Rheem Tankless Water Heater Error 78 — Combustion Fan Fix

What Does This Error Mean?

Rheem tankless Error 78 means the combustion blower motor RPM is wrong — either it's not spinning at all, spinning too slow, or spinning fast against an obstruction.
The control board reads the motor's tach signal continuously; if it doesn't match the commanded speed, the unit shuts down and shows Error 78.
Most cases trace to a blocked vent, a failed motor bearing, or a loose wiring harness.

Affected Models

Rheem RTGH series condensing tankless
Rheem Performance Platinum tankless
Rheem RTG series mid-tier (some units use the same code)
Rheem commercial tankless (RTGH-CM)

Common Causes

Vent pipe blocked — bird nest, snow, leaves
Combustion blower motor bearings failed — squeals or seized
Wiring harness from board to blower disconnected or burned
Bad capacitor on the blower motor (older models)
Pressure switch interpreting normal flow as blocked, indirectly disabling the blower

How to Fix It

Inspect the vent pipe outside.

Find where the exhaust pipe terminates outside.
Look for bird nests, paper waste, snow, or insect screens that have clogged with lint.
Clear anything you find.
Many Error 78 calls in spring and fall are simply nesting birds — an exhaust grille that hasn't been cleaned in a year is a textbook nest spot.
Listen to the blower at startup.

Turn off the unit, then back on, and trigger demand by running hot water.
You should hear the blower spin up smoothly within 5 seconds.
If you hear nothing, the motor isn't running.
If you hear grinding, squealing, or rattling, the bearings are failing.
Either symptom means a service call.
Reset the unit completely.

Turn the unit off at the front panel.
Turn off the breaker for 30 seconds.
Restore breaker, turn unit on, retry.
This clears any soft fault state.
If Error 78 returns within one cycle, soft reset isn't going to help — the hardware is the problem.
Have a technician check the blower harness and motor.

A plumber will remove the front panel and look for a disconnected harness, burned connector, or visibly damaged motor.
The blower motor is a replaceable part; for most Rheem tankless models it's $120-$220 plus an hour of labour.
Don't keep cycling a unit that fails Error 78 — running with weak combustion airflow risks incomplete combustion.

When to Call a Professional

Diagnosing Error 78 means accessing the blower assembly behind the front panel of a gas appliance.
If the simple checks below don't clear it, call a licensed plumber.
A failing blower can also indicate a partial vent obstruction — that's a CO risk worth taking seriously.

Frequently Asked Questions

Is Error 78 dangerous?

The unit shuts itself down — that part is safe.
The risk is if you keep resetting and forcing it to run with a partial vent block or a struggling blower.
Both lead to incomplete combustion, which produces carbon monoxide.
If your CO alarm has gone off near the unit, treat that as the priority — open windows, leave the building, and call the gas company before troubleshooting Error 78.