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Estwing magic
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Discussion Starter · #1 ·
I am going to be doing some work for a customer this week. One thing he needs me to do is fix the wiring for his interlock wall switch. He took it apart for some reason and ended up blowing the circuit board on his furnace. I don't want to be an idiot and blow it again.

I seem to recall seeing these wired to control the lo vo side of the transformer. I'm not sure why. How he got 120 to the board is another mystery. I didn't take it apart when I was there the first time.

Help please.
 

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I have done this a couple of different ways. Essentially you need to put a 3-way on the fan in the uppermost bathroom exhaust (if it meets the CFM requirements). The second 3-way is supposed to be located by the T-Stat. That was in Ontario anyway… An other approved option was a kit that you buy to insert into the kitchen hood fan. Not sure if this is still a requirement anymore… I have not done one in years; this is all building code and not electrical code.

In both cases the intention was when the exhaust fan was activated the furnace blower motor would run drawing in fresh air in. I have not done a new house in a while now, so not sure if there is new methods / technology out there for it; in both cases we used a relay although you could simply use the 3-way as mentioned above. I believe we used it to close the white and red terminals on the fan circuit board (going from memory so please check) basically the same as putting the T-Stat on the “fan only” setting.

We ran a low voltage wire from the furnace to the relay (kitchen / bathroom or both) so when they were activated it closed the contacts on the circuit board and ran the furnace. Used the power supply from the furnace for the contacts and the power from the exhaust fan to close the relay. Like I said, there might be an actual device for this now, it was a bit rodeo back when we did it.

Hope this is what you were asking for.

Cheers
John
 

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I am going to be doing some work for a customer this week. One thing he needs me to do is fix the wiring for his interlock wall switch. He took it apart for some reason and ended up blowing the circuit board on his furnace. I don't want to be an idiot and blow it again.

I seem to recall seeing these wired to control the lo vo side of the transformer. I'm not sure why. How he got 120 to the board is another mystery. I didn't take it apart when I was there the first time.

Help please.
What switch are you talking about? The switch that turns on the exhaust fan?
 

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Anyway, here is what you need to do. You should have a relay with a 120 volt coil between the switch and the exhaust fan. There should be an 18/3 from the relay to the furnace. The G terminal on the thermostat should, instead of going to the G terminal on the furnace, connect to the black wire of your 18/3 going to the relay. That black wire will go on a NC contact on the relay. The green wire from the 18/3 will go on the G terminal on the furnace and depending what kind of relay you are using, will go on either a common terminal or on the other NC contact along with the black wire and then also jumpered to a NO contact. The red wire from the 18/3 will go on the R terminal on the furnace and the NO contact on the relay. Hope this helps.
 

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If I did it a long,long time ago then there is a 4-11/16 box above the furnace with a relay and wiring diagram on the cover. :)

If I did it a long (one long) time ago the inspectors wanted us to start using a 'Hoyme Adaptor' It's a little relay thingy manufactured not too far from you. I used to get them from Sinclair Heating Supplies for $55. A 'Hoyme ADP-1101-05A'...
http://www.hoyme.com/index.php?option=com_content&view=article&id=151&Itemid=173

^^ Edit: Not done yet, hit the wrong button... wait for it.



A diagram I sketched for another guy back in 2010. He was trying to figure it out and plying me with beer and rye. Finally I sketched this and stumbled home. I've no urge to make it neat. The fan switch is ofcourse in the bathroom with a 2 wire to the hall switch and a 2 wire to the Hoyme box.

So the deal is... The house wants to have a 'ventilation switch' in the hall. I usually put mine above the stat(different height than other switches). When turned on it would start the big fan and the furnace fan. That's it.

Many electricians start the furnace fan when they turn on the bathroom fan. I had an old guy reading his Playboy mag in the bathroom on day when she was -33 outside. :eek:

Of course his house was warm and the stat was... wherever. Hitting his bath fan switch and setting down it only took a few moments to realize the furnace was pumping in -33 degree air to the little bathroom. So I wire as above. Just send a 2 wire down to the Hoyme and the NO contacts to R&G on the furnace board.

You will have to take stuff apart to see how things got blown up.
 

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Estwing magic
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Discussion Starter · #7 ·
Anyway, here is what you need to do. You should have a relay with a 120 volt coil between the switch and the exhaust fan. There should be an 18/3 from the relay to the furnace. The G terminal on the thermostat should, instead of going to the G terminal on the furnace, connect to the black wire of your 18/3 going to the relay. That black wire will go on a NC contact on the relay. The green wire from the 18/3 will go on the G terminal on the furnace and depending what kind of relay you are using, will go on either a common terminal or on the other NC contact along with the black wire and then also jumpered to a NO contact. The red wire from the 18/3 will go on the R terminal on the furnace and the NO contact on the relay. Hope this helps.
Thanks. Any idea how he could have blown the board? He took out the wall switch, presumably to paint the wall, and then smoked the board when he hooked it back up again.
 

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Did he change the switch? Maybe he had a 2-pole switch with 120 on one side and 24VAC on the other and got them mixed up?

Never have seen that before, but maybe it was an approach used when this first came out as a requirement.

