Quick Answer:
If your toilet refills or runs briefly whenever the water heater fires, it’s usually due to thermal expansion and a closed plumbing system or a leaky toilet fill valve. Start by watching the toilet while the heater cycles, look for an expansion tank on the water heater, and check whether other fixtures show signs of dripping during the same cycle. If you can’t find the cause or the house pressure rises noticeably, call a plumber.
Why This Happens
When the water in the heater warms, it expands. In an open system that allows water back to the street or storage, that extra volume can escape. In a closed system—one with a check valve, backflow preventer, or pressure-reducing valve—there’s nowhere for the expanded water to go, so pressure in the home increases.
If the toilet’s fill valve or flapper isn’t sealing perfectly, that small pressure spike can push water into the bowl or cause the fill valve to top up briefly. A weak or missing expansion tank on a closed system will let that pressure transfer into plumbing fixtures instead of being absorbed.
Step-by-Step What to Do
1. Reproduce and observe
- Flush the toilet and then run a hot water tap or trigger the heater so it fires. Watch the toilet tank and bowl closely while the heater is heating.
- Note exactly when the refill happens (immediately, after a delay, or multiple times during the cycle).
2. Look for an expansion tank
- Inspect near the water heater for a small tank attached to the cold inlet. That is an expansion tank. If you have a closed system and no expansion tank, thermal expansion will pressurize the house plumbing.
- If an expansion tank is present, check that it looks intact (not severely corroded or waterlogged). A waterlogged tank won’t absorb expansion.
3. Check the toilet’s internal parts
- Open the tank and look at the flapper and fill valve. If the flapper doesn’t seat well or the fill valve is slowly letting water through, a small pressure change can cause a refill.
- Replace or tighten parts if they are clearly worn or loose. For testing, gently press the flapper to see if the running stops.
4. Observe other fixtures during the heater cycle
- Watch faucets, showerheads, and any visible supply valves while the heater runs. If multiple fixtures drip or move, that points to a pressure/expansion issue rather than a single faulty toilet part.
- Make a short checklist: which fixtures show activity, and when it occurs relative to the heater firing.
5. Check for a check valve or pressure-reducing valve on the main
- If you have a PRV (pressure-reducing valve) near the main shutoff or a backflow preventer, the system behaves like a closed system. That makes an expansion tank important.
- If you’re unsure whether your system is closed, note whether the water meter or street connection prevents backflow. If in doubt, document what you see and discuss it with a plumber.
6. Test after small fixes
- After replacing a worn flapper or servicing the fill valve, repeat the heater cycle test to see if the refill stops.
- If replacing toilet parts doesn’t help and other fixtures are affected, the likely issue is system pressure from thermal expansion.
What Not to Do
- Do not change the water heater temperature to stop the toilet from refilling. Adjusting the thermostat won’t fix a pressure or expansion problem and can create scalding risk or inefficiency.
- Do not ignore the problem if multiple fixtures behave the same way; pressure issues can cause leaks elsewhere.
- Don’t attempt major water heater repairs or remove the tank’s pressure relief valve yourself unless you’re qualified. A plumber is appropriate for diagnosing closed-system expansion and for installing an expansion tank or adjusting backflow devices.
When to Call a Professional
- Call a plumber if you confirm other fixtures move or drip during the heater cycle—this suggests a system pressure issue that may need an expansion tank or valve adjustment.
- Call a plumber if you don’t find a clear toilet-part failure after basic checks, or if you’re not comfortable inspecting the heater area or plumbing valves.
- Call immediately if you notice continuous high pressure, leaking water at joints, or any signs of active leakage from the water heater or supply lines.
Safety Notes
- Be cautious around the water heater: hot surfaces and scalding water are hazards. Don’t touch the tank or pipes when the heater is running if they feel very hot.
- If you shut off the main water supply or any valves, know how to restore them and relieve pressure safely.
- If you suspect a failing pressure relief valve or visible leaks at the heater, shut off the heater’s power source (electric breaker or gas pilot per manufacturer instructions) and call a professional.
Common Homeowner Questions
- Why did this start all of a sudden?
Often because a check valve or PRV was installed previously (making the system closed) or a toilet part has worn; thermal expansion becomes noticeable when parts age. - Can I fix it by replacing the toilet flapper myself?
You can try replacing the flapper or fill valve as a first step; if multiple fixtures are affected, the fix is likely at the system level and needs a plumber. - Should I install an expansion tank?
If your system is closed and you have thermal expansion, an expansion tank is the typical solution—have a plumber size and install it.
Related topics: if this started after a water shutoff, see Toilet runs after city outage. If the issue is limited to one level of the house, see Toilet runs only upstairs bathroom.
For more related articles, see the nan hub.