Atwood heat pump cycles on and off

[cavie]

Originally Posted by s1njin

Atwood Air Command w/ Heat Pump - iRV2 Forums

They were discussing some of this hear. Apparently the unit turning on for a few seconds every minute or two is for it to evaluate the ambient temperature of the coach.

Continued running after compressor shut off for one cycle is very understandable. Continued cycling is not acceptable.

The ambient air is sensed by the t-stat. If I had one doing that I'd be less one shotgun shell.

[s1njin]

They are saying there is some sensor in the unit itself.

The documentation on how this system works leaves something to be desired.

[s1njin]

There are in fact multiple temperature sensors in the Atwood as evident by the service manual for one; page 12.
http://www.bdub.net/manuals/AtwoodRoofACRepair.pdf

[s1njin]

Another forum, another person saying in AC mode, the fan runs 100% of the time. I think both conditions you have are by design.

https://www.rv.net/forum/index.cfm/f...print/true.cfm

[cavie]

Originally Posted by s1njin

There are in fact multiple temperature sensors in the Atwood as evident by the service manual for one; page 12.
http://www.bdub.net/manuals/AtwoodRoofACRepair.pdf

That manual is written for tech experts or very intelligent readers. The average Joe can not understand it.

[westerm52]

Ok, I am an HVAC guy and have worked on heat pumps most of my life. I just bought a 2401W MBS with the Atwood unit. The way I understand it, the Tstat sensor governs the propane heater. The return air sensor in the Atwood unit governs the Heat Pump/ Air Conditioner. In the Heat pump mode it turns the fan on every couple of minutes to sense room temp. In cooling the fan runs constantly. The problem that I see is in the heat Pump mode the hot air comes out the vents and gets sucked right back to the return air grill causing the causing the return air sensor to turn the compressor off because it thinks it's warm in the coach when in reality it is still very cool lower than about six inches from the ceiling. When you check the temperature of the sensor in the unit by pressing and holding a button on the thermostat the sensor reads either warmer in heating or cooler in cooling than the actual room temperature. It seems to me there is a problem with design. When I talked to Atwood they would not put me through to a tech until I take it in to a dealer. I mentioned in a previous post that there is a baffle between the supply air and the return air that needs to be in place. In mine that is fine. I found no air leaks that would cause the sensor to sense false temps. The only other thing I can do is take an ohm reading of the sensor to make sure it is within specs. My gut tells me it is ok and the sensor is just not in a good location for our situation.

[s1njin]

Is yours a ducted system? I can understand your description of the problem being in a non-ducted system where the supply and return are a short span from each other.

???

[5picker]

Originally Posted by westerm52

Ok, I am an HVAC guy and have worked on heat pumps most of my life. I just bought a 2401W MBS with the Atwood unit. The way I understand it, the Tstat sensor governs the propane heater. The return air sensor in the Atwood unit governs the Heat Pump/ Air Conditioner. In the Heat pump mode it turns the fan on every couple of minutes to sense room temp. In cooling the fan runs constantly. The problem that I see is in the heat Pump mode the hot air comes out the vents and gets sucked right back to the return air grill causing the causing the return air sensor to turn the compressor off because it thinks it's warm in the coach when in reality it is still very cool lower than about six inches from the ceiling. When you check the temperature of the sensor in the unit by pressing and holding a button on the thermostat the sensor reads either warmer in heating or cooler in cooling than the actual room temperature. It seems to me there is a problem with design. When I talked to Atwood they would not put me through to a tech until I take it in to a dealer. I mentioned in a previous post that there is a baffle between the supply air and the return air that needs to be in place. In mine that is fine. I found no air leaks that would cause the sensor to sense false temps. The only other thing I can do is take an ohm reading of the sensor to make sure it is within specs. My gut tells me it is ok and the sensor is just not in a good location for our situation.

I'd say you intuitions are correct and Atwood warns of this in their manual... (page 18) and I agree it's not so much the internal duct/baffle but the placement of the ceiling ducts/vents in proximity to the intake.

