BTU SizerWhen do I switch the house over?
The system that fails in the first heatwave is almost always the one nobody tested in May.
One comfort check a month
MonthlyOne thing a month worth measuring, cleaning or switching over before the weather turns — the maintenance side of sizing, in two sentences. No product pitches.
Free to use — Pro is for keeping it
This tool, like every other one here, is free and needs no account. A free account keeps three setups and three email alerts. Pro takes the ads off every site we run and lifts that to 200 setups and 50 alerts, adds a public page for any result and lets you embed the tool without our badge.
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How this works
What changes over, and when
The two dates a year that decide whether the system starts the season working properly.
| When | Cooling side | Heating side |
|---|---|---|
| Spring | Filter, coil clean, condensate drain | Turn heating thermostat down, book service |
| Early summer | Test run before the first heatwave | — |
| Autumn | Cover or clear the outdoor unit surrounds | Filter, bleed radiators, test run |
| Early winter | — | Check backup heat and thermostat schedule |
Both changeovers are worth doing before the weather needs them. The first hot week is when every contractor is booked.
Frequently asked questions
Why twice a year?
Because a house has two systems that each sit idle for half the year, and equipment that has not run for six months is exactly the equipment that fails on the day it is needed. Testing in mild weather is the whole point.
What should I actually do at a changeover?
Change the filter, run the side you are switching to for twenty minutes, listen for anything new, and check that every room gets air. That is fifteen minutes and it catches most of what goes wrong.
When is the best time to book a service?
Spring for cooling and early autumn for heating — before the season starts. Contractors are available, prices are calmer, and a fault found then is an appointment rather than an emergency.