BTU SizerWhen is the filter due again?
Nobody forgets to change a filter on purpose. They forget because there is no date attached to it.
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
How often each part needs attention
Typical intervals for the maintenance that keeps a correctly sized system performing like one.
| Job | Interval | Why |
|---|---|---|
| 1-inch filter | Every 1–3 months | Blocks fastest, cheapest to replace |
| 4–5 inch media filter | Every 6–12 months | More surface area, longer life |
| Outdoor coil clean | Once a year | Dirt on the coil is lost capacity |
| Professional service | Once a year | Refrigerant charge and electrical checks |
| Condensate drain | Twice a year | A blocked drain is a water leak |
A blocked filter is the most common reason a properly sized system stops keeping up. It is also the cheapest thing on this list.
Frequently asked questions
How often should I really change the filter?
A one-inch filter every one to three months, faster with pets or building work. A four or five-inch media filter every six to twelve. If the filter looks grey rather than white, it is already costing you capacity.
Does a dirty filter really matter?
It is the single most common cause of a system that no longer keeps up. Restricted airflow lowers capacity, raises running time and, on an air conditioner, can freeze the coil. Nothing else on the maintenance list is this cheap or this effective.
What about the annual service?
Set a second schedule at yearly intervals. Booking it in spring for cooling or early autumn for heating avoids the queue that forms during the first hot or cold week.