temperature rise in furnace

[lkaufman]

Checking out the furnace and I took some temperature readings, at the discharge, about 24" above the burners, I was reading 162F, inside the return air cabinet at the blower (with the door closed) I was reading 78F so a temperature delta of 84 degrees. Is this normal, if not too little or too much air flow?

Thanks

[amd]

You have to take the temperature rise on the trunk line since the thermometer can pick up radiant heat from the heat exchanger.

8F so a temperature delta of 84 degrees. Is this normal, if not too little or too much air flow?

Check the furnace's rating plate.

Old conventional units generally have a rated rise of 70-100F.

New mid and high efficiency models tend to be rated at 30-60F.

If the temp rise is above the rated, you don't have sufficient airflow and/or the furnace is overfired.

[lkaufman]

Thanks, how far down the trunk line should I check? I did check the name plate and it says 75F max rise. Since the furnace was installed I have known they undersized the ductork, cuasing the higher static on the return. We did some mods to help this but the ultimate solution is to re-do the trunk lines. I may have my tech check the gas pressure and if needed, slghtly de-rate it.

[amd]

The thermometer shouldn't be able to "see" the heat exchanger.

I would do it a couple of feet into the trunk line.

It might be possible to increase the blower speed.

[amd]

Since the furnace was installed I have known they undersized the ductork

I've got the same problem - http://hvac-talk.com/vbb/showthread.php?t=919152

[catmanacman]

sounds like a big furnace on a small duct system. a good place to start is find out how much furnace you need . also it is not a good idea to lower the gas pressure to reduce the temperature rise as you are changing the design conditions of the furnace

[rowens]

Have your tech test manofold pressure to see if it is set correctly.