gets close to 20* switch over to steam.
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@scv900 Great work!

I followed the instructions on the website using a few different temperatures/designs (65 vs 60HDD, using a few options for a design temperature (5F vs 20F)). The range I got depending on my math puts the whole house between 46,000 and 68,000btu/hr of heat loss. (281.4 therms used, 1098 - Base 65, 950.3-base 60).

Using the Base 65 and 5 degree design temp, I get 64kBtu (281.4*100,000/(1098*24))*(65-5). Is the 281.4 therms input or output? If it's input, then 80% of that gets you 51257btu/hr for the whole house, 72kBtu with an oversize factor.

I suppose I could push that installer on a 40k BTU furnace instead, but with a modern high efficiency/variable speed unit, what's the lowest output it could give? Would a 60,000BTU max furnace scale down to <20,000 BTU if it turns out we are very oversized?</div>

Nope, furnaces struggle with turndown. The top of the line 60kBtu Trane can only turndown to 22,800, which is better than nothing, but doesn't really live up to the marketing brochure.
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Thats the output - actual therms was 335 therms. I also assumed efficiency of 84% because thats what the furnace claims, but in retrospect I'd probably use something like 75-78% because we have gas hot water and a stove too. I think i'm in the ballpark, and I tend to trust the 46,000heat loss calculation more. Seems like the 20k was way too aggressive.
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pecmsg said:

Keep the steam,  Use the heat pump during the shoulder seasons. Once the outside gets close to 20* switch over to steam.

Yes -- this is the correct thing to do, and will give you the most comfort for the least expense. There seems to be an impression that just converting steam to hot water is simple. It's not, and it almost never works out well, and always costs almost as much as a whole new system would (sometimes more).

Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 9
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@Jamie Hall - I totally get that. I do think thats probably the right solution, and we're likely to keep the steam for now and run it only when needed. That said, we're putting a heat pump/air handler in our attic space anyway for AC, so we asked for the cost of a furnace too, thinking maybe we'd go hot water for the first floor zones in the future. The benefits of zoning the house and having a system that runs a little quieter are big for me. (I'll say the steam doesn't knock or anything - just have a couple of pipes in the walls that rub on something when they get hot so its been impossible to eliminate noise).
The other consideration for us is the space the boiler and steam mains take up in the basement . I'd love to get that back with a wall hung unit, plus then I don't hit my head on the pipes in the basement.
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the low hanging pipes seem to come up often here. Love the steam, hate the head bangers
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 15,710
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2 tons is about right for the second-floor square footage in Mass. Around here the heating load is usually double the cooling load and the cooling load is half the heating load.

Just rules of thumb.

An accurate heat loss is best.
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hot_rod said:

the low hanging pipes seem to come up often here. Love the steam, hate the head bangers

Just convert to a minitube steam supply system with radiator returns. A 1 inch copper main would probably supply that system. You could even leave the radiators as is and just drain the water off the current one pipe supplies with a trap at each drip.