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Baseboard heating system - one zone always on & other frustration

Posted by dohertyc9 (My Page) on
Wed, Oct 19, 11 at 19:09

I just purchased this house six months ago and it has a baseboard water heating system that is gas-fired. One zone is always on. I can shut it off manually on the return side. I understand that this is most likely a zone valve failure. I had an HVAC guy who was recommended to me look into this as soon as I moved in. He said he'd never seen these valves before and he'd find a replacement. Well, I waited all summer and we talked on the phone several times and, long story short, even when he showed up in September he had forgotten all about looking into it.

This is the valve:

Honeywell MZV series motorized zone actuator. I don't see a model number anywhere, but there are some patent numbers.

Then he said it was just twisted on wrong and the valve should work fine. Of course, a day or two later I realize it's not. It's always on.

The valve says something about twisting the knurled bronze part until the pointer is over the red dot. Most of my valves are not over the red dot. How much can I twist this without them coming off?

I had him repair a leaky baseboard pipe, which required that the system be drained. Then he took forever reflushing the system.

Well, now the main level zone is not even heating up. The upper level zones do heat up. Is this an air pocket? If I follow the pipes from the heater, they're warm until they come to a 90 degree downward bend, then they're room temp from there.

Is a system flush something I can do myself? I asked him about this when he was here, but of course he said he'd rather I call him (and pay him $98 an hour) to do it.

I'm willing to 1. replace the valve and 2. flush the system if you think it's something a mildly handy person could do. What do you think?


Follow-Up Postings:

RE: Baseboard heating system - one zone always on & other frustra

Incidentally, couldn't I test the valve for continuity? Or is this not worth it? The HVAC guy did not do this.

RE: Baseboard heating system - one zone always on & other frustra

Sounds like you have an air pocket at the elbow, as you suspected. Do you hear gurgling when the heat is on? Depending on your specific system, you may or may not have bleeder valves at the baseboards themselves. You may have to bleed from the boiler with a hose.

I've also posted a link for testing zone valves. You could have a stuck zone valve, or a stuck thermostat. 24V is applied to the zone valve to open it. When the thermostat is not calling for heat at that zone, you should not have 24V, and therefore, the valve should close. You can try to move the lever up and down to free the valve. If the motor is bad, you can replace just the operator, or motor itself. If the valve is the problem, you may have to drain the system.

You'll probably be able to get some better help at by posting on their main wall. There are many more hydronic heating professionals there than here, and they are always more than happy to help people out.

Here's a link to the manual for what looks like your zone valve:


Here is a link that might be useful: Testing Zone Valves

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