Boiler AFUE during Summer DHW

[k2qo]

Hi Gang,

I'm trying to complete some energy calcs to convince myself to go hydronic plus separate AC vs. FHA and AC. But one "deal killer" occured to me.

Assuming an 84% AFUE gas boiler, what will the REAL AFUE be during June-September when I am only needing abuit 2.5M BTU input to make DHW? This is for a 50MBH IBR gas boiler. Any ideas? Seems that the summertime AFUE is going to be kinda low comparted to a chinzy standard tank.

TIA,
Mark

[Noel Murdough]

AFUE is ANNUAL Fuel Use Efficiency.

Combustion efficiency is a whole 'nother animal.

Noel

[k2qo]

Noel,

I understand the problem. Combustion efficiency goes down when cycles are shorter. This will effectively lower the AFUE. I know its apples and oranges, so let me rephrase as a statement , you fill in the blank.

For DHW production only, a gas water heater with an EF=??? is the equivalent to a 50MBH IBR boiler that operates at 84% AFUE with a high quality indirect tank.

Better?

Thanks,
Mark

[rich pickering]

An ef of .4 is equal to about 40% afue.