A replacement must be listed for its intended installation and follow the manufactured-home design and model instructions. Fuel-burning heaters need combustion separated from the living space. Storage tanks need a drain pan, and relief piping must go outside or to a permitted indirect waste receptor.
What does the federal standard say about water heaters?
The plumbing rules require a hot water supply system that includes "a listed water heater." The rules for heat producing appliances add that appliances and their vents "must be listed or certified for residential use by a nationally recognized testing agency," and that "the installation of each appliance must conform to the terms of its listing and the manufacturer's instructions." So the first rule is to use a unit that is listed, and to install it the way its listing and manual say.
The standard has three more requirements that decide whether an ordinary heater fits.
### Must a fuel-burning heater be sealed off from the living space?
Yes. The standard says "all fuel-burning appliances, except ranges, ovens, illuminating appliances, clothes dryers, solid fuel-burning fireplaces and solid fuel-burning fireplace stoves, shall be installed to provide for the complete separation of the combustion system from the interior atmosphere of the manufactured home." That leaves gas and oil water heaters in the rule. There are two ways to comply:
- 1A direct vent (sealed combustion) heater. It draws combustion air from outside and vents outside.
- 2An enclosure. The heater goes in a compartment that separates its combustion and venting from the home's interior. The standard says "there shall not be any door, removable access panel, or other opening into the enclosure from the inside of the manufactured home," and any opening for ducts, piping or wiring "shall be sealed."
The standard also says fuel-burning heat producing appliances shall be "of the vented type and vented to the outside." A heater drawing combustion air from the living space does not meet this separation rule. Exhaust must be vented outside; an atmospheric-draft model may be suitable only in an approved enclosure with the required outside air and venting. Keeping the combustion system separate from the interior is the point of the rule. If your home has a water heater compartment, do not swap in a heater without checking that the compartment is still sealed and that the new unit is listed for it, and have a licensed professional handle the gas connection.
### Does a manufactured home need a drip pan?
Yes. The standard says "a corrosion-resistant water drip collection and drain pan must be installed under each storage tank-type water heater" and that it must "allow water leaking from the water heater to drain to the exterior of the manufactured home, or to a drain."
### Where does the relief valve pipe go?
The standard says every water heater must have an "approved and listed fully automatic valve or valves designed to provide temperature and pressure relief." It says the relief valve outlet "must not be directly connected to the drainage system," and that its discharge "must be piped full size separately to the exterior of the manufactured home, not underneath the home, or to an indirect waste receptor." The installation standard adds that surface water runoff from a water heater drain must be directed away from under the home.
### What about electric water heaters?
Electric heaters have no combustion, so the sealed-combustion rules do not apply, but the other rules do: listing, a drip pan, and a relief valve pipe. The standard also sets a limit on standby loss for automatic electric storage heaters and requires gas and oil storage heaters to meet a minimum recovery efficiency and standby loss. In short, buy a unit listed for the job, and check that the model works with your compartment.
### Does a tankless heater work?
The standard we read speaks to storage water heaters in its efficiency and drip pan rules. Tankless units and heat pump water heaters are listed products with their own installation rules. Whether one fits your home depends on its listing and the venting and gas supply in your compartment, and a plumber has to check that before you buy. We would not assume it is a drop-in swap.
What else does the standard say about relief valves and airflow?
The relief valve rule is specific. The valve's temperature sensing element must be immersed "in the hottest water within the upper 6 inches of the tank," and it is set "to start relieving at a pressure of 150 psi or the rated working pressure of the tank whichever is lower and at or below a water temperature of 210 °F." The relief discharge must be piped full size separately to the exterior, not underneath the home, or to an allowed indirect waste receptor inside. It must not be directly connected to drainage piping.
The installation standard also says "dryer vents and combustion air inlets must pass through the skirting to the outside." That means a heater that uses outside air for combustion depends on the skirting openings being kept clear, and skirting that blocks those openings works against the design.
### What should I ask the installer?
Bring a short list to the plumber or the dealer.
- Is this model listed for installation in a manufactured home, and does its manual say so?
- Is it direct vent, or does it go in a sealed compartment, and which does my home have?
- Where will the relief pipe discharge: outside or to an allowed indirect waste receptor? Where does the drip pan drain?
- Does the unit meet the efficiency rules? The standard requires automatic storage water heaters to comply with the federal efficiency requirements in 10 CFR part 430.
- Who will connect the gas or electric supply, and will they test the connection?
A good installer can answer each of these, and the answers are worth having in writing.
What should I check before I buy a replacement?
- Fuel type. Match what the home has now. The standard says a fuel-burning appliance shall not be converted from one fuel to another unless converted "in accordance with the terms of their listing and the appliance manufacturer's instructions."
- Whether the closet is sealed. Look for a door to the outside and no opening into the home.
- The listing. Read the label and manual for whether the model may be installed in a manufactured home.
- Size. Our water heater size page covers capacity, and the compartment has to hold the unit.
- Drip pan and relief pipe. Check both are present and connected as described above.
- Access. The standard says each appliance must be "both accessible and removable" for servicing and replacement.
If your heater is old or failing, see repair or replace a water heater. If you smell gas or the pilot goes out, see water heater pilot light out and gas smell for what to do.
When should I call a plumber?
Call before you buy. A manufactured-home water heater swap needs the right listed unit, correct combustion air and venting, compliant relief discharge and the appropriate utility connections. The installation standard says only qualified personnel familiar with local requirements should make utility connections.
HQ Plumbing & Air is licensed and insured, gives free estimates and upfront pricing, and answers 24/7.