A/C Thermostat Question

[Jim Blackwood]

Seems to me the problem might be what you're doing with the cold air. Sort of like my Dyson fan I keep beside my recliner in the house. If I run it on high all the time I get cold even when the house is warm because it blows right where I sit. But if I turn it down or even off, I can find a setting where I'm comfortable. What I mean is, your setup is great for cooling you down any time you are in the middle of the RV, but does it work as well if you are off in a corner somewhere, or say, in the bathroom? Finding ways to circulate the air more evenly through your living space and less directly through the center area might be all the remedy you need. Might be a challenge to do that, ideally you would suck in air from several locations up high and in stagnant corners for instance, and blow it out towards the more remote and warmer parts of the rig to get a more even temperature gradient. That would circulate the humidity laden air through the evaporator and I guarantee you your AC unit is the best dehumidifier you could have, if you just get the moist air to it.

Or, a bunch of small fans might work. But I don't see running the compressor longer as the solution it might look like. If you get the warm, moist, stagnant air to it, it will run longer anyway.

Jim

[richard5933]

Seems to me the problem might be what you're doing with the cold air. Sort of like my Dyson fan I keep beside my recliner in the house. If I run it on high all the time I get cold even when the house is warm because it blows right where I sit. But if I turn it down or even off, I can find a setting where I'm comfortable. What I mean is, your setup is great for cooling you down any time you are in the middle of the RV, but does it work as well if you are off in a corner somewhere, or say, in the bathroom? Finding ways to circulate the air more evenly through your living space and less directly through the center area might be all the remedy you need. Might be a challenge to do that, ideally you would suck in air from several locations up high and in stagnant corners for instance, and blow it out towards the more remote and warmer parts of the rig to get a more even temperature gradient. That would circulate the humidity laden air through the evaporator and I guarantee you your AC unit is the best dehumidifier you could have, if you just get the moist air to it.

Or, a bunch of small fans might work. But I don't see running the compressor longer as the solution it might look like. If you get the warm, moist, stagnant air to it, it will run longer anyway.

Jim

Yeah, these Custom Coach a/c units will pump out cold air, but they are lousy as far as circulation goes. Just one intake and one outlet, with no ducting. Obviously not easy to add ducting at this point. Perhaps a few well placed fans, but with the limited floor space we have they'd have to be wall mounted to really be convenient. Some thing to think about though.

[Jim Blackwood]

What'd be the chance of adding in some small overhead cabinets that you could route the air through? Not that it'd be great having the air blow over your dishes and things but it could be a pretty simple remedy.

Jim

[richard5933]

What'd be the chance of adding in some small overhead cabinets that you could route the air through? Not that it'd be great having the air blow over your dishes and things but it could be a pretty simple remedy.

Jim

Not really possible without destroying the interior.

[Jim Blackwood]

I see what you mean. But what about some sort of translucent or transparent panel above the right hand (in the photo) cornice board that would let the indirect lighting shine through? That could carry the air forward and distribute it through slots or holes. If you get a significant amount of it to the front and release it there it should help quite a bit.

Jim

[richard5933]

I found what I think is a relay that will work on this project. It's adjustable from 3-300 seconds delay in opening a circuit. Right now my system can run the compressor for as little as 30 seconds if the outside temps are not too warm, and I'm hoping that by extending the time by a couple of minutes the cooled air can more fully circulate in the space and the humidity can be removed better.

My thermostat is really nothing more than a 120v on/off switch which opens and closes contacts according to the temperature. This relay will provide for an extension of the time the thermostat keeps the contacts closed, so if I set it for 120 seconds then it will extend the run time by two minutes.

If I understand the wiring on this correctly, the following needs to be done (refer to pdf for diagram):

A) Terminals 2 & 10 will have a constant 120v to run the timer inside the relay. I will tap from the fan power circuit since it's energized whenever the system is turned on.)

B) The thermostat will be connected to terminals 5 & 6 which are labeled 'ext. switch' in the diagram. The thermostat will then be controlling the new relay and not the existing compressor relay.

C) The wires which currently connect to the thermostat will now be connected to terminals 11 & 9. When the thermostat closes the contacts between 5 & 6, the relay will close the terminals between 11 & 9. The relay will then hold them closed for the pre-set time once the thermostat contacts open.

There were simpler relays available, but they worked in the opposite manner by preventing the thermostat from engaging the compressor for a set time after a shut down. That type of relay seemed more like it was designed to prevent the compressor from cycling on too soon after it shut down, but that is not my problem. Right now it will run for 30-45 seconds and then shut down, and it might be 10 minutes or more before it starts again.

(When the temps are warmer, the compressor will run normally and for extended times. That's why I am not worried about something wrong with the system keeping the compressor from running longer).

