# Calculating Heating Load for Air-Source Heat Pump

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Sizing an ASHP from gas furnace consumption data

I am trying  to determine the optimal air source heat pump to replace our existing gas furnace before we begin soliciting contractors.

Our gas utility offers each customer the ability to download their daily gas consumption and weather data over the past year from their web site.  So, I downloaded our gas usage and maximum and minimum outdoor temperate for every day of  the past heating season.  Since we use very little hot water  (our summer gas consumption is nil) and our gas furnace is 96% efficient, I multiplied each day’s gas consumption by 0.96 and divided by 24 hours, then multiplied by 100000 BTU/therm to calculate our home’s average heat loss rate in BTU/hr for each day of the heating season.

I then divided each of these values by the difference between our thermostat temperature setting (65F) and the average daily temperature to arrive at the average thermal heating coefficient in BTU/hr-F for each day.   I then averaged all these values together to get the best estimate of our home’s average heat loss coefficient.

Finally, to calculate our home’s heating demand load at our location’s 99% design temperature, I multiplied the average heat loss coefficient determined in the previous step by the difference between our location’s design temperature (0 F) and the standard 70 F rating temperature.  I did the same using design temperatures of 5 F, 17 F, and 47 F to determine the energy consumption at standard ASHP outdoor temperature ratings.

My questions are:
1) Is this an acceptable method of calculating the heating demand load  for an ASHP?  Is there something I am doing wrong or overlooking?
2) Would you consider this more or less accurate than a Manual J calculation?
3) I’ve seen elsewhere to multiple the Manual J heating demand load by 1.4 when selecting an ASHP.  Why is this necessary?  Should I do the same for this method?

Thanks, Jim

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### Replies

1. Expert Member
| | #1

Since this method relies on direct measurement I would say it is more accurate than a Manual J which relies on estimates.

The one caveat is if your house has a lot of solar gain this method will underestimate your 99% heating load. Your gas usage is tempered by the solar gain but you can't assume the sun will be available on your design day.

1. | | #2

In retrospect, you're right. I've essentially duplicated Dana's method.

Using degreedays.net data is more accurate than averaging the low and high temperature for the day, since they integrate over every temperature reading.

Good point about solar gain. Days with weather conditions containing the word "Overcast" had an average 13% higher heating demand load coefficient (in BTU/hr F) than days containing only the word "Clear".

1. Expert Member
| | #5

Simply looking at the weather conditions is a clever and elegant solution that I hadn't thought of. Clearly solar gain is a factor, the only question is exactly how much correction to apply.

2. | | #3

Only downside is it provides averages, not peak usage
You kind of care about peak usage for a design day

3. | | #4

DC is right. I will add on to your 3rd question as there’s some important nuance there:

“I’ve seen elsewhere to multiple the Manual J heating demand load by 1.4 when selecting an ASHP. Why is this necessary? Should I do the same for this method?”

It’s not unique to ASHP and it’s to cover thermostat setbacks, so if you think you’ll have setbacks (on the coldest day of the year), you can use it. However, if you’re using this ASHP to cool as well, I’d try to size to that load, which is likely smaller unless you live in the south. That way your AC isn’t 2-3x oversized, which can be a problem for humidity control. Sizing for the cooling load can be easily accomplished if you have some sort of supplemental heat, like resistance strips or a backup furnace or boiler.

4. | | #6

Be aware that heat pump register outputs are typically 10-20F lower than from gas furnaces. Therefore you will need more airflow through your ducts. Failing to check this out before switching heat sources can lead to modest comfort and noise issues. It did for me.

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