We have a 4 year old townhouse that has a hydronic/radiant floor heating system. A tankless water heater is the heating source for both the hydronic system and our regular household hot water. Whenever any of the thermostats are turned on in the house (which turns on the hydronic heating system) the hot water pipes in the garage (the ones pretty close to the tankless water heater) starting knocking very loudly, and they continue to knock while the heater is on. However, they stop knocking right away if either 1) the heating system is turned off or 2) any hot water faucet in the house is turned ON. Turning on the hot water alone does not cause the knocking, the pipes only knock when the hydronic heating system is turned on. Any ideas? Thanks.

Whose idea was it to use a tankless hwh for heat there is not enough volumn of water to start circulating water they heat up so fast it hits the cold water in the pipes there is no time to mix you might need a storage tank with a ciculator so there is a supply of mixed water so its not such a shock. Good luck Mikie D.

The system that you have with the tankless heater is a very good system and has a very high efficiency rate. I have used a tankless heater/boiler in my own home since 1981 but my system uses hot water fan coils instead of radiant floor heat. Sounds to me like you have a air entrapment problem or the expansion tank bladder has reptured internally. What is the exact brand and model of the water heater? Can you post a picture of the heater and control valves?