A cooling system maintenance calendar

A cooling system in Phoenix does not fail at random. It fails in the hours when the load is highest and the margin is thinnest, and by then the household is committed: no contractor is free, no part is in stock, and the house is at 90 °F indoors with the forecast unchanged for a fortnight.

The answer is a calendar driven by the heat record rather than by the calendar on the wall. This article is a checklist in that order: what the equipment is doing, and what to do about it before it is doing it.

The load, by month

Start with what the system is actually up against. The figure below is the normal number of days each month that reach 100 °F at Sky Harbor.

That shape sets the whole schedule. The system earns its keep between the end of May and the middle of September, works lightly in April, May and October, and in the cool months does almost nothing. Maintenance belongs in the quiet periods and the checking happens in the loud one.

February and March: the equipment in reverse

In the two coolest working months of the year (normal lows of 49.0 °F and 54.5 °F), a heat pump is still heating. That matters for one reason worth knowing about before spring: a heat pump that ran through the winter has been reversing cycles all season, and the changeover valve, the defrost cycle and the outdoor coil are the parts a spring service should look at.

• Book the pre-season service now, not in June. The window between the last cold snap and the first 100 °F day is the one time of year when a contractor has room in the diary.
• Replace the filter before the season, and buy a season’s worth. A spare filter in the garage is the difference between a weekly habit and an excuse.
• Walk the outdoor unit and clear its clearance. Gravel, leaf litter, stored items and the growth of the winter yard all matter more than they look like they do.
• Check the condensate drain’s termination outdoors. If the line discharges somewhere that has been quietly filling with yard debris since October, the first week of heavy running will find it.

April and May: before 31 May

The normal high reaches 100 °F on 31 May. Treat that date as the deadline rather than the beginning.

• Have the service done and the findings written down. Compressor amp draw, refrigerant pressures, temperature split across the coil and static pressure. Numbers taken in April are the baseline that makes a June comparison meaningful.
• Clean the condenser coil properly. A coil washed from the outside pushes dust further in. Manufacturers specify the direction for a reason and the manual for your unit has it.
• Flush the condensate line. Algae grows in a warm, wet line, and a blocked drain is the most common cause of a mid-July water stain on a ceiling.
• Confirm the thermostat’s schedule. Whether the household wants a modest setback or a steady setting, decide it in April and leave it. Changing it in July usually happens out of panic.
• Look at the ducts you can reach. Disconnected joints in an attic run are common, and the Department of Energy’s guidance puts duct sealing among the highest-return maintenance tasks in a hot climate.
• Test the changeover to cooling before you need it. The first cooling call of the year is the one that finds a reversing valve that will not reverse.

The first week of June

Once the century days start (9.0 in May on average, 24.6 in June), the schedule changes from preparation to observation.

• Check the filter weekly, not monthly. Replace it when it is loaded rather than when the calendar says so. This is the single task where the manufacturer’s manual and the Department of Energy’s guidance agree most plainly.
• Listen to the outdoor unit from a few feet away. A new rattle, a change in pitch or a hard start are the observations that catch a failing capacitor before it fails on a 110 °F afternoon.
• Watch the condensate pan. If it is standing water rather than a trickle, that is the drain again.
• Note the temperature split once, in the heat of the afternoon. A reading taken during the worst hour of a July day is the one that tells you about margin.

Mid-June to late August: the 105 °F core

The normal high stays at or above 105 °F from 18 June to 22 August, which is 66 days, and in a normal July there is no hour of the day below 86 °F. Nothing in this stretch is a good time to discover a fault.

• Keep the filter habit weekly.
• Keep the outdoor unit’s clearance clear of monsoon growth. A vine that grew through July is a coil running against a wall by August.
• Do not stack anything against the air handler. Return-air restrictions are the quiet cause of low airflow.
• If the system cannot hold a temperature it held in June, call it in as a fault rather than waiting for it to fail. The distance between marginal and failed in this month is often a single afternoon.

Before and during the monsoon

Storm season is also cooling-equipment season, and the damage is usually electrical rather than mechanical.

• Surge protection on the equipment and the panel. SRP’s storm guidance includes surge protectors in its household preparation list, and a lightning strike on a feeder is a common way an air conditioner fails in a summer it was otherwise handling.
• Know how to reset what needs resetting after an outage, and know what not to touch. Both utilities publish what to switch off during an outage and what to leave alone.
• After a storm that dropped the power, check the system deliberately. Listen, look at the drain, and give it an hour before deciding it is fine.
• Clear the outdoor unit of storm debris and dust. A haboob deposits a layer on a condenser coil that a normal week does not.

September and October: the shoulder

The last day the normal high reaches 100 °F is 18 September, and October’s normal high is 89.2 °F. The system is still working, but it is no longer working flat out.

• Have the post-season check done while the system is still warm. A technician diagnosing a cooling fault in October can run the system under load; in December, they cannot.
• Note what the summer cost you in repairs and decide about replacement before next spring. Replacing equipment is a load-calculation question, not a like-for-like swap, which is where the trade’s own standards come in.
• Clean and cover what should be covered, and leave ventilation alone. A cover on an outdoor unit that traps moisture is worse than no cover; the manual for your equipment is the authority.
• Service any heating function before December. The keeping a house cool at 115 degrees article covers the load side of the summer; the winter side of the same equipment is a separate check and belongs before the first cold night rather than after it.

What belongs to a technician

Draw the line in the same place in every month of the year.

The line is drawn at the equipment’s casing and at the disconnect. Filters, clearances, drain lines, thermostat settings and the observation of how the system behaves are household work.

Everything inside the sealed refrigerant circuit, every electrical connection in the equipment, gas work and any load calculation used to size or replace a system are not. A refrigerant charge that is wrong by a small amount will run for a season and cost efficiency for all of it, and the reason it is a licensed task is that the diagnosis requires instruments and the handling is regulated.

What this checklist cannot cover

Six things, stated plainly.

The list is generic. A package unit, a mini-split, a swamp cooler and a heat pump have different components and different service points, and the manufacturer’s manual for the equipment you own overrides any interval here. This site does not print service intervals as fact because they are not its figures to print.

It says nothing about cost. There is no service price, no equipment price and no savings estimate on this page, deliberately. What a service costs depends on the contractor, the equipment and the diagnosis.

It does not size anything. That requires a room-by-room load calculation, which is a trade task performed on site.

It cannot see your ducts. Leakage in an attic run is invisible from the thermostat and shows up only as a system that works harder than its rating suggests.

It assumes a house, not an apartment or a manufactured home, where the equipment arrangement changes the tasks.

And it does not replace a manual. If your equipment came with a maintenance schedule, that schedule is the correct one, and this page is the calendar around it.