What causes temperature differential inside the plenum?
My air handler's plenum has 3 flex ducts connected to it: 16" main supply to 3 rooms + hallways, 10" duct to a room at other side of the house, and 8" duct to utility rooms.
I disconnected the 10" and 8" ducts to check the air flow and noticed that the air coming out of the 10" is warmer than the air coming out of the 8" and the 16" ducts. The temperature differential is about 4 to 6F.
What would cause this to happen? I would thought with the air velocity and pressure inside the plenum, the temperature of the air should be uniform when going into the ducts.
air will go where it wants. That's the simple answer.
Type of metering device.
Position of the tap may not be allowing the air to mix in the plenum.
The coils are not the same temp from one end to the next.
Even if all the items beenthere mentioned are normal, there will always be a bypass factor in a supply plenum, and uneven mixing of air due to turbulence within the air handler and plenum. There is always a certain amount of air that passes through the coil that is unaffected by the coil. This is known as bypass factor.
Also, when you removed the two ducts to observe this temperature difference, removing the ducts changed the dynamics of the airflow in the plenum and across the coil. You would have been better off leaving the ducts in place and placing a temperature probe through the ducts to observe what differences there might be in temperature. I would remeasure the ducts with them in place, not removed.
Actually, I only opened one duct at a time and measured the air temperature from the mouth of the collar, so this should not affect the dynamics of the airflow too much, would it?
Originally Posted by shophound
What is the best way to achieve this? Do I have to poke a hole through the duct and then insert the pen thermometer's tip through it? Thx
Last edited by toohothere; 07-30-2008 at 01:35 PM.
Like I sai. Air goes whrer it wants is the simple answer
Removing just one duct affects the static pressure of the entire system. Where that duct originally offered resistance to airflow, with it removed there is no resistance. Therefore the entire system is affected as the lack of resistance on the take-off with the removed duct will flow air more freely.
Originally Posted by toohothere
Go a few feet away from the plenum and drill a hole into the duct just large enough to insert a temperature probe. Patch the hole with foil tape when you're done. The few feet away will assure you're measuring the mixed air properties entering that duct run, but minimize heat gain should these ducts in question be in the attic.