Pilot on but no heat: the 4-part diagnosis

What "pilot light on" actually tells you

A pilot light is a small flame that stays lit so the burner can ignite when the thermostat asks for heat. That is its only job. It does not prove the burner is firing, it does not prove gas is reaching the burner, and it does not prove water is being heated. It proves one thing: gas is getting to the pilot assembly and the thermocouple or thermopile is holding the safety valve open.

So when you have no hot water but the pilot is on, you are not looking at a pilot problem. You are looking at a break somewhere between the pilot flame and hot water coming out of the tap. There are four places that break. Work through them in order, because each one tells you whether to keep going or stop and call.

1. Is the burner actually firing?

Stand near the water heater and have someone turn on a hot tap. Listen. A working gas water heater makes a low rumble or a soft roar when the main burner lights. You may also see the flame through the sight glass at the bottom of the tank, if your unit has one.

If you hear nothing and see no flame, the burner is not lighting, and the pilot is doing nothing but sitting there. That points to a few specific parts:

The thermocouple or thermopile has weakened. It still holds the pilot, but it no longer generates enough voltage to open the main gas valve.
The gas control valve or its internal thermostat has failed.
The burner orifice or the burner tube is clogged with dust, rust, or debris.
The gas supply pressure is low, which can keep a pilot lit while the main burner starves.

If you hear the burner light and it runs for a minute or two then shuts off, that is a different failure. That is usually a flame sensor or a venting problem, and it belongs in step 3.

Symptom	Most likely cause	Typical part cost	Repair time
Pilot lit, burner never lights	Thermocouple or thermopile	$10 to $30	30 to 60 minutes
Pilot lit, burner never lights	Gas control valve	$100 to $300	1 to 2 hours
Burner lights, then shuts off	Flame sensor or venting	$15 to $80	30 to 90 minutes
Burner runs but water stays cold	Dip tube or sediment	$15 to $60	1 to 3 hours
Burner runs, water warm then cold	Sediment buildup	$0 to $150	1 to 4 hours

2. Is the burner running but the water still cold?

This is the case that confuses people most. The flame is on, you can hear it, the tank is clearly doing something, and the water at the tap is cold or barely warm. The pilot and burner are both fine. The problem is inside the tank or at the dip tube.

Every tank water heater has a dip tube, a plastic tube that carries cold incoming water to the bottom of the tank so it gets heated before it rises. If the dip tube cracks or breaks off, cold water shoots straight to the hot outlet and you get lukewarm water no matter how long the burner runs. Dip tubes on some models made in the 1990s and early 2000s were recalled for exactly this. A broken dip tube is cheap to replace and easy to diagnose once you know to look for it.

Sediment is the other culprit. Over years, minerals settle on the bottom of the tank and form a layer between the burner flame and the water. The burner runs, the tank feels hot on the outside, and the water inside stays cool. You may also hear popping or rumbling sounds, which is water trapped under the sediment boiling. Flushing the tank helps if the buildup is light. If it is thick, the tank is usually near the end of its life.

3. Is the burner shutting off early?

If the burner lights and then cuts out after a minute or two, the pilot stays lit but you never get a full tank of hot water. Three things cause this.

The flame sensor is dirty or failing. It tells the control board that the burner is lit. If it cannot sense the flame, it shuts the gas off as a safety measure. Cleaning it with fine steel wool fixes this more often than replacing it.

The venting is blocked. A gas water heater needs to push exhaust out through the flue. If the vent connector is clogged, disconnected, or has a bird nest in it, the unit overheats the flue and shuts down on the high limit switch. This is a safety issue, not just an inconvenience. Do not keep resetting a unit that shuts off on venting. Carbon monoxide is the risk.

The air intake is blocked. Some units pull combustion air from the room. If the closet or utility space is sealed tight, or if you have recently added insulation or weatherstripping, the burner can starve for air and shut off.

4. Is the thermostat or control failing?

Sometimes the pilot is on, the burner is capable of lighting, and the water is cold because nothing is telling the burner to light. That is the thermostat or the gas control valve.

On a standard tank water heater, the thermostat is built into the gas control valve at the bottom of the tank. If the thermostat reads wrong or the valve sticks, the burner never gets the signal. You will not see a flame, you will not hear the burner, and the pilot will sit there lit all day.

You can test this in a limited way. Turn the temperature dial up and see if the burner kicks on. If it does, the thermostat is working and the problem is elsewhere. If nothing happens at any setting, the control is suspect. Replacing a gas control valve is not a homeowner job. It involves disconnecting gas, and the valve has to be matched to the tank.

One more thing worth checking before you call anyone: the tank itself. If the tank is leaking from the bottom, or if the relief valve is dripping constantly, the unit is done. No amount of pilot or burner diagnosis will save a tank that has rusted through. Look for water pooling under the tank, rust streaks on the shell, or a relief valve that will not stop dripping.

What you can safely check yourself

Before calling, do these three things. They cost nothing and rule out the most common false alarms.

Confirm the gas supply is on. If you have other gas appliances, check that they work. A shutoff valve that got bumped during cleaning will kill the burner while leaving the pilot lit if the pilot has its own supply.
Turn the temperature dial up and listen for the burner. If it fires, the issue is the thermostat setting, not the unit.
Look at the flame through the sight glass. A healthy flame is blue with a slight yellow tip. A mostly yellow, lazy flame means dirty burner ports or poor air mixture.

Do not open the burner compartment, do not disconnect gas lines, and do not attempt to replace a thermocouple or gas valve yourself. Those parts sit behind a gas connection, and a small mistake there is a real hazard.

When to stop and call

If the burner never lights, if it lights and shuts off, if you smell gas, or if you see water under the tank, stop. Those are the four signals that the problem is beyond a homeowner check. A technician can test the thermocouple output, check gas pressure, inspect the flue, and tell you whether the unit is worth repairing or whether you are looking at replacement.

If you have gone through the four steps above and you are still getting cold water with the pilot lit, call (718) 555-0142 for a free estimate or to schedule an inspection. Describe what you heard and saw at each step. That saves time on the visit and gets you an accurate answer faster.