Measuring AC temperature drop - Montana Owners Club

sreigle · 06-26-2005, 10:56 AM

Some of you have mentioned you measured AC output air temperature and found the temperature xx degrees less than the ambient outside temperature. Our AC is not cooling this summer as well as it has in the past. We're sitting here with outside temperature at 92 with 42% humidity, according to the weather website. We have full sunlight, no shade until late evening. The thermostat is at 75 and the temperature holds at 75 until mid afternoon. Inside temperature is currently 81 degrees, AC running fulltime, ceiling fan and two floor fans keeping it tolerable in here. Day/night shades are on night to minimize sun heat intrusion.

1. How do you measure the AC output temperature? Do you use a thermometer directly in the airflow as it comes out of the AC unit itself? Or out of one of the vents farthest from the AC unit? Or do you stick a thermometer into one of the outlets?

2. What is considered an acceptable temperature difference between outside temperature and the measured temperature from above?

3. This unit is just short of three years old, by a few weeks. Is it normal to lose some of the charge or are we likely looking at a leak?

4. I climbed the roof. The fins are clean and not bent. Do I need to pull the cover and clean it out inside?

Thoughts would be appreciated.
Thanks.

trukdoc · 06-26-2005, 11:48 AM

Have not had a problem with Rosie yet, heck she is not a year old yet. Our motorhome was an 89. I took the outside cover off and found dirt dober nests a build up of trash leaves and dirt. Also take off the cover on the inside and make sure the fins are clean there as well. Outside of that it is an issue of how many available BTU's you have to cool vs: how many BTU's are beating on the Monty from ol Sol.

Broome101 · 06-26-2005, 12:13 PM

Steve, As you recall I had several issues such as this. when I picked up our unit before leaving for Myrtle Beach, SC they showed me how to check AC temp output. They out digital thermistor on a front , mid, Adan back AC duct and measured temp at each one. the stat was set for 70 degrees it was about 72 degrees inside when I looked at it , outside temp if I remember correct was about 89-90 degrees humidity around 72%. The temp the AC was putting out was 46 degrees which they said is great. They said they had done nothing to it but the day before that it was putting out hot air and could not the the coach below 83 degrees inside with everything going full blast. So not to take a chance on vacation I installed a 2nd AC which worked perfect. We keep the inside temp about 690 degrees through the day at the beach with both running at the same time.
Dealer told me that diff between outside and AC out put should be about 30 degrees or better anything better than that is great.
Also I was told that the AC units are sealed units and noway to check freon levels.

padredw · 06-26-2005, 02:46 PM

Perhaps I should start another thread, but the answers seem pretty complete to Steve's question, and I have a related question which may fit in here. On our recent trip last week, we noticed a slight 'rattling' noise when our fan was on low speed. We usually run the AC at the high speed to avoid 'freeze up' and to cover up outside noises. But a couple of times in the unusually cool days we had "up north" we ran the fan alone in low speed. That's when we heard the noise. Sounds like a bearing may be going bad or need lubrication. Is there anything I can do about that? I surely would like to avoid expensive repair or replacement. If anyone has experience or knowledge along these lines, I hope you will respond to this topic.

Thanks.

Charlie · 06-26-2005, 03:12 PM

Steve-
I have been told by A/C techs that on the stick house duct temp should run about 50 degrees. I just came in from doing a little test on the Monty. The A/C has not been on for a couple of weeks and I turned it on. Temp inside was 90 degrees and in less than 15 minutes the temp on the duct in the bedroom was 56 degrees, this measured with a mercury themometer. This should give you some idea of your A/C's performance.

DHenry · 06-26-2005, 06:00 PM

Steve, you shoud get a 15-20 degree split from your supply air to your return air temperatures. If you get this reading you would know that your A/C is working properly. So stick your thermometer in the supply register, read it, then stick it at the return air inlet, check the difference. Also have you cleaned the air filter at the return side? A dirty air filter is the worst thing you can have in the system. The A/C system is a sealed system so you should not loose coolant. Typically under the best circumstances you should expect a 20-25 degree difference from the outside temp to the inside temp. You can get better than that if you put the insulating materials in the windows and have fans running. Keep the night shades down also. Hope this information helps.

mazeeff · 06-27-2005, 03:18 AM

Doug has it right. When we refer to "ambient", we mean "inside ambient". What I normally do is use a wireless temperture gauge set, that you can get at wallmart for $20. It has a wall unit, and a remote. You can mount the wall unit at mid point, and this will give you a good inside ambient read. The remote can then be placed up in the outlet grate to measure A/C outlet temperture. You should see 20-30 degrees of differential, when the ambient is 70-80 degrees. You can also use the remote to measure freezer temp or outside temp as desired. I have done a lot of measuring on outside ambient to inside ambient. With two A/C units running inside the trailer, I can maintain 30 degrees of differential. 100 outside and 70 inside. With only one 15K main unit running, I can maintain 20 degrees of differential. 100 outside and 80 inside.

