Variable Speed

How much should the variable speed air handlers effect the temperature split? I had two Goodman systems yesterday that are relatively new in a very well maintained home. The 2 year old system gave a 13 degree split and the 4 year old system 11 degrees. They’re properly sized, no duct issues, newer home, and the house was cold. They had it set to 72 when I got there. Any input? I’ve struggled with citing clear deficiencies on variable speed units wondering if the old rules are now defunct.

So the unit is cooling.

What else do you want to know?

I’ve always relied on the 14 to 22 degree split to tell me the system is operating correctly and it’s served me well. It’s even what the NACHI training says is a good rule of thumb. Saved many customers money too. If it’s outside those numbers I suggest they have it evaluated and every time I get feedback it is to tell me it was in fact low or high on refrigerant or some other component did in fact fail. I just wasn’t sure how much the variable speed systems can change that reading or if they do at all.

Shouldn’t make much of a difference. The main advantage with variable speed it allows dehumidification to take place without hitting the set point temperature too quickly.

That’s what I always thought. I just wasn’t sure if it was similar to the heating mode… If you set the thermostat 2 degrees the system will use the heat pump only, 4 or 5 degrees the heat strips will kick in, etc. I know the variable speed is the same heat pump but wasn’t sure if the speed change would defunct the split altogether, especially based on settings. If anything I would have thought the lower speed would increase the split… same compressor with a slower blower would allow the evaporator coil to get colder?

The old “rules” are just rough rules-of-thumb, not cast in stone, and are largely anecdotal.


Thank you all for the replies. Dominic, I see you are close by. Do you have a rule or it’s ok if it’s cold?

How can you say that it will not make a difference when your changing the sensible heat ratio with the fan speed change?

This guy is measuring with a thermometer (which is useless regardless).
You may not get a capacity change, but you will get a dry bulb temperature change.

The bypass factor changes, so the output readings will also change.

Matt, if you want a real answer to your question; quit trying to analyze the HVAC with a thermometer. The only thing you can tell is if it is running (yes or no).

You say the duct is designed correctly.
If you have the equipment to determine this, why are you measuring the dry bulb temperature and using temperature splits?

I wasn’t saying the ducts are accurately sized. I meant the systems are sized for the home well, and that I didn’t find any duct issues like damage or leaks.


Well how were we to know what you actually meant?:roll::roll:

Ok, I’ll rephrase the question…

You say the systems are designed correctly for the house.
If you have the Expertise to determine this, why are you measuring the dry bulb temperature and using temperature splits?