Find out how cold the spot actually gets
Put a min/max thermometer next to the heater and another by the coldest stretch of pipe, then check it after a few cold nights, including a night after the garage door was left open. Philadelphia Water Department advises keeping the water-meter area above 40°F. That's a target for the meter and nearby pipes, not an operating limit for every heater. Take a photo of the heater's data plate so you or a plumber can pull the right manual.
Insulate the pipes first
Philadelphia Water Department tells homeowners to wrap and insulate all water pipes in unheated areas like basements, pay extra attention to pipes near exterior walls, and repair or cover broken windows, since cold air blowing on a pipe raises the risk of freezing. The Department of Energy's weatherization checklist calls for insulating the first 6 feet of the heater's inlet and outlet pipes, with sleeves sized to the pipe and the seams sealed.
On a gas heater, keep pipe insulation at least 6 inches from the flue. The Building America Solution Center adds that if pipes run within 8 inches of the flue, fiberglass or mineral-wool wrap is the safer choice than foam. For pipes that still get close to freezing, it mentions thermostat-controlled heat tape. Have that installed properly, not run off an extension cord. Our guide on which basement pipes to insulate first covers the rest of the house's piping.
When a tank blanket is OK, and when it isn't
The DOE job aids for both electric and gas tanks start the same way: check the data plate and warning label and confirm the manufacturer doesn't prohibit extra insulation. If a blanket is allowed:
- Leave the temperature-and-pressure relief valve, thermostats, controls and access panels uncovered.
- On a gas tank, don't cover the burner access plate or the combustion-air intake, and keep the blanket at least 6 inches below the draft diverter on top.
- Don't cinch the straps so tight they crush the insulation.
Heat-pump water heaters are the exception. DOE's purchasing guidance says not to wrap them in blankets at all, because they're already well insulated and a blanket can block the airflow they depend on and may void the warranty.
Don't seal off a gas heater's air
A cold draft through a louver or vent near a gas heater may be the air the burner needs. Many gas tank heaters draw combustion air from the room. The Building America Solution Center says that when a standard atmospheric-vented heater sits in a confined space or a tight building, it needs dedicated combustion-air openings, and that tight homes carry a greater risk of backdrafting, where exhaust spills back into the house. Keep stored items clear of the heater too; see how much clearance a furnace or water heater needs.
Before you build an enclosure around the heater or seal up the garage or basement, ask an HVAC or plumbing contractor where the heater gets its combustion air, and whether the change calls for a combustion-safety check (draft, spillage and carbon-monoxide testing).
Heat-pump and tankless heaters in the cold
Building America's code brief says most manufacturers want a heat-pump water heater's surrounding air between about 45°F and 110°F, and typically at least 700 cubic feet of space. Below the minimum, the unit switches to electric-resistance mode, which costs more to run. In a garage that regularly drops below 45°F, expect higher winter bills, and ask the installer what the model does in that range.
Some tankless heaters have built-in freeze protection, but the Solution Center notes it uses extra energy and the water lines leading to and from the unit still need their own protection. Gas tankless units also need a 120-volt outlet for controls and ignition, so ask what happens in a power outage. Comparing types for a replacement? Start with tank or tankless: what decides a water heater replacement.
If the water stops in a freeze
Ask a neighbor first. If they have no water either, Philadelphia Water Department says a main break is the likelier cause; if they do, the problem is in your pipes. To thaw, PWD says not to put direct heat on the pipe; warm the area with a space heater or hair dryer and open the faucet so melt water can drip out. No torches. If a pipe has split, shut the house-side valve near the meter, close the valve on top of the water heater if the break is on the hot side, and call a licensed plumber.
Permits if you replace or move the heater
In Philadelphia, L&I requires a Plumbing Permit for plumbing replacement work, overseen by a licensed master plumber. The water-heater line on its fee table is $31 for a one- or two-family home. Our guide to which Philadelphia plumbing jobs need an L&I permit has the details. In Bucks or Montgomery County, check with your township's code office before replacing or relocating a heater.