A conventional gas tank with a standing pilot usually keeps heating in a power outage, because the pilot's own heat powers its control. Power-vent, electronic-ignition, electric, heat pump and tankless water heaters stop. A full tank still holds its stored hot water, roughly three average showers in a 50-gallon tank. Run generators outdoors only, and connect them through an electrician-installed transfer switch.
- Standing-pilot atmospheric gas tanks usually work without electricity; anything with a fan, igniter, element, compressor or circuit board does not.
- A full tank keeps its hot water for many hours, but every draw is replaced with cold water that cannot be reheated.
- Connect a generator only through an approved transfer switch installed by an electrician, and never back-feed through a wall outlet.
- Tankless units and heaters in unheated spaces are the freeze risks in a long winter outage; drain them rather than let them crack.
- Refill an emptied tank before restoring its power, and keep generators, barbecues and camp stoves outside.
01 /Does a water heater work without power?
It depends on how the heater lights its burner and moves its exhaust. A conventional atmospheric gas tank with a standing pilot usually keeps heating through an outage because nothing in it needs household electricity. Anything with a fan, an electronic igniter, heating elements, a compressor or a circuit board stops when the power does.
Natural Resources Canada's water heater guide explains the exception: the continuous pilot heats a thermopile that makes a small amount of electricity to run the control and safety circuits, and flue gases rise up the chimney on their own. The same guide lists the inability to make hot water during outages as a drawback of power-vented gas heaters.
| Heater | Heats without power? | Why |
|---|---|---|
| Atmospheric gas or propane tank, standing pilot | Usually yes | Pilot-powered control and natural-draft venting |
| Atmospheric tank with electronic ignition or a plug-in control | No | The igniter or control needs 120 V |
| Natural-draft direct vent with a standing pilot | Often yes | Depends on the control; check the manual |
| Power vent or power direct vent | No | Blower, pressure switch and igniter all need power |
| Electric tank | No | The elements are electric |
| Heat pump water heater | No | Compressor, fan, controls and backup elements are electric |
| Gas or electric tankless | No | Fan, controls and ignition, or the elements, need power |
| Indirect tank or combi system on a boiler | Usually no | The boiler's controls and pumps need power |
A quick test: a power cord, a wire running into the unit, a fan on top or a digital display means it stops in an outage. A gas tank with a knob marked PILOT, a sight glass and no cord is the kind most likely to keep going. The gas vs electric water heater guide weighs outage resilience against running cost.
Hot water also needs water pressure. A private well stops when its pump loses power, and upper floors of some apartment buildings may depend on booster pumps, so a working gas tank helps little until the water flows again.
02 /How long does a full hot water tank stay hot without power?
An undisturbed tank stays hot for a long time, because it is an insulated drum of water. It cools slowly as heat escapes through the jacket and, on gas tanks, up the central flue, so the water is usually still warm many hours later. What empties a tank in an outage is use: every litre drawn is replaced by cold water that cannot be reheated.
Roughly three-quarters of a tank is usable hot water, because incoming cold water mixes into the rest. With the representative values the Hot Water Morning Simulator uses, water stored at 55 °C and 7 °C water from the main, an 8-minute shower at 7.6 litres a minute takes about 42 litres of stored hot water.
| Tank size | Usable hot water, about 75% | 8-minute showers, roughly |
|---|---|---|
| 40 US gal (151 L) | About 113 L | 2 to 3 |
| 50 US gal (189 L) | About 142 L | 3 |
| 60 US gal (227 L) | About 170 L | 4 |
| 80 US gal (303 L) | About 227 L | 5 |
Colder winter water, longer showers and baths shrink those numbers. To stretch a tank, keep showers short, skip baths, and leave dishes and warm laundry until it can reheat. Use the stored hot water for washing people first.
03 /Can I run my water heater on a generator?
Yes, if the generator is connected safely and is large enough for the heater. A power vent is a small load and an electric tank is a large one, and every one of them has to be supplied in a way that cannot push generator power back into the house wiring.
The Government of Canada's Get Prepared guidance advises getting a licensed electrician's advice before installing a generator, and says a backup generator may only be connected to a home's electrical system through an approved transfer panel and switch installed by a qualified electrician. It also says never to plug a generator into a wall outlet. Back-feeding a house that way energises wiring everyone assumes is dead, including utility lines crews may be repairing.
| Heater | Electrical load | Safe way to power it |
|---|---|---|
| Power vent or gas tankless | Small: blower, igniter and controls on 120 V | Its circuit on an electrician-installed transfer switch; its own cord plugged into the generator only if the manufacturer allows it |
| Electric tank | Large: commonly one 240 V element at a time, around 4.5 kW on many tanks | Transfer switch only, with a generator an electrician has confirmed can carry it |
| Heat pump water heater | 240 V compressor, plus elements in hybrid or electric mode | Transfer switch; ask about compressor starting load |
| Standing-pilot gas tank | None | No generator needed |
Power vents need one more caution. Many electronic ignition controls sense the flame through a tiny current that depends on correct polarity and a solid ground, which some portable generators do not provide, so a heater that runs fine on utility power can lock out on a generator. Ask the electrician to put the water heater's circuit on the transfer switch and confirm the generator suits electronic controls.
