T-Stat Control Wire Shorted - Twice

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Old 01-06-12, 11:45 PM
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T-Stat Control Wire Shorted - Twice

First let me mention I have a Hydroair system. 3-zones, (2 heat, 1 hot water heater) house is only 6 years old.
A week ago I noticed that my circulator pump was constantly running and I had blown through 1/2 tank of oil in one month. The weather in the North East has been very mild, 40-50.

I did some troubleshooting and I found that the zone for the 2nd floor was calling for heat and the zone valve was open. This in turn was turning on the circulator pump. I went upstairs and the thermostat was not calling for heat and the fan was not running. I turned off the thermostat. The circulator continued to run.

Up to the attic I went. I opened up the air handler and removed the t-stat wires from the relay that calls for heat. Back down to the basement and the circulator pump was still running. I disconnected the basement end of the t-stat wire. Zone valve closed and circulator stopped. Using my ohm meter I rang out the t-stat wire. The white and red were shorted read 0 ohms and got my continuity beep. Since the wire was a 3 wire I decided to check the green. Also shorted.

So it appeared that the t-stat wire became shorted. I chalked it up to a screw or nail or pinched wire that finally shorted after several seasons.

Double checked my theory by running a t-stat wire from the basement, up 3 flights of stairs to the attic. Connect everything back up and it appeared to work. After spending a couple of hours trying to find a route to pull a new t-stat wire I caved and decided to steal a spare security wire that went from the attic to the basement.

Hooked this all back up and everything was working fine. That was Wednesday. Tonight I was back in the basement working on the kids pinewood derby car and I noticed the circulator pump was running. Check the zone valves and 2nd floor was calling. Up to the 2nd and Shut the t-stat off and began troubleshooting. Long story short the new wire is now shorted.

My guess is the system pulling a lot of current and melting the t-stat insulation, thus causing the wires to short out. Does anybody have any ideas? How much current would a zone value pull through a relay? The only thing I could find for my TACO zone values is less than 1 AMP.
 
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Old 01-07-12, 12:07 AM
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Animals eating thru the wire.
Or.
The wire is melting against a heat pipe.
Can you see if it is 60*C wire or 90*C.
I've noticed the 90*C stuff is harder to find these days.

Also put the ac amp meter around the wire an d see how much current is being drawn.

Peter
 
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Old 01-07-12, 12:24 AM
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Hi Peter,

I thought maybe an animal originally but when the second wire shorted I'm doubting it. The two wire runs take different paths to the basement and the 2nd one only worked for 2 days!

I did measure the current when I connected the 2nd wire but never saw anything over 30mA. I didn't leave it on for a full cycle so I might try that tomorrow. Good thing it's warming up again!

I'm probably going to run the t-Stat cable up the three flights of stairs again and then keep an eye on it. The wife won't be happy but I need to figure this out before I start cutting holes to run a new wire.

I'm wondering if anybody else ever had a problem like this. I also wonder if the security cable wire gauge was to small. 22AWG vs the 18AWG of the t-Stat wire.
 
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Old 01-07-12, 04:50 AM
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Gonna take an awfull lot of currect to melt thru like that.
Do you have a Taco ZVC? They are internally fused.
Short term for 22 ga is about 7 amps and well over 10 for 18ga.
How can all that current come from a lttle 24v transformer?
Did you find the temp rating of the cables?
When you run the wire up the stairs, keep the amp meter hooked up to it.
Might be a good idea to go to radio shack and get a slow blow inline fuse.. 2 amps maybe?


Peter
 
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Old 01-07-12, 06:22 AM
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Hi Peter,

I agree it would take a lot of current to melt the wire. The wire type is CL2 I'm not sure on the temperature rating it isn't on the cable.

To your original point about running near a hot pipe the t-stat wire takes a completely different route to the attic and isn't anywhere near any pipes except where I can see them in the basement.

My TACO valve is a 555-050RP.

I assume the transformer is 40VA, I don't see any markings on it. The measured voltage is 26V. 40VA/26V = 1.5A This should be well under what the wire can handle.

I tried measuring the current last night just at the boiler with shorted wire disconnected. The zone valve came on but I didn't seem to get a reading. I'll try again once I run the wire up the stairs. I'll need to double check my meter to make sure it is OK too.

The fuse is a good idea. I wonder if I can get one at Lowes?

I'm also going to check the airhandler and make sure it isn't some how back feeding my t-stat wire with it's 24V supply or 120V supply. My guess is something in the circuit is causing a large power draw at some point. It's interesting that my 1st floor zone doesn't have any problems.
 
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Old 01-07-12, 07:45 AM
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I'm also going to check the airhandler and make sure it isn't some how back feeding my t-stat wire with it's 24V supply or 120V supply. My guess is something in the circuit is causing a large power draw at some point
Sounds like a plan.
Hot enough to melt insulation of wires, is about hot enough to start a fire.
I hope you get this figured out.

Peter
 
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Old 01-07-12, 08:19 AM
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Me too!

I bought an inline fuse holder and a couple 2AMP and 3AMP fuses. I measured the zone valve current this morning and it is pulling 1.05 when it opens.

My plan now is to run the wire up the stairs, connect in the fuse, attach the AMP meter and see what happens.
 
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Old 01-07-12, 08:35 AM
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Hello John,
If, by chance, you have another meter, hook it up to watch the voltage also.


Peter
 
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Old 01-07-12, 12:21 PM
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My TACO valve is a 555-050RP.
That head appears to have been replaced at least once already. That's the replacement part (RP) number.

I would watch that current draw for several minutes. About a minute or two after the valve opens, you should see the current drop to zero. Then, after about a half a minute it should go back up to about 1 amp for 10-15 seconds, then drop again. It should repeat this the entire time that the valve is open. It should NOT be a continuous current draw. There is a temperature switch inside the valve head that controls this.

If 18 gauge wire, you should be able to safely pass at least 2 AMPS CONTINUOUS with no problem. The 1 Amp draw of a Taco valve is well within the limits.

20 gauge is just within the limits... if your wiring is anything smaller, then it's too small.
 
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Old 01-08-12, 02:38 PM
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So I installed a TACO zone control box today that has a built in fuse. I haven't seen the valve draw more than 1.05AMPS. Everything seems to be working fine with the wire running up the stairs. Now I'm wondering if the 22AWG wire just melted down with it pulling an AMP.

I'm going to run the system this way for a week or so and see what happens. Then I'll have to cut some holes in the wall and snake a new wire up to the attic.
 
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Old 01-08-12, 03:04 PM
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I have seen the thermostat wire wrapped around the line wire to the boiler and in damp conditions bad things happen,
 
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Old 01-08-12, 03:08 PM
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I think 22 ga is only good for about .6 Amps continuous.

The Taco box will isolate the zone valve current draw from the thermostats... but the wiring between the Taco and the zone valves should be 18 ga.

Thermostat wiring should really be at least 18 ga, in my opinion. If you are replacing, don't use 22 again. If you google for "Thermostat wire" you will see that nearly all the hits you get will be 18 ga.
 

Last edited by NJT; 01-10-12 at 05:42 PM.
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Old 01-08-12, 03:29 PM
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I think 22 ga is only good for about .6 Amps continuous.
And AWG 22 may not have sufficient physical strength to withstand draping, pulling, and twisting it during installation.
 
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Old 01-08-12, 03:46 PM
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Yeah, I didn't want to use the 22AWG wire originally but with the 18AWG wire being shorted out and no other way to fix the problem, I grab the spare security wire in the attic. All the wiring for the system is 18AWG and when I get the new t-stat wire snaked to the attic it will be 18AWG also.
 
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