Electric Floor Heat

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Jim Becker
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Electric Floor Heat

Postby Jim Becker » Wed Dec 24, 2014 11:51 am

I have a GFI that occasionally trips on an electric floor heat circuit. I've done all the poking and prodding that I can think of to identify the problem, thought I'd see what the collective wisdom here might be able to add.

First off, the way it is set up: There is a 240 volt thermostat with a built-in GFI. (Not a typo, the whole thing is 240 volts.) Cable one goes from the breaker box to the line side of an electrical contactor (essentially a relay). A second electrical cable is connected to the same contactor terminals and runs line voltage to the thermostat. A third cable connects the thermostat output to the control terminals of the contactor. A fourth cable goes from the contactor output lugs and connects to the heating lines in the floor. To slightly complicate the picture a little more, the power is on a curtailment system. The power company can, via radio receiver and a switch in the middle of cable 1, cut power to the whole thing.

The symptom: The GFI in the thermostat kicks out on occasion. This doesn't happen very quickly, might take 2 hours or so. I have only been present to see the GFI light come on once. It happened as the contactor either closed or opened, not sure which but probably closed. The GFI reacts properly to the self test button.

My observations and experiments: Given the wiring, the only part of the circuit actually protected by the GFI is the cable (#3) from the thermostat to the contactor and the contactor itself. No other electricity flows through the GFI. It seems highly unlikely that a ground fault anywhere else (like in the heating elements) would be detected at the thermostat.

The contactor is by itself in a box which is attached to a wooden stud. I theorized that any ground fault in the contactor would have to be to the contactor box. I removed the bare ground wire from the contactor box and connected a multimeter between the box and ground wire. It is an analog meter and the needle never even wiggled, whether on a 15 volt or 30 milliamp scale. Unfortunately, I have yet to be looking at it as the GFI trips. I left it with the ground wire off the contactor box and it seemed to go a long time without tripping the GFI, but eventually did so. I want to say the longer time was meaningful, but it could have just been random variation. Then I regrounded the box and disconnected the control cable (#3) at the contactor and capped it off. So the thermostat is now sitting there controlling on open circuit. It has been that way for at least 12 hours and the GFI has not tripped. So I at least know the GFI isn't failing with no load.

My conclusion: The problem has to be either the contactor or the GFI itself. I see three options on what to do next. 1) guess it is a bad contactor and replace it (about $50), 2) guess it is a bad GFI and replace the thermostat (about $130), 3) call in an electrician (will probably cost as much as options 1 and 2 combined). My gut wants to blame the GFI as it is a fairly complicated circuit protecting one simple component. I also know that doing either options 1 or 2 are guesses, if wrong I will then end up doing the other. After doing both, I still may be stuck with doing option 3 and ultimately paying for all three choices.

Anybody have some thoughts on this? Am I looking at something the wrong way? Any other test I can do with simple tools?

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Re: Electric Floor Heat

Postby Bezirk » Wed Dec 24, 2014 12:19 pm

Jim, if you haven't already I would check all connections in box and slightly loosen them then re tighten with breaker off.
Also if any of the cables are al instead of copper make sure they are coated with anit oxidation compound. Also explore possibility of a defective breaker, particularly if it s be en service for a long period of time. You may have a high resistance connection that is causing a slight current flow and tripping the gfi breaker.

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Jim Becker
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Re: Electric Floor Heat

Postby Jim Becker » Wed Dec 24, 2014 2:51 pm

I had already tried to tighten each connection on the contactor. Since your post, I went back and loosened, wiggled the wire and retightened each of them. All cables are copper. I have manually opened/closed the breaker multiple times while checking things out. The installation is about 10 years old, probably including the breaker in the main box.

After doing the above, I reconnected the #3 cable so everything is back to the original configuration and turned it back on. When I rechecked it after an hour, it had already tripped the GFI.

I think I will wait until Friday and get a new contactor unless I discover something else in the meanwhile.

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Re: Electric Floor Heat

Postby John *.?-!.* cub owner » Wed Dec 24, 2014 3:48 pm

Have you tried opening the circuit breaker and used and ohm meter to check for a high resistance reading to ground? Is the GFI that is tripping the one in your T'stat or one built into the circuit breaker?
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Re: Electric Floor Heat

Postby Jim Becker » Wed Dec 24, 2014 6:58 pm

With the breaker open and all wiring in place, each of the 6 lugs on the contactor show open to ground. Same results analog or digital meter. The circuit breaker does not have a built in GFI. The one tripping is in the thermostat.

