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Old 04-01-2015, 09:29 PM   #1
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1979 31' Excella 500
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Heating and Air Cond. - Which wires are which?

Anyone with heating/cooling experience, I'd love to get your input!

I'm in the midst of renovating our '79 AS Excella 500 31' and am at the point where I need to purchase a new digital thermostat to replace the original 2 analog thermostats (one for Furnace and one for Air Conditioner). Only, there's a problem... I can't figure out what the wires coming out of the furnace and A/C are for! I need to know this in order to get the correct thermostat since some just have "heat, cool" while others have additional functions like "fan" and "heat pump".

Here's a picture of the wires coming out of the furnace (2 blues, 1 yellow and 1 red):

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Anyone know what each of these furnace wires does? Or how to test to see what each does?

The A/C unit is a bit more convoluted. I made a diagram to try to determine what each wire does using an old schematic I found inside the A/C unit:

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There are 4 wires coming in (Green, Black, Brown and Red), 1 (the black one) is disconnected for whatever reason, and 3 connect to the AC unit. Once inside the AC unit...
  • The Green wire turns Yellow, and I think connects to the "Cool Coil"
  • The Brown wire turns Orange, and I think connects to the "Heat Coil"
  • The Red wire turns Pink, and I think connects to the "24V Transformer"
Does the fact that there's a separate wire for "Heat Coil" mean this A/C unit has a "heat pump" function? The A/C unit says "Armstrong 14" on it, but the only manual I could find (linked here for future googlers who find this post: Armstrong 14 Air Condition Manual) didn't have much to help me.

If this A/C unit does have a heat pump, do I need a special kind of thermostat? Was hoping I could use a Dometic thermostat (like this one on Amazon), some of which have a "heat pump" option.

Any input or guidance would be greatly appreciated!
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Old 04-01-2015, 11:05 PM   #2
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On your furnace thermostat, the two blue wires are the ones for the thermostat. They are interchangeable. The red is +12 volts, yellow is -12 volts.

On your old Armstrong AC unit... it is not a heat pump but may have a 1500 watt resistance coil which can be used for heating. I don't know the remainder of your questions on the Armstrong.

I expect you would be better off maintaining the two thermostat system your rig had originally because of the mixture of 12 volt DC for the furnace and 24 volt AC controls for the AC unit. Now, something could be fabricated I am sure, but it might be difficult and messy. The new Dometic thermostats are generally electronic and integrated with the AC and furnace systems they make. I am not sure they are easy to use with older systems like yours. And the one on my 2014 FC is a difficult to program and understand unit. The prices are also high.

Just my thoughts on the matter.
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Old 04-02-2015, 06:43 AM   #3
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Thanks Idroba! One follow up question...

Quote:
Originally Posted by idroba View Post
On your furnace thermostat, the two blue wires are the ones for the thermostat. They are interchangeable.
If the wires are interchangeable, does that mean I connect both, or just one? I don't know what the wiring on that back of a thermostat looks like yet, so that might answer my question...
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Old 04-02-2015, 06:56 AM   #4
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Quote:
Originally Posted by trekerboy View Post
Thanks Idroba! One follow up question...



If the wires are interchangeable, does that mean I connect both, or just one? I don't know what the wiring on that back of a thermostat looks like yet, so that might answer my question...
After the Red (12V+) and the Yellow (Neg) are energized. If you complete the circuit between the two blue wires the furnace will run....
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Old 04-02-2015, 10:33 AM   #5
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Quote:
Originally Posted by trekerboy View Post
Thanks Idroba! One follow up question...



If the wires are interchangeable, does that mean I connect both, or just one? I don't know what the wiring on that back of a thermostat looks like yet, so that might answer my question...
As Ohiolou says, the blue wires, connected together will run the furnace. So, any thermostat which you use is just like a switch, and either blue wire can connect to either side of the thermostat switch.
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Old 04-02-2015, 12:28 PM   #6
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Quote:
Originally Posted by idroba View Post
On your old Armstrong AC unit... it is not a heat pump but may have a 1500 watt resistance coil which can be used for heating. I don't know the remainder of your questions on the Armstrong.
Any ideas on how I could test to see whether or not the unit has a 1500 watt resistance coil? Not exactly sure how I could trigger that function given the wires provided...

Quote:
Originally Posted by idroba View Post
I expect you would be better off maintaining the two thermostat system your rig had originally because of the mixture of 12 volt DC for the furnace and 24 volt AC controls for the AC unit.
I had heard of folks using 120V thermostats that also can run off of some AA batteries in RV's. I wonder these units are able to effectively "not care" what sort of voltage they're connected to (12V for Furnace, and 24V for A/C unit). Again, I know practically nothing about these systems, so any advice is appreciated!
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Old 04-02-2015, 06:34 PM   #7
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I would keep the stock set up unless there is a compelling reason to change it. As soon as I figured one was an VAC controller and the other VDC, I didn't want to second guess the HVAC folks at AS. If it ain't broke.....
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Old 04-02-2015, 10:08 PM   #8
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Quote:
Originally Posted by trekerboy View Post
Any ideas on how I could test to see whether or not the unit has a 1500 watt resistance coil? Not exactly sure how I could trigger that function given the wires provided...

