Checking AC with temp differentials.

I do it the same way . Write the split example 57/72 and give my recomendations example outside coil should be leveled and move on .

I recently inspected a home for a HVAC company owner. I was taking splits at registers and returns. He laughed and showed me the laser thermometer he had brought in his back pocket. He said he checks them the same way. I used to use thermometers at the air handler but found the closest register and return got right at the same results.

The deal with today’s was that it was a sellers inspection. He just had the AC and the furnace installed last week prior to my inspection. I got a temp differential of 12 degrees from one bedroom, and 9 from another (return in hall). The AC guy who did the install claimed that I did it in the wrong place, and that I should have done it over the coils. His diff was 18 degrees…

From what you guys are telling me, something else was up, and he’s blowing smoke to cover his but.

The only thing that counts is the coil temp. The supply air from the register will vary depending on the length of the duct as well as many other factors (like insulation). Don’t worry so much about temp splits between the supply and return because they vary. If you just start the unit the return temp will be much higher than it is after the unit has run for an hour. You want a 40 degree evap coil or something close to that after the unit has been operating for some time.

And where exactly would you take that 40 degree temperature at?

The 40° temperature that he is talking about is saturation temperature of the refrigerant within the coil. You need an oil filled well to take this temperature at the outlet of the evaporator coil or you can convert pressure to temperature (i.e. 70 psi G; R. 22 refrigerant is equivalent to about 40 degrees saturation).

None of this you’re going to do because you need a EPA certification to touch the refrigerant.

As for your other question, your temperature differential is supposed to be taken across the coil.

The reason that your measurement is not accurate is because you are gaining heat from the air duct system which passes through unconditioned spaces and the air gets warmer as it travels down the pipe due to ambient temperatures as well as friction.

And as always; taking these measurements with the dry bulb thermometer (or a laser thermometer which is even less accurate) provides you with nothing unless you know what the baseline operating conditions of that designed equipment was one it was initially installed.

You need a hygrometer and a psychrometric chart to even come close.

An air air-conditioners primary work is done on latent heat (cannot be measured with a dry bulb thermometer).

The dry bulb temperature will not fall until the proper amount of latent heat is removed first before this can happen.

So you have absolutely no idea how many BTUs per hour the equipment is removing because you cannot measure latent temperature with a dry bulb thermometer.

So how does a home inspector test the ac?

Turn on the heating system and let it run until everyone complains about how hot it is.

Then turn on the cooling system and let it run until everyone complains about how cold it is.

Obviously the system works, so when the new owner calls in six months, remind them about their complaints, tell them to call an HVAC tech, and move on.

I’ve only had one person call me in 11,000 inspections, and she didn’t move into the house until nine months later. She did a non-secured rentback to the previous owner. Stupid. But she learned.

Mark -

As a past HVAC Tech, taking the TD across the coil verifies for me that my equipment is doing (or not doing) what it should be.

Taking TD’s at RA and SA tell me if its getting out into the room where your customers LIVE. If I’ve only got 9-12 degree TD’s at my RA and SA sources thats as good as it gets for the people living there - It only goes downhill from there once it hits the room.

At 9-12 degrees TD at RA & SA, they’re losing the conditioned air between Point A & Point B. At this point, they can have a COMPETENT hvac tech try to improve their lot in life OR the homes duct system, etc.

As Dan points out, this test can be used as an indicator if the compressor is even running.

The problem is that all too often this process is used as a diagnostic tool to analyze the operation system.

Just this week I got a 70 page home inspection report that condemned three HVAC units because of Delta T. High relative humidity was in place and the Delta was low.

Delta can change with fan speed. Who’s to say from our end, what is right and what is wrong. System design may be to handle humidity more than temperature. We don’t know this. The equipment is still functioning as intended (or in some cases unintended), but it still is operating to full capacity though you’re not getting the proper Delta that you feel you should achieve.

The best way you can test an HVAC system is like what Russel pointed out. Turn it on till they scream!
I still use this RR recommendation (many years later), and every time they scream I smile and think about Russel Ray! :wink:

I have posted in the past how you can test the HVAC system without using any tools. All I got was grief about it. The preferred method is the SOS put out by home inspector diploma mills.

Many years ago I had a service manager tell all of us that we must take the Delta before we leave a service call and record it on the invoice. I told him that I would start tracking mud through the client’s living room to take those measurements when he explained to me how this useless process was going to be utilized. He couldn’t, so I didn’t. There are way too many variables involved and unless you fully understand all of the variables that affect the reading if you’re getting they are worthless to you.

I always had mud on my shoes because I was one of the few service technicians that took coil cleaner and a water hose out of the truck before anything else and clean the unit (when needed) so that I could properly analyze its operation under factory design conditions (clean).

Granted, a large majority of the units tested will have an adequate Delta , which is in fact an indication they are performing. But unfortunately you will find that all too many of the units that fail your test are actually performing and you are generating an unnecessary additional expense to the buyer and/or seller. These additional expenses are starting to filter back to the home inspectors when HVAC contractors find nothing wrong.

HVAC contractors are thanking you though!

Give me a call Mark. If I’m cruising the interstate we can discuss HVAC testing.

After reading about Delta T for nine years now, I still have absolutely no idea what it is.

I’m sticking with my tried, tested, and proven method.

Please do and teach those that think otherwise!

I did three energy audit/thermal evaluations today.

Everyone was “Hot under the collar”.

All three had HVAC equipment working their @ss off but could not perform.

All three had one or more HVAC professionals (alleged) there prior to my arrival.
They found nothing.

So much for deferral to those that should know!

This is the method that I use. I’m looking more for consistent temperatures coming out of the resisters. If I’m at one end of a house and all the resisters in that area are reading at 64-69 degrees and I find one at 74 degrees, I call it out for further evaluation.

To hear some tell it, if the Client complains because their Unit does not produce cold enough air at the registers, the customer just is not smart enough to know that, that is just how the system “Works”, and when fall rolls around they will be just fine :wink:

PS and the RR “Technique” is a Delta T, inside vs outside. :stuck_out_tongue:

I do exactly that.

And as well, I have only had one “question”–two months after the inspection.

“Do you remember what your mother said about it being so cold…”

I take the temperature difference. I also feel the suction line and liquid line.
Remember boys and girls the old beer can method . It is not our job to design or engineer. I also take the Outside unit in and out temperature .

Checking the temp at the coil is the most accurate way to see if the unit is functioning properly however homeowners do not live in the unit. If you are seeing differential temps consistently less than 12 degrees on a system that has been running for at least an hour there is something wrong. To all those coil readers only, go ahead and tell that buyer “well the coil temps were okay, I am sorry you can’t get the bedrooms under 85 degrees” let’s see what that gets you. Checking the input and outputs in the home is the best method for the buyer in my opinion. Maybe the lower number is because the duct runs are too long, well that is a problem isn’t it. I have tested thousands of systems and have two close relatives with over 30 years experience each in the HVAC business they agree with my procedures. I always check the temp differential in the home where the person lives. And I check every register I can get to.

Scott -

As my earlier post indicated you’re EXACTLY right on.