Basement Air Vent to return temperature difference

[azloafer]

I took a temperature measurement at the first air vent in the bedroom and a measurement at the return. The difference was exactly 10 degrees. I did this at two different times of the day with the same result...a 10 degree difference. Is this reading acceptable? Is there any real rule that a person should go by? It seems to be working fine, but I think that I may have read a post somewhere that said there is a specific temperature difference reading that indicates "all is well." Please help if you are sure of a rule for basement air in Winnebago's. Thanks, Joe

[Bug512]

10 degree differential sounds low..

Read this documentation on RVP's website. Read through it, it will explain the procedure.

Click here

[azloafer]

Originally Posted by Bug512

10 degree differential sounds low..

Read this documentation on RVP's website. Read through it, it will explain the procedure.

Click here

I'll have to run the test again because the unit was on low cool. Joe

[AFChap]

10 is definitely not good. But what I read in the above reference is talking about a roof unit with a single compressor. That is not what we have in the basement.

I have seen a 20 degree differential quoted before as "normal" and "good". HOWEVER, I have checked my basement unit several times with an infrared thermometer. I check air temp at the return, then I point the infrered into a ceiling vent, not hitting the vent but going inside to the styrofoam lining, and mine is consistently 20 different with one compressor running, and right at 40 different with both compressors running. That 40 degree differential is true regardless of outside temp ...even 100+. I get the same differentials in heat mode, unless outside temps are close to the unit lock-out point normally in high 30's. When it is several degrees or more less than 40, I know it is time to retape the duct. My problem area has been where the duct out of the unit turns upward to go up inside the rear cap.

I believe if you are content with only a 20 degree differential in your basement unit, 1) your unit has a duct leak or other significant problem, and 2) you won't be happy with it's cooling ability.

[McDuff]

AFChap - you must have gotten the only "wednesday" unit ever built, on a monday,wednesday,friday work schedule.

I haven't measured my new unit yet, my old one was ~17-18 before it failed.

Since you have my identical coach; where do you hit the return air? Do you hit the return grate at the end of the bed? Shoot thru the grate into the space under bed?

How far do you hold the IR unit from target?

I will try and recreate your exact target scenario and see what I get. But I am sure it is nowhere near a 40 diff.

[Bug512]

Please do not use a inferred thermometer. The readings con be incorrect because they read surface temperature, not air temperature.

My biggest problem is that they call the unit a two ton unit. It runs at 600 CFM, that is only 1.5 tons.

[AFChap]

where do you hit the return air? Do you hit the return grate at the end of the bed? Shoot thru the grate into the space under bed? How far do you hold the IR unit from target?

I point it through the grate and hold it until the temp stabilizes, then move it slightly and hold again to see if it changes any, and compare the reading to the thermostat room temp reading, which is normally within 1-2 degrees, so that seems to be a fairly accurate reading. Then I hold the infrared thermometer right at the ceiling vent toward one side so it shoots inside the air duct, and as much as possible pointing toward the ac unit (rather than directly up at the ceiling, or at the side of the duct). Again, I hold it there until the reading stabilizes, which can take 15 seconds or so. Then I move it slightly to see if the reading changes. Sometimes it will go down or up a degree or two. I have compared readings at vents close to the rear where the ac unit is, and right above the driver seat. The readings are normally the same or very close ...the front vent might be 1-2 degrees warmer than the rear one.

I did a check just a bit ago where we are here NW of Saginaw TX. Outdoor thermometer read 83 at the time. With one compressor running mainting room temp of 75 the air at the intake read 73. The vent over the dirver seat read 54, right at 20 difference. Then I lowered the thermostat to 69 so the second compressor would kick in and waited a couple of minutes. Then again holding the infrared thermometer pointing into the ceiling vent until it stablized, I eventually got a reading of 39. That's apx at 35 degree difference. I think the lowest output air temp I have ever seen was 38 when the input air/room temp was 78 on a 100 degree day. I cannot imagine the ac being able to put out air close to freezing temps, so 38-39 may be as low as it can go.

If one compressor can do a 20 degree difference, it only makes sense that two compressors can do considerably better, no?

Please do not use a inferred thermometer. The readings con be incorrect because they read surface temperature, not air temperature.

