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HVAC Zoning Basics – Bypass Dampers and Dump Zones

HVAC Zoning Basics – