When a storage water heater fails, replacing it with the same fuel is usually the simplest job, because the gas line and vent, or the electrical circuit, are already in place. Switch fuels only when a site-specific quote and your own utility rates show it’s worth the extra installation work. In either case, size the new unit by its first-hour rating, not tank gallons. This page covers conventional gas storage and standard electric-resistance storage tanks. Heat-pump and tankless water heaters are different categories.
Size by first-hour rating
DOE defines the first-hour rating (FHR) as an estimate of the most hot water, in gallons, a storage heater can deliver in an hour that starts with a fully heated tank. It combines stored water and recovery, so two 50-gallon tanks can have different FHRs. Add up the showers, baths, dishwasher and laundry loads that can overlap in your busiest hour, and pick a model whose FHR covers them. Efficiency is listed as the uniform energy factor (UEF), and the yellow EnergyGuide label estimates annual energy use.
What changes with the fuel
| Item | Gas storage | Electric-resistance storage |
|---|---|---|
| Site needs | Gas supply and shutoff, combustion air, a vent approved for the model | Dedicated circuit and disconnect as the model and local code require |
| Heat source parts | Burner, gas control, pilot or igniter, vent; some have fans and pressure switches | Heating elements, thermostats, high-limit switch, wiring |
| Safety concern | Combustion: carbon monoxide and gas leaks | Line voltage inside the cabinet |
| Shared by both | Tank, anode, drain valve, temperature-and-pressure relief valve, expansion control | Same |
Estimate running cost from your bills
- Record each candidate’s model number, FHR, UEF and EnergyGuide annual energy use.
- Apply your actual gas and electric rates. Include a fixed charge only if it would actually change.
- Check rebates and tax incentives against the exact model and eligibility date.
A fuel that’s cheaper per unit can still cost more once efficiency and installation are counted.
When the old one acts up
| Symptom | What to ask the technician |
|---|---|
| Water around the base | Is it a fitting, drain valve, relief discharge, condensation or the tank itself? |
| Not enough hot water | Is demand above the FHR, is the setpoint right, is the burner or element working? |
| Rusty or smelly hot water | Is it only on the hot side, and what does the manual say about the anode? |
| Relief valve discharging | Is temperature or pressure too high, or is thermal expansion uncontrolled? |
Never cap or plug a relief valve discharge. Get a written diagnosis that names the failed part and test result, separates parts, labor and code corrections, and states warranty coverage. A leaking tank means replacement. A failed thermostat or igniter usually doesn’t.
Safety
- Gas smell: leave right away and call the gas company or 911 from outside. Don’t touch switches, phones or appliances inside (CPSC ).
- CO alarm or symptoms: get to fresh air and call emergency services. CPSC identifies malfunctioning fuel-burning appliances, including water heaters, as CO sources (CPSC CO fact sheet ) and recommends professional installation and inspection.
- Electric units: keep covers on. Leave element testing and wiring to a qualified person, and don’t energize a unit that has been wet.
What a replacement quote should list
The exact model and FHR, permits, gas or electrical work, venting, drain pan, discharge piping, expansion tank, any required seismic straps, removal and disposal, and labor warranty.
Related guides
Sources
Frequently asked questions
Which type is cheaper to repair?
There is no reliable fuel-wide answer. The failed part, access, parts availability, warranty status and local labor decide the bill. Compare a written diagnosis with the cost of replacement.
Can I switch fuels when I replace a water heater?
Often, but it can add gas piping, venting, panel or circuit work, permits and inspection. Get a site-specific scope before buying the heater.
Should I buy the highest first-hour rating available?
Buy enough first-hour rating for your busiest hour. Oversizing adds purchase cost and may not fit the space.