9: During heating, the unit cycles regularly, the temperature display indicates the room temperature to be
between 75⁰F and 82⁰F (24⁰C to 28⁰C) however the average temperature in the cabin is much lower.
This condition can be caused by a faulty duct which is causing the warm air to short cycle.
To see if this is the cause, disassemble the fascia from the inside plenum (4 screws) and examine
carefully the extension duct for leaks, particularly where it joins the horizontal duct.
A leaking duct will short cycle warm air onto the return air sensor causing the compressor to close
down prematurely. Repair any leaks and reassemble.

[westerm52]

My RV is only 25 ft long with multiple vents in the ceiling. When the compressor is running it puts out lots of warm air but that air stays mostly up at the ceiling. The compressor ends up shutting down before the room temperature at sitting level even gets close to the set temp. It will then cycle on the fan every minute or so until the sensor in the return air cools to room temp and then turn the compressor back on but only for a minute tops. Again the ceiling is nice and warm but sitting level is cold. I have turned closest vents off and directed other vents away but the situation is still basically the same. My next step will be to relocate the return air sensor to a place not directly in the return flow.

[chroniekon]

Originally Posted by westerm52

My RV is only 25 ft long with multiple vents in the ceiling. When the compressor is running it puts out lots of warm air but that air stays mostly up at the ceiling. The compressor ends up shutting down before the room temperature at sitting level even gets close to the set temp. It will then cycle on the fan every minute or so until the sensor in the return air cools to room temp and then turn the compressor back on but only for a minute tops. Again the ceiling is nice and warm but sitting level is cold. I have turned closest vents off and directed other vents away but the situation is still basically the same. My next step will be to relocate the return air sensor to a place not directly in the return flow.

This seems like it might actually work. Keep us posted

[Duckogram]

Well, I took it in to the Dealer in Ft. Myers and demonstrated the issue to the service writer.
He documented it and had a technician looking at it in an hour or so. Four hours later I went back to the service desk to see if I should go have lunch or wait. At that point the technician informed me that he could not diagnose the unit because the ambient air temp was too high. He suggested that I could bring it back when the ambient temp was between 50 - 65 degrees. I mentioned to him that we are in SouthWest Florida and it is the end of March. It is unlikely that we will have ambient air temps at that level during your hours of operation until next January! Then I asked him if he was suggesting that I move North of the Mason-Dixon to get a diagnosis that the factory/manufacturer would accept?
Needless to say, the need for diagnosis at cold temps needs to be addressed.
Basically, all I accomplished was to document the issue and take the first step in the process. So, next step is to elevate this to FR. I have a customer service number however if anybody has a direct point of contact it would ne appreciated.
TIA,
Ducky

[cavie]

Originally Posted by Duckogram

Well, I took it in to the Dealer in Ft. Myers and demonstrated the issue to the service writer.
He documented it and had a technician looking at it in an hour or so. Four hours later I went back to the service desk to see if I should go have lunch or wait. At that point the technician informed me that he could not diagnose the unit because the ambient air temp was too high. He suggested that I could bring it back when the ambient temp was between 50 - 65 degrees. I mentioned to him that we are in SouthWest Florida and it is the end of March. It is unlikely that we will have ambient air temps at that level during your hours of operation until next January! Then I asked him if he was suggesting that I move North of the Mason-Dixon to get a diagnosis that the factory/manufacturer would accept?
Needless to say, the need for diagnosis at cold temps needs to be addressed.
Basically, all I accomplished was to document the issue and take the first step in the process. So, next step is to elevate this to FR. I have a customer service number however if anybody has a direct point of contact it would ne appreciated.
TIA,
Ducky

Call a local residential AC service company.

[westerm52]

Ok, I worked on my Atwood Heat Pump today. The return air sensor (thermistor) is located in the return air mounted with a bracket to the coil. I took it off the bracket, and through trial and error, located it on top of the existing wiring in the return air but out of the main flow of air. The unit worked great. I set it at 69 and watched as it ran the temp up. It stopped about 70 degrees. I did'nt get a chance to let it come back on by itself so I don't know how big a temperature swing there is. At this point, it is a great improvement. The air still comes on every minute or so once the compressor stops to sample room air temperatures. Definitely a design issue on someone's side.

