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Old 10-14-2012, 03:03 PM   #1
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Need help understanding a 240V circuit for radiant ceiling heat


I just dug out the old digital volt meter and took off the thermostat for testing. The house has settled and I think I have a break in the overhead grid.


Here's the thermostat specs:

Thermostat model SW22
22 amp, 125 - 250 Volt AC
Non Inductive, 60 sycle

(each room in the house has an independent thermostat for the ceiling heat in that room)

The system is by Swan mfg, Vancouver, Washington, they closed in 2005

DVM results for the thermostat:

0 ohms when turned up
infinite resistance when turned down
temperature accurate for room temp


The thermostat has just two wires running to it. I can only assume that the heat coil is hardwired into one side, and the t-stat offers either ON or OFF on the other (I don't know whether the t-stat is to ground or to hot)

I'm going to push my luck, put on some rubber gloves and turn the breaker box back on, then test for voltage at the two wires.... here goes....

OK - I have voltage on the black wire
No voltage on the red wire

No voltage registers when I test voltage from the black wire using the red wire as the ground



Breaker back off, then:

I then checked for resistance. I put the DVM on the most sensitive setting (2000K) and got no change from infinite when testing across the two wires

Next I checked continuity to the little metal box the thermostat bolts in to (hoping it is a ground)


Red wire to the wall t stat box - open/infinite (1)
Black wire to the thermostat box - shorted (0 ohms) This is where I get confused. Why would a wire that is DEFINETLY carrying 240 V when the breaker is "on" be showing 0 ohms to ground when off?

This is baffling to me.

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Old 10-14-2012, 04:08 PM   #2
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Need help understanding a 240V circuit for radiant ceiling heat


With power on and the thermostat turned up (so it clicks on) you should measure 240 volts from red to black at the heating unit. If not then there is definitely something wrong in the power supply wiring.

Also measure voltage across the two breakers (or/ie the two screws of the double wide breaker unit) in the panel that supply this heating circuit. If you don't measure 240 volts there, you need to fix that before examining the heater grid further.

You also should measure 120 volts from black to ground and 120 volts from red to ground but if you measured 0 volts for both then you need to take that with a grain of salt. (Don't draw any conclusions yet.)

If you measure 120 volts from black to ground and 0 volts from red to ground (or vice versa) then there is a problem in the line that measured 0 volts to ground.

Turn off the power and unhook at least one of the terminals to the heating grid. Now, measure continuity or resistance between the two heating grid terminals. Infinity means the heating grid is bad.

Unhook everything from at least one of the wires or screws to which you will touch the meter probes before doing continuity/resistance tests.

Always turn off the power before doing continuity/resistance tests.

Always put labels on all wires that you take apart so you can put it back the way it was.

Quote:
Why would a wire that is DEFINETLY carrying 240 V when the breaker is "on" be showing 0 ohms to ground when off?
1. There is another appliance on that circuit that is still plugged in and turned on.
2. If you used the main breaker to turn off the power then there is another appliance or a light on some other circuit that is still plugged in and turned on.

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Last edited by AllanJ; 10-14-2012 at 04:22 PM.
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Old 10-14-2012, 04:19 PM   #3
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Need help understanding a 240V circuit for radiant ceiling heat


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Originally Posted by AllanJ View Post
With the thermostat turned up (so it clicks on) you should measure 240 volts from red to black at the heating unit. If not then there is definitely something wrong in the power supply wiring.

You also should measure 120 volts from black to ground and 120 volts from red to ground but if you measured 0 volts for both then you need to take that with a grain of salt. (Don't draw any conclusions yet.)

If you measure 120 volts from black to ground and 0 volts from red to ground (or vice versa) then there is a problem in the line that measured 0 volts to ground.

I measure good voltage to ground on the black side. As I don't mess with hot AC circuits much I simply verified voltage with my probe - the actual number was bouncing around but it is "hot". The thermostat is a simple on-off switch (the contacts are the size of screwdriver blades). I measure 0 volts to ground on the red side, and infinite to ground for ohms on the red side.

A simple "tap-tap" test by briefly touching the red and black tstat wires to each other also confirms that there is no current flow without the thermostat in the circuit. No sparks when gently tapping -

See if I have this right - it is a dedicated 240V system on a 30A breaker. There are only TWO wires to the thermostat, as the third wire runs straight back up the wall to the other end of the grid. The thermostat switches the black side of the circuit into the grid up the red wire and completes the circuit.

