Air Conditioner Performance

bsheet2 wrote:

Kamphiker wrote:
As far as testing the AC units:

Measuring Intake Air Temp And output air temp (at closest air registrar if you cant measure in the main Plenum) you should see 15 - 20° differential.

Thanks for the replies everyone.

I do want to comment on the 15-20 degree temperature differential though. I am an engineer, sorry I can't help it. I have heard this temperature differential quoted a few times. However, I do not think this is accurate. I think we should be seeing 18 to 20 degrees temp differential -- minimum. This is an educated guess right now based on a knowledge of how home AC is designed. I have been trying to find design info about our RV AC units but have not laid my hands on any yet. But I think most AC units are designed to achieve a 20 degree temp differential. Then they allow for a 10% (or 2 degree) loss of cooling due to ducting or other losses. I am certain this is the basis for a home AC.

Background - if you want it.
Several years ago when we purchased a new home the AC was not working well at all. After a saga with the contractor he finally sent a consultant over to take a look. The consultant was a retired lead designer that had worked for Carrier. Everything changed as soon as he walked onto the site. I learned the basis of how the AC system is designed. It should achieve a delta T of 20 degrees measured immediately after the blower fan before going into the ducts. They then allow for the air to gain 2 degrees of heat in the ducts running through the attic. So, you should see at least 18 degrees delta T at each and every duct outlet. If not, something is wrong.

The issues we found in my house:
- The attic was too hot. Not enough ventilation.
- Not enough flow in some ducts. Slow flow allows more heat input from the attic and not enough volume of cool air in that room. Larger diameter ducts needed.
- Some main ducts were very long. Long length allows extra heat into the duct. Larger diameter and double insulation was the cure.

By the end of this saga I learned a lot about AC system design and what we should be seeing to meet what it is designed for.

The question is, is the design basis the same for RV AC's??

RV roof top systems are all sealed systems and ALL models of the same model number have the exact same performance. NO variables except for correct install. This is from the Service manual of RVP(Coleman). Doug
•An ideal cooling system should give you an output temperature range of 16-22 degrees lower than the temperature taken in at the filter. Humidity is a significant determining factor in this temperature difference.

PS, you cannot take a residential or commercial design and install and relate it to a RV roof top AC. Totally different design and charging aspects. Residential systems vary in size of the Evap and outside tonnage of condenser. Then you must charge with the Supercool method as there is NO exact point of lbs of coolant like on a RV AC unit. As you found out, having a knowledgable installation of any residential is critical to optimum performance.