Actual measurement of heat loss

Old 02-21-19, 03:09 PM
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Actual measurement of heat loss

I've been through a few heat loss calculators tools for my garage build and they give out very very different numbers (up to a 3 fold difference with same inputs).

I'd like to understand how I could do an actual measurement as follows :
- pick a cold day
- shut heat off to the rest of the house so it doesnt bleed into garage and makes measurements incorrect
- wait a couple of hours for heat mass of radiators in house to dissipate then measure temperature drop for the next 10 hours vs outside temp (could do this in each room to size my next project as well).

I dont have the math formulas to actually size the radiators after that. I'm guessing it shouldn't be overly complicated.

If someone can pitch in that'd be greatly appreciated.
Old 02-21-19, 05:08 PM
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Welcome to the forums.

I don't have an answer but I do have a couple of questions.
Are you planning on installing baseboard heat in a garage ?
Does it have regular overhead doors ?
How many doors ?
Sheetrocked ceiling or open to the roof ?

A very effective garage type of heater is a fan forced hydronic unit. It would typically hang from the ceiling. It gets connected to your boiler on its own loop and is fan forced. It would be operated by its own thermostat.

Just an example.......
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Old 02-21-19, 06:16 PM
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I politely suggest that you're approaching this incorrectly.
This is not a finely balanced "calculate the heat loss" situation.
This is an "install the biggest heater you can afford" situation where you want overkill.

A bigger heater will have a thermostat to shut off when it's warm enough, but will bring you up to temperature quicker, and take less running time to maintain temperature.
In contrast a smaller just-balanced-to-heat-loss heater is going to be on constantly and struggle to keep up.
Old 02-21-19, 08:16 PM
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Thanks for your inputs, the house originally had a garage when built, that got converted to a family room by previous owners, and I'm now turning it back into a garage for my needs.

This means sheetrocked and R38 batts on top, insulated walls, and I'll choose an insulated garage door when I open that wall to the outside.

I want to keep the space in the low fifties at all times, and be able to crank it to 60-65 when I work on the project car in the winter. Heat will come from 2 hydronic panels fed from a condensing boiler.
Sizing of those panels is key to making sure the water stays at a low temperature (thus running the boiler at a good efficiency).
To your point, Hal_S this is a situation where I believe you'd rather have the boiler run constantly than cycle.
I could go overkill and take the biggest panels I can, but I'd rather size it right and leave room for larger tools.

The calculation would also be used to size panels for the rest of the house in 2020 when I move away from baseboards (designed to run at 180) towards hydronic panels.

Garage is small, less than 300 sqft, ceilings are 8ft, and this is in New England, my current guesstimate is 13000 BTU/h, but again, I'd like to measure it to size it right for the coldest days.
Old 02-22-19, 05:23 AM
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Hi Zak, I'm a retired energy auditor and if you are getting a wide variation from different heat loss calculators, it probably isn't the calculators. They can vary a bit on the input details but all basically use the same equations:
Q = U x A x HDD x 24 for annual loss or q = U x A x delta T for heat load.
Source Residential Energy by Krigger and Dorsi.
If you want to make it easier this web site has a fill in the blanks using those equations.

Old 02-22-19, 05:50 AM
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Thanks Bud9051, this gives me 12815 BTUs so this confirms my prior calculations, main loss through the slab (ranch, slab on grade uninsulated built in the 50s).

I'll get something that can keep up with this number with 140 water, and if it get's way colder, the boiler will just crank the water temp up.
Old 02-22-19, 07:16 AM
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If you rented one or more space heaters from Home Depot and ran a test in the garage, would that help? I was wondering if you ran the space heater(or heaters) with known output over a period of time, would that help gauge the number of Btu’s that you would need to keep the garage at your target temperatures. Maybe confirming your calculations.

But on the other hand, I guess maybe the space heater(s) would be too different from the panels that you will be using and thus wouldn’t help much in verifying your calculation. But is it possible you would still get something useful from such a test?

Just a thought.

(ahh! just re-read. I guess the garage is under construction. Thought it was done. My bad.)

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