Temperature Rise Location (hot side).

[deejoe]

To actually arrive at a true "temp rise" of a forced air furnace where is the supply temp taken?

The incoming COLD return air location is no problem, as it can be taken just about anywhere just before the blower's compartment,(approx 66 F.)

As for the location of the HOT supply, if you take it approx 8" above the top of the exchanger you get maybe 160 f.(or more)
If you take it higher up in the plenum it may be 140 F
If you take it 24" (downstream) from the supply plenum it could be 120. (or less)

These variations can make quite a diffence in the actual "rise" calculation.

Does anyone know precisely where this location is on the outgoing 'hot' supply air side?

[karsthuntr]

As close to the plenum as you can get before an runs and not directly in sight(the temp probe that is) of the HX.

[deejoe]

thanks for your reply, but itt will not "pin-point" the location of a probe.
you saying..."not in sight of the exchanger"... can in some layouts be as much as 6' ,which will make an considerable difference in the temperature.

[karsthuntr]

You will get radiant heat if the probe can see the HX. If you have an A/C coil on the furnace you can take it right above that, because the coil will block the radiant heat.

[deejoe]

Originally posted by karsthuntr
You will get radiant heat if the probe can see the HX. If you have an A/C coil on the furnace you can take it right above that, because the coil will block the radiant heat.

Assuming your above statement is correct, a furnace with an ac coil in it and a probe taken just above this coil could have a temp of ,say, 160 F.

And then again, an exact identical furnace andlayout,etc,, with no ac coil, the probe would be placed further (out of sight) from the exchanger, hence a considerable lower temp,say, 140 F and that would be an entirely different rise from the other identical furnace.

We still have no precise location in obtaining the true rise.

[coolwhip]

When checking temperature rise, your probe must be far eough down stream so that the thermometer is not exposed to radiant heat from the HX.

[karsthuntr]

Originally posted by deejoe
As for the location of the HOT supply, if you take it approx 8" above the top of the exchanger you get maybe 160 f.(or more)
If you take it higher up in the plenum it may be 140 F
If you take it 24" (downstream) from the supply plenum it could be 120. (or less)

Your first two measurements are probally reading the radaint heat so the 120 is most likely the right number, 54° delta T.

[RoBoTeq]

As long as the temperature is taken before the first takeoff and is out of sight of the heat exchanger, it is going to be accurate.

If you are getting erratic temperatures, you are doing something wrong.

[jack david]

I was taught that if a proper place to insert your probe wasn't readily available, the next best place is where the hi limit is. I even keep a small rectangle of sheet metal with a hole all ready drilled in it in the back of my van. I can use it over and over again. I replace the hi limit with my blank of sheet metal, the probe to my Cooper temp. meter, a couple pieces of tape to seal it, and I'm good to go.

[RoBoTeq]

Even limits need to be shielded from the radiant heat from the heatexchangers, so that is not a good place to check for temperature rise readings.

[hvac-tech-lane]

Originally posted by jack david
I was taught that if a proper place to insert your probe wasn't readily available, the next best place is where the hi limit is. I even keep a small rectangle of sheet metal with a hole all ready drilled in it in the back of my van. I can use it over and over again. I replace the hi limit with my blank of sheet metal, the probe to my Cooper temp. meter, a couple pieces of tape to seal it, and I'm good to go.

Thats only to check the temp that the hi limit is subject to and would need radiant sheilding to mimick the performance of that hi limit , not the heat rise of the furnace.

[deejoe]

RoboTeq; the readings are not erratic,Just naturally getting lower as you move further downstream.that's obvious.
What I'm asking is ....WHERE is the best place to take this temp. It would seem like the best place (according to some)is just out of sight of the HX.

[RoBoTeq]

Originally posted by deejoe
RoboTeq; the readings are not erratic,Just naturally getting lower as you move further downstream.that's obvious.
What I'm asking is ....WHERE is the best place to take this temp. It would seem like the best place (according to some)is just out of sight of the HX.

The air temperature should not get appreciably lower within just a few feet from the furnace unless the ducting is uninsulated and running through a freezer. Having a 20º difference between two measurements in that short of a distance with no take offs is quite erratic and needs to be addressed as to why there is a difference.

Hell, most times the air will not decrease 5º from the plenum to the regesters.

[jack david]

Thanks RoBo and hvac-tech-lane. I wasn't going to post, but now I'm glad I did. Always eager to learn. Thanks.

[tinknocker service tech]

i normaly go high on the pleanum in line or as close to the main trunk take off
even still why are you getting a 40 degree differance in only a few feet

robo has a point 5 degrees from point a to regs

[Jultzya]

Originally posted by deejoe
To actually arrive at a true "temp rise" of a forced air furnace where is the supply temp taken?

[RoBoTeq]

For the supply, I agree with that diagram. For the return, I prefer being closer to the blower, especially if there is a filter near the blower where air leakage into that area could change the average temperature hitting the heat exchanger.

[RoBoTeq]

Sometimes, for simply unknown reasons, we will hit hot of cool spots in the plenum or ducting. What it would take to pinpoint the reason for the hot/cold spot is just not worth the time or effort in the field.