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Gas Furnace No Show In Mode Selection

October 30, 2017, 01:37:30 pm

My four button, two zone thermostat allows one to choose type of service in forward or rear zones. Got a quick cool front through in Destin and decided we should use the furnaces. Zone 2 modes scroll through and the display shows "Furnace", wait a second a away she goes. Perfect. Press the two buttons simultaneously and switch to Zone 1. Hmmmm. No "Furnace" selection in modes. Do some quick calling and I'm told " you lost the 12 volts to your furnace. Take out the MTM and I find:

1. Battery voltage at wire connection in to 10 amp circuit breaker.
2. Continuity through circuit breaker.
3. Battery voltage coming out of circuit breaker.
4. Battery voltage to timed relay. No other power readings at other 3 positions on relay.
5. Contuity at rocker switch.

As I understand it, the timed relay ALWAYS has power and waits for a signal from the thermostat to close. The relay has three other wires. Coming from outside the box is a blue wire that connects to the relay then continues in the blower with a white wire (ground?) then opposite the blue wire is from outside a yellow wire that connects to the relay and co to use on to a connection on the narrow part of the aluminum blower housing and seems to complet a circuit with a red wire that returns to the fourth position on the timed relay (sail switch?).

Anyway, the problem seems to be that a 12 volt signal FROM the furnace is not making it to the thermostat. Thus, the tempo stat eliminates "furnace" from the mode choices.

How do I get hat signal? Where would I find that circuit to check for bad connection or??

Thanks in advance.

Alan

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Re: Gas Furnace No Show In Mode Selection

Reply #1 – October 30, 2017, 04:48:17 pm

There is  reset mode.  Control "lost" the furnaces?

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Re: Gas Furnace No Show In Mode Selection

Reply #2 – October 30, 2017, 05:41:15 pm

Found the reset procedure in my files. Went through the process with the validating "62 and 72" indicating reset had occurred.  Still no recognition of "furnace" in Zone 1. Zone 2 works like a champ.

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Reply #3 – October 30, 2017, 06:02:38 pm

Alan, remove the air filter, look for the gray phone cable and the connectors, unplug and replug them , if you have dilectric grease apply a little and reconnect, there should be 2 connector blocks.

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Re: Gas Furnace No Show In Mode Selection

Reply #4 – October 30, 2017, 06:45:26 pm

On  the bay door in front of your furnace is a micro switch to shut the furnace off if the door is open some times the little arm will get bent.

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Reply #5 – October 30, 2017, 07:57:28 pm

Bill, this the filter in the cold air return for the furnace? Or in the AC unit on the ceiling?  I do have dielectric grease.

Bruce, the Bay door which if opened would cover the external hot air discharge vent?

Dark now, try in the am.

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Re: Gas Furnace No Show In Mode Selection

Reply #6 – October 30, 2017, 08:09:05 pm

Yes the outside bay door that if opened would block the front heater exhaust. There is a micro switch at the top of the opening

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Reply #7 – October 30, 2017, 08:20:04 pm

Alan, it is in the return air on the A/C look for the 2 connector blocks.

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Re: Gas Furnace No Show In Mode Selection

Reply #8 – October 30, 2017, 08:50:27 pm

Alan,

You might check the fuses on the furnace board and both air conditioner boards see if they are good. Then do the thermostat reset - always do the reset last after any changes, including changing the dip switches on the air conditioner boards..
Also check to see if your thermostat has furnace programmed on one zone and cool on the other.  That is a conflict for the thermostat.
You need either cool or furnace showing on both zones for it to work properly.

A bad furnace board or its connections certainly would be a cause.
When was the last time it all worked normally? Clearly the usual problem of the word furnace showing on the thermostat means there is something wrong with the furnace or wiring to the air conditioner board. Failure to light could be no propane or it not igniting, but you would think that would not cause the word furnace to disappear on the display. The wires from the furnace go to the air conditioner board, not directly to the thermostat.  If you have two furnaces, then a pair of wires (often versions of blue and white) will go to each air conditioner board...for zone 1 and zone 2.
You also could check the push-terminals on the air conditioner boards for corrosion, etc.

And check for "open" connections of the data cable colored wires to their plugs. Not usually the problem.
As is often said "you will find the problem in the last place you look".
Jim

2002 U320

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Re: Gas Furnace No Show In Mode Selection

Reply #9 – October 31, 2017, 05:40:24 pm

Cleaned and reset the "phone cable" connections in the forward ac. Checked the kill switch on the door that could block the furnace exhaust. Cleaned and reset a few grounds. Still no furnace selection on Zone 1. Will keep poking around. It's finally warmer in Florida panhandle..p gives me time to explore.

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Re: Gas Furnace No Show In Mode Selection

Reply #10 – October 31, 2017, 05:48:21 pm

Alan,

If the thermostat works OK on air conditioning (cool) on both zones then the problem undoubtedly is not the thermostat. Normally the reset procedure will capture the services on the RV, so there must be something not right about the furnace or its wiring. - even at / in the air conditioner boards.
Sounds like a toughie.
Jim

2002 U320

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Re: Gas Furnace No Show In Mode Selection

Reply #11 – November 01, 2017, 03:17:42 pm

DIP switch on roof circuit board must be set for furnace on that zone.  If boards were changed, or wall thermostat changed, DIP switches are something to check out.

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Re: Gas Furnace No Show In Mode Selection

Reply #12 – February 27, 2018, 08:30:44 am

To close this out for records,

1. Checked dip switches, cleaned contacts, furnace still a no show in the mode selection of Zone 1.
2. Switched circuit boards from rear unit and, voila, furnace in Zone 1 appears.
3. Tried to run down circuit board replacement, ended up buying all new circuit boards for both units and switching to a five button thermostat. Get rid of that pesky four button model anyway.

Wanted to make sure the solution was in the database for future use.

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