There is no reason to suspect deterioration of the wiring of the house. My dad built to last - and he built this one for himself. We've had zero electrical problems since new. I've also tested and swapped the 30A breaker for one that is on a working circuit.

I have not yet tested the white wire, as it has no wire nut, it just loops into the box and up through a cutout toward the ceiling. If necessary I'll be happy to penetrate it to test in-place.

Last edited by imautoparts; 10-14-2012 at 04:30 PM.
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Old 10-14-2012, 05:57 PM   #4
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Need help understanding a 240V circuit for radiant ceiling heat


keep in mind you don't have 2 hot wires going to the stat that will read 240V it is only one side of the 240V for the heater the other side of the 240V from the CB panel goes into the unit directly..2-wire line voltage stats only break one side of the resistance heat so never be up on a ladder digging into it without the CB OFF then it is a solid 240V with the stat closed..why are you ringing out grounds on a 240V control circuit..those black and red wires are the same 115V one side of 240V at the stat if you removed them off the stat and touched them the heater would run slight load spark also
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Old 10-14-2012, 06:13 PM   #5
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Need help understanding a 240V circuit for radiant ceiling heat


OK - thanks to a friend on the HVAC side (and your input here) I think I've got it sorted out.

1. Black is hot, red is dead, white is most likely hot but I'm reluctant to strip wire to check since we have noticeable ceiling cracks. I'm assuming that if I did check white, it would show hot to ground just like the black side.

2. Red is the wire that connects the grid to the thermostat, and carries the hot from the black when the tstat kicks on

3. The tool I need is called a "cable toner". DIY quality toners are around $40

So - my next step is to acquire the toner, and hook the transmitter to the red lead (with the CB off of course). Using the toner/sniffer device, I should then be able to follow the wire until the signal stops. I hope sniffers have adjustable gain controls so I can make sure it isn't so sensitive I have to de-plaster more ceiling than necessary.

Do you see any obvious flaws or missing links in this plan?

I've already located correct butt connecters and single-wire radiant wire that is compatible with the existing system.
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Old 10-14-2012, 07:01 PM   #6
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Need help understanding a 240V circuit for radiant ceiling heat


the white is a neutral the black and red are 240V hot from the CB panel...when you wire the heater in the ceiling you run the pair of stat wires up to the heater and make the connect there to one wire coming in from the CB panel color does't matter.the color of the stat wires doesn't matter(size does/same as 240V feed) again all your doing is breaking one side 115V of that 240V to cycle the heater.....
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Old 10-14-2012, 07:22 PM   #7
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Need help understanding a 240V circuit for radiant ceiling heat


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Originally Posted by biggles View Post
the white is a neutral the black and red are 240V hot from the CB panel...when you wire the heater in the ceiling you run the pair of stat wires up to the heater and make the connect there to one wire coming in from the CB panel color does't matter.the color of the stat wires doesn't matter(size does/same as 240V feed) again all your doing is breaking one side 115V of that 240V to cycle the heater.....
The wires from the CB are not red, black and white - they are copper, black and white. 30 amp breaker.

The wire that I presume is the lead down the wall from the grid is red. It goes to the 'stat.

The black wire from the wall goes to the 'stat. It is hot (with what looked to me like way over 120V.... if that is possible)

Anyway, it throws a spark. It is hot. My volt ohmeter leads are trash so I get some funky readings at times.

The white wire comes out of the romex and vanishes into a hole in the top of the 'stat junction box without any connection or wire nut. It just loops up and away.

Now you're saying that is a NEUTRAL>? What does that mean? I only have an understanding of DC - so I've never heard of a neutral.

I thought the black wire was hot and the white wire somehow could feed another 120 in, to create a 240 V circuit.

Is it possible the house ceilings are 120V?

Sorry for all the questions. I'm trying to not only find the break and repair it, but to truly understand the system.
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Old 10-16-2012, 07:05 PM   #8
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Need help understanding a 240V circuit for radiant ceiling heat


Is the white wire connected to the 30 amp circuit breaker? If it is it should have been taped with a color other than white, gray or green. One wire on each ploe of the circuit breaker would be 240 volt, with no neutral! Answer that and we can go from there.
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Old 10-16-2012, 07:28 PM   #9
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Need help understanding a 240V circuit for radiant ceiling heat


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Now you're saying that is a NEUTRAL>? What does that mean? I only have an understanding of DC - so I've never heard of a neutral.
As you're discovering, AC is not DC.

The neutral wire is the return part of the circuit. Voltage on the hot wire (commonly black) causes current to flow thru the load; after the load the current is returned (to complete the circuit) thru the neutral wire (commonly white).

