I had a tech out to review my furnace that's not keeping up – a Trane XV80. They poked around, and saw that the plenum temperature is 95F. They then told me that it should be around 140F, which is clearly, substantially more. The thing that confused me is that in the same breath, he said that the temp delta between the return and the plenum should be 20-30F, which would mean a return temp of 110-120, which is impossible.
So – which is it? And what causes a low plenum temp / how can it be remedied?
Best Answer
Look on the furnace data sticker. Every furnace since at least the 1980s will list the delta on the name plate. It will be a little different for each furnace but will always be a range such as 30-60 degrees. Measure the supply air temperature and subtract the return air temperature. This is basic furnace commissioning 101. Too cold and you risk condensation and rust. To hot and you risk cracking the heat exchanger and damaging other components. A low delta T is typically caused by low gas pressure but there are many other reasons it could be. I always do a combustion analysis when starting up a new furnace. That will tell if the burners are firing properly but is beyond what a home owner could do.