Gas Burner On but Still No Hot Water

The Short Answer: If the main burner is actually firing and the house is still cold, the tank is heating and the water is not getting out — or heat is not reaching the water. Dip tube, sediment blanket, mixing valve, a closed hot isolation valve, or a crossover. This is not an ignition job. Do not crank the thermostat to 140°F as a test.

Search Intent & The Real Problem

You opened the combustion door. There is a real flame under the tank, not just a pilot. Faucets still run cold, or you get a thirty-second slug of hot and then ice. Forums told you the gas valve is bad. A gas valve that is holding a main flame is doing its job. Heat is either trapped under lime, dumped at the top by a broken dip tube, mixed back to tepid, or shut in by a valve someone left closed after a swap.

If the flame is not staying on, you are in the wrong article — pilot holds, main drops or will not stay lit.

What other guides get wrong

They treat “burner on, no hot water” as a failed thermocouple. They tell you to raise the setpoint to 140°F, which scalds and hides a mixing valve. They skip the tank-outlet vs faucet temperature split, so a closed hot-side ball valve after an install gets a new heater. They also confuse the pilot with the main — a standing pilot is not “the burner on.”

Step-by-Step Troubleshooting

Step 1: Prove the main is on, then where the heat is

• What to look for: Rating plate, a real main flame (full burner ring, not a 1” pilot), then a contact reading on the hot outlet nipple, not the faucet.
• Required tools: Contact thermometer, flashlight, a helper at a fixture.

A tankless on the wall with a code is not this job. Photograph the code and hire a tankless tech.

Step 2: Isolation valves and the post-install closed hot

• What to look for: Cold inlet open, hot outlet open — handles parallel to the pipe. After a replacement, the hot ball valve left at 45° or fully shut is the entire diagnosis: burner runs, tank goes nuclear, nothing useful at the tap. Flex connectors kinked or a 1/2” whip on 3/4” ports will also starve fixtures while the burner looks busy. Same-day low flow after a swap has its own pattern — low pressure after install.
• Required tools: Eyes on both tank valves. No gas wrenches.

If the tank is in a rental closet, confirm nobody shut a downstream valve — apartment shutoff works both directions.

Step 3: Mixing valve, recirc, and crossover

• What to look for: Thermostatic mixing valve on the hot outlet (three ports). Tank-out 130–140°F, mixed-out 105°F and the shower matches mixed-out = the mixer is doing its job and the complaint is setpoint. Tank-out 140°F, mixed-out 90°F = failed mixer (hot port closed or cartridge stuck). No mixer, tank-out hot, one fixture cold, others fine = that cartridge is crossing. Recirc pumping 24/7 into the cold inlet will strip a tank and keep the burner lit with lukewarm fixtures.
• Required tools: Thermometer at tank-out vs mixed-out vs the worst fixture.

Do not “fix” a 105°F mixer by turning the gas control to Very Hot. That is how people get scalded when the mixer later fails open.

Step 4: Dip tube — short slug of hot, then cold

• What to look for: A burst of hot, then tepid, with the burner still running and the upper jacket hot. Cold supply is dumping at the top of the tank because the plastic dip tube split or dropped. White polypropylene shards in faucet screens. Common on aging tanks; I pull them with the cold inlet isolated, tank cooled, energy off.
• Required tools: Cold inlet off, a way to relieve pressure, needle-nose for the broken tube if you are pulling it. Licensed if the nipple is frozen or the dielectric is weeping.

A dip tube is not a gas-valve part. Replacing the control will not put cold water back on the bottom.

Step 5: Sediment — flame on, water barely heating

• What to look for: Popping and kettling on every recovery, rotten-egg flush, a tank that has never been drained, hard water, 8–12+ years. Lime stacks on the bottom head so the burner is heating rock, not water. Drain valve that will not pass water is the same pile — won’t drain. A wet jacket or rust at the seam is a vessel, not a flush — leaking from the bottom.
• Required tools: Hose, pan. Energy off. If there is no floor drain, use this drain path. Do not restore gas until the tank is full.

A one-time flush can save a tank that still has lining. A rumbling 12-year tank with rusty drain water is a replacement conversation, not a weekend vinegar project.

When DIY Ends: Calling a Licensed Pro

Stop if you smell gas, the burner yellow-tips or bangs, the jacket is wet, the mixing valve is sweated in a place you cannot isolate, or the dip-tube nipple is about to round off. You may confirm both isolation valves, compare tank-out vs faucet temperature, and drain a healthy tank if you can kill energy. You may not raise the control to 140°F as a diagnosis, cap a T&P, or open gas unions. A licensed plumber will prove the flame is transferring heat, pull a failed dip tube, service the mixer, or cut the tank out when the bottom is a lime brick.

Frequently Asked Questions

Q: Why is my gas water heater burner on but I have no hot water? A: The main is heating the tank, but a closed hot valve, failed mixing valve, broken dip tube, crossover, or sediment blanket is keeping usable hot water from the fixtures. Confirm the main flame, then temperature at the tank outlet.

Q: Can a bad dip tube leave the burner running with cold showers? A: Yes. Cold water enters at the top, you get a short hot slug, then lukewarm, and the burner keeps trying to catch up. Plastic bits in aerators are the tell.

Q: Should I turn the gas control up if the burner is on but water is lukewarm? A: No. That scalds and hides a mixing valve or dip tube. If the tank outlet cannot make about 120°F with the burner firing, the heat is not reaching the water — hire it.