What the annual service actually is
Roughly twenty separate checks, sold as one appointment. Knowing which is which is worth money in both directions.
The three kinds of task
An annual maintenance visit is not a single procedure. It is a list, and the items on it need very different things from whoever does them.
About half need nothing but a hose, a screwdriver and an hour. Washing the outdoor coil, clearing the condensate drain, changing the filter, cutting back the shrub that has grown against the condenser, testing the carbon monoxide alarms, clearing snow off a flue termination. None of these need training. All of them get charged for.
A few need an inexpensive instrument and tell you more than everything else combined. A thermometer gives you the temperature split on an air conditioner and the temperature rise on a furnace. A manometer — which costs less than a single service call — gives you static pressure, and static pressure is the most diagnostic number on a residential system.
The rest genuinely need a technician. Refrigerant work requires EPA Section 608 certification in the United States and venting refrigerant is illegal. A run capacitor holds a charge that can kill after the power is off. Combustion is diagnosed with an analyser, because carbon monoxide has no smell and a flame that looks fine can still be producing it.
Do the cleaning before they arrive
A maintenance visit is usually sold at a fixed price for a fixed hour. If forty minutes of it goes on washing a coil you could have washed, the measurements get the remaining twenty — and the measurements are the part you cannot do.
Do the hose-and-screwdriver items the weekend before. The visit is then spent on charge, combustion and electrical, which is the work only they can do, and you get considerably more for the same money.
The five numbers that are the service
If you want one test of whether a contractor was thorough, it is whether they measured anything. Ask for these afterwards. A good technician will already have written them down.
Superheat or subcooling, with the conditions. Superheat on a fixed-orifice system, subcooling on one with an expansion valve, and the outdoor and indoor conditions they were taken in. A figure with no conditions attached cannot be checked against anything.
Total external static pressure. Supply side plus return side. Most residential equipment is rated at 0.5 in.wc and a great many installed systems run at double that, which is a blower fighting the duct it was given.
Temperature split or temperature rise. On cooling, return minus supply. On a furnace, supply minus return against the range printed on the data plate — outside that range is a fault, not a preference.
Capacitor microfarads against the rating. Printed on the can. A capacitor that has drifted below tolerance is the most common failure in the trade and the cheapest thing on the system to replace before it strands you in August.
Air-free carbon monoxide, on anything that burns. Air-free is the figure that matters, because a diluted sample can be made to look clean simply by letting more air into it.
The two mistakes that cost the most
Treating a small temperature split as low refrigerant. It is the diagnosis reached most often and wrong a good share of the time. A small split also comes from too much airflow, from a compressor that is not pumping, and from a coil that has iced and is no longer passing air at all. Static pressure and airflow should be checked before a gauge goes on. And a system genuinely low on refrigerant is leaking — topping it up without finding the leak is an appointment to do the same thing again next summer.
Buying efficiency to fix a maintenance problem. Run the numbers on replacing a working system for a higher SEER2 and the payback is usually measured in decades. Meanwhile a system running at half its rated airflow because of a filter and a crushed duct is losing more than the difference between two efficiency ratings, and fixing that costs a fraction of a new unit.
When to stop
Any smell of gas — leave and call the utility from outside. Any soot, any yellow lazy flame, any carbon monoxide alarm going off. Ice on the refrigerant lines in cooling weather, which means shut it down before the compressor takes liquid. Oil stains around the outdoor unit, which is refrigerant leaking along with the oil that carries it. A breaker that trips more than once. Water where there has not been water before.
None of these are things to work around.
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