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Old 10-13-2007, 03:26 PM   #1
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I just installed an ATWOOD furnace in my trailer. It needs a thermostat, so went to home depot and all that they have are 24V AC types ... was told that the thermostat will still work for 12 V DC applications ... true? Further, how do I determine it is working once it is hooked up? (it's just going to be a dial thermostat, non-digital ... if the furnace doesn't start of course))

Tim
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Old 10-13-2007, 04:35 PM   #2
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Any non-digital dial type thermostat should work. All it has is a set of contact points that open at the tempreture you set it at. When you hook it up to the heater with the temp set up high it will close the contacts between to 2 wires and the heater should come on. When the temp has gone up it will open the contacts and the heater should shut off. If you want to test the heater before getting the thermostat just connect the 2 wires together and heater should come on.
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Old 10-13-2007, 04:40 PM   #3
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Since a basic thermostat is just a temperature-controlled switch (as Mike explained), with no electronic components, I can't think of any reason why the voltage would matter, as long as it is no higher than the rating for the device. That is, the 24V unit should be fine for the 12V furnace.

Although the factory-stock Honeywell thermostat in my Boler says "Suburban" on it (matching the furnace brand), I assume that it is just a normal residential unit, not anything special for 12V or RV use. The markings inside include "MAX 30V".

I compared the Boler's Honeywell to the White-Rogers in my house, and noticed only two differences:
  • the White-Rogers has a mercury switch, while the Honeywell has open contacts (the mercury switch is an upgrade, as far as I am concerned)
  • the Honeywell has an extra switch component which turns it off - regardless of temperature - if the control lever is pushed all the way to the low-temperature end; this is good to keep the furnace from running when you don't need the heat on, but doesn't really apply in a house which must always be kept habitable
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Old 10-13-2007, 04:44 PM   #4
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I've had good luck with the Hunter digital thermostat sold through WalMart and others. It does require batteries, but they should last a long time. Seems to keep the temperature within quite a narrow range, which is good.

It is also quite cheap, and if you should want to monkey around with the circuit board in it you can hack in easily and install remote switches for it and mount them right where you sleep. You can then precisely change the temperature settings by touch without even getting out from underneath the covers. Extreme, eh!
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Old 10-13-2007, 06:40 PM   #5
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Per, Brian, Mike .... Thanks for the reply guys ...

I ended up buying a Honeywell today ... Walmart didn't stock the Hunter brand here .. don't know of they were out or if I was just blind.

Interestingly, the furnace didn't come in when I hooked up the thermostat (temp on high), but when I took the cover off and put the metre to it (to see if there was DC voltage from the battery, it kicked the fan on, though the furnace it didn't light. I suppose the battery from the meter is providing the required juice and the battery is not for some reason ... but I'll need to think about that for a bit.

Playing with this stuff is a bit of fun eh?

Oh and Per ... I have a 2 year old that will get me up in the trailer long before the need for heat ... got a remote switch idea for her?

Tim
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Old 10-13-2007, 07:36 PM   #6
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If your Atwood is like my Atwood the furnace burner "wood" come on only after a delay. Fan first, then after some discussion and further nudging the burner will light. Similar disagreement in reverse when shutting down.

As for your 2-year old she probably has the remote switch to her daddy all figured out, no extra wiring needed.
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Old 10-14-2007, 02:48 PM   #7
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OK ... enough tinkering.

I have installed my new ATWOOD 8500 IV furnace under the sink of my Surfside. I ran the two requisite ducts ... one fore and one aft. I used a Honeywell T87K thermostat (no mercury switch, 24 VAC) and ran the two wires from the furnace to it as per the instructions. I then ran the power directly to the battery (I will be installing an isolation switch once I get one one tomorrow).

Here's the problem. When I turn up the thermostat, nothing happens ... ever. If take off the cover of the thermostat and put a meter on the connections for a second, the furnace fan kicks in and runs for a few minutes then shuts off. The fire never ignites. The gas is on.

Could it be as simple as the wrong thermostat? I thought it should work with this system.

Thanks for the help in advance.

Tim

Bye the by ... aside from it not working right now, the installation was easy and it fits perfectly ... hidden away under the sink in space that I never use. Much better than the original Hydroflame that I had before, though it will draw around 5 Amps/hour.
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Old 10-14-2007, 10:21 PM   #8
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Tim,
To troubleshoot this problem, jumper the two wires from the thermostat together at the furnace. This simulates the thermostat signaling the furnace to run. The furnace should turn on and run continuously. If it does not, the problem is in the furnace.

As mentioned above, the fan runs for several seconds before the flame lights. Don’t forget to turn the propane on. My furnace has a small window that you can look through to see the flame.

(In fact, if you wanted to install a switch near the bed to turn on the furnace without getting out from the warm covers on a cold morning, you would connect it just like the jumper.)

