This is the most common cooling complaint we get in a Sacramento heat wave, and it is different from an AC that has stopped. The system runs, air comes out, the air is even somewhat cool, and the house sits at 78 or 80 and will not come down. Here is how to work out which of the five causes you have before you call anybody.
First, a number that decides most of this
Put a thermometer in the return air grille and another in a supply register near the air handler. Run the system for fifteen minutes, then compare. A healthy system produces a temperature split of roughly 16 to 22 degrees between return and supply.
- Split is normal, house still hot. The equipment is producing cooling and something else is losing it. Look at duct work, insulation and heat gain. Go to sections two and three.
- Split is low, under about 14 degrees. The equipment is not producing enough cooling. Look at airflow, the coil and the refrigerant charge. Go to section four.
That one measurement separates an equipment problem from a house problem, and it is the first thing we do on these calls.
Cause one: the system is doing its job and the house is beating it
On a 105 degree afternoon a correctly sized air conditioner is designed to hold roughly a 20 to 25 degree difference from outdoors. At 108 outside, a system holding the house at 80 to 83 may be performing exactly to specification. It is not broken, it is outmatched.
This is worth knowing because a contractor can sell you a larger system to fix it, and a larger system will short cycle, dehumidify worse and cost more, while only partly solving the problem. The cheaper and better fixes are shade on west-facing glass, attic insulation, and sealing the duct work.
Cause two: the duct work is cooling your attic
This is the one we find most often in Sacramento. In tract housing across Natomas, Elk Grove, Rocklin and newer Roseville, the supply ducts run through an attic that reaches 130 degrees in July. Flex duct that has come loose at a boot, crushed runs, or joints that were never mastic sealed dump conditioned air into that space.
The signs: the rooms farthest from the air handler are the worst, the house does fine in June and fails in August, and the system runs continuously without ever satisfying the thermostat. Duct leakage and static pressure are measurable, and they should be measured before anyone sells you equipment. Our article on duct sealing versus replacement covers what the fix involves and costs.
Cause three: airflow is restricted
Low airflow across the indoor coil reduces capacity and eventually freezes the coil solid. Check, in this order:
- The filter. If you cannot see light through it, that is your answer.
- Closed or blocked supply registers, and furniture against the return grille.
- Whether the indoor coil is iced. If the refrigerant line at the air handler is frosted or the unit is dripping heavily, you have a frozen coil, and that has its own list of causes.
Cause four: the equipment is genuinely down on capacity
If the temperature split is low and the filter is clean, the likely causes are a dirty condenser coil outside, a dirty evaporator coil inside, a low refrigerant charge from a leak, or a compressor that is losing efficiency.
The outdoor coil is worth looking at yourself. If the fins are packed with cottonwood, grass clippings or dust, the unit cannot reject heat and its capacity falls off exactly when the outdoor temperature is highest. That is a cleaning, not a repair.
A low charge is a leak, because refrigerant is not consumed. What that costs and why it is rising is in refrigerant costs and the R-410A phase-out. A compressor losing capacity is covered in compressor replacement cost.
Cause five: the system was never the right size for the house
Undersizing is less common than oversizing in Sacramento tract housing, but it happens after a remodel, an addition, a converted garage, or when someone replaced a system by matching the old tonnage without recalculating. If the house has never kept up in any summer since you bought it, the load calculation is the place to start rather than the equipment.
What we do
We measure the temperature split, static pressure and refrigerant charge before we say anything about your equipment, because those three numbers tell us whether you have an equipment problem, a duct problem or a load problem. Our diagnostic is a flat $69 and it is credited toward the repair.
If the house is genuinely unsafe in the heat, emergency and same-day repair explains how we prioritize. Related: AC blowing warm air, upstairs hot and downstairs cold, and seasonal maintenance.
Frequently asked questions
What temperature should my AC air be?
Measure the return air and the supply air after fifteen minutes of running. A healthy system shows a difference of roughly 16 to 22 degrees. Below about 14 degrees points to an equipment or airflow problem. A normal split with a hot house points to duct work or heat gain instead.
Is it normal for my AC to run constantly when it is 105 out?
Yes, and it is usually a good sign rather than a bad one. A correctly sized system is designed to run nearly continuously at design conditions. What is not normal is running continuously and still losing ground, with the indoor temperature climbing through the afternoon.
Why does my house cool fine in June but not in August?
Because the system is marginal and the extra heat exposes it. The usual culprits are leaking attic duct work, a dirty condenser coil and a slowly falling refrigerant charge. All three are invisible in mild weather and decisive at 105 degrees.
Should I just buy a bigger air conditioner?
Usually not. Oversized equipment short cycles, dehumidifies poorly and costs more to run and to buy. Duct leakage, static pressure and the actual load should all be measured first, because the fix is frequently the duct work rather than the tonnage.
My AC is freezing up. Can I keep running it?
No. Turn the cooling off and set the fan to ON so the coil thaws, which takes two to four hours. Running a system with a frozen coil can send liquid refrigerant back to the compressor, which turns a small repair into a large one.
How much of a difference does shade make?
More than most people expect. Shading west-facing glass and keeping the condenser out of direct afternoon sun both reduce load meaningfully. Neither replaces a correctly performing system, but together they can be the difference between keeping up and falling behind on the worst days.