Diagnose in this order
Flow, then gas or power, then ignition, then the exchanger. Heaters are conservative machines: almost every safety interlock exists to stop them firing without water moving through them, so a heater that refuses to run is usually telling the truth about something upstream.
Where an amateur stops
Gas valves, burner trays, orifices and flue components are licensed work in most jurisdictions. Diagnose freely; open the gas train only if you are qualified to.
No flow, or not enough
A dirty filter, a half-closed valve, a clogged pump basket or a pump running too slow on a variable-speed schedule will all trip the pressure switch. Clean the filter, open the valves, and raise the pump speed above the heater’s minimum — many heat pumps and gas heaters need substantially more flow than a low-speed filtration setting provides, which is why heaters “fail” the week a variable-speed pump is installed.
If flow is genuinely fine, the pressure switch itself may be stuck or mis-adjusted. That is a part, not a mystery.
It will not light
| Behaviour | Usual cause | Safe to check? |
|---|---|---|
| No display at all | Breaker, transformer, fuse | Yes |
| Display on, no ignition attempt | Flow switch, thermostat, high limit | Yes |
| Sparks, no flame | Gas supply, valve, igniter gap | Gas supply only |
| Lights then drops out | Flame sensor, grounding, gas pressure | Sensor cleaning only |
| Repeated lockouts | Venting, condensate, control board | Vent path only |
Error codes differ by manufacturer and model. Read the label inside the door — it is the only authority on what a code means for that unit.
Heat pumps behave differently
A heat pump that runs but barely warms the water is often working exactly as designed: output falls with air temperature, and below roughly 50 °F ambient most units make very little heat. Check the air temperature before you condemn anything. Ice on the evaporator, restricted airflow into the fan, or a filthy coil produce the same complaint and are all cheap to fix.
The failure that ends a heater
A split heat exchanger is where most gas heaters die, and it has two causes: freeze damage from a heater that was not drained, and years of low-pH water eating the copper from the inside. Water appearing under the cabinet when the heater runs, or a heater that keeps flooding the burner tray, is the tell.
Exchangers are the single most expensive part in the box. On an older unit the repair usually costs a serious fraction of a new heater, which is the point at which the conversation turns into which type to buy next.
What it costs to run, and to fix
Running cost is arithmetic once you know the fuel price. At the EIA’s published residential figures — 17.30 cents per kWh for electricity in 2025 and $24.09 per thousand cubic feet of natural gas in June 2026, about $2.32 a therm — the sizing calculator prices a heat-up on either fuel.
Repair cost is labour plus parts, and labour is the bigger half on a heater. Price your own hour honestly rather than guessing: the BLS median for general maintenance and repair workers was $23.84 an hour in May 2025, and a shop rate is a multiple of that once vehicle, insurance and unbillable time are in.