[Lin]

If your compressor is shutting down after only 30 seconds, it would seem that there is a "short circuit" between the cold air and the return.  How many degrees is it cooling the whole cabin in 30 seconds?  If cooled air is getting sucked into the return it will cycle like that without sufficiently cooling or dehumidifying the room.  Also, the slower the air speed, the more dehumidifying.

There used to be a device at Camping World that was wired in to manage the AC's when there was limited power supply.  If activated, it had a timer that alternated between the front and back AC units.

[richard5933]

If your compressor is shutting down after only 30 seconds, it would seem that there is a "short circuit" between the cold air and the return.  How many degrees is it cooling the whole cabin in 30 seconds?  If cooled air is getting sucked into the return it will cycle like that without sufficiently cooling or dehumidifying the room.  Also, the slower the air speed, the more dehumidifying.

There used to be a device at Camping World that was wired in to manage the AC's when there was limited power supply.  If activated, it had a timer that alternated between the front and back AC units.

The output for the rear a/c is what you see on the wall above the couch. The thermostat is above it. The intake for the a/c is on the horizontal surface above & behind the television. The evaporator unit is inside the upper cabinetry above the TV.

I've tried running it with higher fan speeds and low fan speeds. With the higher fan speeds the cold air makes it across the room enough to bring down the temp across the room. When I run it on slower fan speeds the air seems to short circuit like you describe and go from the outlet right back into the inlet. Medium speed seems to be the ideal.

If I move the thermostat across the room, then it will be in the direct path of the airflow when the fan is on high. Where it is now it's above the airflow.

Not sure that there is an easy solution, since the unit is really over-powered for a room that's only about 7' x 9' in size.

[Jim Blackwood]

If you can picture the way water flows in a trout stream, apply the same ideas and see if you can find an "eddy" in the airflow where the warm air recirculates and doesn't get sucked back into the inlet. Then put your thermostat near that area so it will be out of the main air flow areas. That should work better than your relay idea. The opposite side of a cabinet for instance. (Since the refrigerator makes heat that might or might not be a good spot.)

Jim

[chessie4905]

How about replacing that thermostat with a newer or different one. Maybe it is the culprit.Or if it is mounted further away.

[richard5933]

How about replacing that thermostat with a newer or different one. Maybe it is the culprit.Or if it is mounted further away.

Original thermostat was a Penn Controls line voltage unit. I replaced it with a brand new one. Penn Controls was bought by Johnson Controls (they put the 'Controls' in Johnson Controls' name), so although the units are identical the new one has the Johnson Controls name on it. These are pricey units, over $100 for the thermostat, but they are good thermostats.

One thing that I did try to do was set the differential so that there would be a gap in temps between calling for cooling and shutting off the compressor, but that is preset on these thermostats and is pretty tight.

I've thought about moving the thermostat, but not sure if it will do any good. There is a thermometer on the wall across the room from the a/c unit, and it usually shows the same temp as the setting on the thermostat. I'd have to hunt around the small room for a spot which is slightly warmer.

My hunch is that the unit is simply too powerful for such a small space when the outside temps are only slightly above the inside temps, which is what's causing the compressor to run for such a short time.

Our household York a/c unit with the fancy touch screen has a setting which can add run time to the compressor beyond what's necessary to hit the temperature set point. They have it listed as a feature to help with humidity control. This is where I got the idea to add the delay relay to the compressor circuit and extend the run time on the compressor just a bit.

I'll be installing the relay in the next week or so when I have time & patience to do it right. I'll report back once it's installed to let you all know how it works.

[chessie4905]

Another easy one is to run a small electric heater at the same time. Vehicles with automatic hvac do it to temper the air for the temperature set. Just get a small one with 3 heat outputs and/ or fan speeds with thermostats.

[richard5933]

Another easy one is to run a small electric heater at the same time. Vehicles with automatic hvac do it to temper the air for the temperature set. Just get a small one with 3 heat outputs and/ or fan speeds with thermostats.

Funny how similar this sounds to how the OTR a/c system tempers the air - compressor runs all the time and the heater core is used to control temp.

This would work when I'm on 50-amp service. The a/c units draw 19 amps each on startup, so I wouldn't try it on 30-amp service, not even with just one a/c unit and one heater, as that would leave me no overhead for the lights, battery charger, etc.

[Lin]

Before the energy crisis in the 70's, it was common to design commercial AC systems with what was called "Reheat".  As such that air would be over-cooled and than reheated to the desired supply temp thereby extracting more moisture and providing more control.  This practice was stopped because it was considered wasteful in an era of acknowledged limited resources.

Have you tried just shielding the thermostat from the airflow a bit so it will react more to ambient temperature rather than convection?  I am not sure what would work best, but even just wrapping it in aluminum foil might slow the reaction time.  Just guessing though.

[Lin]

Just wanted to mention another possibility.  The thermostat sensor should be in the return air stream.  If the sensor is across from the blower and in the supply stream, it will act like you have described.