Repair-or-replace math, sizing, efficiency ratings, equipment tiers and the honest trade-offs for Michigan homes.
Repair the air conditioner when it is under 12 years old, uses R-410A, and the failed part is a capacitor, contactor, fan motor or minor leak. Lean toward replacement when it is 13-plus years old, runs on R-22, needs a compressor or evaporator coil, or the repair would cost more than about a third of a new system. In Southeast Michigan the compressor and R-22 questions settle most cases.
Repair a furnace when it is under 15 years old and the failed part is an ignitor, flame sensor, pressure switch, inducer or blower motor. Replace it when the heat exchanger is cracked, when it is 20-plus years old and needs a major part, or when a repair would cost over a third of a new furnace. A cracked exchanger is never a repair; it is a carbon monoxide hazard.
AFUE is the share of the gas you buy that becomes heat in the house over a season. An 80% furnace sends 20 cents of every heating dollar up a metal flue; a 96% condensing furnace recovers most of that by cooling the exhaust until water condenses out, venting through PVC. In Southeast Michigan's 6,000-plus heating degree days, the 96% usually earns back its premium, but not in every house.
Central air conditioners are sized in tons, each ton being 12,000 BTU per hour of cooling. In Southeast Michigan most homes need roughly one ton per 600 to 800 square feet, so a 1,500-square-foot ranch is usually a 2- to 2.5-ton system and a 2,400-square-foot colonial a 3- to 3.5-ton. The right number comes from a Manual J load calculation, not square footage, and oversizing is the most common mistake.
In Southeast Michigan a furnace needs roughly 30 to 40 BTU of output per square foot of heated space: about 35,000 to 45,000 BTU for a tight 1,200-square-foot ranch and 60,000 to 80,000 for an older 2,000-square-foot two-story. Output is input times AFUE, so an 80,000 BTU 96% furnace delivers about 76,800. A Manual J calculation replaces the rule of thumb and usually finds the old furnace was too big.
In Southeast Michigan the best answer for most gas-connected homes is dual fuel: a heat pump that heats efficiently down to roughly 25 to 35°F and a gas furnace that takes over below that. A heat pump alone works with cold-climate equipment and a tight house, but on DTE rates a furnace is usually cheaper below freezing. All-electric makes the most sense where there is no gas line.
A cold-climate heat pump uses an inverter-driven variable-speed compressor and enhanced vapor injection to keep delivering most of its rated heat at 5°F, and useful heat down to minus 13 or lower. For Southeast Michigan, look at the published capacity at 5°F and 17°F, not the nameplate tonnage, size it to the heating load at the local design temperature, and keep backup heat for polar-vortex nights.
Single-stage equipment runs at full capacity or off. Two-stage runs at about 65 percent most of the time and full on the coldest or hottest days. Variable-speed (modulating) adjusts continuously between roughly 25 and 100 percent. In Southeast Michigan, two-stage is the sweet spot for most homes: longer cycles, better humidity removal and quieter operation at a modest premium. Variable-speed earns its cost in larger or uneven houses.
There is no single best furnace brand. The major lines share components, parts quality is similar across tiers, and the installation, sizing and commissioning matter far more than the badge. What differs is parts availability and pricing: Carrier, Bryant and Payne share parts; Goodman and Amana are value-priced with common parts; Lennox uses more proprietary components. Pick the contractor first and the brand second.
In Southeast Michigan a tankless water heater must raise 40°F winter groundwater to 120°F, an 80-degree rise that cuts a typical unit to roughly 4 to 5 gallons per minute, enough for one shower and a sink. A 40- or 50-gallon tank is cheaper to install and simpler to repair; tankless saves space and standby loss but needs a larger gas line, sidewall venting and yearly descaling.
Call or text and you reach Al, not a call center. Same-day service whenever possible, flat-rate pricing quoted before any work, and the same rate at 2 a.m. as 2 p.m.
