boiler sizing question
i have used a heat loss calculator to get a BTU/hr estimate for my house. (to be conservative i am using 0 degree design day for philly, and 70* indoor temp whereas really 10* and 65* would be fine. plus i will add 20% fudge factor)
which spec should i use to compare?
- DOE heat capacity
- IBR net rating
Well for starters, you're very heavily adding to the load on the system. Decreasing the design temperature outdoors and increasing it indoors is already a 'fudge factor' without adding another 20%. So if all else is accurate, then I'd eliminate the 20%.
I=B=R ratings are for 'pick-up' load on the unit. For a water boiler, that's about a 15% allowance and for a steam boiler it's about 33% allowance. "Pick-up" is the term used to initially heat the boiler and the water in the system with no load; that is, no radiation attached to the distribution system, just he water in the system. The excess Btu's are available to heat the home through the connected load. If you're being conservative, use the I=B=R numbers and you'll be just fine. These days, using small volume distribution, such as pex tubing and copper finned baseboard, pick-up is not a big deal. If you've got an old home with big pipes and cast iron radiators, then it's a different thing and I'd definitely figure on pick-up in such a situation.
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