Pump not pumping? Air in the line?
I have a new hydronic heating system consisting of a Takagi tankless heater (gas powered) connected to three baseboard heaters through a manifold and a Taco pump which is controlled by a thermostat and controller. This is all part of a DHW system, so all of the piping is copper and there is a mixing valve to introduce cold water to the 180 degree hot water for the DHW system.
All works well except sometimes the thermostat calls for heat, the pump comes on and nothing happens. The Takagi doesn’t sense flow so no heat is created. I don’t know if there is any flow going on but I suspect not. I am guessing that there is a slug of air that gets in the pump and stops the pumping from happening. (The pump is pumping vertically, in a downward direction, in the return from the heaters).
The plumber who installed it all came out and bled the pipes once and that fixed it. This weekend it stopped working again. I messed around with it, bled some water out and now it’s working again. Perhaps there was some residual air in the pipes.
Should there be a valve to bleed this air out or is this just because it’s all new?
Replies
I probably can't answer your question, but where is the mixing valve in relation to the pump and WH?
I saw a thread on HVAC-TALK.com about a system that had 2 mixing valve and under some combination it would block water flow.
The pump may be cavitating for some reason.
Well, it's been working fine lately. I think there is an air bubble in there and if it is at the pump when it is switched off, it can't start up again. When it's running, if the bubble goes by the pump, the inertia of the water will push it on by. If it stops working again, I'll bleed the line again.
Don't know how much this may help, but when my new furnace was installed, one of the new fittings was an air scavenger valve. This automatically releases air built up in my hot-water baseboard heating lines. As water may carry air in suspension, released during the heat/cooling cycle, such a fitting may prevent future lock-ups...
Has anyone else heard of this air scavenger valve? Would I just put it in anywhere in the loop? Sounds good! Automatic!
I would think that you would need an air scavenger (air release valve). Granted, your system is slightly different than our old one, but with a typical system with a hot water heater (not a boiler), you have an expansion tank, because as water heats up, it expands. It also holds less air as it heats up, so there is an air release valve on the expansion tank to bleed off the air as it builds up. The problem is worst with a newly filled system, as the incoming cold water has air in it; after a while of heating cycles, the air is driven from the water.
I would expect that you are getting a build up of air over time, so you need to have an air release valve of some sort.
Yes, that sounds right. My system is not a closed system. That is, the water in the heating loop returns to the inlet of the heater where it could also go to the house faucets. Also new water will flow into the heaters every day, with more entrained air. That air can build up. I do have an expansion tank but no air valve.
Is that a common system? I always understood, and I could be totally wrong here, that the water in a heating system should be kept totally separate from domestic hot water....but that may be a function of the type of heating system in use, I don't know.
In this type of system, using only copper pipes, you can have DHW and heating in the same circuits. This is in Southern California, where we don't have to worry about freezing pipes, so no anti-freeze is needed in the heating circuit pipes.
I have been emailing the heater manufacturer, Takagi. They suggested cleaning the in-line filter of the heater. I'll try that this weekend.