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For most homes the answer is 30 gallons. The typical mobile home water heater size is capped not by how much hot water you want but by how much tank fits in the closet the factory built for it — and unlike a site-built house, you cannot simply buy a bigger one and squeeze it in.
There is also a legal constraint that catches people out, so read the second section before you buy anything.
The mobile home water heater size that actually fits
Site-built houses commonly run 40 to 60 gallon tanks. Manufactured home closets are generally built around a 30-gallon maximum.
A rough guide, assuming a conventional tank:
| Household | Typical capacity |
|---|---|
| 1–2 people | 20–30 gallons |
| 3–4 people | 30 gallons, run carefully |
| 5+ people | 30 gallons plus habits, or a tankless conversion |
Notice what is missing: an option to go up. That is the honest situation. If you have a family of five in a single-wide, the tank is not going to solve it — sequencing showers will, or a tankless unit will.
Measure the closet before you shop. Height to the underside of any shelf, width, depth, and the door opening. A 30-gallon tank that is an inch too tall for the closet is a 30-gallon tank you are returning.
The part people get wrong: it has to be HUD-approved
This is not a preference. A gas water heater installed inside a manufactured home must be a sealed combustion (direct vent) unit with HUD approval — and here is the regulation that says so.
Sealed combustion means the burner draws its air from outside through a concentric vent and exhausts outside through the same assembly, with no opening into the living space. A standard residential water heater draws combustion air from the room it sits in. In a small, tightly sealed manufactured home, that is a carbon monoxide risk, which is exactly why the requirement exists.
The consequences of ignoring it are not theoretical:
- It violates the federal standard the home was built to — the one named on your data plate — (see HUD’s own homeowner resources).
- It voids the manufacturer’s warranty on the unit.
- It can give your insurer grounds to deny a claim after a fire or CO incident.
If a contractor offers to save you money by fitting a standard heater, that is the moment to find a different contractor.
Three more things that are different
The connections are on the side
On a standard residential heater, the cold inlet and hot outlet are both on the top. On many mobile home units the cold inlet is on the side and the hot outlet on top — and on some models both are on the side.
This matters because the existing supply lines in your home were run to reach those positions. Buy a heater with top-only connections and you are re-plumbing, in a closet, with no room.
Check the photo and the spec sheet, not just the gallon figure.
Gas heaters convert between natural gas and propane
Mobile home gas water heaters commonly ship with an adjustable orifice so the same unit runs on natural gas or propane. That flexibility is useful — plenty of manufactured homes are on propane — but it does add to the price compared with a fixed-fuel residential unit.
If your home is on propane, confirm the conversion kit is included and that whoever installs it actually performs the conversion.
Electric is often the simpler answer
An electric mobile home water heater has no combustion, so no sealed-vent requirement and no CO concern. It is usually cheaper to buy and much cheaper to install, since there is no vent assembly to cut through the wall.
Against that: running costs are typically higher than gas, and recovery after a long shower is slower.
If you are replacing like for like, stay with the fuel you have. If you are replacing a failed gas unit and your home has the electrical capacity, price the electric option before you commit — the installation saving is often substantial.
What to check before you buy
- Measure the closet. Height, width, depth, and the door opening.
- Photograph the existing connections. Top or side, and which side.
- Confirm the fuel. Natural gas, propane or electric.
- Look for the HUD approval on the unit you’re buying. If the listing does not say it, assume it is not.
- Check the drain pan and the drain line. Manufactured homes concentrate a lot of damage into a small area when a tank lets go, and the pan is often missing or the line blocked.
- Check the floor under the tank while the old one is out. This is the single most common place for rot in a manufactured home.
When to replace rather than repair
Replace if the tank itself is leaking — a leaking tank is not repairable. Repair is worth considering for a failed thermocouple, thermostat, heating element or anode rod.
A conventional tank typically gives eight to twelve years. If yours is at that age and the recovery has slowed, replace it on your own schedule rather than on the day it floods the hallway.
Common questions
Can I put a 40-gallon tank in a mobile home?
Only if the closet physically accommodates it, which is unusual. Measure first; do not assume it can be made to fit by removing a shelf.
Does the mobile home water heater size change in a double-wide?
The closet is sometimes larger, so occasionally yes. It still follows the closet, not the home.
What about tankless?
It solves the capacity problem and frees the closet. It needs adequate gas supply or a significant electrical upgrade, and the install is more involved. Worth pricing if capacity is your actual complaint.
Can I install one outside?
Some homes have an exterior compartment designed for it. Inside the living space is where the sealed-combustion requirement applies.