A/C air temps

[Jimalberta]

It was obviously a failed attempt at a bit of humor. If I offended anyone I apologise.

 

I do find it Interesting that no one messaged me.....if I am offending anyone I want to know about it not be talked about behind my back.

 

Cheers

[RangeMaggotBob]

Don't have an IR thermometer. Don't even have a digital stat. If it's a hundred degrees out, I can turn on both ACs and in about an hour, it's down in the 70s. Extremely comfortable. 13.5 unit in the bedroom and 15 in the living room.

[TXiceman]

I really get a chuckle from the volume of misinformation from the internet on something as simple as an A/C unit. I spent my working career designing, building and working on large industrial refrigeration systems. My education was in mechanical engineering with a specialization in applied heat transfer and thermodynamics. I did work with some small A/C system, but tended to stay away from these systems. The concepts are the same, just a smaller scale

 

These are concepts that you cannot touch and feel like you can a bridge structure and simply are not comprehended by a lot of people, even engineers.

 

The 20 degF "rule" is simply an estimate or guide. A lot of things can influence the temperature difference, so these are just a starting point to see if the unit is operating anywhere near where it should. For the most part, if you are operating at a 20 degF delta on the evaporator, plus or minus a couple of degrees, you are doing about all you can do.

 

Things that can influence the temperature difference are:

-Mechanical issues with the motor or compressor.

-dirty evaporator or dirty filters.

-internally bypassing air from the outlet to the inlet.

-low on refrigerant (not seen very often as these are hermetic sealed units)

 

The other thing that has an effect on the readings is the dew point of the air (relative humidity).

 

The design engineer for the A/C unit can control the coil delta temperature and the air off temperature by the design of the coil and the evaporator temperature. To match the capacity of a compressor, the evaporator and condenser are all balanced to provide a given capacity at a given set of air temperatures (inside and outside). The design of the coil is influenced by the number of rows deep, the fin spacing and fin design, the coil circuiting and the airflow.

 

The cooling load on the coil in based on two loads. One is sensible cooling which is the actual reduction of the air temperature. The other part is the latent cooling which is not a temperature change, but the removal of the moisture from the air.

 

In a high humidity area, the latent load is the largest part of the cooling load, so you will see less actual drop in temperature across the evaporator. With less humidity to deal with, the latent load is lower and there will be a larger sensible temperature change.

 

So, with so many variables, you can see that 20 degF is not a fixed number, but for comfort cooling, it is a guide or starting point. With this temperature reading, it is a starting point for additional diagnostics by a person that has to have a basic understanding of the system design and theory.

 

I was luck enough to get to spend part of my career in the field working with the technicians in trouble shooting problem jobs.

 

As an aside, one photo presented above showing the two temperatures, your inlet filters are dirty and need to be cleaned.

 

Keep cool folks.

 

Ken

[hemsteadc]

I really get a chuckle from the volume of misinformation from the internet on something as simple as an A/C unit.

Such as..?

[TXiceman]

Such as..?

Just too may to take the time to list.

 

Ken

[AFchap]

I have a question for the career air-conditioner techs. Type of unit in question is rooftop package type, dual compressor unit (the Winnebago basement mounted unit). The question is simply, why two conpressors? If one compressor brings output air temp to the desired role of thumb of 20 degrees below return air temperature, why add a second compressor? What does it add to the performance? The second compressor runs only when the called for temp is 3 or more degrees below ambient temp. There is no additional blower, so no added air flow. What can/does the addition of the second compressor add to the performance of the unit?

[Ray,IN]

For the Coleman-Mach (Airxcel) basement unit, the installation instructions explain how supply and return air duct size affects efficiency and temperature differential. The instructions also explain why this unit goes through so many filters so quickly.

If ours ever reaches the point of "beyond repair" I will replace it with a new unit because it cools and heats our MH very well.

 

I expect most of the explanations also apply to a ducted roof-mount unit.