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Community and Q&A

Zoned HVAC and dampers — Opinions?

JC72 | Posted in Mechanicals on

Townhome has a zoned (3-levels) HVAC system. This system contains four trunk lines off the supply plenum one of which contains a weighted (manually adjusted) damper that was originally routed back into the supply plenum. This weighted damper is designed to provide bleed off air when less than 3 zones are in use.

You would adjust the damper by so that is operated as follows:

3-zones in use (damper is basically closed)
2-zones in use (damper partially open)
1-zone in use (damps is slightly more open vs 2 zones in use).


A few years ago our builder-grade compress blew. The HVAC company hired by our 3rd party home warranty said that the failure was due to freon returning to the compressor as a liquid instead of a gas. The cause for the liquid freon was due to the recirculation of conditioned air back into the supply plenum behind the evaporator coil.

Solution: Disconnect this trunk line and allow the excess air to be vented into the attic space.

We’ve left our system set up this way for the last six years and whenever we have an HVAC company inspect it they shake their heads saying that we’re wasting money on our power bill, but they can’t tell me that our reason for disconnecting this trunk line isn’t sound.

So what is it? Should we reconnect this trunk line so that it routes back into the supply plenum or leave it to vent into the attic?


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  1. GBA Editor
    Martin Holladay | | #1

    Zoning forced-air systems requires unsatisfying work-arounds and inefficient performance situations, but yours takes the cake. This is unacceptable. Time to get an intelligent HVAC contractor on board to reconfigure your system.

    I don't like zoned forced-air systems. Any chance you can use your existing system for just two of the existing three zones -- turning those two zones into a one-zone system? Then install a ductless minisplit in the third zone.

  2. JC72 | | #2

    The 3rd floor is non-permitted finished attic and rarely used (Thermostat off most of the year). With us planning to build or move within the next 24 months I plan to demo the sheet rock and insulation at the knee wall and then disconnect the trunk line feeding the floor.

    HVAC guys apparently don't want to mess with the by-pass damper or the trunk line. Something about it the line being brittle or too easy to tear and difficult to repair.

  3. GBA Editor
    Martin Holladay | | #3

    A contractor who says that a duct is "too brittle or too easy to tear and difficult to repair" isn't being helpful. If you want to solve this problem, there is always a solution. Of course, the solution may not be cheap. If you are willing to pay for the work, however, the problem can be solved.

    It's possible (in light of the fact that you are planning to sell the house in 24 months) that you may not be motivated to pay a lot of money for repairs.

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