166 Degrees coming from a register?

[Scott Gilligan, CMI]

Was doing an energy audit today because the client was concerned about one room in the house being consistantly colder than all the other rooms. I had my weather gauge with me so I decided to see if there was a restriction in the ducting by measuring the cubic feet per minute and temperature at the registers in the home. When I got to the family room, I almost burned my hand when I put the meter over the vent. It read 590 cfm and was pumping out 166 degree air. This vent is close to the heating system in the basement and is likely the first in line off the main trunk. There were signs at the heating system that there might be a cracked heat exchanger, but I do not have the equipment to do a combustion analysis to determine that for sure. The rest of the house was measuring approximately 300-350 cfm at about 120-130 degrees.

I am thinking that these temps are pretty high and could be an issue. There was also ducting running through the walls and the wall surfaces measured 97 degrees. Anyone have an idea what would make a heating system pump out air at such a high temperature?

It was a Carrier forced air system that is nautral gas fired. System is rated at 125000 btu/hr.

[Brian A. MacNeish]

Originally Posted by sgilligan1

Was doing an energy audit today because the client was concerned about one room in the house being consistantly colder than all the other rooms. I had my weather gauge with me so I decided to see if there was a restriction in the ducting by measuring the cubic feet per minute and temperature at the registers in the home. When I got to the family room, I almost burned my hand when I put the meter over the vent. It read 590 cfm and was pumping out 166 degree air. This vent is close to the heating system in the basement and is likely the first in line off the main trunk. There were signs at the heating system that there might be a cracked heat exchanger, but I do not have the equipment to do a combustion analysis to determine that for sure. The rest of the house was measuring approximately 300-350 cfm at about 120-130 degrees.

I am thinking that these temps are pretty high and could be an issue. There was also ducting running through the walls and the wall surfaces measured 97 degrees. Anyone have an idea what would make a heating system pump out air at such a high temperature?

It was a Carrier forced air system that is nautral gas fired. System is rated at 125000 btu/hr.

Too low air flows causes by low fan speed, dirty filter, too small return ducts, blockages in ducts.

Have a few questions though:

What do you call a "weather guage" and how accurate is the air flow function?

What was the size of the supply register that had 590 cfm? That is very high for a regularly sized register of 4x10 or 12". What was the air velocity at that register outlet? Was there "air rush" noise at the register from too high velocity?

Some thoughts and comments:

The delivery air temperature is a bit high. Most fuel-fired hot air heating unit temp controls are preset at 150 deg F fan "On" settings with the safety hi-limit "Off" set at 200 deg F. You can see that the temp you measured is still within the operational temps allowed by the controls. If the duct takeoff is closest to the supply plenum or on the supply plenem then you can expect higher temps.......but these should be adjusted downward by analyzing and correcting the above possible causes.

[Michael Roberson]

Yea, agreed. My average per register (depending on the system and location of course) is 100-130 deg F. Sounds like an over sized furnace?