Sizing hot water boiler for for use with hot water coils?

paul_43

I would like to know if anyone can tell me how to properly size a hot water boiler to feed 3 hydronic coils , in 3 seperate air handelers? The boiler must be a direct vent boiler, due to the fact tht there is no chimeny.
Coil # 1- 107,000 btu
Coil # 2- 107,000 btu
coil # 3- 60,000 btu
Open for suggestions
Thanks in advance

bob young

heat loss

should probably be sized to heat loss calculation

Ron Schroeder

Hi Paul,

The BTU output of each air handler will also be dependant on the air handlers air flow and the entering water temperature. Getting the air flow low enough and the water temperature low enough to just meet the structures heat loss at the design temperature will will save fuel. The boiler's output should be just barely larger than the structures heat loss at the design temperature.

Brad White_9

As Bob suggested

you should make sure your coils are based on calculated heat loss for starters. If they are, it is a matter of adding the coils and selecting the smallest boiler that meets the load plus a margin for piping and warm-up. (I suggest 10% assuming insulated pipe within the heated envelope.)

Ron's points are also excellent.

That said, you can get coils (if not already purchased) with two to six rows for maximum heat extraction (for use with low water temperatures). There is a "rule" that says coils need 130 degree F water. I do not necessarily buy into that. It is a function of leaving air temperature. How much air are you moving across each coil? Your BTU ratings will vary depending on what the entering air temperature is. If you know the rating parameters you can extrapolate within reason to what your leaving air temperature will be.

Even if just warm enough to heat the room (a lot of air at a lower temperature) the room may not feel as warm. (More air at a lower temperature will carry the same heat as less air at a higher temperature). Distribution is key also. If supply is low at the perimeter and return toward the interior, that is best. If heating from above from the interior, that is the worst for comfort. Enough variables for now... What is your situation?

Brad