Help with Tonnage

Well this is real interesting. They give you a graph to adjust with Rh. If you have the tool to determine Rh, why are you taking Sensible Heat Delta-T with a thermometer?

Another thought; Air ducts leak about 30% on a good day. Might this effect the Delta-T when taken at a supply and return register?

I am not saying you can’t use a Delta-T to determine if the unit is heating or cooling, and that 'way out of range readings do not indicate a problem. But I see all the time that Delta-T’s are fine and get called out all the time, costing everyone $$$.

I was hoping to give perspective of what is involved and should be considered before making a call like this.

Mr. Non-Member, it is in fact a science. Plumbing, Electrical, Structural, it’s all is dictated by science. And HVAC is a science that is often the reverse of what is perceived through ‘common sense’.

Feel free to present and discuss what you disagree with. But beware, someone may call you an egotistical guru if your right!


David, AHRI (p. 665) calls for checking the return air system for leaks—with an allowable max of 2 deg between return air at the grill and that entering the coil—but isn’t this something the tech would discover after the inspector recommends servicing based on Delta T? The tech would then probably make a straightforward recommendation of insulating or sealing the return. Not trying to be argumentative, just arguing that HI’s can easily measure and report Delta T, provided they can also measure for RH and adjust accordingly. If, for example, the Delta T is 5 — 10 degrees, the system isn’t cooling as it should.

This is my point: 100%
The key is that no one here ever talks about Rh. No one knows how to read a psychrometric chart. Besides, why guess when you have the IR option?

It has been my experience in all my years that the tech will never test for duct leakage. They are concerned about equipment operation. If the refrigerant pressures are right, air flow is sufficient, they will tell the customer it’s working fine. When was the last time you saw an HVAC Tech pull out a duct blaster, or recommend Aeroseal?


I find this 80% of the time.
Do you know the cause?
You can spend all day trying to seal the return plenum and duct and not fix this. If you do not remove the filter register and seal the sheet rock where it connects to the rafter or joist it will leak behind the register and no about of duct seal or caulk will stop it.

If you can not read IR scans properly, this indicates two types of leakage that require two types of mitigation. One is ceiling construction and one is duct leakage.

Yep! That’s the way it works in the Great White North Take off. …It’s a beauty way to go!
I inspected a home not long ago where 8 tons, 2 - 4 ton was used in 2,700 sq ft. It was not effective. The owner was head strong.

Joseph, David is one smart cookie. I learn from him often. :smile: