Yo:
I have a two story house with central air conditioning. In the summer when the downstairs is at an even comfortable temperature the upstairs gets too hot and uncomfortable. As the evening approaches it gets worse. At night we have to kick the air up to the point where the downstairs is like a freezer to get the upstairs tolerable.
I suspect what happens is our roof acts as a giant solar collector all day long (dark roof shingles) and then at night it cooks the top floor.
The house was built in 1993 and I think there are enough vents in the roof – they are all just standard vents – no power vents or anything.
What I was wondering is if I put a whole house fan at the top of the stairwell leading up to the second floor do I need to put more or bigger vents in the roof or will the existing vents be enough to exit the extra air pressure from the whole house fan? My theory is that if I can push out all that hot air faster the upstairs will be cooler and it will also pull up mor AC from down below or even from out side if we open the windows.
I would assume if there is not enough vents in the roof there will be a problem with a big fan shoving air in there too fast (or is it?).
Any advice and counsel is appreciated!
Replies
You have a common problem that is cause by all the insulation in the attic.
The insulation will actually store the heat during the day or slow the transmission of heat. In the evening when the outside air is nice and cool, it will be hot on the 2nd floor cause the insulation radiating the heat to the second story.
Except for the electricity consumption (less than A/C) and outside dust coming in, a whole house fan work great. By being in the ceiling of the 2nd story and blowing into the attic, it will cool down both the attic and the whole house. Just open windows to cool certain rooms. The attic does need dormer vents to exhaust out the hot attic air. I suspect you do not have them in the upper portion of the attic, thus trapping a lot of hot air.
By the way, those wind turbines will actually slow down the flow of heat, a hole in the roof draws out more hot air and a chimney of several feet (hot air siphon) will draw out even more.
No problem with too fast, except in the winter time.
Thanks.
I have been reading past posts and have learned much. I have blown in FG insulation in the attic floor (roof joists) and nothing on the rafters (I can see the roof deck). If I could have I would have put cellulose in it instead - but I was unaware of what the contractor was doing. The rafters are 2 X 4 so not much point in trying to put insulation in there.
Also, the attic is very low and small - VERY difficult to move around in and it would be almost impossible to try and put a reflective panel or something on the rafters back to the soffits.
If I install a whole house fan, I would need to cut the hole at the top of the stairs as there is no oepning there now - access is through a bedroom closet ceiling hatch.
I would put it on a wall switch for manual control and not have it on any thermostat or automatic control.
How would insulation "store heat", except for a tiny amount ? I would think that fiberglass batts, for example, have so little mass that they couldn't store much heat at all. Isn't it just that as the attic gets hotter and hotter toward the end of the day, more heat is eventually transmitted through the insulation ?
I don't know where the heat is stored.And I don't remember all of the details, but the Florida Solar Institute has done a number of studies on attics, insulation, types of roofing, venting, radiant barriers, etc.One of the reports was published in JLC a couple of years ago.Anyway they show that with some designs that there was a large increase in heating load in the late afternoon, early evening from stored energy.IIRC that was from "insulation". But I don't remember the type and if that was combined with any of the other factors such as light color roofing or radiant barriers.
> The attic does need dormer vents to exhaust out the hot attic air. I suspect you do not have them in the upper portion of the attic, thus trapping a lot of hot air.
Also consider venting the ridge, like Shingle Vent II. Hot air rises, so the best thing is to let it escape at the highest point.
-- J.S.
In general combining a whole house fan with AC is bad news.
It will have to open windows downstair to allow air into the house. Otherwise it will back draft through frunaces, water heaters, etc.
And you will be bring in hot air from the outside and pushing the cooled inside air out through the attic.
And if you are in a area with high humidity it will be even worse.
Two things too look at to improve the situation.
One is to put in insulation in the attic that blocks infra-red such as blow celuslose.
The other is to have a return air vent in the 2nd floor. Problem there is none so the hot air sits there and when the AC blows cool air out of the vents it just flows down the stairs to where the returns are.
There are return air vents in the upstairs, but it does not seem to make much difference.
You concern about the AC is one that I have as well, however, my intent would be to run the whole house fan primarily in the evening with the windows open. In the evening (like right now for example) the outside temp is quite pleasant and there would be no need for the AC if I could just get the outside air in and the attic to cool down.
