Cold water sandwich (temperature swings at fixture)

What is going on

The unit cycles on and off at low flow, so cold water gets pushed into the hot line and the outlet temperature swings.

Likely causes

• Flow rate below the unit's minimum (less than 1 gpm at fixtures)
• Undersized plumbing causing pressure drop
• Faulty or flapping check valves
• Debris in the fan blower wheel
• Clogged inlet water filter
• Inlet gas pressure surging
• Fluctuating inlet water pressure
• Multi-unit system (MSA/MSB) cycling, possibly from a clogged filter on unit 1
• Faulty valve diaphragm or check valve bleeding cold water into the hot side
• Frequent short hot water draws without recirculation
• Demand or recirculation system cycling the unit
• Loose electrical connection or power dropouts
• Low-flow shower heads, especially with high ground water temperature

Check in this order

1. Open the fixture and record the flow rate on the temperature control pad.
2. Shut off the hot and cold feed lines, remove the inlet filter, check for debris, clean it and reinstall. Turn the supply back on and recheck the flow rate at the same tap.
3. If flow is below 1 gpm, remove the restrictor and clean the fixture strainer. Confirm at least 1 gpm at all fixtures.
4. Install swing check valves with the hex cap horizontal and facing up.
5. Check inlet gas pressure while the unit runs for surging up or down.
6. Confirm the gas system is sized properly.
7. Check the whole plumbing system upstream and downstream of the fixture for cold water bleed-over.

More tankless water heater guides

• Checking PC board fuses and thermistors
• CO sensor / combustion air fault (code number not printed on these pages)
• CO sensor voltage check (PCB connector C1)
• Code 12 or 19 appears immediately after water flow starts (no flame in burner box)
• Hot water still flowing with the unit off (bleed-down or cross-over)
• Water leaking from pump housing