Estimating Cost

Jason_13

Is there a way to estimate the cost of using indoor combustion air as opposed to outdoor combustion air?

Brad White

The Value of Infiltration

Hold your hands about 18 inches apart, then move them back and forth between shoulder-width and describing the fish you wish you caught. That is about the range we are taking about here. (No, I am not being a "nickname for Richard" here, I am serious!)



Are you comparing say, an atmospheric boiler (huge gulps of air and excess air), or a power burner type, versus a mod-con with sealed combustion? That is why there can be such a range.  Also is the "from space" combustion air supplied by a ducted damper arrangement or are you relying on natural infiltration? Another wide range there.



If you are looking at "directly ducted combustion outside air to the boiler" versus "combustion air from natural infiltration into  the room", it gets easier.



If this is the case, here is how I would calculate this:



1) Find out from the manufacturer what their combustion air ratio is (draft fan cfm rating, basically). Also, while you are at it, find the modulation range, how far down it goes in RPM/CFM. 



You may have to make assumptions as to how many hours it operates at what speed and at what outdoor temperature of course. But for a ballpark number, I would take 70% of the full load number as a seasonal average and would include DHW generation. This is for modulation figuring, not hours of use.



Now make an assumption of how many hours per year the system runs. I would probably take any constant average number and cut it in half. This is "modeling on the fly", but all things being equal, different approaches will at least be proportional, right?



Say the manufacturer gives you a 150 cfm full capacity number. At 70%, that is 105 cfm. Typically, I multiply this cfm x 1.085 then by the design delta-T between the outside air and the basement where the boiler is located. Call it 70F, who knows.



So that 105 cfm at say 70 degree rise, I get 7,975 BTUH heat loss for an hour's operation on your coldest day.



From that number, I would apply the Heating Degree-Day (HDD) formula for your area. Say that is 6,000 HDD and the boiler runs at 90% From that and a 0.60 Cd factor, I get 304 therms of gas per year at the above figures if running all the time. But at 50% running time, this drops to about 152 therms. If you pay $1.80 per therm, that is $274 per year. When conducted into the boiler directly, this would represent what you might save.



Mitigating factors:

1) You are making an assumption that without ducted OA your home infiltration will otherwise be constant- Not necessarily so. When an open boiler fires, it starts the induction of outside air above what you would otherwise have naturally. Also, the open flue, even with a damper, draws air up the chimney all of the time. With direct vent or sealed combustion and the chimney gone, this loss disappears. This is your biggest benefit in one place.



2) If you have ducted dampers, unless the house is well air-sealed, this amplifies your basic infiltration, whether the boiler is firing or not.



Sorry to ramble, just some thoughts to get out. I hope they help.

Robert O'Brien

Very interesting topic!

I've been trying to quantify this with oil fired boilers