sreigle · 06-27-2005, 05:31 AM

Thanks to all for the great replies. I guess I need to get a thermometer. Our human temperature thermometer is a digital so that might not work for this. Vicki went to Topeka today so I am without transportation to do this today. However, I've added to my list of items to get done today in prep for leaving next week to pull the AC cover and check for debris, mud daubers, etc. Haven't seen any mud daubers around here so hopefully they won't be in there. They are not among my favorite live things to deal with. I suppose I should wait until evening to check this so I can turn off the AC. Vicki doesn't want me on the roof when she's not here (I guess she doesn't want to miss the crash as I hit the ground) but I may do it anyhow. As hot as it is I'll be beat by the time I get the other things done outside today so will probably wait until evening.

Thanks again. This unit used to do a really good job of keeping us cool so hopefully the problem is simple and will clear it up.

Padre, sorry, but I have no answers for you. Maybe someone else will come along with an answer. Or maybe a separate topic will be more visible?

Montana_2785 · 06-27-2005, 05:47 AM

Just a data point.

We had a full sun site a week ago. Our 3575RL is 36.5 feet long with three slides that have slide toppers. On a 95+ degree day our 15,000BTU A/C unit would only maintain a 15 degree difference with the outside. The interior differential (A/C intake vs output of vent) was almost 25 degrees so it was working just fine. As soon as a cloud would hide the sun, the interior temperature would drop a couple of degrees in just a few minutes.

The sun supplies a lot of BTU's!!!! I would think that in a shady location our A/C would likely maintain a 20-25 degree difference with the outside.

Eric

mazeeff · 06-27-2005, 05:59 AM

Quote:

quote:Originally posted by sreigle

Vicki doesn't want me on the roof when she's not here (I guess she doesn't want to miss the crash as I hit the ground) but I may do it anyhow. As hot as it is I'll be beat by the time I get the other things done outside today so will probably wait until evening.

Steve. Since Vicki does not allow you to do anything fun while she is away, the remote thermostat may provide some cheap safe entertainment. I bought three remotes with mine, and I can sit for hours watching it cycle through the outside, inside, freezer, and A/C outlet!

sreigle · 06-27-2005, 06:35 AM

Wow, Mike. You have such an exciting life! I'd have to walk several miles to find a remote thermostat, though, since Vicki took the truck with her today. We're near downtown Independence so I may just walk down there and see what I can find.

Eric, yours sounds like what ours is doing, except I haven't measured temp drop yet. Vicki says in our prior stays here we were in a site with larger trees to the rear but that doesn't account for the afternoon before the sun reaches the trees. You're right the sun really heats the place up, though. But this unit used to hold it nice and cool in here. I'm about done reading through MOC and will go outside and enjoy the heat and humidity for awhile. I'm sure it will feel really nice in here when I return. Right now it's only 87 (humidity at 57%)outside so the AC is holding at the 75 degree thermostat setting. I'm waiting to see if having the fan on constantly will make much difference as it warms up.

mazeeff · 06-28-2005, 02:53 AM

One way to get the trailer a little cooler, is to NOT use the "quick cool" function. When you use the quickcool function the cold outlet air is drawn back into the inlet, since they are right next to eachother. This is not very effiecent, due to the fact that you want the warm air being drawn into the return. I have found that closing the quickcool vents and routing the cold air to the normal vents, yields another 2-3 degrees of differential.

Dustytuu · 06-28-2005, 03:13 AM

That is a great idea to use the thermometer remote to check temperatures! I never thought of that! That is one of the first things Don bought when we bought the Monty.

mazeeff · 06-28-2005, 05:00 AM

Quote:

quote:Originally posted by Dustytuu

That is a great idea to use the thermometer remote to check temperatures! I never thought of that! That is one of the first things Don bought when we bought the Monty.

Dusty. What brand of W/D is that in your photo album. Looks like a tight fit. Does the door close?

sreigle · 06-29-2005, 04:38 PM

Mike, I know you're right but standing right under the Quick Cool sure feels good sometimes!

Today it hit 97 outside, very humid, we kept the fan on On all day. Thermostat at 75. It got up to 79 in here, which is better than it did before. I pulled the cover off the AC on the roof today. It was not nearly as dirty as I expected. Just a light coating of dirt on the inside covers and a little dust but not much. No mud daubers. All fins are clean and straight. So no problem there.

We leave here Monday and by mid-July will reach our destination, which had a high today of 69. Looks like they're going to have a heat wave the next couple of days, though. They're predicting 77. I'm ready.

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