Cooling down the house
Q: How long should it take to cool down a house? We had a new air-conditioning coil replaced yesterday afternoon — it was an extremely hot day — and by last night the house had only cooled off to around 81. Shouldn’t it have gotten cooler quicker than this? I want to be sure this new unit is working properly.
A: I talked to some energy researchers who say this effect is called “pull-down,” and under normal conditions it should take four or more hours to reach the set-point as long as it is not set too low.
Generally, an air conditioner should be able to provide an interior temperature no less than 25 degrees F lower than the outdoor temperature. As this is approached, the time is takes to reach the desired set-point becomes longer, especially if the unit is started up in the afternoon.
Let me suggest that if you feel the unit is not cooling off enough after a few days of use that you get someone to inspect your ductwork if you haven’t had that done already. You may wonder why you’re seeing so many ads these days for companies to do duct cleaning and duct inspections, and it’s simply because research has found that leaky ducts are among the biggest culprits in wasting energy and making a home’s interior uncomfortable. Even properly installed and sealed ductwork can tear over time, causing your cooled or heated air to leak out into the attic and other unconditioned spaces.
Hire a contractor to do this work since I don’t recommend it as a do-it-yourself project. I’ve talked to too many people who tried inspecting or cleaning their ducts themselves and quickly found out that they caused more problems than they fixed.
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Q: This summer’s heat really pushed our cooling system to the limit, but what bothers us the most is that part of the house stays cool but part never seems to cool off. I was ready to clean the filter in our range hood today when I felt that it was very warm, so I opened the cabinets over the hood and almost burned my fingers on the hood’s venting. I inspected the system a little and I think the range hood vents into the attic. Can this be possible?”
A: Unfortunately, it sounds like some corners were cut when your home was built or when a do-it-yourselfer put in the range hood, and something like you describe doesn’t even meet fire code, let alone common sense. Get someone out soon to continue the pipe stack to the roof line and have a vent installed.
Exhaust fans in the kitchen and bathroom are very important in getting heat and humidity out of the house, so it is essential that they vent to the outdoors. Putting this heat into the attic means it will only get back into the living space, probably accounting for the home never really cooling off in hot weather.
Much of the success of keeping a home comfortable is protecting the thermal envelope around it. This means that you seal cracks, holes and other unwanted openings and you have adequate insulation levels in the walls and ceiling. But this also means that you need to use vents to get rid of heat and humidity that is created inside the home or it will stay there. Research at the Florida Solar Energy Center has found that fully one-third of the load on the air conditioner is generated within the residence, largely caused by heat and moisture given off by people. Appliances add to this load, particularly the refrigerator since it runs all the time. Cooking and bathing are two other load sources. Vent fans sending internal heat and moisture outdoors are key ingredients in making a home comfortable.