Heating, Cooling & Hot Water Sheet

Bigger is not safer

An oversized air conditioner cools the room to the thermostat setting quickly and then shuts off, which sounds like exactly what you want. The problem is that pulling moisture out of the air takes running time, and a unit that is done in four minutes never gets there. The room hits the right temperature and still feels damp, and the compressor wears out faster for the short cycling.

About twenty BTU per square foot is the cooling baseline. Add for a sunny room with big south or west windows, add for a ceiling over eight feet because you are conditioning the volume and not the floor, and add roughly 600 BTU for each person past two.

Insulation: measure what is already there

The number that matters is not what you are adding, it is what you end up with. Push a ruler down through the existing insulation to the ceiling drywall and read the depth — not to the top of the joist, which is higher. Multiply by the R per inch of whatever is up there: old blown fiberglass is about R-3.2, cellulose about R-3.5, batts about R-3.7.

Subtract that from your target and you have the R-value still to add, which divides by the R per inch of the new material to give a depth, which times the area gives the cubic feet, which divides by the bag coverage to give the bags. The sheet runs that chain; the Department of Energy target is R-49 to R-60 for most of the country and R-30 to R-49 in the warmest zones.

Hot water is an hour, not a day

Everyone sizes a water heater by household size, and it is the wrong question. What matters is the busiest single hour: two showers back to back, the washer running, someone doing dishes. The tank refills between those bursts, so a day's total tells you nothing.

List what actually happens in that hour and add up the gallons. A standard shower is about 20 gallons of hot water, a low-flow head 14, a rainfall head 30. A filled tub is 25. A dishwasher is 7, a front-loading washer 15.

Then shop on the first hour rating printed on the yellow label, not on the tank size. A 40 gallon gas heater recovers fast enough to beat a 50 gallon electric one over an hour, which is why two tanks with the same number on the side are not the same appliance.