What Size Drain Pan Do I Need?

Capacity is a rough proxy for diameter and a bad one at the margins. Short and tall variants of the same capacity have different diameters; heat pump units are often wider than a resistance tank of the same gallons. Measure the tank, or read the diameter off its specification sheet.

The sizing table

How to measure

1. Read the tank's specification sheet for diameter. The A. O. Smith Signature 100 50-gallon, for instance, is published at 22 inches.
2. If the tank is in place, measure across the jacket at its widest, including any insulation blanket you intend to keep.
3. Measure the alcove floor, including the door swing or the opening you have to get the tank through.
4. Check where the pan's drain fitting will sit relative to the tank base and the wall. This is the dimension people discover on install day.

Depth, and what it actually buys

The pans here are 2-1/4 to 2-1/2 inches deep. The Camco 11400 at 26 inches is the deepest at 2-1/2 inches.

A 22-inch pan 2-1/4 inches deep holds roughly two to three gallons. That is genuinely useful for a weeping seam, a dripping relief valve or a slow condensate overflow. It is irrelevant to a tank failure, which produces the tank's whole contents in minutes.

So the honest framing is: a pan plus a drain handles slow failures, which are the common ones. Fast failures need an automatic shutoff valve or a tank with one built in, like the Rheem Gladiator.

The drain connection

Every pan here includes a PVC drain fitting, and the connection has to run somewhere by gravity:

• A floor drain in the same space. Best case, and usually what makes a basement install straightforward.
• A laundry standpipe, where one is close enough and code permits the connection.
• To the exterior, where code permits a termination outside.
• Nowhere — in which case the pan buys you a couple of gallons of grace and a visible puddle. Still better than nothing, but say so honestly: it is a detection device at that point, not containment.

Which is why a pan without a drain pairs so well with a cheap water sensor sitting in it.

The pans these sizes point to

Pick 1 — 50 gallon tanks

Camco 20841 aluminum pan, 22 in

The 22-inch aluminum pan: the right size under a standard 50-gallon tank, which is itself usually 22 inches in diameter.

Published specifications for the Camco 20841 aluminum pan, 22 in

Inside diameter	22 in
Depth	2.25 in
Material	Aluminum
Drain fitting	PVC

What is good

• +Matches the 22-inch diameter of the common 50-gallon tank with room for the fitting
• +Aluminum construction
• +Standard PVC drain connection
• +Fits the alcove a 50-gallon tank already occupies

What is not

• −A 22 in pan under a 22 in tank is a tight fit — check the drain boss clears
• −2.25 in depth
• −Requires a gravity drain path

Skip it if: Skip it for a heat pump water heater. Those are usually wider than a resistance tank of the same capacity and also produce condensate continuously, which means a larger pan and a committed drain, not a shallow pan.

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Specifications above from Pacific Plumbing listing, Camco 20841; HD Supply listing, Camco 22 in ID pan. We have not tested this unit ourselves.

Pick 2 — 80 gallon and heat pump tanks

Camco 11400 plastic pan, 26 in

Twenty-six inches across and 2-1/2 inches deep in plastic — the size that actually fits under an 80-gallon tank or a wide heat pump unit.

Published specifications for the Camco 11400 plastic pan, 26 in

Inside diameter	26 in
Depth	2-1/2 in
Material	Plastic
Drain fitting	PVC

What is good

• +26 in clears the larger diameter of 80-gallon and heat pump tanks
• +Deepest pan in this registry at 2-1/2 inches
• +Plastic will not corrode in standing condensate the way aluminum can
• +Takes the same PVC drain connection as the metal pans

What is not

• −Plastic can crack in cold weather or under the weight of a dragged tank
• −26 inches may not fit the alcove the old tank came out of
• −Still only a pan, not a shutoff

Skip it if: Skip plastic if the installer is going to walk a full tank across the pan. On a replacement day a thin plastic pan takes the weight of a 500 lb full tank being rocked into position — put aluminum under that.

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Specifications above from Do It Best listing, Camco 26 in plastic pan. We have not tested this unit ourselves.

Pick 3 — 30 and 40 gallon tanks

Camco 20800 aluminum pan, 20 in

A 20-inch inside-diameter aluminum pan with a PVC fitting — the correct size under most 30- and 40-gallon tanks, which measure around 18 inches across.

Published specifications for the Camco 20800 aluminum pan, 20 in

Inside diameter	20 in
Depth	2-1/4 in
Material	Aluminum
Drain fitting	PVC, included

What is good

• +Aluminum does not crack when a tank is dragged across it the way thin plastic does
• +20 in ID clears the typical 18 in diameter of a 40-gallon tank
• +Pre-fitted PVC drain connection
• +Cheap enough that there is no argument for skipping it

What is not

• −2-1/4 in deep holds a leak, not a tank failure
• −Aluminum corrodes where it sits in standing water with dissimilar metals
• −Needs a real drain or the pan just delays the damage

Skip it if: Skip it if your heater is a 50-gallon or larger tank. Those measure 18 to 22 inches across and the clearance the pan needs for the drain fitting means a 20-inch pan will not take it.

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Specifications above from Auer Steel listing, Camco 20800; HD Supply listing, Camco 20 in ID aluminum pan. We have not tested this unit ourselves.

The heat pump case is different

A heat pump water heater produces condensate continuously, so its pan is not waiting for a failure — it is handling a working fluid. Three consequences:

• Go up a size. Heat pump units are frequently wider than resistance tanks of the same capacity. Verify against the model's specification sheet.
• Prefer plastic. Standing condensate for years is where aluminum can corrode, particularly with dissimilar metals in contact.
• The drain is not optional. Both the condensate line and the pan drain need a route. A blocked condensate line overflowing into an undrained pan is a slow, quiet flood.