Takagi Tankless Water Heater Problem

rijianwang

I bought a new Takagi tankless water model T-H3S-DV-N in Oct 10th 2023 to replace the existing water heater in my basement. The vent is the existing 3-in PVC directly vented to outside. The water heater worked nicely for several weeks but suddenly got a error code 711 which means "gas valve failure" referred to error code 111 or 121. Trouble shutting found the "High Limit Switch" was triggered. Checked everthing is fine and reset the high limit switch and the water heater started to work. A few days later, the code 711 appeared again thus I have to find the cause of the triggering. Interestingly, I find the water heater fired up and stopped frequently without water flow when the water treatment system started to work that is uptream of the water heater about 25 feet away. Since there is no water flow, the heat accumulated in the heating core trig the error code 711. I have a private well with a pressure tank. The working water treatment system causes the water pressure fluctuate from 40 to 60 PSI. My question is how the water pressure fluctuates trig the Takagi water heater to fire up and how to avoid it? Any help is appreciated.

Jamie Hall

What kind of water treatment system? I suspect the problem is something to do with the water supply as a whole -- interaction between the pump, the pump expansion tank, and the water treatment system -- dropping the flow to zero from time to time.

Can you create and post a diagram? Maybe some photos?

rijianwang

@ Jamie Hall. Thank you for the comments. My well water contains high levels of Fe and sulfa hydrogen thus I have to use the "air pock/resin" water treatment system to oxidize and filter the Fe and S. The water treatment system involves "backwash" (20 minutes) and fast forward wash (10 minutes) to "regenerate" the resin every 2 days. I set the regeneration process at 12:00 am mid nights. Then the treated water will supply the whole house through a 1" PEX pipe. The 1" PEX Pipe will branch to supply every faucets including the water heater (1/2" PEX). When the system go through the regeneration process, the water flow (not related to the water heater) causes the pressure fluctuate between 40 and 60 PSI. At 40 PSI the well pump will start and it stops at 60 PSI. I found the water heater starts and stops frequently when the water pressure changes. I am sure there is no water flow through the water heater at this time but for sure the water heater will sense the pressure change. Is it clear and any advice? Thanks