We are about to close on a new condo and we had a two zone system put in. The problem is when the heat is off upstairs and the temp is set around 70 in the lower level the temp in the upper level reaches 74 or higher and you can feel the heat coming out the vents. The contractor says this is normal and there always is BLEED BY in the dampers. This is not what I had imagined. This is a forced gas system with one furnace with a damper system on the ducts. It is an open stair well but the heat is not near the stairs but in the master bedroom at the other end of the house.

If this is not correct where can I get help to prove to my contractor.

Thanks for your help.

This is a very hard one to explain since I'm not there to see how he did it, so all of this is based on assumptions.


First of all a zone controlled module should have been used to control each corresponding damper, thermostat and furnace communication. Then, if at all possible, a damper had to be placed in each trunk leading to the second floor and first floor. Often the first floor will branch off of the plenum in 2 directions, requiring 2 zone dampers just for that floor.....and so on.
Now, airflow considerations must be met.

Since the old system duct design was to use all the existing ductwork.
It is not likely either zone, independent of the other, will accommodate the BTU requirements of the equipment.
Two things can be done.

A bypass loop of ductwork can be made to allow needed cfm from supply duct back to the return duct to accommodate the equipment.

Or. I have a feeling, he just used the closed position stops in the dampers to always keep them slightly opened.

In other words, he made it so the stats open the damper all the way when needed, but when the other stat is off some air still gets past the other damper. He has a "prevent close stop" set a little bit open, so he can move enough air for the system to work, and not overheat or freeze up.