[s1njin]

Good to know ! Let us know how it behaves with more use cycles.

[xgarys]

Originally Posted by westerm52

Ok, I worked on my Atwood Heat Pump today. The return air sensor (thermistor) is located in the return air mounted with a bracket to the coil. I took it off the bracket, and through trial and error, located it on top of the existing wiring in the return air but out of the main flow of air. The unit worked great. I set it at 69 and watched as it ran the temp up. It stopped about 70 degrees. I did'nt get a chance to let it come back on by itself so I don't know how big a temperature swing there is. At this point, it is a great improvement. The air still comes on every minute or so once the compressor stops to sample room air temperatures. Definitely a design issue on someone's side.

It seems odd that they wouldn't have the space temp sensor in the thermostat. Do you have any idea why the might have done that?

[westerm52]

The people at Atwood told me it is considered a self contained unit. My guess is that is why. Also, heat pumps run on a temperature pressure relationship. They may very well be trying to protect the heat pump from high pressure problems.
I really don't think it was meant for our application. The guys at Forest River told me it was a recent change to Atwood.

[Duckogram]

Originally Posted by westerm52

Ok, I worked on my Atwood Heat Pump today. The return air sensor (thermistor) is located in the return air mounted with a bracket to the coil. I took it off the bracket, and through trial and error, located it on top of the existing wiring in the return air but out of the main flow of air. The unit worked great. I set it at 69 and watched as it ran the temp up. It stopped about 70 degrees. I did'nt get a chance to let it come back on by itself so I don't know how big a temperature swing there is. At this point, it is a great improvement. The air still comes on every minute or so once the compressor stops to sample room air temperatures. Definitely a design issue on someone's side.

Any chance of getting a picture of what you did? It would be greatly appreciated.

[westerm52]

I'm sorry but I would have to take it all apart again to take pictures, but I will try to describe it better.
If you take the indoor cover off which is held on by four or maybe six screws you will be able to see up into both the return air and the supply air. Check to make sure there is a partition in place separating both compartments. It is movable and mine was not very secure. Looking in the return side, you will see the power wire and the control wires all running across the air space. Above that, on the coil itself ( looks like a radiator), you will see a small plastic bracket that holds a small black sensor connected to a couple wires from above. That is the return air sensor. It is very sensitive to temperature change. I unhooked the sensor from the bracket. Actually took the bracket off. It is just held in the fins using pressure. I placed the sensor in different spots within the airflow, each time running the unit and checking the sensor temp by pushing the sleep button and holding it for five seconds on the thermostat. If it is in too much of a direct airflow the temperature would rise too quickly and not reflect the actual air temp in the coach. The hot air at the ceiling level was being directed right at the sensor. Finally ended up placing the sensor above the wires running through the return air and zip tied them to the wires. This blocked the direct flow of air to the sensor but still allowed it to sense the room temperature. I ran the thermostat up two degrees past the room temp and watched as the unit came on and ran. Before it sensed the hot air too quickly and registered the room temp as being way warmer than it was. Now, it went up more gradually and ended up turning off the unit at the right temperature. After the compressor shut down the sensor temp actually raised a couple more degrees because of heat in the return air. That is why the fan comes on every minute or so to sample the room air. It cools down the housing and brings it to actual room temperature. The only thing I couldn't do, because of time, was let the unit cycle back on by itself. I want to make sure that it cycles back on without too great a temperature swing. RVs typically have a large swing because of their construction so I'm not too concerned. All said and done, I believe the issue deals with the size of the unit vs the size of the space being heated or cooled and the location of the vents distributing the air. After I get to try this out on the road I will call Forest River and talk to them about it.

[Duckogram]

Thanks for your description western52. I think I have a work around under the circumstances.
I’m just going to use the dump on the indoor cover to try to get the heat closer to the floor. Then I’m turn the HP off and use a ceramic space heater to maintain it while setting the thermostat to furnace heat only as a backup.
The useful heating band for the HP is fairly narrow so I dont expect any re-engineering efforts from FR that would or could actually fix anything beyond what you accomplished.

[westerm52]

Yes, Living in Florida you really won't need it much anyway unless it affects the cooling also. Time will tell.