Takagi KD-20 Error Code 21

[chiron47]

I have a Takagi KD-20 that works for a bit then stops and flashes an error code 21. We haven't had any problems with it before. Today it is -10degrees F with -30 windchill - the only thing that seems different from most days.

The manual just says "vent pipe".

any ideas on what is going on here?

when running inlet temp is 43 and out is at 122 until it stops...so it doesn't appear to be anything freezing.

[chiron47]

To add a little bit to this:
The furnace and water heater have independent exhaust lines, but common air intake lines (the WH line tee's off of the furnace line).

The water heater will work fine when the furnace is off, but stops with an error code 21 shortly after the furnace starts.

[Dana]

Presumably it's installed indoors where it's well above 5F, and the backflow preventer is installed on the vent, in which case the electric heat-exchanger heater can and will protect it. The incoming air for combustion can be quite cold and not cause a problem, since the only time it's drafting over the heat exchanger with out the flame is on flue purges, and the air has far less thermal mass than the water in the HX & internal plumbing.

I too would suspect frost/ice restriction on the air intake under super-cold conditions, especially if the intake is near the exhaust vent to the furnace.

[zl700]

You cant common vent the intake of a heater that has been adapted to 2-pipe.
The furnace draw is affecting the venting pressure switch of the Takagi.
Follow the instructions by doing a correct install.

[Tecno]

Make sure the backflow preventer is opening all the way..I have a TKJR and it will work with the backflow preventer in the closed position IE it was stuck shut from small particles that had grown on the vent pipe inside.this was causing a pressure Differential some of the time but it must have been very close to the trip point of the Pressure Sensor. I was getting the Pressure switch code 21 I removed the vent and cleaned the inside of the pipe and it fixed the problem..