# Thread: I there a relation between HDD and heat loss calc ?

1. Regular Guest
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Aug 2008
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Seattle, WA
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Originally Posted by beenthere
You asked at what temp is heatloss relevant. Its relavent below 65°F.

Questions ask in a vague context, will often get an incorrect, or unexpected answer.
Oh a ball buster, eh? I'm in good company

So when a contractor says "yo dude, I calculated your heat loss at 17 MBTU", at what outside temperature should I expect to realize that amount of heat loss?

2. New Guest
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Oct 2008
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Thunder Bay, ON
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Isnt the heat loss calculated at something like the 90% factor temperature.. like where the temp is higher than that point, 90% of the time (in the heating season) ?

Anyhow.. Im still reading this thread (Im not a post and vanish kind of guy).. Im just having a hard time finding that 'design temp' for this area to redo my loss. Nothing via google yet, but Ill keep looking. (Its Thunder Bay, Ontario in case someone has a table etc).

3. For Seattle Washington.
Winter outdoor design temp is 26&#176;F.
Thats your 97&#189; design dry bulb temp.
And that is what your heat loss shold be based on.

4. Thunder Bay, Ontario, Air Port.
And according to my manual, you have 10,405 HDDs under 65&#176;F.

5. New Guest
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Oct 2008
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Thunder Bay, ON
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Thanks, beenthere

So, for -24F, the Slantfin app says I need 75000 btu/h.

My avg oil use seems to be about 2400L, or 87Mbtu (and at 80% eff 70Mbtu actual heat.) (heh,i forgot to incl my actual energy used in my first post).

So, is it possible to say set up a graph, with one axis +65 to -24 (or hdd's), and the other axis 0-70.. ? What shape would the line be ? Sunny days like you say would put you lower (btu input required) than the line and cloudy or night would push you above it ?

Actually, now that i think of it.. the higher the required input (high hdd's / low temp), the more the boiler is running and the more efficient it is (less standing stack loss too).

Sorry if Im gettin kinda weird on yas.. this stuff interests me.

6. It would be a straight line.
Since its temp only based, there would be no curve.

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