04 /Can a water heater freeze during a winter outage?
A tank in a heated house rarely freezes in an ordinary outage. It holds a large mass of water that cools slowly, usually in a basement that stays above freezing longer than the rest of the house. The real risks are pipes on exterior walls and in crawlspaces, heaters in unheated garages, cottages and utility rooms, and tankless units.
Tankless units are the most vulnerable. Many have built-in freeze-protection heaters that run on household power, so an outage disables them just when they are needed, and a unit on an exterior wall can freeze quickly in deep cold. Manuals commonly describe how to drain the unit when it may freeze without power; follow yours. A frozen heat exchanger can crack, and the damage often shows up as a leak when the water comes back.
Get Prepared advises draining water from the plumbing system because pipes can freeze during winter outages. If the house will drop below freezing, or you are leaving it, drain the water heater as part of that job.
- Turn the heater off
Gas control to OFF, or the breaker off for an electric tank or heat pump. Switch power-vent and tankless controls off too, so they do not try to start when power returns.
- Shut the water
Close the main water valve, or at least the heater's cold supply valve.
- Let air in
Open a hot tap on the lowest floor, and others above it, so the tank and pipes can drain.
- Drain
Run a hose from the drain valve to a floor drain or outdoors and open the valve. The water may still be scalding, so keep people and pets clear of the hose end.
- Label it
Leave the valves open and tape a note to the heater and the panel: EMPTY, DO NOT TURN ON.
05 /What should I do when the power comes back on?
Bring the house back gradually, then check the water heater before relying on it. Get Prepared suggests turning heating thermostats up first, reconnecting the fridge and freezer, and waiting 10 to 15 minutes before reconnecting other appliances. Switching an electric water heater back on can wait for that later step.
| Heater | What to do |
|---|---|
| Standing-pilot gas tank | Look through the sight glass. If the gas was also interrupted and the pilot is out, relight it by following the label. |
| Power vent | It should restart on its own. If the light shows a lockout, power-cycle it once per the label; if it stays locked out, see power vent troubleshooting. |
| Electric tank or heat pump | If it was drained, refill with a hot tap open until water runs steadily with no air, then switch the breaker on. Some heat pump units may come back in a default mode; check it. |
| Tankless | Turn the water on and look for leaks at the unit before restoring gas and power. Any sign it froze means a technician inspects it first. |
| Any heater that was under water | Do not use it. Submerged gas controls, burners and electrical parts need inspection and usually replacement. |
After a winter storm, make sure the vent termination is not buried before a power vent or tankless unit tries to fire; power vent water heater in winter covers clearing it. Expect the first hot water to take a while, because the tank starts lukewarm and needs a full recovery.
06 /How do I keep carbon monoxide out of the house during an outage?
Outages raise carbon monoxide risk because people bring fuel-burning equipment inside or too close to the house, and Get Prepared warns against using outdoor equipment such as barbecues and camp stoves indoors. The water heater plays a part too: a standing-pilot tank keeps burning while the house is sealed against the cold, and storm snow can bury sidewall vents.
- Keep generators, barbecues and camp stoves outside, well away from doors, windows and vents.
- Test CO alarms; a hard-wired alarm keeps working in an outage only if its backup battery is good.
- Health Canada advises inspecting exhaust vents during and after snowstorms to make sure they are not blocked.
- If an alarm sounds or anyone has a headache, dizziness or nausea, get everyone outside and call 911 from there.
The carbon monoxide and water heaters guide explains how water heaters spill CO and where alarms belong. If outages are frequent where you live, the choosing a water heater checklist weighs outage resilience against efficiency, and you can request quotes for a replacement.
FAQQuestions people ask
Does a hot water heater work in a power outage?
A conventional gas tank with a standing pilot usually does, because the pilot's heat powers its control. Electric, heat pump, power-vent, electronic-ignition and tankless heaters stop, though a tank keeps the hot water already stored in it for a while.
Does an electric water heater work when the power is out?
No. The elements need electricity. The insulated tank keeps the water already in it warm for many hours, so you can use that until the hot layer runs out.
Can I plug my power vent water heater into a generator?
Only in a way the manufacturer and an electrician approve. The safest route is a transfer switch that includes the heater's circuit. Never back-feed the house through a wall outlet, and expect some electronic ignition controls to lock out on portable generator power.
Should I turn off my water heater during a power outage?
Switching off electric, heat pump, power-vent and tankless units lets you bring them back one at a time, and it is essential if you drain them. A standing-pilot gas tank can keep running unless you are draining it or leaving the house cold.
How long will water stay hot in a water heater without power?
An undisturbed tank loses heat slowly and usually stays warm for many hours. Use is what runs it out: a 50-gallon tank holds roughly three average showers of stored hot water with no reheating.
Can a tankless water heater freeze in a power outage?
Yes. Many depend on electric freeze-protection heaters that stop in an outage. In deep cold, especially on exterior walls, drain the unit as its manual describes.
Why won't my water heater work after a power outage?
Power vents and tankless units often lock out when power flickers, so power-cycle once per the label. Also check for a tripped breaker or GFCI, a pilot that went out when the gas was interrupted, a snow-blocked vent, or an electric tank switched on before it was full.