hydronic airhandler

dmmechanical

help! I have a customer with a gas water heater that supplies a hydronic air handler. the system has been in for years. I went there on a service call tonight because there was cold air blowing from the diffusers. supply pipe was hot, but return was coming back cold. I tried purging the loop, I replaced the zone valve, and the circulator pump. The return will come back hot if I disconnect the fan and just let the pump circulate. As soon as the fan comes on, the return pipe goes cold. Any suggestions would be appreciated! Thanks!

Ironman

Water Heater

What type of water heater do they have and where are the hydronic lines connected to it?

dmmechanical

hydronic airhandler

it is a gas fired 40 gallon water heater and the hydronic lines are just piped in with tees off the domestic lines

HDE

DIP Tube

Probably a failed dip tube in the tank water heater. When the fan is off the water just circs through, but with fan on and tube failed cooler water returns to tank and right back out the hot. This continuous cycle with water short circulating on the top of tank will do this.

The return line should have been teed into bottom drain connection

Does the tank do the domestic also? Dangerous but oh so often done. If so the DHW would go cold fast also I'm sure.

Just a guess though but seen it happen.

Very inefficient system though

dmmechanical

water heater

The water heater does do the DHW also. I will check tomorrow to see if the DHW runs out quickly. Thanks for the advice.