Question How to tell thermostat to use itself for temp and not thermistor?

[Neal]

Hating the thermistor setup for zone 1 (front) in my 2017 Newmar Ventana motorhome. I'd like to set the front Dometic A/C to use the thermostat for its temperature sensor instead of the thermistor. Anyone know how to make this change? My guess is a dip switch on the roof top unit and that's what I'll start hunting down but wanted to present the question to the forum to see if anyone knows and will report back what I find.

 

[Rich W.]

you probably have this and it might not address the issue, but just in case…

https://www.dometic.com/assets/47/55/operating%20manual_64755.pdf

 

[Neal]

I'm not sure the attached document is for the unit I have but it came up in a google search and it seems the same. It appears the P4 wire connection is the one. Zone 1 doesn't require an internal temp sensor but anything other than zone 1 does per this doc. I want to disable the internal temp sensor and see how things work using the TSTAT as the sensor.

 

[Neal]

My fear though is per the purpose of the external sensor (thermistor). The TSTAT would be fine where I am here but with it being on the wall next to the door, if I had direct sun there I'd have a problem. I don't know if there is a way to calibration thermistors, I don't see a way other than offsetting temps for the TSTAT itself, think I'll have to just life with keeping the bedroom door half closed to block the zone 1 thermistor from zone 2 temps. Will go on the roof and see what the wiring looks like and how hard it would be to try if low risk. Curiosity is all there is for now.

 

[Tnedator]

My fear though is per the purpose of the external sensor (thermistor). The TSTAT would be fine where I am here but with it being on the wall next to the door, if I had direct sun there I'd have a problem. I don't know if there is a way to calibration thermistors, I don't see a way other than offsetting temps for the TSTAT itself, think I'll have to just life with keeping the bedroom door half closed to block the zone 1 thermistor from zone 2 temps. Will go on the roof and see what the wiring looks like and how hard it would be to try if low risk. Curiosity is all there is for now.

My zone one is the Thermostat on the outside wall (I'm assuming same location as yours) and, you are right, it reads hot in most situations. I just keep zone 1 higher than the thermistor based zones (two and three).

 

[BigFunBus]

Neal -
I thought about making the same adjustment but decided to take a different approach. I used a box extender (like those on GFI outlets) and drilled holes around the sides to allow for airflow. Then mounted it in place of the thermistor adding some thin foam insulation between the wall and the holes. Then mounted the thermistor on a blank plate screwed to the extender.
Now coach air flows behind/around the thermistor; seems it reads the temps closer to actual than before.
joe

 

[Tnedator]

Neal -
I thought about making the same adjustment but decided to take a different approach. I used a box extender (like those on GFI outlets) and drilled holes around the sides to allow for airflow. Then mounted it in place of the thermistor adding some thin foam insulation between the wall and the holes. Then mounted the thermistor on a blank plate screwed to the extender.
Now coach air flows behind/around the thermistor; seems it reads the temps closer to actual than before.
joe


View attachment 14470

Interesting. Your's is a bit of a different problem than Neil with the Thermistor. His Thermistor problem is that it's located to close to Zone 2 (sill placement by Newmar, which I have as well, but with Zones 2 and 3). His and my thermistors are on inside walls.

However, it's possible the same trick you have there, but with the thermostat, instead of thermistor, might help keep the thermostat from reading so high.

 

[Rich W.]

My zone one is the Thermostat on the outside wall (I'm assuming same location as yours) and, you are right, it reads hot in most situations. I just keep zone 1 higher than the thermistor based zones (two and three).

This makes sense. My system works just fine, but both my thermistors are on interior walls. Maybe someone should mention this to Newmar.

 

[J&JD]

Neal -
I thought about making the same adjustment but decided to take a different approach. I used a box extender (like those on GFI outlets) and drilled holes around the sides to allow for airflow. Then mounted it in place of the thermistor adding some thin foam insulation between the wall and the holes. Then mounted the thermistor on a blank plate screwed to the extender.
Now coach air flows behind/around the thermistor; seems it reads the temps closer to actual than before.
joe


View attachment 14470Reasonable solution. Makes sense to me.

 

[Buly]

I have the same problem. Tried insulating the sensor with foam pad. Not much difference. Finally took it off the wall and now it is hanging loose away from the wall
( I was frustrated).
.BTW I did read the install manual. Dometic explicitly instructs the sensors NOT to be installed on outside walls and no higher than 50” (I think) from the floor.

 

[Tnedator]

This makes sense. My system works just fine, but both my thermistors are on interior walls. Maybe someone should mention this to Newmar.

Neils are on interior walls as well. His issue, which might only impact those with half baths, is that Newmar puts zone 2 (in two zone systems) thermistor on the half bath wall, inside the bedroom. They put zone 1 (in two zone systems) on the half bath wall in the hallway, but these two thermistors are only 3-4' apart, and the zone 1 thermistor is closer to and more effected by the rear air, then it is the front air.

So, what he's trying to overcome is not a thermistor on an exterior wall, but just a really stupid placement that Newmar uses on the 4037 and 4369, at the least. I have the same issue with my zone 2/3 that Neil has with his zone 1/2, they are placed too close together, and too far from the living room area.

 

[DavidL]

So, where in the RV is the best place that the thermistor should have been placed? "someone" should come up with a kit where the thermistor is battery / wireless that periodically wakes up and sends it's current temp to a receiver that plugs into the standard connection that the old thermistor wire plugs in....

My issue with Dometic: who the heck designs a thermostat that doesn't tell you the current temp?
Weird...

Anyway, replacing the stupid factory system is on my to do list. I pulled the Dometic off the panel to start sniffing the data traffic....

 

[AbdRahim]

I just adjust each zone according to feel. Sometimes zone one is comfortable at 75 on AC, sometimes higher or lower depending on outside temperature. When up front zone two supplements zone one, a few degrees cooler. On heat nothing is ever above 71-72.