# Estimating Heat Loss

| Posted in Mechanicals on

I’ve been building a Pretty Good House for the last 9 months and have been doing sheetrock for the last month. With that, I’ve kept the building at a minimum of 70 degrees using (3) 1500w space heaters. I’d like to estimate my heat loss from the electrical consumption for the month of December which was 1,412 kwh for 31 days.

We had 1336 heating degree days for the month of December in my zip code.

Can anyone advise on how to make this calculation?

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### Replies

1. | | #1

You first convert kwh to Btus - 1412kwh x 3412 btus/kwh = 4,817,744 Btus. This is all the input energy, which with an electric space heater = output energy.
Then divide that by 1336 HDD to get 3,606 Btus / HDD.
Assuming that 65 was used as the base temp for the HDD, you then multiply (65-design temp)*3606/24.

So if design day has an average temp of 0, it’d be a heat loss of 9,766 btu/h. This is assuming the space heaters are keeping the place at 70 when you’re not there.

Also, you have kWhs that are not being used by the space heaters, which could be subtracted out and lower the result.

1. | | #2

I think you're going to end up with a very optimistic heat loss value using this method. For one, it's ignoring the rather significant passive heat gain during the daylight hours. Maybe there's a rule of thumb on how to account for this. It would make more sense to me to just monitor the energy usage over one cold night, starting well after the sun has gone down and ending when it comes back up. Divide this by the number of hours, convert to BTU/H, divide by the deltaT and prorate it for the local design temperature.

Virtually all electrical energy usage gets converted to heat, so I don't think you'd want to subtract any of that. The only exception would be exterior lights or any other appliances outside the thermal envelope.

1. | | #3

During December, I'm not particularly worried about solar gain, since the days are shortest. I think it's worth subtracting out lights, refrigerators, etc. because they'll be there once the space heaters are gone and a permanent heat source is installed and sizing the heat source is usually the goal here. It's hard to know what electricity is being used excluding the heat since the house is under construction, so in this case maybe keeping all kwhs makes sense. Either way, Michael is dealing with a very low heat loss, which is great news for him.

1. | | #4

Hey guys, thanks so much!

I would say that for the month of December, the only additional electrical load would have been power tools, tablesaw, charging stations, vacuum, etc. It is likely quite negligible and I expect that most of the electric use is going towards heat.

The only potentially significant variable is that I kept the house at 70 most of the time but during the drywall mudding I would turn it up to 80+ for several days at a time to aid in drying.

Either way, I am just trying to pencil out a rough heat loss to get a worst case scenario.

Where I am at in Wisconsin, I had to use the design temp of -26. Is there a way that I can convert those numbers to tell me heat loss at design temp of -26? That would help me check heat loss against our original calculations.

1. | | #5

Yup: 65 minus -26 = 91, so 91*3606/24 ~ 13,670. How does that compare? Now, HDD of 91 isn't the same as a design hour of -26 (a HDD of 91 is a daily average temp of -26, so some hours above, some hours below), but it's close enough. How does that compare to the initial calculations?

2. | | #7

If you want worst case scenario, you need to account for the solar heat gain. The lower your load, the more prominent a factor it becomes. In my house, solar heat gain covers the entire load during sunny daylight hours, even at 0F. I still say the most accurate would be to take the average power consumed over a cold night to give you the heat loss per degree and multiply that by your design delta T.

I did this measurement and calculation recently, and it came out surprisingly close to modelled heat loss.

3. | | #9

Just finished a month (Dec 15-Jan 15) of 1343 HDD with ASHP usage of about 1550 kWh (total electric was around 1900 kWh, running at about 300-350 kWh non-ASHP per true power meter). House is not at the PGH standard. You may be looking at ASHP usage under 1000 kWh/month for a ~1300 HDD/month once you switch to your ASHP.

2. | | #14

Hey Paul and Trevor,
I hope you're well. Wondering if you might help me again.

We've got another month under our belt of keeping the house heated. It would have been kept around 75-80 until January 14th and then around 65 after that (I know that is difficult to estimate). We had drywall going on and they had to crank the heat for everything to dry.

That said, the electric bill was pretty sizeable this month! Still relying on space heaters to keep the place heated as I work to get our heat pump hooked up.

