Water Heater and Furnace Both Not Working
On this page (6 sections)
The Short Answer: When the furnace and water heater stop at the same time, they almost always share one cause: the gas or propane supply, a blocked shared chimney or vent, or power both depend on, such as the furnace emergency switch or a shared outlet. If you smell gas or a CO alarm sounds, leave and call from outside before checking anything.
What do the two appliances share?
| Shared link | Signs | What it points to |
|---|---|---|
| Gas meter or utility service | Range and dryer also won’t light | Utility shutoff, meter problem, or service interruption |
| Propane tank | All propane appliances out, often after a cold spell | Tank empty or regulator iced or failed |
| Common chimney or vent | Both appliances vent into one chimney or liner; soot or exhaust smell | Blockage (nest, debris, collapsed liner, ice at the cap) |
| Furnace room emergency switch or shared outlet | Furnace display dark, power-vent or tankless heater dark too | Switch turned off, tripped breaker or GFCI |
| Shared condensate pump | High-efficiency furnace and water heater drain into one pump | Pump full or failed; both shut down on drain or pressure faults |
| Power outage just ended | Both need electricity for ignition or controls | Control lockouts after the outage |
A standard atmospheric gas water heater with a standing pilot does not need electricity. If it is out at the same time as the furnace, the gas supply or the vent is more likely than power.
What you can check safely
- Other gas appliances: if the stove and dryer also won’t light, call the gas utility. If you have propane, check the gauge on the tank and call the supplier if it is low. After an empty propane tank has been refilled, the supplier or a technician usually needs to check the system before appliances are relit.
- Appliance supply valves: confirm the gas valve on each appliance’s supply line is open (handle parallel to the pipe). Don’t loosen any fittings.
- Power: check the furnace emergency switch (often a red cover plate at the basement stairs or by the furnace) and the breakers. Reset a tripped breaker or GFCI once only.
- Outside vent terminations: from the ground, look for snow, ice, nests, or debris at sidewall vents. Don’t climb onto the roof to inspect a chimney.
- Condensate pump: if both appliances drain into a small pump, check whether it is full of water or overflowing.
- Displays and lights: photograph any furnace status light pattern and any water heater code before resetting.
Why a blocked chimney is serious
When a furnace and a gas water heater share a chimney or vent, one blockage stops both from venting. Many appliances have spill or rollout switches that shut them down when exhaust comes back into the room. That shutdown is protection, not a nuisance fault. Relighting or resetting while the vent is blocked can push carbon monoxide into the house. If one appliance keeps shutting down a few minutes after lighting, see water heater shuts off after 5 minutes.
What the technician will check
A gas technician or heating contractor will confirm gas supply pressure at the appliances, inspect the shared vent or chimney and test draft, check that safety switches have not tripped from a real blockage, and look at shared electrical and condensate connections. If the gas supply was interrupted, they handle the safe restart, which includes leak checks and relighting each appliance.
If only one appliance is out
If the furnace works and only the water heater is cold, or the other way around, the cause is probably in that appliance. For the water heater, start at no hot water or pilot and ignition problems.
Frequently Asked Questions
Q: Can a power outage stop a gas water heater? A: Only if it uses electricity, as power-vent, direct-vent fan, and tankless models do. A standing-pilot tank keeps working through an outage. See gas water heater not working after a power outage.
Q: Both came back on but the water heater pilot keeps going out. Why? A: After a gas supply interruption, air in the line can make the first lighting attempts hard. A pilot that keeps going out after it has lit can also mean a draft or venting problem. If it doesn’t hold after you follow the lighting label, have it checked rather than relighting it again and again.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
- CPSC: carbon monoxide fact sheet (opens in a new tab)Carbon monoxide hazards and emergency response.
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