Boiler sizing from gas usage

curiousburke_2

Hello All, I posted a recent thread on boiler sizing in the case of exposed steam mains. As a sanity check I am trying to calculate the necessary boiler size from my monthly gas usage and local monthly average weather records. I read that this can be done, but not that it could necessarily be done with monthly averages.

What I did is calculate an hourly BTU usage for each heating month that I had good data (only 5 months). I first removed my summer usage from each month, and I scaled the remaining usage by 129/210, which are the NET IBR output and BTUH input of my current boiler. I plotted this versus the monthly mean temperature and found the linear trend line. Then I used this equation to calculate the NET IBR BTUH as a function of outdoor temperature. My result is: BTUH = 51646 - 912*temp, so at zero Fahrenheit I use 52K BTUH, which seems low but at least ballpark reasonable.

I think this is a linear system, so, I think, I can use monthly averages to calculate the BTUH as a function of outdoor temperature, and that this tend line is valid for daily temperatures as well. What do you think?

Snowmelt

first you have to get your degree days for the month for the city you live in

curiousburke_2

In fact, that is how I calculated the average monthly temperature, from the degree days. And, amybe that was a mistake if every day that month was not a heating day. If I restrict the linear fit to months that would have had heating every day I get a slightly different answer:

BTUH = 59538 - 1140*temp

and I should have enough power in the 92k boiler for days as cold as -30F. Actually, I figure even more, since that includes a pipe/pickup factor of 33%, which would be available if the boiler is running 100% of the time

curiousburke_2

Ah, I see why you would want to use degree days, because then you can solve for the fuel usage per degree day directly and not have to worry about whether every day the boiler was running. This wasn't as intuitive to me, but the results are about the same: 857 BTUH per degree day, so around 60K BTUH on a zero degree day.

curiousburke_2

Thanks, I see that and you're right, it makes more sense to calculate the actual heat loose. However, I've been shopping by IBR so I wanted to convert it anyway.