Thermostat wire - continuity test fails after heat call causing valve to stay open

pant_deepak

I am experiencing a weird problem.

I have ecobee 3 thermostat and Taco ZVC 405-4 Five Zone Valve control. I do not have a C wire and I am using a wall plugin 24v transformer to power the thermostat.

The zone valve stays open after the thermostat stops calling for heat. I replaced the ecobee with old honeywell mercury thermostat and the behavior is same. The call for heat and Zone valve opening and closing works for sometimes say an 1 hour and sometimes for as high as 24-48 hours. And after that zone valve just stays open.

1. If I switch off the whole system for say 10-15 minutes. The continuity test passes when I disconnect both the R and W wires that runs from Thermostat to the Taco ZVC relay board.

2. The continuity test fails when Zone valve has stayed open even after thermostat is no longer calling for heat. The zone valve closes only when I disconnect R from the relay board. I noticed that the thermostat wire is running close to the forced hot water pipe. Is it possible that close proximity of thermostat wire to the hot water pipe causes a short, which causes zone valve to stay open.

mattmia2

Continuity where? What brand/model zone valve is it?

pant_deepak

Zone Valve are three wire brown color taco zone valves. I tried changing the zone valve as well. The new valve stays open even after thermostat stops calling for heat. I did continuity test in thermostat wires to see if there is any short

109A_5

Hello @pant_deepak,

You disconnected the R wire at the Orange circle ? If so the thermostat wire has a short or the thermostat is bad.

Jamie Hall

Continuity test? From where to where on which wire? And have you checked for shorts? Either between wires or to ground?

pant_deepak

109A_5 said:

Hello @pant_deepak,

You disconnected the R wire at the Orange circle ? If so the thermostat wire has a short or the thermostat is bad.

Thank you. Yes you are correct the short is somewhere in the T-stat wires. But the issue is if there was really a short then the call for heat should always be ON. I have a old mercury thermostat in place right now. If the wires were short somewhere then it should be always calling for heat. What happens is the thermostat and zone valve work nicely for say 3-4 hours and then valve just stays open. At that point only option is to disconnect the red wire from relay board. I was following the wire in the basement and saw that it is close proximity of a hot water pipe. Thats why I was asking if that close proximity causes wires to short due to heat.

pant_deepak

Jamie Hall said:

Continuity test? From where to where on which wire? And have you checked for shorts? Either between wires or to ground?

1. I did the continuity test between red and white wire from thermostat end to the wire that is near the relay board. It passes when its cold.
2. The continuity test fails after 2-3 hours of running when call for heat stays on.

109A_5

Hello @pant_deepak,
OK, I guess what is not clear is what Red wire ?

From the thermostat to the Taco ZVC 405-4
OR
From the Taco ZVC 405-4 to the Zone Valve

When the Zone is stuck on what do the status indicators show ?

pant_deepak said:

1. I did the continuity test between red and white wire from thermostat end to the wire that is near the relay board. It passes when its cold.
2. The continuity test fails after 2-3 hours of running when call for heat stays on.

The short certainly could be heat induced especially if the wire is compressed to the hot pipe with a zip tie or the like.

Jamie Hall

The damaged wire or wires could very well be heat induced or otherwise time sensitive. Wouldn't be the first time I'd seen that (if you've ever worked on a British car with Lucas electrics, you'd know what I meant).

The tell that it isn't the thermostat or the control is this: the thermostat can control the heat for a while, and the zone valve and boiler respond correctly. Then you check the wire after two or three hours and trouble.

Try replacing that whole thermostat cable. Unless someone got really tidy and stapled it down, you may even be able to use the old cable to fish a new one.

HomerJSmith

How many ecobees and ZVs? Your problem may be in the ZV itself. Disconnect the wire to the #1 terminal on the ZV when the lockup occurs and see if the zone valve closes. It will take awhile for the thermo wax to cool.

pant_deepak

HomerJSmith said:

How many ecobees and ZVs? Your problem may be in the ZV itself. Disconnect the wire to the #1 terminal on the ZV when the lockup occurs and see if the zone valve closes. It will take awhile for the thermo wax to cool.

My life was good with three zones connected to three old slider style honeywell thermostat. It became a mess when I introduced two ecobees with fast-stat common maker and third ecobee power using plug-in transformer. This plug-in transformer supported one does not work correctly. Even when I switch back to the old clider thermostat, it works for sometimes an hour, sometimes for days and the heat stays on.

pant_deepak

109A_5 said:

Hello @pant_deepak,
OK, I guess what is not clear is what Red wire ?

From the thermostat to the Taco ZVC 405-4
OR
From the Taco ZVC 405-4 to the Zone Valve

When the Zone is stuck on what do the status indicators show ?

pant_deepak said:

1. I did the continuity test between red and white wire from thermostat end to the wire that is near the relay board. It passes when its cold.
2. The continuity test fails after 2-3 hours of running when call for heat stays on.

The short certainly could be heat induced especially if the wire is compressed to the hot pipe with a zip tie or the like.

