Subject: From: Date:

HVAC Zoning Judson Gary Mull Sun, 27 Sep 1998 12:14:20 -0400

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NOTE:  This post was on the HomeVision List.  I thought it worth sharing here.
       It was posted by Bill Neukranz

Beyond the automation subjects, I think there's some engineering principles
that should be considered, at least from my experience.

Keep in mind that zoning means that you'll quite likely run all of your air
handler into one zone.  If you split your system into many small zones,
you'll overwhelm your HVAC system as you try to pump (for example) 5 tons of
AC, or 120,000 BTUs of heat into one-fourth or one-fifth of your home.
Thus, for any given air handler unit, I recommend you don't split it into
more than three zones without adding additional safety features.

I learned this when I split my home into 5 zones, thinking that this would
give me really nice control of AC and heat, resulting in rooms that only
have the AC or heat as needed.  While that did happen, actually very nicely,
I didn't realize the problems I caused for myself by designing a system that
forces (in my case) 5 tons of AC into two bedrooms.

If you do split into many zones, here are some safety designs to consider,
and actions that I implemented to ensure adequate safety:

(1) Run a large "bypass" duct from the air handler output (pressure side)
back to the input "return" side.  Put a pressure valve in the duct that only
opens when pressure is high.

(2) Monitor the temperature of your liquid freon line to avoid damaging the
AC compressor.  (In my case, I open up all of the dampers if the line gets
too cold).

(3) If you have automatic thermostats, monitor the outside temperature such
that you don't turn on your AC if it's too cold out (could damage
compressor), or turn on furnace if it's too hot out).

(4) Monitor the temperature of the air handler output to avoid icing up the
AC coil and/or overheating the furnace.  (In my case, I downshift the AC
compressor first, and then if needed, open up all of the dampers, if the
temperature gets below 55 - I have a two stage compressor; I open all of the
dampers if the temperature gets above 120).

While X10 is popular, particularly because it's an inexpensive
communications medium (and for example I use it extensively for lighting), I
don't recommend it as part of implementing the above solutions.  Hard wire
it to minimize any safety concerns.

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