Yes, infrared thermometers do read surface temp if pointed at a solid object. But we have to work with what we have, and infrared appears to be the best I have found for the job with the equipment I have. Seems to me in this case any inaccurate temp read by reading a hard surface temp would be a hotter than air temp rather than colder ...I certainly don't see how the surface temp on any part of the vent/air duct/etc could be colder than the air temp. That is why I slightly re-aim the infrared and see if it changes slightly, to see if I am hitting a difference surface temp. The inside of the air ducts is styrofoam, but if I AM reading a cold surface temp, I know for sure the ac unit made it that cold! How do you suggest reading air temp? I have tried a meat thermometer probe and found it didn't do much at all. I suppose we could insert a small medical thermometer into the air vent, make sure it is not touching any surface(?) and wait 10 minutes or so for it to stabilze...

[wa8yxm]

The answer is: You did not take enough temp readings.

Takes six or 7 readings

Inlet (return air) temp, wet bulb, dry bulb..
Outlet (cold air) Wet and dry
Outside ambient (inlet of condenser) Wet/dry
Outside, condenser outlet, Dry only.

Then you get the charts from the Air Conditioner maker and you decide.

IF the basement compartment the unit is mounted in is full sun and 120 + degrees ambient (Condenser inlet) then the difference inside will be.....reduced.

[azloafer]

A 40 degree difference is unlikely. The info provided by Bug512 came right from the company that makes the basement unit. *The company states that with the unit on high cool and tested only after running at least 30 minutes, the difference in temperature between the return and the nearest air outlet should show a difference [with a digital thermometer] of 16 to 22 degrees if the unit is operating properly. Since mine showed only 10 degrees difference, I will consult with the RV service department. I'll post the results when I get them. Joe

*For more info check out the link Bug512 provided in his post.

[Bug512]

Being that I am in the HVAC buisness I have a couple different measurment devices including Fluke 80 Series, a Fieldpiece duel temperature meter and other clamp on thermocouples.

You can accomplish the same thing with a indoor / outdoor thermometer. You can find one at a radio shack / lowes / home depot for under $20
This is also good item to keep in the toolbox.


First to check it's accuracy, when you purchase one put a fresh battery in it, place it in a area away from sunlight and vents / moving air and keep the remote sensing bulb close to the unit (indoor sensor) the reading should be within 1 degree or less.

After that place the unit (indoor part) on top and center of the air filter, place the remote sensing bulb (outdoor part) in the vent closest to the rear of the coach (or closest to the unit). Open all the diffusers (vents).

Let the unit run for at least 30 min and log your temperature differential.

Make sure your condenser and evaporator are clean along with a clean air filter.

Good luck with your readings.

Didn't want to upset anyone regarding the use of inferred thermometers, it's just their reading can be very screwy. We do use them to "shoot" pipes and diffusers to see if we have temperature differences when we turn on reheat coils. Very handy for that.

Let us know what your reading are.

[Bug512]

Azloafer, can you confirm both compressors were running ?

You might want to give the manufacture a call. I'm sure their technical department can help you out.

[azloafer]

Originally Posted by Bug512

Azloafer, can you confirm both compressors were running ?

You might want to give the manufacture a call. I'm sure their technical department can help you out.

Yes, they are both on when you set the thermostat to call for a lot of cool. The way to test it is to let the AC run while calling for just a few degrees and then set it to call for ten degrees or so. You MUST be running it with the generator on for this to show the amp difference between one compressor and two at the OnePlace Panel in Winnies. I don't recall the amp reading now, but it does show a difference. Thanks, Joe

[TXiceman]

The return air temp and the leaving air temp should have a difference of 18 to 20 deg F with the unit operating on high fan and both compressor running. Let the unit run for at least 10 to 15 minute and not cycle off. If necessary, leave some windows and door open to keep a good load on the unit.

First thing to do is start looking for air duct or return air leaks or internal air bypassing.

Ken

[azloafer]

The service center that I use said 20 degrees is correct. They will examine the unit and report what they find. I'll post the results when I get it. Joe

[Bug512]

Thanks Joe.. Let us know what they find.

[azloafer]

Turned out to be an intermittent wiring malfunction causing the fan to NOT be on high cool all the time. Fixed-no charge.

[Bug512]

Great!

Did you find out the differential ?

[azloafer]

Originally Posted by Bug512

Great!

Did you find out the differential ?

When fixed it was 25! Now with the supplemental roof air, we had installed last summer, we are all set for this weekends high temps! Thanks, Joe

[Bug512]

Great information to have.

Glad everything is in working order for the summer months...