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Re: Electric Floor Heat

Postby Bus Driver » Wed Dec 24, 2014 8:18 pm

So the contactor coil is on one circuit and the supply to the heating elements is on a separate circuit? Or is the supply to the elements themselves on the same circuit but bypassing the GFCI? Evidently the load was considered to be greater than the rating of the thermostat, thus the need a for contactor.
Not the usual setup as the typical GFCI protection is for the in-floor elements. A breaker-type GFCI in the panel would add the GFCI protection for the elements.
I have two bathrooms with electric radiant in the floor and those elements are GFCI protected. Self-installed about 10 years ago and zero problems.
The GFCI could be developing a problem, but of every GFCI failure I have observed, the GFCI would not reset.
I suspect that your capabilities for diagnosing this problem are the equal to or better than of 95% of the electricians you might contact. This one is far out of the ordinary.
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Re: Electric Floor Heat

Postby tst » Wed Dec 24, 2014 9:03 pm

I would lay my money on the GFI
Merry Xmas
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Re: Electric Floor Heat

Postby John *.?-!.* cub owner » Wed Dec 24, 2014 10:16 pm

tst wrote:I would lay my money on the GFI
Merry Xmas
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At this point I would put my money with Tim. Are the floor elements wired so they can be disconnected one at a time?
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Re: Electric Floor Heat

Postby Jim Becker » Thu Dec 25, 2014 12:37 am

There is only one breaker feeding the whole thing. Bus Driver, the rest is exactly the way you spell it out. The thermostat is designed to directly feed the heating elements but is limited to 15 amps. I measured the resistance of all the elements combined at 12.7 ohms, which should come to about 18 amps. Everything in the system other than the thermostat is sized for at least 20 amps, which is what the breaker is. Since the thermostat is only driving the coil in the contactor, that is the only part protected by that GFI. As you stated, a GFI breaker in the panel would cover the whole system, including the heating elements. The elements should be GFI protected (according to the manufacturer). By the way, thanks for the vote of confidence on figuring this out. Part of the problem is figuring out whatever non-standard approach was used by the last guy. At this point, most electricians probably would just slap in a new contactor and see if it fixes it. I think that is my next step.

My gut still wants to say it is a faulty GFI. But the fact it didn't trip with no connection plus the fact that it will reset seem to suggest otherwise. I also have to wonder if I am chasing some weird inductance problem. Maybe the fix is to reposition the wires.

The floor elements (there are 3) are wired so that they can be electrically isolated. I guess one option would be to bypass the contactor and hook up one element. I could swap out one element for another and see if any of them make it kick out.

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Re: Electric Floor Heat

Postby Bus Driver » Thu Dec 25, 2014 7:32 am

Assuming that this thermostat controls the temperature of just one room/area, the calculated amperage indicates about 450 square feet of floor area, assuming about 10 watts per square foot. I thought you lived in TX but the present indication is MN. Double-pole GFCI breakers are available.
About how long are the conductors from the thermostat to the contactor and are those conductors accessible?
Does your thermostat read the temperature from the air or from a sensor in the floor? If from the air, a functionally-equivalent replacement thermostat should be easy to find. Grainger probably has the largest selection of thermostats pictured on their website.

Mine in the bathrooms have a sensor in a small conduit buried in the floor with ceramic tile over the heating mats. The conduit runs from the floor to the wall box. The sensor is easily replaceable from the wall box where the thermostat is mounted.

We do not know the type flooring nor the use of the room in question. I can find no requirement in Article 424 of the 2014 NEC for GFCI protection of interior floor electric heating. But the UL Listing and manufacturers instructions might require it and if so, it becomes mandated by the NEC. NEC Article 90.7and 110.3(B). I consider it important in bathrooms due to the potential for flooding. Under wooden flooring in normally dry areas, it seems much less necessary.
Last edited by Bus Driver on Thu Dec 25, 2014 9:29 am, edited 1 time in total.
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Re: Electric Floor Heat

Postby Bus Driver » Thu Dec 25, 2014 9:15 am

In some settings, the noise of the contactor closing might be objectionable. If changing of components is in the plan. Grainger offers a "silent relay", their 2E346. The same part may be lower cost elsewhere. It has a 24 volt transformer for the thermostat. The 24 volts supplies an internal heater which actuates a bimetallic strip contact arm. No noise, but about 45 seconds operating delay.
So with one of these, your thermostat could be a 24 volt unit and the contactor could be silent.
I have very limited experience with solid state relays so have no specific recommendation for those.
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Re: Electric Floor Heat

Postby Jim Becker » Thu Dec 25, 2014 10:13 am

You hit everything pretty well on the mark again. I have moved from Texas to Minnesota (owned this house about a year but didn't finish moving until last summer).

