Baffled

[dannyb]

How can my home be warm inside on a cool morning? The 1,980 sq.ft. house is under three years old with a Bryant 13.5 to 14 SEER system. It has a 4 ton two-speed condensing unit with a 5 ton air-handler. The walls have sprayed in cellulose. The attic has blown fiberglass at R38. I have pretty good aluminum windows, if there is such a thing.

The temperature outside at 7:30 A.M. was 72. The inside was 77 with a thermostat set at 76. How can the A/C be running? This makes no sense to me.

[mr big]

If your tstat is set at 76 & its 77 in the house; sounds close enough to me. Your structure has a certain amount of built up heat in it that enters your house along with your lighting & cooking etc.

[Shophound]

A combination of interior generated heat and thermal flywheel effect of the building structure. The R38 insulation in your attic is still subject to radiant heat from the roof decking. Insulation slows the transfer of heat but does not stop it. Eventually the radiant heat that beats on the insulation fibers all day long warms them up, along with ceiling joists. At night as the attic begins to cool, some of the heat built up in the insulation and framing members will transfer to the cooling attic air, but it will also migrate into the structure via conduction and radiation. This migration will occur as long as there is a temperature difference between your living spaces and the attic. Night cooling of the structural members and attic air will offset this somewhat, but if the attic is slow to cool down at night, as many are, this built up heat will continue to radiate into the house all night long, where you find yourself waking up in the morning to a house stilll warmer than outdoors.

One thing to bear in mind; if you live in a humid climate, your outdoor temperature may be below indoors in the morning but the humidity level will be a lot higher than the evening before. If you opened the windows to cool the house you would introduce a lot of humidity into the house, making you feel uncomfortable and inviting other high humidity induced problems into the house.

Interior generated heat comes from computers, appliances, lighting, showering, laundry, cooking, cleaning, people, etc.

[CottyGee]

Don't forget insulation works both ways!

Is opening a window an option? Otherwise, you could get something added to your system - forget what it's called - that would allow you to pull fresh air inside when the temp outside is lower than the temp inside and you're in cooling mode.

[dannyb]

Thanks for the feedback! I had to look up thermal flywheel effect. I learn something every day.:)

[gonekuku]

To support what these guys stated - if it's 77F inside and 72F outside in the morning, the thermal mass of your home inside the insulated envelope is still hot from the day before. If the differential is 5F between inside and outside, it can take 6-12 hours to equalize in a well insulated home.

You could use a whole house fan to suck air out, which would result in fresh cool air. I've experimented with this, and my conclusion is that the moist air this time of year (in Dallas) decreases comfort. I would rather spend $1 on HVAC while I sleep. Also, sucking 68F air through a 76F house has very little effect for anything outside the direct airflow path. You need turbulence so that the air absorbs more heat. Applying aluminum heat sinks on the walls would help.

I think the whole house fan can work well in a dry climate if you are willing to open / close many windows for the night, otherwise, it's not worth the trouble. Add rain, and you'll have water damage on your window sills.

Also, keep in mind that if it was 72F at 7AM, 4AM may have been 74F, 1 AM, 76F.

No free lunch(cooling) - That's what I think.

[subcooled_]

Don't forget insulation works both ways!

Is opening a window an option? Otherwise, you could get something added to your system - forget what it's called - that would allow you to pull fresh air inside when the temp outside is lower than the temp inside and you're in cooling mode.

Sounds like your talking about an economizer...not practical for most residential applications due to install costs, large outdoor air intake on outside of house, and lack of available "free cooling" (how often is a house warmer than the outside and you want cooling)?

[CottyGee]

Sounds like your talking about an economizer...not practical for most residential applications due to install costs, large outdoor air intake on outside of house, and lack of available "free cooling" (how often is a house warmer than the outside and you want cooling)?
Ah - yeah, that's the deal. Not cost effective huh? Didn't know that.

In my part of the world, there are many folks with dual systems - evaporative cooling and air conditioning.

[subcooled_]

Ah - yeah, that's the deal. Not cost effective huh? Didn't know that.

In my part of the world, there are many folks with dual systems - evaporative cooling and air conditioning.

Well, there I go again assuming everything from the Minnesota stand point.:eek: HERE it's not a practical residential setup.