# Heating system sizing.

Posted Mar 8, 2018 5:06 PM ET

Tags:

1.

"Is it possible to calculate the size of a heating system based on past energy usage?"

Yep! But it's not about the average hourly use, it's about what it needs at the coldest hours, using a linear approximation.

See:

Posted Mar 8, 2018 5:13 PM ET

2.

Using 28,000 BTU/hr at a mean temp of +6F, which is (65F-6F=) 59F below the presumptive base 65F balance point is a linear constant of (28,000 / 59=) ~ 475 BTU/degree-hour, give or take.

-19F (is this near Hibbing?) would be (65F- -19F =) 84F heating-degrees...

...x 475 BTU/degree-hour is an implied heat load of 39,900 BTU/hr @ -19F.

Heat load isn't EXACTLY linear with temperature- window losses are extremely non-linear, but that would be the right range.

So that's less than half the capacity of his electric boiler. ASHRAE recommends 1.4x oversizing as the compromise (that gets you through Polar Vortex coolth without suffering, but not ridiculously oversized), which for a ~40K load would be 56,000 BTU/hr, or 16kw.

That's the gist of it, but run your kwh/HDD numbers carefully, and use both base 60F and base 65F linear approximations to bracket it.

Is the electric boiler sub-metered, or was that the entire power use on the premises, including outdoor lights, hot water that ended up going down the drain, etc?

Posted Mar 8, 2018 5:25 PM ET

3.

Thanks Dana, that was exactly what I was looking for.

Posted Mar 8, 2018 5:37 PM ET

4.

The boiler is sub-metered at a rate of \$.0765 per KW. Total usage for the time period was 5890 KW. A high bill complaint brought me to his house. He usually heats with an outdoor wood boiler. Your close on the location, about a half hour west of Hibbing.

Posted Mar 8, 2018 5:41 PM ET

5.

A smaller electric boiler isn't going to be cheaper to run than an oversized electric boiler- the effieincy is the same either way.

If using this data to size some other heating equipment it has to be a period where auxiliary heat from the wood boiler wasn't in use.

Posted Mar 8, 2018 5:48 PM ET

6.

Got it, thanks again.

Posted Mar 8, 2018 6:23 PM ET

7.

> Heat load isn't EXACTLY linear with temperature

It can can get fairly in-exact when wind variation is factored in.

Posted Mar 8, 2018 9:39 PM ET

## Other Questions in Mechanicals

### Conventional heat pumps with mini-split low temp efficiency?

In Mechanicals | Asked by Mai Tai | Jun 22, 18

### Fastening Exterior pine trim to an ICF home

In Green building techniques | Asked by Wayde | Jun 23, 18

### Dirty electric from minisplit ductless heat and air conditioning

In General questions | Asked by Arleendv | May 30, 18

### Bubble-wrap air space 2" below in-joist radiant floor heating

In Energy efficiency and durability | Asked by user-6765846 | Jun 23, 18

### Tyvek reinvent battens

In Green building techniques | Asked by user-7088022 | Jun 23, 18