Digital to Analog Thermostat?

[twowhlrcr]

Good afternoon, I have a 2017 Forest River Stealth. My question is regarding replacing the digital thermostat with an analog thermostat.


It has:
(1) Dometic B59516.XX1CO Duo-Therm Brisk Air II Air Conditioner
(1) Suburban SF30FQ Furnace
It originally had a Dometic 3313192 Thermostat
It currently has a Dometic Capacitive Touch Thermostat


My problem with the original thermostat is that the furnace would stop working. I would leave the trailer parked up in Northern AZ for weeks at a time. We would set the heater to 50 degrees to keep things from freezing inside the trailer. When we’d return, the temperature inside the trailer was cold and the furnace would not be working even though the thermostat was still set at 50 degrees. Once we turned the thermostat off and then back on….the furnace would kick right on.

We took the trailer in for warranty and they replaced the thermostat with the Capacitive Touch style. We are still having the same issue. Not only with the furnace, but also with the A/C. Last weekend while using the trailer, the A/C worked fine throughout the day with the thermostat set at 76 degrees. During the night, the A/C would stop working even with the thermostat set at 76. As soon as we turned the thermostat off and then back to cool…the A/C fired right up.

So my question….Can I replace the digital thermostat with an analog version to see if that will solve the problem? Is there a direct replacement? Are extra parts required?

I called Dometic this morning and they said the analog thermostat would require 3 extra wires be run. Has anybody had any experience with what I’m trying to do? I appreciate your help. Thank you much.

[Bama Rambler]

What Dometic told you was partially correct. Not only will it require more thermostat wires to be ran, it will also require you to replace the control box in the A/C.

There is no direct analog replacement for those thermostats.

[Splitshaft]

I may not have a clue what I am posting here. However, I don’t understand why the furnace cannot be taken off the electronic thermostat and put on its own mechanical bimetallic thermostat? Per Suburban: “The thermostat controls the operating circuit to the furnace by reacting to room temperature to open and close a set of contact points which allows current to flow to the ON and OFF switch then to the relay.” Leave the AC on the digital thermostat as it is likely required to control the AC. The Suburban furnaces I am familiar with only require two wires for operation. They are either on or off and not modulation fan speeds. You would lose timer operation and night setting the might automatically lower temperatures that a digital thermostat can offer, but a bimetallic thermostat without batteries or circuit boards is either calling for heat or not. It is as simple and basic as it gets.

[Bama Rambler]

You are absolutely correct that you could add a bimetallic thermostat (or battery powered programmable one if you wanted to) to control just the furnace. Provided you have access and an easy way to route thermostat wires to a suitable location.

[archer314]

I have a 2019 grey wolf with the same issue as far as the AC,,both times out sa.e thing happened. Ac just stopped ,,it would just quit even with the thermostat set to 71,,it would be almost 80 in the trailer. Wouldn't start up till you turned thermostat off,,then back on, then it wod run
I have an appt.to have it looked at under warranty next monday

[Splitshaft]

Originally Posted by archer314

I have a 2019 grey wolf with the same issue as far as the AC,,both times out sa.e thing happened. Ac just stopped ,,it would just quit even with the thermostat set to 71,,it would be almost 80 in the trailer. Wouldn't start up till you turned thermostat off,,then back on, then it wod run
I have an appt.to have it looked at under warranty next monday

I wonder if they now build these things to meet some new government energy standard whereby stuff just shuts off without periodic user input? My TV does the same thing after several hours unless I provide remote control input.

[Skip12]

We have the same problem with one of our two thermostats, so I don't think it's working as designed or per regulation. Looking forward to hearing how twowhlrcr and archer314 solve this problem.

[DustyRoads]

We have had to re-initialize our thermostat every so often. We have had run time issues. This especially happens when we have had all power disconnected including the batter bank.

Here are the instructions from the manual:

System Initialization
A system initialization will need to be performed by the installer after
the system is installed.
• Make sure the Single Zone LCD thermostat is in the Off condition.
See page 4, “Quick Reference To Control Buttons”.
• Press the “+” button and, while holding it, also press and hold
the On/Off Mode button for three seconds. LCD will show
― ― . Press the On/Off Mode button again to turn system off.
This completes the initialization.
The furnace On/Off temperature differential should be set at this time.

[twowhlrcr]

First off, Thanks for all of the input. Keep it coming if you have anything to share.

Here's my plan next time I'm up at the trailer (should be within a couple weeks). It's hot out here in AZ, so I'm sure this will only relate to the A/C:

1) Dismount the thermostat but leave the wires hooked up.
2) Set the thermostat to turn on the A/C.
3) Measure the voltage on the wires to see what the thermostat sends to the control unit.
4) When the A/C turns off (when it's not supposed to), measure the voltage to see if the thermostat is still sending that same signal to the control unit.
5) If there is no voltage, I'm assuming the thermostat is the cause.
6) If the voltage is the same, then possibly the control unit?

I grew up installing car stereos and my Father In Law owns his own electric company so we have the basics of RV wiring covered.

Let me know if I'm on the right track.

Thanks again for the help.

[twowhlrcr]

DustyRoads, I will try what you posted first and hope the problem never comes back. Thanks.

[XenoRev]

If the ac shuts down when the temp difference is high, it's possible that the coils ate icing and head pressure sensor is causing the system to shut down. Especially if the humidity is high.
Dirty filters or plugged return can cause this as well.

