Paloma tankless water heater shows 05 error code

Try simple things first. Whenever your Paloma tankless water heater shows the error code, like 05 in this case, do the following:

Close the hot water tap, turn the switch off on your Paloma remote, wait for several minutes and turn the switch back ON. If the code 05 or any other error code disappears, great, if not either call the technician or try to fix it. I will try to guide you trough the troubleshooting process.

Your Paloma water heater, PH-28 as you said, is using the ICAD technology, Incomplete Combustion Avoidance Device. This is a safety feature that allows Paloma sophisticate system to monitor and optimize combustion in the gas firing chamber. If there are problems like the incomplete combustion and presence of the carbon monoxide, the unit shuts down.

Now, what can cause this problem?

If there is an obstruction in the vent system there is a risk that the flue gases are going back into your home. Check for the blockage and leakage and ventilate the area. If you have other appliances installed in the same room as your Paloma tankless water heater, they might contaminate the air or cause the lack of air. In both cases provide sufficient air to the room.

Air intake should also be without obstructions.