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Old 09-04-2011, 12:26 PM   #1
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Keyair's Avatar
1984 34.5' Airstream 345
Foothill Ranch , California
Join Date: Mar 2010
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Upgrading to a Digital Thermostat.. Need some help!

1984 345 Motorhome.
2 Furnaces, and 2 Roof AC units.
Rear furnace Thermotsat is faulty, and switches furnace on at random, draining the batteries.
Pic of old unit:

Did some research and found the ModmyRV site and article on upgrading to a Hunter Digital Thermostat, here.
Bought the same unit, and looked at the Owners and Install Manual here.

Issue is that upon removing the old Thermostat, I find only 2 wires... Both the same color!
The Hunter Manual says for a 2 wire heat only system, connect one wire to Terminal W(Heat relay or valve), and the other wire to Rh, then Bridge Rh and Rc.

My concern is how do I know which wire to connect where?
Should there be 12v at one?

Also, a more complex question, is it possible to wire our older systems with rooftop AC into the Thermostat?

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Old 09-04-2011, 12:44 PM   #2
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2002 25' Safari
1977 20' Argosy 20
northern valley , new jersey
Join Date: Nov 2008
Posts: 510
since it's an existing two wire 'stat, W and Rh, either wire to either terminal should work for the heat. if it does not (don't worry, there's nothing her you can break), swap the wires on those two terminals.

the answer for the AC is "it depends" on whose AC and what controls are available for that particular unit. start by identifying the manufacturer + model, then ask that mfr, or one of their dealers what's available to you. you also may have a small challenge getting the wires to/from the AC to your thermostat location.


Joseph & Gabrielle

Simplicity is the ultimate sophistication - Leonardo da Vinci.

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Old 09-04-2011, 12:52 PM   #3
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1974 Argosy 20
2014 20' Flying Cloud
Kooskia , Idaho
Join Date: Jul 2009
Posts: 4,592
It should not matter which wire you attach to the two called for posts in the thermostat. The digital thermostat uses an internal relay to control the power, and that is not voltage or terminal sensitive. Yes, you will have 12 volts + on one of the wires from the furnace, but that does not matter. The digital thermostat you use MUST have it's own power supply (a battery) and cannot be the standard home thermostat which uses 24 volts AC to power it.

When I had my 83 310 motorhome, the AC units had their own internal thermostats and there was no practical way to convert them to low voltage thermostats as you would like to do. Yes, could be done, No, not easy or practical, in my opinion. Your 84 would probably be the same or similar.
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Old 09-04-2011, 01:09 PM   #4
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1984 34.5' Airstream 345
Foothill Ranch , California
Join Date: Mar 2010
Posts: 3,376
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Thanks Peeps!
Superfast replies!
I am laid up on the Couch with a bad back today... and frustrated as hell, so doing research on my plans!
The Hunter I have has 2 AA batteries, so thats what I will be using to power it..
I bought one as a test piece, but will buy another the same if it works good for the front furnace... maybe the one with the remote.
Pic of mine..

I will look at what units I have both for the Furnaces and AC, and post it later.
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Old 02-16-2016, 08:26 AM   #5
2 Rivet Member
2007 19' International CCD
The Woodlands , Texas
Join Date: May 2011
Posts: 39
Thermostat Upgrade

I am upgrading to the Hunter 42999b on a two wire heat only. Per the wiring diagram, 1 wire to W and the other to Rh with the provided jumper between Rh and Rc. After wiring and restoring power, the fuse controlling the furnace blows. Any suggestions? Switch the W and Rh wiring even though the manual says it doesn't matter?
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Old 02-16-2016, 09:52 AM   #6
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1979 31' Excella 500
Charlevoix , Michigan
Join Date: Aug 2014
Posts: 258
Not sure if this helps, but I had to do thermostat upgrade that turned out to be more complicated than I thought, so I documented the results here:

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