Alex Frost
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Also, how can I figure out how many CFM's woud each room require? I know that is the BTU requirement per room but can't figure out how to come up with CFM's.
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Assume the output of the mini-split is going to be something like 105F-115F, and that the return air leaving the room is 65F, a 40-50F delta-T.
The specific heat of dry air is about 0.018 BTU per cubic foot per degree F.
At a 40-50F delta-I, every cfm it will be delivering at least 40F x 0.018 x 60 minutes/hour= 43 BTU/hr to (50F x 0.018 x 60=) 54 BTU/hr.
Say an unoccupied basement bedroom with a design heat load of say, 1500 BTU/hr, would have a heat load of 1500- 250 BTU/hr per sleeping human. You don't really care what the temp is when no one is there, so call it 1250BTU/hr. That an absolute minimum requirement of (1250 / 50 =) 25 cfm to (1250/43=) 29 cfm, but it's fine to give it quite a bit more for better air mixing, or to account for return paths that aren't pulling air from near the floor. As long as the cfm ratios to the different rooms are proportional to their heat loads it'll do the right thing.
If you follow ASHRAE 62.2 ventilation specs without a heat recovery ventilation system the ventilation alone could represent a heat load that big or bigger if sourced directly from outdoors, not air from the rest of the house that is already being heated with the main mini-splits.
The specific heat of dry air is about 0.018 BTU per cubic foot per degree F.
At a 40-50F delta-I, every cfm it will be delivering at least 40F x 0.018 x 60 minutes/hour= 43 BTU/hr to (50F x 0.018 x 60=) 54 BTU/hr.
Say an unoccupied basement bedroom with a design heat load of say, 1500 BTU/hr, would have a heat load of 1500- 250 BTU/hr per sleeping human. You don't really care what the temp is when no one is there, so call it 1250BTU/hr. That an absolute minimum requirement of (1250 / 50 =) 25 cfm to (1250/43=) 29 cfm, but it's fine to give it quite a bit more for better air mixing, or to account for return paths that aren't pulling air from near the floor. As long as the cfm ratios to the different rooms are proportional to their heat loads it'll do the right thing.
If you follow ASHRAE 62.2 ventilation specs without a heat recovery ventilation system the ventilation alone could represent a heat load that big or bigger if sourced directly from outdoors, not air from the rest of the house that is already being heated with the main mini-splits.
