Originally Posted by

**SD4US**
I'm trying to follow Dana's very nicely detailed (many thanks for it) heat load calculation in post #5 (using my own numbers) but there is a step there, where something seems wrong:

*"so with a maximum of 780 BTU/degree- hour, that means your heat load at + 5F is less than or equal to 60 x 780= 78,000BTU/hr as an absolute max, which is pretty close to your online heat load numbers"*

Either you factored in something unknown into the above equation or it could be a simple mistake, as 60 heating degrees x 780 BTU per degree-hour = 46,800 BTU/hr, which is no longer very close to DPJ's online heat load figure (75,000).

You felt (I guess correctly) that 78,000 BTU/hr was a significant over-estimate and further lowered it based on DHW and old boiler efficiency and came up with an end result of 55,000 BTU/hr (which you felt was still on the high side). If the quoted calculation was indeed incorrect, that final number would be only 33,000 BTU/hr.

Does this make any sense?