I am thinking that with the sun down (or just about) in the evening and windows open and whole house fan on, the house would cool down fast and stay cool all night. I might only have to run the fan for an hour to nake that happen.
Right now my ceiling fan is cranking and the windows are open. At about 2 or 3 in the morning the upsatirs is just about right. I am hoping a whole fan appraoch as I indicated would make that happen faster.
Where are you located?In many area the amount of moisture that you draw in would cause the AC to run several extra hours just to remove the moisture.
I am in mid-Michigan. I would probably not run the AC after putting on the fan in the evening toward night, at least not until the next day when it started to get warmer outside and then inside the house. I would endeavor not to run the AC and the fan together at all.
You may have already thought of this, so I don't mean to come across as a smart-a$$ in saying it. But have you thought about simply closing some of the supply air vents downstairs?Cool air naturally drops, and warm air rises. So the warm air goes upstairs and the cool air stays down. Since it sounds like you only have one HVAC system, there's no way it can handle both heating and cooling 2 floors evenly without some adjustments. That could be a simple no-cost fix to your problem...
I don't want yes-men around me. I want everybody to tell me the truth even if it costs them their jobs. [Samuel Goldwyn]
Several possibilities come to mind.
You say there are roof vents, but are there adequate intake/soffit vents to accompany them? If so, are these soffit vents perhaps covered over by the blown FG and cannot provide intake air?
Presuming forced air heat and AC distribution………have you tried rebalancing the system (dampers in ducts) for the cooling season? Choke down the first floor ducts (thermostat on first floor, yes?) to cause more flow to the second floor. If you find the right balance, get out a Sharpie and mark the proper setting (position of damper handle) on those ducts for the summer and winter.
Is the first floor thermostat so close to a wide open register that it gets satisfied too soon? Close off or seriously choke down that register.
Duct work running thru the attic space? Insulated? Have you checked the joints for leakage? If so, seal those up with mastic.
Open staircase to aggravate the situation? Not much you can do about that.
We have tried the vent option but it does not seem to help much. We have shutoffs on the basement vents and we close the first floor vents.
I did check the soffit vents from the outside of the house and they are pretty lame (spaced about every 3-4 feet and not very large). I wouldn't be surprised if the dolt that built the house plugged them up when he blew in the FG insulation. When winter comes I may go up and check but it is pretty hard to get back there.
Sounds like my house. I'm also in Mid-Mich (Fowlerville) and we have a Bungalow. Our upstairs was 10 degrees warmer than downstairs.
All I did was close EVERY register and damper for the downstairs to force all the cold air upstairs. That seemed to help considerably.
I'm also thinking of moving the thermostat to the stairs because that's a more median temperature area for the whole house.
Customer complaint sounded very similiar to your problem. HVAC man found missing collar clamp had allowed insulation to slip and expose 2" of return air plenum to attic heat.
If you only have one A/C unit for the whole house this situation is pretty much unavoidable because the cooler air will always fall to the lower level. Any thermostat setting that makes the downstairs comfortable will leave the upper level 5-10 degrees hotter. A dual zone unit with separate thermostats and air returns will help if you want to pay for the additional equipment.
In the late afternoon, heat radiates from the attic structure and into the rooms below. A layer of radiant barrier film on top of the existing insulation might cut down on this heat gain. Make sure it's the perforated type so moisture doesn't get trapped.
I would assume if there is not enough vents in the roof there will be a problem with a big fan shoving air in there too fast (or is it?).
Depends on a bunch of things. Like what temperature is the air? If it's 140º in the attic, it will take quite a lot of 90º outside air to cool it.
Cooling the attic with vents is a bit like cooling you car by cracking the windows--until you get to full open and over 45mph, there's not much real change (only about 5-10º in my car, for example).
Now, an attic fan would be drawing air you paid to aircondition up into the attic to cool the attic which is being heated by the shingles & roof deck mass. Now, there are some climates where moving the 80-85º upstairs air int othe attick works. Except for one thing--where does the air to replace what just went into the attic come from? Infiltration of 90º outside air is probably not the best answer.
What you probably need is some sort of thermostatically controlled fan to recirculate the air through the existing return air system. Or, you need two zones on the a/c system. One can be as expensive as the other--makes it a hard call. You could also foam the bottom of the roof right over the bottoms of the rafters, too--but that's going to be a dime or two, two.
Lots of answers--no hugely good ones, I'm afraid.