Thread: How do you check an ac unit?

Originally Posted by Vivian Deaton

I just purchased a Lennox XC17 air conditioning unit (and heating unit also). Anyway, the air conditioning doesn't even cool my upstairs as well as my old unit. The unit is twice the size, and they made some mighty big guarantees on satisfaction. They came once and adjusted the guages and spoke of 100's of different setting changes to make this right. Whatever he did was meaningless and changed nothing. Next they came and added a return air vent in the basement out in the hallway from the unit. This really made no impact either. There could be a slight change but nothing notable. I paid 12,000+ for this unit. I've made it clear that I am not really interested in energy saving...my energy bills are quite nice. I wanted a unit like they described...that would cool evenly and continually, and circulate the air continually. Shouldn't it be cooling my upstairs as well?? What could be the problem?

Vivian - I'm not an expert, HVAC tech or even home inspector, but I am a homeowner who has recently done a lot of learning in preparation for my own furnace replacement, with similar concerns as yours (properly conditioning a finished attic space).

What I have learned is this: especially with modern, high-efficiency furnaces (and extra-especially for AC), perhaps the most important factor is sufficient airflow to the various living spaces. Many (perhaps most) homes, especially older ones, just do not have sufficient duct capacity, especially on the return side, to work optimally. For example, if your upstairs has no (or not enough) return air from the upstairs, it may never be able to really cool upstairs well.

Another thing I've learned is that many (perhaps most) HVAC techs are git-er-done hacks who do not really understand the science behind their trade. Sadly I felt like I knew more about building science than most of the HVAC techs I had out to bid on my furnace.

Because of this lack of understanding, it's unfortunately very common to grossly oversize systems (especially AC), which leads to a myriad of problems. Obviously I'm just guessing from here but if your original problem was lack of sufficient airflow to the upstairs then doubling the capacity of the AC would make the problem worse, not better, because (as Tom mentioned above) the unit will run for less time before it satisfies the thermostat (which is, presumably, downstairs), resulting in less cooling upstairs as well as less dehumidification.

I apologize if I'm on a rant here, but I wanted to share my experience. Unfortunately, you may be about to learn exactly what that satisfaction guarantee is worth... in my experience those are often a red flag for a lot of hot air. (no pun intended!)

All that being said (and no offense to the very respected and knowledgeable folks collected here), this probably isn't the best place to ask for advice. Please allow me to suggest that you go to the forums at HVAC-Talk: Heating, Air & Refrigeration Discussion which is a forum for HVAC pros and where I learned a great deal.

I would never hire or recommend an inspector who doesn't do TD and include the numbers in the report. That's why most home inspections are a joke......the inspectors don't do anything that an untrained person could do themselves and likely do a better job.

Originally Posted by James Duffin

I agree that 18 degrees is ideal and 8-10 is marginal but that is my cut off for repair. Some older system are not capable of 18 degrees so using 18 degrees would mean replacing lots of old systems that are still working to some degree and are performing their intended function albeit not as good as a newer system would. And that is why I agree with Scott that you don't put TD in the report. I'm sure you disagree!

- - - Updated - - -

I would never hire or recommend an inspector who doesn't do TD and include the numbers in the report. That's why most home inspections are a joke......the inspectors don't do anything that an untrained person could do themselves and likely do a better job.

Originally Posted by James Duffin

I agree that 18 degrees is ideal and 8-10 is marginal but that is my cut off for repair. Some older system are not capable of 18 degrees so using 18 degrees would mean replacing lots of old systems that are still working to some degree and are performing their intended function albeit not as good as a newer system would. And that is why I agree with Scott that you don't put TD in the report. I'm sure you disagree!

[QUOTE=Brian C;244718]I would never hire or recommend an inspector who doesn't do TD and include the numbers in the report. That's why most home inspections are a joke......the inspectors don't do anything that an untrained person could do themselves and likely do a better job.

TD are useful and important but should NOT be relied on as a pass or fail. The prevailing wisdom among the uninformed and HI schools is to check the TD, report it and move on. This is a short sighted view which leaves many other aspects undone. I can show you a half dozen scenarios with "proper" TD readings but the units are NOT operating properly. A more detailed inspection is needed and chances are the inspector that includes the TD in his report has done little more than shoot the TD just to include it in the report. I have formal HVAC training and worked in the industry for many years and I do NOT include the TD in the report but rare is the system that I find that has no problems.
If I include the TD in the report it just gets in the way of the fact that service is needed by giving fodder to the arm chair quarterbacks who argue that the system works just fine based on the TD and no service is needed or argue that the inspector took the TD at the wrong location, the wrong time of day, etc.
"I" would never base a hiring or referral recommendation on something so trivial as TD splits appearing in a report. To do so is a joke.

Originally Posted by Brian C

I would never hire or recommend an inspector who doesn't do TD and include the numbers in the report. That's why most home inspections are a joke......the inspectors don't do anything that an untrained person could do themselves and likely do a better job.

Originally Posted by Jim Luttrall

TD are useful and important but should NOT be relied on as a pass or fail. The prevailing wisdom among the uninformed and HI schools is to check the TD, report it and move on. This is a short sighted view which leaves many other aspects undone. I can show you a half dozen scenarios with "proper" TD readings but the units are NOT operating properly. A more detailed inspection is needed and chances are the inspector that includes the TD in his report has done little more than shoot the TD just to include it in the report. I have formal HVAC training and worked in the industry for many years and I do NOT include the TD in the report but rare is the system that I find that has no problems.
If I include the TD in the report it just gets in the way of the fact that service is needed by giving fodder to the arm chair quarterbacks who argue that the system works just fine based on the TD and no service is needed or argue that the inspector took the TD at the wrong location, the wrong time of day, etc.
"I" would never base a hiring or referral recommendation on something so trivial as TD splits appearing in a report. To do so is a joke.

I do not even call a TD taken the way it is taken for home inspections as being useful.

A TD is not really indicative of any other than the temperature of the air - and TD taken with an IR thermometer is only giving the temperature of the surfaces within circle size being averaged into the reading shown on the IR readout.

Someone saying that taking a TD would be a basis to recommend an inspector or not quite possibly does not have a clue to what is going on and makes their other actions suspect and less than professional.