[Bama Rambler]

Just remember that the thermostat you have sends digital signals to the control box so I'm not sure checking for voltage is going to tell you anything, other than it's there or it isn't. It's not going to tell you what it's trying to 'tell' the control to do.

[dieselguy]

I agree with XenoRev as to a possible scenario going on with your A/C. Many people set the A/C fan to auto or low for varied personal reasons. Low air flow caused by the auto or low setting over the evaporator coils will cause them to ice up and trip the sensor mounted in those coils to shut off the unit. By the time you return to turn the thermostat off and back on … the ice has melted and the unit starts right back up. Just for grins, keep your A/C fan set on high and report back in a while.

As for leaving the trailer's heater on for "weeks at a time" … unless you have some whopper of a tank … it will run out of propane during "several weeks at a time".

[twowhlrcr]

Xeno, that's good to know. Is there any way to check to see if that is the cause? If it was the head pressure sensor, would the A/C start right up after simply shutting off the thermostat and turning it right back on?

Here's an exact scenario:
1) Outside temperature 95 degrees.
2) Thermostat set to 78 degrees.
3) A/C will run good for quite a while (6 hours or more). Cycling on/off as it cools down the inside of the trailer. When the A/C is off, it typically stays off for about 10-15 minutes.
4) We can tell when the Thermostat/A/C is about to act up because it will turn off and then immediately kick right back on for about 15 seconds and then turn off again. When it does this, it is off for good.
5) The thermostat is still set for 76 degrees but the inside temp of the trailer will read 80+ degrees (thermostat turned off and pressing the temp button provides this).
6) I turn the thermostat back to cool and the A/C immediately kicks back on.

Bama, that it what I want to check. The signal from the thermostat to the control unit. If that signal is still there when the A/C turns off then it probably isn't the thermostat...correct?

Dieselguy, I will try that and report back. As far as the heater, we only do that during the winter. Doesn't get terribly cold in Northern AZ. Lows typically overnight in the High 30's. We set the thermostat to 45-50 degrees just to help prevent freezing of the water lines. There are 2 large propane tanks on it. Haven't run out yet.

[XenoRev]

Twowhlrce
Is the stat set to fan auto?
I'm not really an HVAC guy so I'm not sure if it is head pressure, I just have seen this be the problem.
Interesting that your stat shows you the temp, mine only shows the set point. I put a thermometer in our TT to see actual room temp.
With my unit having the fan on auto it will cycle on and off, and change fan speeds. The fan shuts of for 10 sec when changing speeds.
If it is icing up, you could pull the cover down and look at the coils, the ice would be visible. You may want to pull that cover and look anyway. I've read here of scenarios where the tinners left metal or tape in the duct or in the mounting blocking air flow.
I'm not ruling out a bad stat either. If I knew the wiring better i could tell you how to shunt the stat, calling for cooling continuously and then see if it cuts out.
As I'm typing I thought of another possibility that would be likely, low freon. That will cause the other side if the coil to overheat and call for a shutdown.
This could be getting complicated. Maybe a beer and some contemplating time.

[twowhlrcr]

Xeno,

Yes, T-Stat set to Auto.

Which T-Stat do you have? Mine will show the inside temp of the trailer if you do the following:
1) Turn T-Stat to the OFF Position
2) Press either the Temp Up or Temp Down Button

[XenoRev]

Originally Posted by twowhlrcr

Xeno,

Yes, T-Stat set to Auto.

Which T-Stat do you have? Mine will show the inside temp of the trailer if you do the following:
1) Turn T-Stat to the OFF Position
2) Press either the Temp Up or Temp Down Button

Mine is one of the newer domestics. No switches. A power touch button and two arrows.

[archer314]

My camper is brand new ,,2019 grey wolf,,,I have the same thermostat,,,power button and 1 up and 1 down arrow

[twowhlrcr]

Good afternoon all, so just got home from Prescott. Spent 2 nights in the trailer. Of course....... the A/C didn't act up all. Almost like it knows I'm talking sh&% about it? The weather was awesome though. Didn't need the A/C at night at all.

Played around with the thermostat today. 3 wires coming off of it. Red, Black and Green. So the Red & Black are pretty basic....Just 12 volt negative & 12 volt positive. Easy to test. The Green wire is a communication wire. Not sure what type of signal it sends or how to measure it?

With that being said, I'm going to pursue a possibility that Xeno brought up. I'm going to focus on the Freeze Sensor. If you remove the access cover and the air filter, you can look up in the void and see the evaporator (I think that's what it's called?) along with a freeze sensor that's mounted to the base of it. The freeze sensor has 2 small white wires coming off of it and they go to a small blue plug on the side of the control unit. When I unplug that blue plug from the control unit, the A/C goes into shut down mode. When I plug that blue plug back in, it takes about 2-4 minutes for the A/C to kick back on. Keep in mind while I'm unplugging and plugging that blue plug back in, the thermostat is on the entire time. That blue plug causes the A/C to act up like it has been acting up on us lately.

So....who knows how this freeze sensor works? Is it a normally open or normally closed type of switch? Is it supposed to read a certain resistance? What is the best way to test it?

My plan is to test the freeze sensor while the A/C is operating normally and record the reading. Then, if the A/C turns off before it has reached the temperature set on the thermostat, I will test it right away to see if the reading is different. That should tell me right away if it's related to my issue.

[Bama Rambler]

I believe that freeze sensor is actually a thermistor, so it's not an open or closed switch.

You could have had a bad connection in the blue connector.