measuring heat loss-check my math?

Spudwrench

Ok, here's my situation. I am trying to emprically calculate my heatloss... at 140* water temps, my boiler will run just about continuously at 13 deg. outside temps. Heat output from all my convectors @ 140 deg is 23,619 BTU. (I'm only heating 1 floor of a 2 flat with this system) A design day here in Chicago is -4 deg, so rather than the temp diff being (68*-13* =55 deg) it would be (68*- -4*=72*)... would that make my ACTUAL heating load on a -4deg day around 30,919 BTU? Thanks in advance for any insight!
Nathan

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The base for degree-days is 65, not 68 (no big deal). Your calculation is basically fine, but you are taking your calculator's display too seriously; you cannot calculate these numbers exact to the last digit! Round it up.

I had understood that the design day in Chicago was -10F. But I've also heard 0F. In pratice it must depend on distance from the lake.

How do you know the heat output of your convectors? If your boiler is running all the time, is the boiler's output also 23,619 (!!) BTU? So the boiler's rated input is about 30,000BTU? If not, there is an inconsistency.

Spudwrench

aah, I see...

Thanks for your reply. Actually, the "boiler" is in fact a Takagi tankless hot water heater with an electronically modulated burner. The input ranges from 19K BTU to 140K BTU dependant on the delta T sensed by the heater. The size of the boiler isn't directly relevant to my question, though... 'cause it can't output more BTUs to the house than the radiation can emit, right? (at least in a steady-state condition.) A 50K BTU Munchkin would probably be a much better choice, but at 4X the cost, hmm. Someday, I guess.
I know the heat output of my convectors (Myson panel rads) from the manufacturer's specs...the BTU outputs are spec'ed @ 175* AWT but they provide a deration factor for lower water temps.
I suppose I fully deserved the ribbing about my failure to round the BTU numbers...I just thought it would be easier for someone to understand how I arrived at my calculations if I didn't round.
I didn't know that the indoor design temp for heat losses was 65*. Seems odd...most people would (I imagine) like to maintain an indoor temp of 68 on a design day. Anyway, so far the system has kept up fine with a 0 deg day here, though I do have to raise the water temp to 180* to keep up when outside temps are below 13deg. As for -10* temps, I guess we will see someday! Judging by the non-continuous duty cycle at 0 deg, I think it'll do fine.
Thanks for your thoughts.
Nathan

Chuckles_3

The 65F figure comes from gas company data. Demand is a combination of a weather-independent part and a part that varies linearly with degree-days. The linear part extrapolates to zero at 65F. Why is it not 68F or 70F? Who cares, that's the data. Internal heat gains (people, cooking) maybe. Anyway it is for the total demand of many homes and may not apply to your home, which is one more reason why the numbers should be rounded up.

hr

I'm not convinced

you will ever see that 140K out of a tankless like that,in a heating application. I'm not sure you can pump that coil!

I know other tankless brands, innthe output size, limit the output to somewhere around 60K due to the small coil.

hot rod

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