Marv
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Old 10-15-2007, 09:50 PM   #9
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Marv,

Thanks for the advice .. not sure why I didn't think of it. The furnace came on and ignited, I smoked myself out of the trailer (as I hadn't sealed any of the holes and put on the exterior vents yet ... smoke detector and CO detector started screaming .. (at least they work) .. quite a scene.

Guess the thermostat is not up to the task. I'll replace it tomorrow.

Once again, many thanks,

Tim
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Old 10-16-2007, 08:19 PM   #10
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Quote:
Marv,

Thanks for the advice .. not sure why I didn't think of it. The furnace came on and ignited, I smoked myself out of the trailer (as I hadn't sealed any of the holes and put on the exterior vents yet ... smoke detector and CO detector started screaming .. (at least they work) .. quite a scene.

Guess the thermostat is not up to the task. I'll replace it tomorrow.

Once again, many thanks,

Tim
Good Job!

You might be able to save that thermostat. It should be a simple make/break switch. As you move the lever, you should hear a "tink" when the bi-metal switch transfers. Pull the cover off and everything should be right if front of you. You should be able to watch the switch open and close.

Don't forget to seal that puppy well! Think of that combustion chamber like Las Vegas: what happens there, you want to stay there, not come into the trailer.
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Old 10-17-2007, 05:58 PM   #11
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Quote:
I've had good luck with the Hunter digital thermostat sold through WalMart and others. It does require batteries, but they should last a long time. Seems to keep the temperature within quite a narrow range, which is good.

It is also quite cheap, and if you should want to monkey around with the circuit board in it you can hack in easily and install remote switches for it and mount them right where you sleep. You can then precisely change the temperature settings by touch without even getting out from underneath the covers. Extreme, eh!
PER ... your digital thermostat ... does it require AC power for the switch? My Honeywell I have just realized is a digital thermostat, and still doesn't work... but if your works on DC ... this one should too right?

If not, what model is it? I gotta get me one of those ...

Marv ... any suggestions before this brand new $39 becomes a piece of junk in my shed?

Tim
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Old 10-17-2007, 07:42 PM   #12
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Tim:
Its official marketing title is "The Hunter Just Right Digital Thermostat, Model 42999B." (I think they take themselves quite seriously).
No AC required, just 2 alkaline AA batteries. It does warn you if the batteries are about to run out, and they should last about a year.
Now if you should want to know how to hack into one of these for that bed-side control, just let me know and I might be able to share my secret. Self-indulgence knows no bounds.
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Old 10-17-2007, 11:35 PM   #13
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Are you sure it's digital and requires 24 Vac from the furnace? If that's the case put it on Craigslist and buy one that is either mechanical or runs on internal batteries.
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Old 10-18-2007, 04:48 PM   #14
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Quote:
Are you sure it's digital and requires 24 Vac from the furnace? If that's the case put it on Craigslist and buy one that is either mechanical or runs on internal batteries.
No not sure, but I've tried everything to get it to work ... I connected the wires in accordance with the instructions (three connection locations ... use the top two) and dial the temp up, nothing happens even if I wait a while. I thought maybe since it was digital it needed an external source of AC power ... hmmmm ... think I am wrong? Maybe I should ask Honeywell ...
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Old 10-18-2007, 06:19 PM   #15
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It's not digital, but according to Honeywell, the T87K is not a simple pair of contacts on a bimetallic spring, like mine:
Quote:
Power Method - Uses stored power from heating controls.
Sensor Element - Thermistor
and most importantly:
Quote:
Voltage - 20 to 30 Vac
Also, there are at least two versions of the T87, but
Quote:
The T87K heat only works with 2 or 3 wire heat only systems
Unfortunately, it expects one of those wires to be from a 24 VAC transformer.

This is a vastly more complex thing than my simple thermostatic switch, which might be fine if it were powered by its own battery... but apparently it's not.
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Old 10-18-2007, 06:57 PM   #16
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Quote:
It's not digital, but according to Honeywell, the T87K is not a simple pair of contacts on a bimetallic spring, like mine:

and most importantly:

Also, there are at least two versions of the T87, but

Unfortunately, it expects one of those wires to be from a 24 VAC transformer.

This is a vastly more complex thing than my simple thermostatic switch, which might be fine if it were powered by its own battery... but apparently it's not.
Yeah ... I just put in a Walmart special ... it works great ... so much for trying to be fancy eh?

Thanks for the advice all ...
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Old 02-14-2010, 03:34 PM   #17
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Good suggestions on the Hunter digital thermstat. I was having trouble with the furnace in our 13' Scamp, and just replaced the thermostat with this one from Amazon:

Hunter 42999B Just Right Digital Thermostat

It wired right up per the included instructions for a 2-wire installation and works great! Ahh, heat again )
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