Cheers

John
 

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Thanks. Any idea how he could have blown the board? He took out the wall switch, presumably to paint the wall, and then smoked the board when he hooked it back up again.
No idea. The 120 volt and the low voltage should be isolated from each other via the relay. You would have to take it apart to see how its wired.
 

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Estwing magic
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Discussion Starter · #11 ·
If I did it a long,long time ago then there is a 4-11/16 box above the furnace with a relay and wiring diagram on the cover. :)

If I did it a long (one long) time ago the inspectors wanted us to start using a 'Hoyme Adaptor' It's a little relay thingy manufactured not too far from you. I used to get them from Sinclair Heating Supplies for $55. A 'Hoyme ADP-1101-05A'...
http://www.hoyme.com/index.php?option=com_content&view=article&id=151&Itemid=173

^^ Edit: Not done yet, hit the wrong button... wait for it.
It's a newer place so it likely has the Hoyme. Anytime I have done these, I took care of the 120V side and said it was up to the tinbasher to do the rest.

I remember one time taking a wall switch out and seeing a 3 way switch connected to 18 guage wire. Maybe, in this case, there is both 120V and low voltage in the same box (??).

Anyway, I'm just guessing until I break it open. Thanks for the help, guys :) .
 

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Thanks. Any idea how he could have blown the board? He took out the wall switch, presumably to paint the wall, and then smoked the board when he hooked it back up again.
Possible he just blew the on-board fuse ?

Either way, open the furnace breaker while you doing any of this.
 

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Just make sure to use a 3 wire. At first glance it might look like you can do it with a 2 wire by just energizing the fan on the furnace via the R and G terminals, but you end up back feeding the AC compressor coil through the thermostat and it'll turn on everytime the exhaust fan comes on. Not a good situation in winter if the compressor doesn't have a low temp lock out.
 

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Discussion Starter · #15 ·
I figured it out. The clowns who wired this used a double pole, single throw switch with 120V on one side and 24V on the other. The HO was changing his devices and installed a 4 way. When the switch criss-crossed, it sent 120V to the board and fried it.

I know it's the HO's fault for frying the board but it was a $600 fix to the furnace guy plus my time. If it was done to code this wouldn't have happened. And, in case you're wondering, I put in a new DPST switch. The basement was finished and I wouldn't have been able to put in the relay box without doing surgery.
 

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Thanks. Any idea how he could have blown the board? He took out the wall switch, presumably to paint the wall, and then smoked the board when he hooked it back up again.
The guys that wired my Brother in laws house used a 2 pole switch on the primary fan (master ensuite bathroom) they ran loomex from the 2 pole as a switchleg from R to G on the furnace board. Bro in law had no idea, wires weren't marked when he changed out the switch, they gave him a 4 way instead of a 2 pole. he hooked it up and promptly put 120V to the board, frying it. had no idea the furnace was pooched for almost a week because it was spring. He called plumber, new board, had a shower, blew another board. Called me to fix the fan switch still had no idea that the two were related. So if you are anywhere near Drayton Valley, that could be your issue.
 

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Estwing magic
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Discussion Starter · #18 ·
The guys that wired my Brother in laws house used a 2 pole switch on the primary fan (master ensuite bathroom) they ran loomex from the 2 pole as a switchleg from R to G on the furnace board. Bro in law had no idea, wires weren't marked when he changed out the switch, they gave him a 4 way instead of a 2 pole. he hooked it up and promptly put 120V to the board, frying it. had no idea the furnace was pooched for almost a week because it was spring. He called plumber, new board, had a shower, blew another board. Called me to fix the fan switch still had no idea that the two were related. So if you are anywhere near Drayton Valley, that could be your issue.
Not in DV but I am going to put this on my check list for reno jobs. It takes two minutes to inspect and could save the customer a lot of grief.
 

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I figured it out. The clowns who wired this used a double pole, single throw switch with 120V on one side and 24V on the other. The HO was changing his devices and installed a 4 way. When the switch criss-crossed, it sent 120V to the board and fried it.

I know it's the HO's fault for frying the board but it was a $600 fix to the furnace guy plus my time. If it was done to code this wouldn't have happened. And, in case you're wondering, I put in a new DPST switch. The basement was finished and I wouldn't have been able to put in the relay box without doing surgery.
Never saw this post when I wrote my original. Figured only one outfit would be scabby enough to do this.Even if they did that but used Securex or LVT for the 24V that would at least twig the HO that something was different. If there is a neutral in the switch box you should be able to use the existing switch leg and put a 120V relay in the furnace powered from the switch.
 

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Nothing wrong with using the Alberta Standata method, simple and easy to rough in, and easy to connect at the furnace. :wink:

Run a 3-wire from the fan switch to the range hood, (if your not sure which one is primary at first), then a two wire, (run a 3-wire if your unsure, but you shouldn't need it), from one of them, (I like the bathroom fan switch location as It's more accessible), to the furnace area where a LV relay JB is located.

Then use Dark Knight's diagram to connect the LV contacts. :thumbsup:

FYI, basement suite building codes are getting picky on ventilation issues, and furnace installations, which only adds work to our trade.

Borgi
 
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