In most houses in the Mission Viejo area, heating and hot water come from two separate appliances, so one can quit while the other carries on. The furnace working proves only that the home has power or gas at that moment. The tank carries its own pilot or igniter, thermostat, gas valve or breaker, and that is where to look. Wider diagnosis sits in the water heater repair guide.
Why would the heating run when the water heater is dead?
Because they are not linked. A typical Orange County tract home has a forced-air furnace in an attic or closet and a tank in the garage, each with its own controls. They share a gas supply, not a heat source. A pilot that blew out, an igniter that failed, a tripped thermal switch or an off breaker on the water heater leaves the furnace untouched.
That is useful news. If a gas furnace is running, gas is reaching the house, so the meter and main are fine. Your search narrows to the heater's own gas valve, burner, thermocouple or controls. For an electric tank, a working furnace says nothing about the 240-volt circuit feeding the heater, and a tripped breaker is the likeliest cause.
An exception: a combination system, such as a boiler or a combi unit that heats both radiators and taps, does tie them together. In that case the diverter valve or a pump is the usual culprit when heat works and taps do not. That layout is less common here.
What should I check at a gas water heater first?
Check the gas control knob, the pilot window and the manufacturer's label. If the knob sits at off or pilot and the flame is out, the label gives the relighting steps. If the pilot catches yet dies the moment you release the knob, a worn thermocouple is the likely cause. After a couple of failed attempts, stop and book a repair visit.
A whiff of gas ends the attempts. Get everyone out and use 1-800-427-2200 to reach SoCalGas from the yard; 911 is for anyone feeling unwell. No switches, no flames. On newer heaters with an electronic igniter, you may see blinking diagnostic lights; photograph them for the plumber, since the pattern names the fault. The pilot light repair page works through relighting in more detail.
Also look at the flue and the area around the burner. Soot, rust flakes or a scorched look mean stop and call, because poor combustion can produce carbon monoxide.
Where do I look on an electric tank?
Begin at the breaker panel, where a tripped switch is the commonest answer. Next, the red reset button sits behind the upper access panel and pops on overheating; push it with the breaker off. If heat returns, keep watching. If it trips again, suspect the element or thermostat. Testing those parts involves 240 volts and is a plumber or electrician task.
An electric tank that gives only a little hot water, or lukewarm water, usually has a bad lower element. The pair of problems in electric units are covered in the no hot water on an electric heater page and in replacing a heater element. Remember that power must be proven off before panels come off.
Could the problem be the tap and not the heater?
Yes, if only one fixture lacks hot water. A worn single-handle cartridge, a failed mixing valve or a crossed supply line can leave one shower or sink cold while the rest run hot. A heater issue affects every hot tap, and the house will run out fast. A fixture issue is isolated to one spot.
Try this: open hot at the kitchen, then the bathroom sink, then the shower, one after another. All cold sends you to the heater. If only the shower is cold, the cartridge or valve behind it is the suspect. Weak flow on the hot side only is its own symptom, taken up in low hot water pressure.
When is it a tankless heater problem?
Tankless units fail differently. They shut down on error codes, such as a blocked vent, a dirty flame sensor, a plugged inlet screen or scale on the heat exchanger, and a unit that is running but delivering cold water at low flow may not trigger the burner. Minimum flow rates matter, so a trickle from a tap may not be enough.
Write down the display code, then look it up in the owner's manual. Cleaning the inlet screen is often fine for a homeowner, but descaling and burner work belong to a plumber. Imported hard water in this area builds scale in the heat exchanger over time, and units often need periodic descaling. See tankless gas water heater repair for what repairs involve.
How urgent is it?
A cold heater with no other symptoms can sit until a business day. Gas odor, a leaking tank, a burning smell or breakers that keep tripping make it urgent. The urgent water heater page draws that line more finely. Phones here are answered weekdays, nine to five.
Common questions
Can a gas furnace and a gas water heater run off the same pilot?
No. Each appliance has its own ignition, whether a standing pilot or an electronic igniter. They share the gas supply line and, in some homes, a vent chase. When one fails, the other is unaffected, which is why a working furnace does not clear the heater.
Is relighting a pilot a do-it-yourself step?
On a standing-pilot heater, yes, provided no gas odor is present and you follow the instruction label step by step, including the waiting time it states. If the flame will not stay lit, or gas odor appears at any stage, stop. Repairs to gas parts are for a contractor with the state license.
Why are only some of my taps cold?
Because the heater is probably working. One tap can be starved by a tired cartridge, a gunked aerator, a fixture valve left partly shut, or a bad mixing valve. A system problem, like a failed crossover on a recirculating loop, can also do it. Compare taps and note which are affected. That pattern guides the visit.
What does repair cost if only the thermocouple failed?
A thermocouple replacement is among the cheaper gas heater repairs, usually well under a typical tank replacement, with the visit charge the bigger part. Prices vary, so ask for a quote in writing. If the tank is old, compare the repair to replacement costs before deciding.
Does fixing a water heater part require a permit?
Minor parts like a thermocouple or an element usually do not. Replacing the whole heater normally does, and the plumber handles that filing. If your case is unclear, the city's Building Services Division can confirm what applies at your address.
The rest of our guides start from Capistrano Plumbing.
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