Bad design?

[Paul Hovnanian P.E.]

I recently extracted a floor heating thermostat that had probably been
mis-installed and (as far as we can tell) never worked. The story about
the electrician who did the job and left before testing the system is
another sad and long one for some other thread.

The unit is (as far as I can tell) a triac that controls 120 VAC to an
electric mat installed under a bathroom floor. It incorporates a ground
fault sensing circuit so as not to electrocute one stepping out of the
shower. Because of this, the unit has two line input wires (black and
white) and two output wires to the mat (also black and white). See
crummy ASCII art below. We suspect that the electrician initially mixed
up the line and load connections (due to the way he left quickly without
testing the system). I'm trying to figure out how one could damage a
triac by swapping the Line/Load wiring. Due to the crappy color coding,
this seems to be the most probable wiring error.

System (properly wired) as I envision the
thermostat innards:

triac
black |\| black
----o-------+-----|/|---MMM-o---------+
| /|/| | |
| / |GF CT |
| | | |
Line +------+ | Load
|brains|------+ |
+------+ | |
| | |
----o----------+--------WWW-o---------+
white white


Most of the likely errors I can envision would simply prevent the unit
from operating. Am I
missing something?

 

[Robert Baer]

I recently extracted a floor heating thermostat that had probably been
mis-installed and (as far as we can tell) never worked. The story about
the electrician who did the job and left before testing the system is
another sad and long one for some other thread.

The unit is (as far as I can tell) a triac that controls 120 VAC to an
electric mat installed under a bathroom floor. It incorporates a ground
fault sensing circuit so as not to electrocute one stepping out of the
shower. Because of this, the unit has two line input wires (black and
white) and two output wires to the mat (also black and white). See
crummy ASCII art below. We suspect that the electrician initially mixed
up the line and load connections (due to the way he left quickly without
testing the system). I'm trying to figure out how one could damage a
triac by swapping the Line/Load wiring. Due to the crappy color coding,
this seems to be the most probable wiring error.

System (properly wired) as I envision the
thermostat innards:

triac
black |\| black
----o-------+-----|/|---MMM-o---------+
| /|/| | |
| / |GF CT |
| | | |
Line +------+ | Load
|brains|------+ |
+------+ | |
| | |
----o----------+--------WWW-o---------+
white white


Most of the likely errors I can envision would simply prevent the unit
from operating. Am I
missing something?

If the triac gets zapped, it might turn into a very good 2 or 3
terminal wire..
The black box labelled "brains" should take that into account; adding
a fuse or circuit breaker before the triac could them allow a fault open
for safety.

 

[Ross Herbert]

I recently extracted a floor heating thermostat that had probably been
mis-installed and (as far as we can tell) never worked. The story about
the electrician who did the job and left before testing the system is
another sad and long one for some other thread.

The unit is (as far as I can tell) a triac that controls 120 VAC to an
electric mat installed under a bathroom floor. It incorporates a ground
fault sensing circuit so as not to electrocute one stepping out of the
shower. Because of this, the unit has two line input wires (black and
white) and two output wires to the mat (also black and white). See
crummy ASCII art below. We suspect that the electrician initially mixed
up the line and load connections (due to the way he left quickly without
testing the system). I'm trying to figure out how one could damage a
triac by swapping the Line/Load wiring. Due to the crappy color coding,
this seems to be the most probable wiring error.

System (properly wired) as I envision the
thermostat innards:

triac
black |\| black
----o-------+-----|/|---MMM-o---------+
| /|/| | |
| / |GF CT |
| | | |
Line +------+ | Load
|brains|------+ |
+------+ | |
| | |
----o----------+--------WWW-o---------+
white white


Most of the likely errors I can envision would simply prevent the unit
from operating. Am I
missing something?

Your diagram appears to show the GF circuit breaker AFTER the
thermostat/controller.

The Earth Leakage Circuit Breaker (ELCB or GFC interrupter) should be
in the distribution box and the thermostat/controller should be wired
between the ELCB and the in-floor heating element.

This is a typical installation document for in-floor heating as used
on 240Vac in Australia http://www.devi.com.au/DSOT&DSIG.pdf which
shows photos of a typical installation.
Yours should be very similar.

Main in-floor heating page http://www.devi.com.au/slab.htm

 

[Paul Hovnanian P.E.]

Your diagram appears to show the GF circuit breaker AFTER the
thermostat/controller.

Yes. This unit incorporates its own internal ground fault sensing for a
fault in the mat. Based upon the observation that the unit has a GF
fault LED that illuminates following a fault or test (test button not
shown here), I figured it just shuts off the triac and the brains remain
powered.

There's a mechanical on/off switch (also not shown) that is (probably)
upstream of the triac in the hot leg to comply with UL requirements for
a disconnect. This switch is not automatically opened following a GF,
like a GFI receptacle reset is. It must be cycled manually.

My main concern is, given the color coding scheme shown above, could an
electrician swap the two white wires or two black wires and cause a
triac to fail? I don't know what is inside the 'brains'. I didn't open
this thing up due to the fact that 1) it is expensive 2) in isn't mine
and 3) we traded it in for a new one. All I know is that the electrician
left the job site quickly, without informing anyone that the unit didn't
work. Its possible that something else failed internally (maybe a
fusible link).

The one error that I can envision is to connect source hot (black) to
t'stst line black and source neutral (white) to t'stat load black. But I
have yet to meet an electrician dim enough to connect a black to a white
wire.

Since he didn't buy it (my friend did), he could have very easily said,
'Hey buddy, this mat you bought is junk.' We only discovered the problem
later and, prior to isolating the problem to the t'stat, the fault could
very well have been in the parts now embedded under an expensive tile
floor. Fortunately, that was not the case and his failure to alert the
customer is a subject for another thread on professional ethics.