Gas pack exhaust temperature

[ramdino]

Looked at a new gas pack today. I typically don't check the exhaust temperature because usually the plate is so faded it can't be read but this was a new pack so just for grins I checked it. The plate said max output was like 186 for the exhaust, but my ir said I was getting 264. Thoughts? Does 186 seem low or 264 seem high?

[David Meiland]

What IR are you using, what were you pointing it at, and specifically *where* in relation to the box?

[ramdino]

raytek minitemp,poinmted at the exhaust duct coming out of unit right where it exits the unit.

[David Meiland]

Not something I know much about, but if they state that there's a max exhaust temp, is that right at the exit from the box, or could it be further from the box, perhaps at the closest point that vent pipe could enter combustible construction given minimum clearance? And I assume it states degrees F and your are measuring F, to state the obvious. It doesn't seem likely that your unit is that far out of calibration.

Sorry, no real help here...

[Brandon Whitmore]

I didn't know there was a specified exhaust temp. either. I'll have to look into this now, but just out of curiosity. I will probably never check exhaust temp.

[Marc]

What's a gas pack?

Marc

[ramdino]

A gas pack is the type of all in one unit where the furnace and the A/C compressor and the air handler are one unit and typically sit on the outside of the house on the ground. I checked the discharge temp directly at the point of discharge where the exhaust was coming out. Like within 4 " of it.

[Bill Kibbel]

Your IR is not reading the temperature of the exhaust gas. It's measuring the temperature of the surface that it's aimed at.

[Rob Amaral]

bill k is correct (as usual).

[Marc]

But what heats the exhaust pipe if not the exhaust gases?

Marc

[ramdino]

OK but that should be the same as the gases. The gases are not going to rasie the pipe to a higher temp than they are.

[Bill Kibbel]

OK but that should be the same as the gases. The gases are not going to rasie the pipe to a higher temp than they are.

Think of sticking your hand into an oven set at 250F. Now grab a piece of steel that is also 250F. There's a few reasons that measuring the exhaust pipe/duct with IR will not give you anything close to the actual temperature of the exhaust.

• You can't get accurate temperature readings from metal with IR. Your IR gun is measuring infrared radiation. There are some (very expensive) IR devices used for measuring temps of metals in industry, but they compensate for emmissvity of metals.

The metal exhaust pipe will very likely be hotter than the exhaust gases as heat is conducted directly from the heat source (combustion) to the metal exhaust pipe.

The exhaust gas is in motion - there's not enough time for thermal equilibrium between the exhaust gas and the metal exhaust pipe

[David Meiland]

You can get reasonably accurate temperatures from shiny metals with IR if you know the emissivity of the metal and the background temperature of the environment. Some low end IR imagers and even some of the IR thermometers are adjustable. It is very easy if you can put a piece of Scotch 33 electrical tape or even masking tape on the metal as a target. Not sure I would do this with a hot exhaust pipe, maybe just use a thermocouple on it, or drill the pipe and put in a flue probe.

I suppose the pipe could be hotter than the gas inside it due to conduction from the burner, but that would seem to be highly variable with the design of the unit, and would certainly vary a lot with exactly how far from the flue connection you take your reading.

I don't see how the gas being "in motion" has much to do with the pipe being heated. If the gas is at a reasonably steady temperature, then equilibrium will be reached, taking into account the temperature outside the pipe, the emissivity of the pipe, etc.

I'd still like to know more about what exactly the manufacturer means when they say the max exhaust temp = X. Where and under what conditions?

[Marc]

You can get reasonably accurate temperatures from shiny metals with IR if you know the emissivity of the metal and the background temperature of the environment.

I disagree. Emissivity values vary enormously and published values are but approximations.

Marc

[Scottpat]

Use a thermometer with a probe for air temps! The IR is a great tool but it does have its limitations. I use the IR thermometer to check floor and ceiling registers to make sure they have airflow, and to check wall and ceiling insulation.

[Charles46]

For me this is the most relevant question asked:

"I'd still like to know more about what exactly the manufacturer means when they say the max exhaust temp = X. Where and under what conditions?"