Neutral is not ground. That's the bare copper wire (sometimes with green insulation). The only place where neutral and ground are connected is at the main circuit breaker panel. In normal operation, no current flows thru the ground wire. It is there to provide a return path in case of a short (such as a hot lead contacting the metal chassis of an appliance).
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Old 10-17-2012, 07:18 AM   #10
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Need help understanding a 240V circuit for radiant ceiling heat


Here is the wiring in the thermostat box.

I'm beginning to understand - black is hot 120V, white is hot 120V, there is no neutral. The black and white are out of phase, thus resulting in 240V when connected by appropriate load such as the ceiling coil.

Here is the thermostat box wiring:


The thermostat is a SPST, the ceiling grid is an old-style single wire; thus to locate the break I need a non-contact tester or a cable toner - like old timey Christmas lights once broken the entire ceiling goes dead, and I can't trace it with an infrared sensor.

Repairs can only be made with correctly sized good quality butt connectors, and must immediately be reburied in plaster to provide the necessary thermal sink to avoid overheating.

New cable is available, nowadays they use "MI" cable; that won't heat up and burn out if exposed to the air. This type of cable is preferred if repairing any distance of cable or replacing a grid.

I've ordered a decent quality toner, updated my DVOM and am more excited than the dog when the mailman comes up the street!

I will post the results of my inspection of the inside of the breaker box and the use of the toner.

Thanks to you all!
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Old 11-06-2012, 06:39 AM   #11
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Need help understanding a 240V circuit for radiant ceiling heat


OK - so I received my toner in the mail yesterday.

I've tried testing the ceiling grid by disconnecting the red lead, and using the two leads from the toner, 1 lead to ground and 1 lead to the red lead coming down from the ceiling.

The trouble I'm having is that while I can tell where the signal IS and ISN'T on the ceiling, the sensitivity isn't fine enough to actually confirm the exact route of the wire. There are no clues that I can find that lead me to be able to say "here is the location of the break".

So, I'm back to square one. I've confirmed the problem, and know where the live portion is, but can't noodle any way to locate the exact break.

Any ideas short of de-plastering the entire ceiling?
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Old 11-06-2012, 08:16 AM   #12
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Need help understanding a 240V circuit for radiant ceiling heat


Yes, have a differant heating source installed.
That type heating has to be one of the silliest ideas they ever came up with.
Heat rises, so what good does it do to have nice warm ceilings?

I highly doubt that type heating has been installed for many many years.
Heating the floors, now that makes since.
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Old 11-06-2012, 08:20 AM   #13
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Need help understanding a 240V circuit for radiant ceiling heat


I'm not 100% certain that that type of heating is repairable. I know with radiant in floor heating some can be repaired, but you need special repair kits.
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Old 11-06-2012, 12:11 PM   #14
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Need help understanding a 240V circuit for radiant ceiling heat


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Yes, have a differant heating source installed.
That type heating has to be one of the silliest ideas they ever came up with.
Heat rises, so what good does it do to have nice warm ceilings?

I highly doubt that type heating has been installed for many many years.
Heating the floors, now that makes since.
No - It isn't a silly system - since it is radiant, it doesn't heat the ceiling, it heats the objects in the room. We've found it to be reliable and economical and have been in this house since 1968. I've investigated converting to a floor system, but that IS a silly idea... it turns out that you have to be very careful to avoid certain types of furniture, large boxes, etc or you can create overheated spots on the floor. Also, the effectiveness suffers due to the radiant effect having to penetrate whatever floor covering you have.

The problem I'm having is the lack of local installers - in other parts of the country where this type of system is popular there are plenty of tradesmen who advertise to repair these systems, but nobody around here.

Here is a chart of what i've been able to tone-trace so far. I went ahead and opened up the wall next to the t-stat and located the wire nut for the white side of the circuit. Now I'm able to isolate and tone test both sides, and here is the result:

Green squiggles are confirmed strong white-side tone

Red squiggles are confirmed strong black-side tone



So I guess the next question is, how would you recommend I carefully de-plaster this section of the ceiling? Ceiling finish is smooth plaster over the wires with wallboard surface underneath. Any suggestions on tools? Maybe one of those wallpaper scorers would work to carefully begin opening up the plaster layer.
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Old 11-06-2012, 01:00 PM   #15
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Need help understanding a 240V circuit for radiant ceiling heat


I think you need to reduce the sensitivity of the toner to pinpoint the break. Does the sender or receiver have an adjustment for this?

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