(I just don't know the old Armstrong, and can't help)


I had heard of folks using 120V thermostats that also can run off of some AA batteries in RV's. I wonder these units are able to effectively "not care" what sort of voltage they're connected to (12V for Furnace, and 24V for A/C unit). Again, I know practically nothing about these systems, so any advice is appreciated!
see (.....) in quote.

The battery operated thermostats are great, however they are really made for home applications with 24 volts AC as the thing they are controlling. When you want to mix 24 volt AC and 12 volt DC thing can get complex, which is why I don't recommend it. Anything can be fabricated, but it does not sound like you have the knowledge to do that.
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Old 04-03-2015, 06:25 AM   #9
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Most thermostats use dry circuits, meaning it's just opening and closing the circuit with a relay. It doesn't matter if it's 12 or 24.
Rob
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Old 04-03-2015, 06:54 AM   #10
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The modern digital thermostats have a small relay in them that is operated by some AA batteries. This relay can probably control about an amp without burning up. The rating should be in the specs. I replaced my furnace thermostat with one of these digital jobs. It also has an AC side which could be connected to the AC unit but you will need a relay in your AC that can handle the current of the compressor kicking on and off. The thermostat on my Dometic Briskair is a piece of junk and won't allow the compressor to run much below 75 degrees. I may at some point connect it to the cool side of the thermostat I put in for the furnace.

Perry
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Old 04-09-2015, 07:31 AM   #11
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Thanks for the replies everyone, I think I've figured it out, but could use some verification...

I've since figured out that I can use pretty much any household thermostat to control the furnace and A/C unit, however I found out that my A/C unit has a 1500w resistance coil that can be used for heat into it as well (thanks for the tip idroba!). Now knowing this I want to be able to to control both the A/C's heating unit and the Furnace with a single thermostat. Further, I want to be able to control which heating source is used at any given time (A/C's heat or Furnace or both). My thought is that when I'm plugged into shorepower I'd want to use the electric heat form the A/C unit, or if it's really cold I'd want to use both. If I'm boonedocking, I'd only be able to use the gas furnace.

As far as finding a thermostat that supported multiple different heat sources, there are many out there however none of them were configured quite right for my application. For example, there are "multistage" thermostats out there that turn on one heat source then another in succession, but this didn't allow me to choose which heat source should be used at any given point (it was all controlled automatically by the thermostat). There were other thermostats that had an "emergency heat" option, but again this didn't quite meet my needs since enabling "emergency heat" required that the 1st heat source be on as well.

Since my original post, I've determined that the A/C unit uses a 24VAC line and the furnace uses some sort of millivolt setup. In the case of the furnace, connecting the power wires (red and yellow) to a power source (12V+ and 12V-) then connecting the two blue wires together turned the unit on. For the A/C unit, connecting the 24VAC line to either the "heating" or "cooling" wires coming out of the unit triggers the unit to enable that function.

At first I thought I could just attach the "heating" wires from each unit to the "heating" (W) terminal in the thermostat with 24VAC on the RC thermostat terminal jumped to the RH terminal, and also having one of the blue furnace wires connected to the RH terminal. That didn't work because when the thermostat heat relay was open (i.e. heat turned off) both the furnace and the A/C unit would still be on. I eventually figured out that the circuit for the heating wires on both units was being completed through the other heating unit even when the thermostat relay was open. Also, sending 24VAC through the furnaces millivolt system probably wasn't a good thing, but fortunately the furnace still works !

I eventually realized I needed to use some additional relays and on/off switches to accomplish what I wanted to do. Here's a diagram of what I'm thinking of doing:



The Thermostat specs say the relays on the thermostat itself can handle loads up to 1A, so I put a 12 OHM resistor inline from the power source (used to power the new heating and cooling relays). Not sure if that's necessary, but seemed like a reasonable precaution to not overload the thermostat relays.

Can anyone confirm whether or not this will work?
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Old 04-12-2015, 09:35 PM   #12
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Success!

Quote:
Originally Posted by trekerboy View Post


Can anyone confirm whether or not this will work?
Just checking back in with those interested in this topic...

I wired my thermostat, Gas Furnace, Electric Heater (inside A/C unit) and Air Conditioner according to the diagram I noted above and IT WORKED! Wahoo!

It's up for debate whether or not this was the best was to address my situation (wanting to use single thermostat with 2 heaters and 1 A/C) but I can confirm this method does indeed work for other out there seeking to do the same thing.

Ideally there would have been a thermostat out there that would allow the user to connect multiple heaters/furnaces and then selectively enable/disable those according to an on-screen menu or something, unfortunately those don't seem to exist (although "Multi Stage Heating" supporting thermostats, and those that support "Emergency Heat" option come close).
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Old 08-09-2015, 09:37 AM   #13
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We know this is an old thread, but...

We want to give everyone who posted to it a big thank you for all the wiring pics and leg work. Getting ready to go on a trip and needed to put a new thermostat on the AC. PO removed the old furnace from under the stove then re-purposed some of the wires for something else and added some splices. Finding this info really saved our bacon (and will keep it from getting fried while we travel from Texas and into OK.) The below info allowed us to go straight to the correct wiring. Now the AC is turning our AS into an igloo.
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