The most common fault we find is a system that is too large

Almost certainly that the system has more capacity than the house needs. Cooling lowers the temperature quickly, but moisture only leaves the air while the indoor coil has been cold and wet for a sustained period. A system that satisfies the thermostat in a few minutes never gets there, so you end up cold and damp at once. Three things are worth checking before anybody sells you equipment: airflow over the coil, refrigerant charge against the manufacturer’s target, and whether the thermostat runs the fan on after the compressor stops — that last setting re-evaporates moisture off the coil back into the house and it costs nothing to change.

Time it on a hot afternoon. Note how long the outdoor unit runs and how long it stays off. Runs of twenty to thirty minutes are healthy. Ten minutes on and ten off, repeatedly, is short cycling, and if the humidity indoors does not fall while the temperature does, that is the picture complete. Write the timings down — it is the single most useful thing you can hand a contractor, and it is evidence that is hard to talk around.

Because the calculation usually says so, and because bigger is not safer. An oversized unit cools without drying, short-cycles, wears its compressor and contactors out years early and costs more to run. The only thing it does better is satisfy a thermostat quickly, which is the one thing that does not matter. We do the calculation room by room — glass and orientation, insulation, air leakage, shading, occupancy and what the ductwork can carry — and we will show you the working rather than ask you to take a smaller number on trust.

That is a design fault rather than a maintenance one, and no amount of servicing changes the capacity of a machine. But replacing nearly new equipment is usually still the wrong answer, and we would be the people selling you the replacement, so be careful of anybody who leads with that — including us. Ask the installer in writing for the load calculation with its assumptions, the commissioning readings taken on handover, and the permit number. Then look at what can be improved without replacing anything: airflow, charge, return air capacity, thermostat and staging settings, and duct balancing. On a moderately oversized system that fixes a useful share of it, and a separate dehumidifier is usually a better answer than new equipment if it does not.

We will not give you a figure, and we would be careful of anybody who does from a driveway. The saving depends on your ductwork, your insulation, your usage patterns and your rate, none of which are known before somebody measures. What we will do is measure what your system is currently doing, tell you what is demonstrably wrong with it, and let you judge the rest. If bills have risen, it is also worth comparing the same month a year apart in units of energy rather than in dollars — that tells you whether your house changed or only the price did.

Concord, Pleasant Hill, Walnut Creek, Martinez, Clayton, Pittsburg, Lafayette and Pacheco.