Water Heater and Furnace Both Dead at Once

The Short Answer: Two appliances dying in the same hour is one shared problem: a blocked common vent, a utility or LP tank that went empty, a shared 120-volt outlet or switch, or a CO/spill shutdown. It is not two coincidences. If you smell gas or exhaust, leave. Do not tape a vent or jumper a pressure switch.

Search Intent & The Real Problem

The heat quit and the showers went cold in the same afternoon. Forums ask whether a furnace and a water heater are “the same appliance.” They are not. They share fuel, a chimney, a basement emergency switch, or a 120-volt receptacle more often than they share a brain. One bird nest in a common B-vent, one locked gas meter, or one wet condensate pump will take both. You name the shared path before you buy two new machines.

Related vent pages: vent pipe leaking, white smoke vs steam.

What other guides get wrong

They treat this as two separate no-heat tickets, or they tell you the water heater “is” the furnace. They skip the common vent and send you to replace a thermocouple while the chimney is packed. They recommend a bigger gas flex on a tank that never got a chance to fire because the meter is locked. And they still describe holding a pressure-switch hose closed so the burner will run without the inducer.

Step-by-Step Troubleshooting

Step 1: Prove both are actually dead - and name the fuel

• What to look for: Furnace status LED vs water-heater valve LED vs a blank tankless display. A furnace that still has a thermostat call but will not induce is not “the water heater.” A standing-pilot water heater with a live pilot and a dead furnace is not this page.
• Required tools: Flashlight, phone photo of both rating plates and any LEDs. Thermostat set to heat.

If only one appliance is cold, stop using this article. A water heater that turns off by itself is a single-appliance safety.

Step 2: Gas in the air, exhaust in the air - you are done

• What to look for: Rotten-egg mercaptan, soot at a draft hood, a headache, a utility red tag, a CO alarm. Spillage at either hood is the same physics as a leaking vent.
• Required tools: Your feet, a phone outside. No wrenches. No dish soap.

Leave. The utility shuts the meter. A licensed fitter comes after the line is safe. Do not purge a gas line because both units went dark.

Step 3: Shared fuel - meter, LP, cocks - without opening pipe

• What to look for: Other gas appliances also dark (range, dryer). Appliance shutoff handles perpendicular (closed) on both. LP tank gauge on empty after a cold snap. Ice on an LP regulator. Utility lock on the meter. A 1/2-inch kinked whip on a high-BTU tankless that starved the furnace when both called.
• Required tools: Eyes. Manometer work is licensed (typically about 3.5-7 in. w.c. natural gas / 10-11 in. w.c. LP at the appliance - confirm each rating plate).

Two units that will not light after a meter relight are air or a weak supply, not two bad thermocouples. That is a licensed purge and leak test, not a second click.

Step 4: Shared vent, chimney, and CO/spill shutdown

• What to look for: Common masonry chimney or a B-vent wye. Bird nest, crushed liner, disconnected joint in the attic, a cap packed with ice. Tissue at each draft hood should pull in with that burner on - if it spills, shut that appliance down. Power-vent: sidewall termination full of lint or a nest; inducer silent on both if they share a circuit. Condensate trap or pump full; vinyl pressure-switch hose wet.
• Required tools: Flashlight on the termination from the ground. Do not climb a wet roof. Do not tape a joint.

A dripping B-vent is vent pipe leaking, not “condensation, ignore it.” White vapor at a hood is steam vs combustion. A furnace rollout switch and a water-heater spill switch can both open from the same blocked flue in about five minutes - shuts off after 5 minutes.

Step 5: Shared 120-volt, emergency switch, and wet basement

• What to look for: Red emergency switch at the basement stairs off. Shared GFCI or receptacle that feeds a power-vent water heater and a furnace inducer. One double-pole that also happens to feed an electric tank. Flooded pit soaking both cabinets. Hybrid / heat-pump water heater and furnace both dark after an outage - lost a leg of 240 V or a GFCI.
• Required tools: Eyes on the switch and the panel from a dry location. Prove dead before any cover. Need it off? Apartment shutoff.

A power-vent blower that never spins is its own job - do not fire the burner without it. Electric tanks that trip after a few minutes are not a furnace problem.

When DIY Ends: Calling a Licensed Pro

Stop at odor, spillage, a disconnected vent, a locked meter, a wet 120-volt cabinet, and any jumper you saw on a forum. You may confirm shutoff handle position, look at a termination from the ground, and photograph LEDs. You may not tape B-vent, jumper a pressure switch, or relight through a spill. A licensed plumber or HVAC tech will prove draft, measure manifold pressure, clear the common vent, and leak-test. Same-day licensed diagnosis beats two wrong parts and a CO call.

Frequently Asked Questions

Q: Why did my water heater and furnace stop working at the same time? A: They share a vent, a gas meter or LP tank, a 120-volt switch or receptacle, or a condensate path. One blockage or one empty tank takes both. They are not the same appliance.

Q: Can a blocked chimney shut down both the furnace and the water heater? A: Yes. A common B-vent or masonry liner that packs with a nest, ice, or a collapsed tile will spill at both draft hoods and open rollout or spill switches. Leave if you smell exhaust.

Q: Should I relight the water heater if the furnace is also out? A: Not until you know the shared path is clear. Relighting into a blocked chimney is a carbon monoxide event. Call a licensed tech; if you smell gas, call the utility from outside first.