The heated floor is ceramic tile in a basement. The area is 300+ sf. It appears that they used heating wire rather than mat. It looks like this is what was used:
http://www.homedepot.com/p/SunTouch-Floor-Warming-120-sq-ft-240-Volt-Radiant-Heating-Wire-CO240120R/203085054
They may have spaced the wires to the close side of the recommendations, getting it closer to 15 watts per sf. Two of the elements cover about 120 sf each with a shorter third element covering 60 to 80 sf. The temperature sensor for the thermostat is in the floor. I suspect it was stuck directly in the mortar. Since that part is working, I haven't disturbed it. Most of the materials evidently came from WattsRadiant. Their name is on the thermostat, but it looks almost exactly like this one:
http://www.grainger.com/product/HONEYWELL-Line-Voltge-Thermstat-6WY24
Watts has changed their thermostats and they no longer look like mine. Here is the current replacement:
http://www.homedepot.com/p/SunTouch-Floor-Warming-SunStat-120-240-Volt-Programmable-Floor-Warming-Control-500670-SC/100607176

The main breaker box for the house is located at one end of the basement with the contactor box within 2 feet. The power company's curtailment equipment is outside the same wall. The thermostat is on an interior wall about 30 feet away. The cable to the thermostat (previously identified as cable 2) is routed overhead, above a suspended ceiling. Cable 3 returns to the contactor parallel to cable 2. Cable 4 leaves the contactor following a path near the other 2, ending at a junction box mounted about 3 feet from the thermostat. Actual cable length of all 3 is about 40 feet. The 3 elements connect to cable 4 in the junction box. The wall holding both the thermostat and the junction box encloses a small space under the stairs. The wall has exposed studs on the enclosed side. The junction box faces under the stairway. Everything is easily accessible except for the things actually buried under the ceramic tile.

The contactor is not located that close to a bedroom, so the noise isn't a problem. We have adjusted to hearing it when we are in the basement. I really don't want to replace the temperature probe. That limits the choices on thermostat replacement (unless I switch to air temperature control). I'll look some at the options but if I change out the relay, I'll probably just use one identical to the original.

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Re: Electric Floor Heat

Postby Bus Driver » Thu Dec 25, 2014 11:46 am

I would inspect every staple location on the cable. Sometimes staples are overdriven and cause problems. Powering the contactor does not heat the cable, thus we can rule out cable movement from internal heating. Looking for intermittent ground faults is the idea here.
On below-grade concrete, the ground fault protection is quite important. GFCI as used in residential settings is for protection of PEOPLE. Ground fault protection (GFPE) on 3-phase Y-connected transformers is sometimes used in industrial settings for protection of equipment.
Let's hope that moisture from below is not the problem.
Replacing the contactor and thermostat would be the easiest place to start. Do the changes sequentially to see if the problem can be identified.

http://www.elektra.eu/pdf_en/content/floor_heating.pdf

I used an earlier version of this product from Poland for my baths. Found a supplier that charged about 40% of the price of the well-known brands. Saving on the purchase price is one of my favorite hobbies. The thermostat is their earlier version of the programmable one. Wife is not into programming, to say the least. And use of the baths is not always predictable. So the thermostat is always powered and programmed to always be ON at the set temperature. Ordinary wall switch adjacent to the thermostat controls power from the thermostat to the heating elements. So anyone wanting heat simply turns the switch ON and the thermostat cycles as necessary to hold the temperature to the set point. Wife likes that !
Last edited by Bus Driver on Thu Dec 25, 2014 3:51 pm, edited 2 times in total.
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Re: Electric Floor Heat

Postby Bus Driver » Thu Dec 25, 2014 11:56 am

If the problem should ever be finally determined to be a faulty element in the floor, one might consider using Schluter Ditra membrane on top of the existing floor and installing new mats and tile. If raising the floor about 3/4 inches is not objectionable, that might be the easiest solution.

http://www.schluter.com/6_1_ditra.aspx
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Re: Electric Floor Heat

Postby Bus Driver » Thu Dec 25, 2014 3:42 pm

Finally took/found the time to read more carefully the description of the cabling used for all of this.
My mind got off on the floor elements being the problem but they cannot possibly be the present problem with the hookup described.
As you probably have already realized, much of that cabling is simply for the purpose of mounting the contactor at a remote location. It could be simplified.
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