Temperature cycling up and down

by Richard
(Paulden, Arizona)

We have a Takagi T-K2 that's been in use for about 7 years. Recently the temperature started fluctuating up and down and has been getting continuously worse. I've checked all the common problems, crossover, filters, gas flow, venting etc. It has a remote control that monitors the flow rate, incoming water temp and outgoing water temp. The flow rate and incoming temp are steady, the output water temp cycles constantly above and below the temperature setting.
The temperature setting is 122. With a hot water valve open all the way, no cold, the flow of 1.4 gpm, watching the remote monitoring live conditions, about every 15 to 20 seconds the temp will rise to around 140, the heater reacts and the temp drops to about 95 then back up down and so on. The fan, burner, and valves are all reacting.
After about 30 minutes it stabilizes and performs normally.
We live in a rural area and have been unsuccessful at finding a plumber qualified to work on Takagi heaters. I've contacted Takagi customer service and they weren't able to come up with an answer. Just guesses.
I need to add that we are not! getting any trouble codes at the remote or the control board.
Please help if you can.

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