12/29/21 – 01/31/22 we have 3,196 kwh @ .14/kwh = \$447.44
For my location, we had 1887.2 HDD for that period.

If I followed your calcs previously, that is 3196 kwh x 3412 btus/kwh - 10,904,752 / 1887.2 HDD = 5778.27 btus/HDD

X65 = 375,587.55 / 24 = 15,649.48 btu/hr

That is quite a bit different than last month, however, it was colder and it was a very cloudy month.

Can you help me understand what this means and if this is reasonably accurate or better than my heat loss calc still? That would be 26,749 btuh at -26º.

Furthermore, in a climate with 8608 HDD annually, can I just multiple my kwh for January's HDD to get my electric usage for heat annually if I were to use resistance heat? Then, I could divide that by the COP of my Mitsubishi 30k heat pump to understand what I might expect for annual heating cost?

Thank you!

2. | | #6

Hi Paul,
Our manual J estimated 26,749 btuh at -26º but also was based on us achieving a 1.1ACH50 airtightness. Our preliminary blower door test was .4ACH50 and I'd expect that now that our secondary air barrier and sheetrock are all installed, we're likely quite a bit lower than that.

Could that alone have that kind of impact? Of course, currently, the ERV is not running bringing constantly fresh air inside so there is no additional load there.

Either way, it seems likely that the results are quite a bit better than we would've expected. Once we get our ASHP heat system up and running, we'll be taking advantage of a much better COP too.

Thank you

1. | | #8

Definitely contributes, I think you've surpassed Pretty Good House and have created a Very Good House. Once you fully move in, your internal gains will increase I assume, probably coming close to cancelling out the ERV. Tack on an oversize factor for the solar gains Trevor measured and it'll come close but I still expect your real load to come in well under the manual J.

2. | | #10

Michael - what did you end up going with for windows on your build?

1. | | #11

We went with Zola tilt-turn windows.

1. | | #12

Nice. Aluminum cladding with finger jointed pine? Did you get the pine painted or clear? Happy with them?
thx

1. | | #13

Sorry for the delay Warren! I didn't get a notification.

We went with half of them in uPVC for cost reasons (bathrooms, laundry room, basement, etc.) and then we want with all the remaining windows in European White Oak Thermo Clads. Those oak windows are incredible. The doors are also the same and they are the most substantial and well built door I have ever seen.

3. | | #15

Just posting this in the main thread too since I didn't get any responses on the reply.
--
We've got another month under our belt of keeping the house heated. It would have been kept around 75-80 until January 14th and then around 65 after that (I know that is difficult to estimate). We had drywall going on and they had to crank the heat for everything to dry.

That said, the electric bill was pretty sizeable this month! Still relying on space heaters to keep the place heated as I work to get our heat pump hooked up.

12/29/21 – 01/31/22 we have 3,196 kwh @ .14/kwh = \$447.44
For my location, we had 1887.2 HDD for that period.

If I followed your calcs previously, that is 3196 kwh x 3412 btus/kwh - 10,904,752 / 1887.2 HDD = 5778.27 btus/HDD

X65 = 375,587.55 / 24 = 15,649.48 btu/hr

That is quite a bit different than last month, however, it was colder and it was a very cloudy month.

Can you help me understand what this means and if this is reasonably accurate or better than my heat loss calc still? That would be 26,749 btuh at -26º.

Furthermore, in a climate with 8608 HDD annually, can I just multiple my kwh for January's HDD to get my electric usage for heat annually if I were to use resistance heat? Then, I could divide that by the COP of my Mitsubishi 30k heat pump to understand what I might expect for annual heating cost?

Thank you!

1. | | #16

Sorry about that! That's right: 5778.27 btus/HDD x (65-(-26)) /24 is about 21,909 Btu/hr. Probably a bit lower since you kept the temperature high for the first portion of the month.

"Furthermore, in a climate with 8608 HDD annually, can I just multiple my kwh for January's HDD to get my electric usage for heat annually if I were to use resistance heat? Then, I could divide that by the COP of my Mitsubishi 30k heat pump to understand what I might expect for annual heating cost?"

This is totally reasonable - you won't really know your Mitsubishi's annual COP but estimating is fine. It'll probably be between 2 and 4. It'll get you close enough.

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