Residential HVAC 1

[spiridij]

I have a 3 story home with an un conditioned third floor (with an insulated attic above that). I'm obviously losing a lot of heat to the third floor in the winter since all three floors are connected by stairs without any doors separating them. As a result the first two floors are never as comfortable as I'd like. If I were to heat the third floor would that cause less of the heated air from the first two floors to rise to the third floor since there's a lower driving force pushing it up if I set the thermostat at a higher temp on the third floor? I'm thinking about adding a ductless system to the third floor, and I'm wondering if it might actually help make the first and second floors more comfortable as well. Same question for air conditioning in the summer. Any thoughts?

John

 

[IRstuff]

I think that's what the thermostats are for. In most scenarios, the thermostat(s) will eventually adjust to the new conditions, but it does greatly depend on where the thermostats are and how many thermostats there are.

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[spiridij]

I'm not sure what you mean by that's what the thermostats are for. There are currently thermostats on the first and second floor. The ductless system would have a "thermostat" in each of the rooms on the third floor. So my question is if I crank the heat on the third floor would less of the heated air from the first and second floor rise to the third floor?

 

[IRstuff]

Yes, but the thermostats on the other floors would throttle down so that the decrease in heat loss would be compensated by the heaters pumping less heat.

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[Compositepro]

Heating the third floor will not help. Adding heat to the first will help. Have you eliminated air infiltration? If you have air leaks, warm air will leak out at the top and cold air will enter at the bottom. If the top floor gets too warm when heating the lower floors, then you need to correct stratification by moving air from the third floor to the first. Central air could do this with correctly balanced registers, but the balance changes from winter to summer.

 

[spiridij]

So by heating the third floor I wouldn't be eliminating the driving force for the heat from the first two floors from rising to the third floor? Isn't that the second law of thermodynamics?

 

[GT-EGR]

The scenario you described likely has two options for improvement.

Stack effect seems to be what you are leaning towards. But that can be reduced by sealing up the top floor of your house - walls and ceiling. If you accomplish this then you’ll lose the warm air when you intiitally startup, but then that air will be relatively stuck there, and you won’t lose anymore. If your top floor is leaky it is a real concern because stack effect is basically sucking in cold makeup air from the lower floors, creating an additional challenge of having to heat that air too.

From a heating capacity standpoint, if your 3rd floor is unconditioned, but is directly connected, then you’re heating syatem for the connected rooms needs to have been sized for that heat loss as well. So yes, adding another heating syatem with the capacity to heat the third floor will take the burden off your other units.

 

[spiridij]

Thanks GT-EGR, that's exactly what I was thinking and haven't been able to confirm it. Thanks so much for your help.