AC Performance

[01tundra]

Trying to determine if the A/C unit in our Hummingbird is operating correctly.

Took it to the dealership last week and they replaced the "bottom half" of the unit, whatever that means.

Our unit is a 13,500 btuh Coleman unit serving a 17' TT, so it should run away with it in my opinion.

It was 86F outside today and the TT was in the driveway in partial sun/shade. The compressor was cycling on for about 3 minutes and then off for a couple. Our A/C unit has unit-mounted controls (high/low fan and high/low AC and a second knob for temperature - blue/red gradient).

The indoor temp was around 72F with the slide in and the lights off. When the compressor would come on the leaving air temp (supply) was reading around 49-50F and the return was around 68-69F with my infrared thermometer at the A/C air supply/return.

I confirmed that the copper sensing bulb for the t-stat was well within the return air path.

If seems that the compressor runs longer on low cool, probably because we have such a small interior space and on high cool there's a lot of air circulating.

The dealer can't decide if we have an issue or not. I originally thought there was a problem because the indoor blower motor would continue to operate while the compressor cycled on and off even though it was still warm in the TT. Now I know that the indoor blower runs continuously.

So do we have a problem or is this as good as it gets?

Thanks.

[Grumpy]

15-20 temp differential across the unit is about as good as it gets for any air conditioner. Humidity is a big factor and you should always run the fan on high to prevent coil freeze-up.

[01tundra]

Originally Posted by Grumpy

15-20 temp differential across the unit is about as good as it gets for any air conditioner. Humidity is a big factor and you should always run the fan on high to prevent coil freeze-up.

So I was seeing 15-19F delta T depending on how long the compressor was running.


Sounds like temperature differential wise we are in there. Does the compressor (or technically condensing section) typically keep cycling on/off as the TT indoor temp is being brought down?

I'm used to residential/commercial HVAC where you have a thermostat set point and the compressor continues to run until the set point is satisfied. The compressor does not cycle off until the thermostat is satisfied. Of course.....I have no idea what "most blue" correlates with temperature wise. Maybe it is satisfying the thermostat, hard to tell without definitive numbers for set point.

It doesn't help that I was a HVAC service tech for 6 years and an HVAC engineer for the past 26 yrs......I'm probably expecting too much..

I was thinking it would be more like a hotel room PTAC unit where you set it to max cool and you can hang meat in the room within a couple hours.

[Grumpy]

You could have an air leak hitting the backside of the temp control causing the excessive cycling.

[01tundra]

I'm guessing that's why they replaced the bottom half of the unit last week.

Still not sure if it's doing the best it can do or not operating correctly.

Seems to be working the same to me.

[jgonzalez114]

Hey guys. Going to piggyback on this A/C post. I'm using my new 17FD for the first time and it seems the A/C is sending the condensation down the front of the TT. The TT is level, and I'm just not sure if this is as designed or if the A/C should have a drain pipe that may be clogged. I don't want the condensation messing anything up or eventually finding it's way in.

Thanks,

[Marcm157]

Originally Posted by jgonzalez114

Hey guys. Going to piggyback on this A/C post. I'm using my new 17FD for the first time and it seems the A/C is sending the condensation down the front of the TT. The TT is level, and I'm just not sure if this is as designed or if the A/C should have a drain pipe that may be clogged. I don't want the condensation messing anything up or eventually finding it's way in.

Thanks,

My AC drains to the front as well. I consider it normal and definitely better than the alternative (inside)...

[jgonzalez114]

That's good to know. Leaks are my biggest fear. If it won't mess anything up I'm fine with that.

[01tundra]

I updated my TT thread but forgot to update this one -

After talking with Coleman service tech's a few times I now have a good understanding of how the A/C is supposed to operate and why the first question out of anyone's mouth in the know when you're experiencing A/C problems is "did you check to make sure there's no air leakage?".

The upper (outside) portion of the unit houses the evaporator blower fan and passes air down to the part you see inside through a flexible boot. The flexible boot attaches to the indoor portion with double-sided tape. The indoor portion is simply a supply/return air plenum that also houses the 120V power connection, the function selector switch and the thermostat.....and here is where the problem lies.

What was happening with our unit was that we would turn it on when it was hot outside and the TT was hot inside, turn the selector to max cool, turn the temperature to the coldest setting and the unit would energize in cooling mode. The thing was, after a few minutes the condenser portion (outside part that has the compressor and condenser) would cycle off. After about two minutes the compressor who come back on and run for a few minutes and then repeat the cycle off. All along the indoor blower continued to run, as it is designed to do. So basically the compressor was turning off while it was still very hot inside the TT.

The dealer replaced the "bottom half" of our A/C unit, meaning the indoor portion with the thermostat. When I got it home it worked a little better, but would still cycle off too early.

After talking with Coleman's service techs several times we came up with a plan that seems to have worked.

First, kill the power to the unit (I didn't but I like to live dangerously).

To access the magic, you simply pop out both return air grilles and then slide the filter screens out. This will expose four small Philips head screws, one recessed in each corner of the housing. After removing the four screws you hold the white plastic cover and pull down fairly hard on each of the control knobs to slide them off their shafts, this allows the plastic cover to come free and be set aside out of the way.

After removing the cover, you will see a small rectangular opening and the blower wheel up inside the opening.

The first thing to look at is if the flexible supply boot is cut flush with the opening in the lower portion of the unit, meaning the boot should not extend beyond the lower edge of the plastic. I used an exacto knife to cut a few portions of the boot flush.

Next, I found small gaps in the boot where it was supposed to be stuck to the double stick tape, which allows cold supply air to blow into the return plenum, which can cause short cycling. To remedy this, I used HVAC foil tape and taped all the seams in the upper and lower ends of the boot where it attaches to the unit (I taped the boot to the hard plastic on the unit at the upper and lower interfaces of the boot).

Next was the actual metal housing that the thermostat is mounted in (center of the unit). The housing was partially insulated with foam insulation tape, but the lower 1/4" of the sides and the entire bottom cap was not insulated. There was also a gap at the point of the cap where the power wires come in, that allowed supply air to blow right in across the thermostat and copper sensing bulb....

So I used foil tape to close the gaps and then used Armacell 2" wide foam insulating tape from Home Depot to completely insulate the thermostat housing. This is the same type of tape that Coleman uses to insulate the upper sides of the housing......and now they are using it to insulate the entire housing on all of their units, like I did to ours last night.

The last thing to do is to make sure that the copper sensing line from the thermostat is located in the return air plenum and make sure that it is not touching any surfaces inside. Most of the time the sensing bulb is located right at the filter level, but I found that pushing it a couple inches up inside the plenum worked better, I imagine because our TT's are so small that there are cold drafts all around the A/C unit.

Anyways, yesterday I turned the unit on while it was 86F outside with the TT in full sun and it was 94F inside. The A/C unit kept a 20F differential between supply and return air temps until the TT got near set point, and then the delta T rose to about 22F. This is in line with proper operation. The compressor did not cycle off for the first 40 minutes and only cycled off once the TT was around 70-72F inside.

Another thing to note is that full cool on the knob corresponds to a return air temperature at the unit of 60-62F +/- 3F, due to drafts the unit will cycle off when our wall t-stat for the heater reads about 71-72F, but at the return of the A/C unit the air was around 65F, so I feel as though it's doing all that it can do at this point.