HVAC Zoning: What to Do with That Extra Air

Last week we did a zoning basics blog post on zoning for residential homes.  This week on the Fox Family Heating and Air Conditioning blog I want to touch on a little more technical side of the HVAC zoning setup: bypass dampers and dump zones.

Some HVAC installers say you can’t truly set up a unitary ducted HVAC system like the ones most of us in the United States have set up in our homes.  Remember, zoning is for homes that have two thermostats, one upstairs and one downstairs.  They typically allow one HVAC system to heat or cool one zone or the other, but not the whole house at one time.

If you’re an HVAC technician, let me know in the comments down below how you like to set up bypasses and dump zones to get rid of the extra air zoning creates.  I’m sure it’s a little different all around the world, and we’d love to hear about it!

Bypass Dampers on High-End Equipment

American Standard and  Carrier have some nice setups when it comes to their variable speed systems and modulating dampers that open and close strategically, allowing you to really dial in the rooms you want to condition and when.  But buying one of those systems is no joke.  Currently, I’d say only about 7% of the market is buying this high-end equipment.  They really are advanced technology compared to the traditional zoning equipment Americans are used to in their homes today.  But I’m sure this technology will be mainstream soon enough!

Traditional zoning uses two thermostats.  These thermostats can be smart Wi-Fi stats or standard digital programmable stats.  And those two stats talk to a main zone board at the furnace or air handler.  That main zone board then tells the air handler when to come on.  It will trigger air conditioning or heating mode as well as which floor to have come on.

Zoned systems are purposely designed to be about half a ton larger than the largest zone in the house.  Last week’s example of a home with two floors, one at 1150 sq. ft. and one at 800 sq. ft., would be sized at 2.5 to 3 tons depending on insulation levels and other load characteristics.  A system that large can produce 1000 to 1200 cfms.

HVAC Zoning: Directing Extra Air

That smaller 800 sq. ft. zone cools the bedrooms and bathrooms upstairs as well as the laundry room.  But 1000 to 1200 cfms is way too big for 800 sq. ft.  So, what do we do with the extra air?  It should be bled off to another area of the house.

There are a few choices as to where to disperse that extra air:

• We can create a barometric bypass back to the return plenum or return grille.
• A bypass dump zone can be created in another portion of the house.
• Or my favorite, bypass the air to the other zone through dampers set up properly for this.

Option #1 – A barometric bypass straight back to the return plenum

In my opinion, this is the worst way to get rid of the extra air because it sends it immediately back to the return through an 8 to 10” duct with a barometric damper that cracks open with the “extra” air pushing against it.  The more “extra air” there is, the more the damper opens allowing air back to the return plenum.

This superheats the return air in heating mode, and supercools the return air in cooling mode.  How does that affect the system?  In the heating mode, if we have 65-degree air initially entering the return side of the furnace or air handler, it goes through the furnace and gets heated up about 40 degrees to a supply air temperature of about 105 degrees, where that air exits the registers in each room.

If one zone is open and the other closed, the extra air gets sent through an 8 to 12” inch duct immediately back to the return plenum and mixes with that 65-degree air, essentially raising the return air temperature to 70 to 75-degrees.  This air then gets heated up to 115 degrees which now heats up the air in the return plenum to 80 to 90 degrees.

High Temps

On and on this goes, until the system has superheated the return air so high the high limit switch turns off the burners because the supply air is too hot.  And that’s hot because those high limits usually shut the burners off at 165 to 200 degrees.  What does that mean the return air rose to?  125 to 160 degrees!  I see it all the time.

The same thing happens to the evaporator coil.  When the cool supply air gets sent back to the return plenum and recycles over and over, that air gets so cool the evaporator coil eventually freezes.  This blocks the airflow, causing even more problems.

Option #2 – A dump zone

In this scenario we send the extra air through a duct about 8 to 12” to a dump zone, or another section of the house.  I’ve worked on crews that chose to dump the air into a living room, and others that dumped it into the foyer with a 25-foot ceiling!  I’ll admit, that was pretty scary installing that one.  Trusting those ceiling joists to hold as was I was cutting into that 20×20 can was a little intimidating.

I wasn’t the lead installer on those jobs.  In fact, I was just a helper at that time.  Those jobs taught us that the air being dumped in that living area was making those rooms uncomfortably warm or cold depending on the season.

Having learned our lesson, we started dumping that air to the end of the return duct to either a “Y” where the duct meets the can, or a collar cut into the return air can itself, at the ceiling.  I like cutting it into the can because the cold or hot air gets to mix a little more with the return air before being drawn through the furnace or evaporator coil again.  This way the superheating or supercooling doesn’t happen as fast or as easily.

Option #3 – Bleed off to the other zone through dampers

The option that we take at Fox Family is to bleed off the air to the other zone through a small gap left as the damper closes.  We don’t let zone 1 or zone 2’s damper close all the way.  And there are settings on the Honeywell AR Dampers that meter the correct amount the installer decides.

Let’s returning to the house that has 1150 and 800 sq. ft. zones.  If the smaller zone is calling for cooling, the other 400 cfms is redirected to the bigger zone.  This way it won’t be dumped into one single room.  Instead it will get distributed evenly throughout the larger zone through several registers.

The great thing is, this air won’t over-cool or overheat that unused zone.  This allows the system’s static pressure to be regulated at a level that’s closer to manufacturer specs.  This extends the life of the system.

HVAC Zoning Basics can be Complex

Ductless systems are becoming more and more popular in America.  They’re great for zoning individual rooms one at a time.  For those of us who already have supply registers and ducts leading to every room in the house, zoning is still a complicated issue.  Taking care of the HVAC system is the main priority for an HVAC installer.  There are some folks who will just hack it in, and others who try to do it right.

Weigh In

As always, I would love to hear your strategies and comments about how you incorporate HVAC zoning into a house.  All of us are a little different because we work in different parts of the world.  So let me hear from you below in the comments section.

Thanks so much for stopping by, and we’ll see you on the next blog post!