Short answerIn Southeast Michigan, cutting the cooling bill means managing humidity, not just temperature. Run the blower on Auto rather than On, keep the thermostat at 75 to 78 with indoor humidity under 55 percent, keep the outdoor coil clean, shade west windows, shift laundry and dishwashing off DTE's peak hours, and avoid an oversized AC that cools fast but never dries the air.
The Michigan summer problem is water, not heat
A July afternoon in Ypsilanti is usually 86 to 90 degrees. Phoenix laughs at that. What Phoenix does not have is a 70-degree dew point sitting over the Huron River valley for a week, and that is what makes a Michigan house feel miserable at 76 degrees. An air conditioner here spends a large share of its energy condensing water out of the air; a 3-ton system can pull 15 gallons a day out of a Pittsfield colonial in late July. If you want a lower DTE electric bill and a house that does not feel like a locker room, you manage the moisture first and the temperature second. Everything below follows from that.
The fan setting that costs you twice
The thermostat's fan switch has two positions: Auto and On. On keeps the blower running between cooling cycles. It sounds like it should help, and in winter with a furnace it mostly does. In summer it does two bad things. It uses blower electricity around the clock, which on a PSC motor in a 2000s furnace is a few hundred watts, and it re-evaporates the water sitting on the evaporator coil after each cycle, blowing humidity right back into the house. Set it to Auto. If you have rooms that get stuffy between cycles, a short "circulate" mode on a smart thermostat is a compromise.
Thermostat strategy and DTE's time-of-day rate
A setting of 75 to 78 when home and a few degrees higher when away is the usual guidance. The savings are real: each degree higher on the setpoint cuts cooling energy by a few percent. The more important point is that 76 at 45 percent humidity feels better than 72 at 60 percent, so a system that handles humidity lets you hold a higher setpoint comfortably.
Avoid large setbacks in the afternoon, though. Letting the house climb to 85 while you are at work in Ann Arbor and then asking the AC to pull it down at 5 p.m. means the system runs flat out through DTE's peak rate window and the humidity gets ahead of it. A 2 to 4 degree setback is plenty.
DTE's residential electric rates include a time-of-day structure with higher summer prices on weekday afternoons and early evenings, roughly 3 to 7 p.m., and lower prices overnight. Air conditioning is the largest summer load, so this is where the rate matters.
- Precool the house slightly before 3 p.m. and let it drift a couple of degrees during the peak window.
- Run the dishwasher, laundry and the electric water heater, if you have one, after 7 p.m.
- A smart thermostat on DTE's demand-response program will do some of this automatically and earns a small credit for it.
Check dte.com for the current hours and rates, because they have been adjusted since the time-of-day rollout.
The equipment side
Keep the outdoor coil clean. A condenser coil packed with cottonwood or dryer lint cannot reject heat, so the compressor runs at higher pressure and draws more amps for the same cooling. Rinse it in spring and again after cottonwood season. The full list is in prepare your AC for summer.
Keep the filter clean. A restricted filter lowers airflow across the evaporator, which drops the coil temperature, which, counterintuitively, does pull a bit more moisture at first but quickly leads to a frozen coil and no cooling at all. Change it on schedule.
Get the charge right. An undercharged system runs longer and removes less water. A spring tune-up measures superheat or subcooling and catches a slow leak before July. The AC tune-up checklist covers what gets measured.
Blower speed. On many furnaces the cooling blower speed is adjustable. A speed set too high cools the air but moves it across the coil too fast to wring out the moisture. We check this on every tune-up in a house that complains about clamminess, and lowering the cooling speed one tap is often the whole fix.
Insulate the suction line. The big copper line outside should be wrapped end to end. Bare copper in the sun gains heat the compressor then has to remove.
Sizing: why a bigger AC makes a wetter house
An oversized air conditioner, and we see them constantly in 1990s and 2000s subdivisions in Canton, Van Buren and Saline where builders rounded up, hits the thermostat setpoint in 8 minutes and shuts off before the coil has run long enough to dehumidify. The result is a 74-degree house at 60 percent humidity that feels clammy, runs the system in short bursts all day, and wears out the compressor on start cycles. A correctly sized system, or better, a two-stage system that runs on low for long stretches, keeps the coil cold and wet for 20 to 40 minutes at a time and dries the air properly. If your house is cold and damp at the same time, read what size air conditioner do I need. When a replacement is due, this is the main reason we steer people toward two-stage equipment; new AC cost in Michigan shows what that tier runs.
The house
- Shade the west and south glass. A west-facing picture window in a 1960s Westland ranch is a space heater from 3 to 7 p.m. Exterior shade, awnings, or at minimum closed blinds during the afternoon cut the load noticeably.
- Attic insulation and ventilation. An attic at 130 degrees over a ceiling with 4 inches of insulation pushes heat into the house all evening. The same R-49 that saves gas in winter saves electricity in summer.
- Seal the ducts. Supply ducts leaking cold air into a hot attic or a damp basement waste the cooling you paid for, and return leaks in a Michigan basement pull in humid, musty air. Ductwork leaks in older Michigan homes covers the fix.
- Vent moisture at the source. Run the bathroom fan for 20 minutes after a shower and the range hood while cooking. Every gallon you vent outside is a gallon the AC does not have to condense.
- Basement humidity. A damp basement feeds humidity to the whole house through the return ducts and the stairwell. A dehumidifier set to 50 percent in the basement, drained to the floor drain, often makes the upstairs feel better than any thermostat change, which is the subject of Michigan basement HVAC moisture problems.
- Ceiling fans. They cool people, not rooms. Run them when you are in the room and turn them off when you leave; a fan running in an empty bedroom is pure cost.
What the bill should look like
A well-maintained, correctly sized system in a reasonably insulated Washtenaw County house typically adds a modest amount to the DTE bill in June and September and a larger amount in July and August, scaling with the size of the house. If your summer electric bills are double your spring ones and the house still feels damp, something above is wrong, and it is usually one of three things: the fan on On, an oversized or undercharged system, or a wet basement. A maintenance visit that checks airflow, charge and blower speed finds which one.