Check temp split with thermal?

So, I’m watching this video, I don’t know why. I just got sucked in. But this guy uses his camera to determine the temperature split on the furnace. He gets into the thermal part at 41.21. Maybe he knows something I don’t.

I only lasted about 15 seconds ;-):frowning:

It’s no different than using a laser thermometer. Although not accurate, schools teach this method.

Seriously? Why would they teach this?

Because they don’t know any better:shock:

It’s a rule of thumb… and thumbs get hit with hammers.
Especially of they’re in the wrong place at the wrong time… :wink:

No part of that method is accurate for testing HVAC.
Try that on my house and you better have that buy back stuff available!

Never said it was. Just mentioned that the schools teach something inaccurate :slight_smile:

Frank, what is your tool of choice to determine the temperature difference?

I’m going to be a little bit contrarian here and say that you “can” get a fairly accurate temperature of the air moving through the duct if you use the right technique.

Flex ducts have a thin inner membrane with air moving over it rapidly and insulation on the back side, so the temperature of the membrane is going to closely reflect the temperature of the air moving across it (well within the accuracy specification of the device being used by any inspector to measure it). The next challenge is to accurately measure the temperature of the duct lining itself (since an IR imager or point radiometer does not measure air temp, we need to measure duct temp). The reading must be taken close (well within the spot size ratio of the device being used) and should be taken through the register (parallel to the vanes of the louvers) and into the duct to minimize their cross section and influence on the reading. By doing so, the inspector will be taking their reading from well inside the duct (the focus needs to be at the back of the duct). The inside of the duct, itself is a cavity. Cavities have minimal reflectance from outside sources so the majority of incident radiation will be from the duct lining itself, with a small amount coming from the register (the cross-section of the vanes). By doing this, you can get a reasonable (within the tolerance of the device itself) measurement of the air temp coming through the duct.

Does this make it an appropriate way to calculate Delta-T? No. It’s too far from the source and ducts won’t all be consistent temps anyway. I do it with temp probes as close to the coil as possible at the unit (Dave will likely tell you that you can’t draw meaningful conclusions from measuring temp differential alone, anyway, unless you take into account latent heat, at a minimum).

I don’t do it. If I were to do it, it would be with probes as Chuck stated.

Good post Chuck.
I tell clients it is a rough temp split.

Just use a old fashion thermometer Use the kiss method . How ever i do use it for locating outlets . found several covered over in new homes and one covered by a cabinet .

I can’t think of a good reason to do a “rough temp split”. If you’re going to do one, it should be accurate.

Hay Chuck, consider this…

What do you see when you put a plastic bag over someone’s head and scan them with IR?

Hint: I do this for woman who don’t want to bare their breasts during a “boobie scan”.

Consider the transmissivity of the membrane.

You are not measuring the temp of the membrane are you?

Just thinking out loud. We both agree this is only a “The unit is running or not in the heating/cooling mode and the duct is connected along the way to that room”.

The biggest error factor has to do with conductivity of the R-6 insulation of the duct.

Answer to the best tool question: A thermistor is the fastest and best tool to measure fluids (yes, air is a fluid).