1. From the thermostat to the Taco ZVC 405-4
2. When the Zone is stuck on what do the status indicators show ? Both orange/yellow and red are open suggesting call for heat and that valve is also open.
3. I will see if I can find any heat shield or something that I inject where I feel wire is too close to the pipe. Thank you reading and responding

Jamie Hall

pant_deepak said:

HomerJSmith said:

How many ecobees and ZVs? Your problem may be in the ZV itself. Disconnect the wire to the #1 terminal on the ZV when the lockup occurs and see if the zone valve closes. It will take awhile for the thermo wax to cool.

My life was good with three zones connected to three old slider style honeywell thermostat. It became a mess when I introduced two ecobees with fast-stat common maker and third ecobee power using plug-in transformer. This plug-in transformer supported one does not work correctly. Even when I switch back to the old clider thermostat, it works for sometimes an hour, sometimes for days and the heat stays on.

Ah. Oh dear. Well, if it ain't broke, don't fix it. But now that you've fixed it you'll have to live with it. I'm betting that somewhere these is indeed an intermittent short which once in a while makes that connection -- but it may take some inspired sleuthing to find it.

109A_5

Hello @pant_deepak,
OK, if when it acts up, disconnecting the Red wire at the Taco ZVC 405-4 from the thermostat terminates the stuck call for heat. And if turning down the thermostat or disconnecting the thermostat (at the thermostat) never ends the call for heat, it pretty much has to be the wire. Where the damage is to the wire may or may not be where you suspect. But if it always follows the heated pipe I would check that first. maybe just separate the wire from the hot pipe, see if that changes the symptoms. If so replace the wire or just that section.

pant_deepak

109A_5 said:

Hello @pant_deepak,
OK, if when it acts up, disconnecting the Red wire at the Taco ZVC 405-4 from the thermostat terminates the stuck call for heat. And if turning down the thermostat or disconnecting the thermostat (at the thermostat) never ends the call for heat, it pretty much has to be the wire. Where the damage is to the wire may or may not be where you suspect. But if it always follows the heated pipe I would check that first. maybe just separate the wire from the hot pipe, see if that changes the symptoms. If so replace the wire or just that section.

Sure that is what I will try. If that does not work then I will try to fish the wire. The issue is that wire is stapled at in the basement. I can follow it till the point it goes up to the second floor. Thank you very much for prompt responses.

pant_deepak

I also notice that there is air in the hot water pipe and I also hear water flowing and gurgling noise in the room. Can that create this kind of behavior?

HomerJSmith

Remove the thermostat wires at the ZVC R&W connections, set the thermostat at off or very low temp setting and measure for continuity. There should be none. If none the wiring to the thermostat is OK.
If there is continuity and the thermostat isn't calling for heat, the boiler would be firing.

Jamie Hall

HomerJSmith said:

Remove the thermostat wires at the ZVC R&W connections, set the thermostat at off or very low temp setting and measure for continuity. There should be none. If none the wiring to the thermostat is OK.
If there is continuity and the thermostat isn't calling for heat, the boiler would be firing.

yeah... but it's intermittent. Only way to be sure is to look at it.

HomerJSmith

I had a situation where I stapled a thermo cable 18/3 and had a problem. I had actually put one leg of the staple thru the cable. However, I would take a short piece of cable and connect a mercury thermo to the ZVC at the boiler and run up the temp on the thermo and see if the problem occurs. Then you can be sure it's the cable to the thermostat that is defective.
Is the plug in transformer powering that thermostat wired to Rh and C on the thermostat?

EBEBRATT-Ed

@pant_deepak

If you're disconnecting the two thermostat wires at the thermostat and checking continuity between those wires that will get you nowhere if they are connected at the other end.

Take the thermostat off the wall and let the wires hank in the air so they can't touch anything. Then find the other end of that wire at the zone control and disconnect all the wires. I assume you have 2 wires going from the zone panel to the stat. At the zone panel end check for continuity between the two wires you should have none. Go back to the stat end and put the two wires together with a wire nut. Then go back to the zone panel and check continuity. You should have it.

The test is pretty good. There is always a possibility of a staple through the wire or other damage.


Thermostat wire is pretty cheap. Go buy some and run it from the stat down the stairs to the zone panel and hook it up and see if it works that way.

That is one way to tell for sure. if the wire is the issue. If the wire is bad you will have to snake the new wire up through the wall

pant_deepak

pant_deepak said:

I also notice that there is air in the hot water pipe and I also hear water flowing and gurgling noise in the room. Can that create this kind of behavior?

I called a different HVAC Plumbing professional. He reviewed the whole thing. Found nothing wrong with Taco or Thermostat wiring. He noticed the gurgling noise in the forced hot water pipes. He purged it and believe it or not the weird problem got resolved. I waited for 4 full days to see if the problem reoccurs as it used to intermittently reoccur.

So looks like purging the air out of the system fixed this issue. Thank you all very much for your prompt messages and advice. Appreciate it.