I live in a townhouse condo, with a basement and three floors, that has openable skylights on the top floor. I get a nice, if usually mild, chimney effect to clear out stale air by opening the skylights along with windows and sliders on the first and second floors when the outside air is cooler than that inside.
But what should I do when it’s hot and humid outside and the central air conditioning is on? The condo, not surprisingly, gets warmer with each floor up despite the AC and becomes at times stifling on the third floor.
If I crank the skylights open, would that draw cooler air upstairs and circulate out the stuffiness? Would turning on the ceiling fan on the third floor hasten that? Or would opening the skylights allow humidity in and counteract the AC?
A couple of possible problems that could cause this:
Insufficient return air venting. Modern heating/AC relies on one set of vents to distribute the hot/cold air throughout the house, and another set of ‘return’ vents to bring that air back when its heat/cold has been distributed, and then it is reheated/recooled and sent around again. (This saves energy – the returned air is still warmer/cooler than starting from outside air. Many older houses don’t have enough return vents. Sometimes they don’t have any, like in houses built without AC when built.
Insufficient attic insulation/venting. The attic above your top floor is basically at outdoor temperature – no heating or AC in there. And vents to let hot/humid air out of the attic. A temperature in there different from the outside is basically due to leakage from the house. Insufficient insulation will allow heat from the attic to radiate down into the top floor. And insufficient attic vents will keep hot air inside the attic from venting to the outside.
Both of these require a bit of remodeling work to permanently fix effectively.
The condos were built about 30 years ago. The first floor has rooms at the front and back of the unit, with a central core containing a half bath opening to a hallway, and a galley kitchen on the other side of the core; it’s all open, the cats can run rings around the core. The second floor has bedrooms front and back with a full bath between them and a hallway to the stairs up to the dead-end top floor. There’s a large return air intake grill in the first floor core wall facing the hallway, and I don’t believe there are any others in the unit, only floor vents for heating/cooling along the front and back outside walls. I keep the whole place open, no doors shut, for maximum air circulation.
The air flow from the top floor vent seems feeble compared to the lower floors, I think it loses some oomph by the time it gets up there. I just recently had the original air compressor replaced with a new unit; perhaps that will make a difference.
There’s a ceiling fan on the top floor (which is basically a spare bedroom, not used). I could let that run. Would opening the skylights serve the purpose of attic vents?
Pushing air up three floors is hard. I lived in a similar unit - my landlord/roommate lived in the top floor and kept the bottom floor vents closed to help, letting the cold air settle to the bottom level. If you don’t use the top floor regularly, have you considered closing those vents/doors?
I wouldn’t close the top floor off in the winter since there are twin sinks, a shower, and a toilet off the main bedroom up there and I’d be afraid the pipes would freeze. I suppose I could close it off in the summer, although it’s one of the places the cats will flee to when there are STRANGERS!!! in the house.
I’m still wondering whether, all else equal, having the skylights open or closed would work better when the windows are closed and the AC is on.
Remember that hot air rises and cold air sinks. No matter how warm my house gets my basement,which is partially underground, is always nice and cool.
You could try closing some of the return vents on the lower floors in order to force more cool air to the top floor, but opening the skylight windows likely won’t help much, nor will a fan on the top floor. Moving cool air from the bottom to the top floors is difficult. Perhaps it’s time for a upgrade of your A/C unit?
If you open the skylights to vent warm air, air is going to have to come from someplace to replace the air you vent out. As the warm air vents out, it creates a slight negative pressure which draws air in from leaks throughout your condo. That is, the only place air can come from to replace the vented air is from outdoors. This is a good strategy when it’s cooler outdoors than it is inside but not when your A/C is cranking.
It all depends on the dew point of the outside air, which would come in to replace any hot air escaping through the skylight. Right now, the dew point in Boston is 68 °F. My back of the envelope calculations show your AC will work harder cooling and removing that humidity from the incoming air than it would cooling the hot but drier air on the third floor, unless the top floor is well above 95°F.
Any wind will make a difference too. I have several of those skylights and depending on the wind direction, one or the other will do a very nice job of “scooping” fresh air into my house. But if it is really hot outside, sometimes the air smells like the roof. Kinda tar like.
It wouldn’t do much. It just moves the warm air around – that doesn’t cool it, and in fact, the fan motor adds a tiny bit of heat. Moving air – even if warm – cools a human body by promoting evaporation. But if there are no human bodies on the 3rd floor, the fan won’t help any.
[A fan might help if it circulates the air so that it gets to a return air vent, and goes back to the AC. But it’s unlikely that a ceiling fan on the 3rd floor can force the air back down to the 1st or 2nd floors.]
You could put a cat flap in the door that closes off the top floor? The cats would probably not really enjoy going up there if it’s stuffy and hot, but at least it’s still available as their escape hatch.
Thats not a problem. The bricks ,etc of the top level have soaked up a lot of heat from the sun, now you want to blow all that heat away.
Single unit wall mounted aircon’s have a vent for exactly that reason - to vent the inside when you want to vent the inside air - eg better to blow hot air away then to use the aircon to cool it.
Having the top air vent, he can easily let the venturi effect operate… (the air near the wind will go along with the wind… thus leave the building). that takes all the hot air and the heat in the structure away …
Even better if he can open the window at the lower floor a little bit, to be sure that cold air from the aircon will be made to rise.
Dew point ? forget it. He has humid air and that means that even dehumidy mode would cool the room. In cooling mode, The air con works to bring the cold element right down near freezing, and since the air is passing by the very cold element, it dews at the element, always. So the question of whether he is bringing the air down to dew point is moot, cooling mode ALWAYS does. The error with the 68F thing is that the cold element is always much lower than that, in cooling mode. So the idea it has to work “harder” to achieve it is totally wrong. Its always working that hard to dew … That is the reason for the dehumidify mode on air cons… Instead of running the compressor until the cooling element is down near 35F , what it does is run the compressor to be just enough to keep the cold element at just a little below the rooms current temperature… just enough lower to dehumidify… the change in temperature that would reduce 90% relative down to 60% relative and the people won’t be sweating sitting still. If the people want drier air to avoid sweating, then they just set it to cooling mode and lower the air temp.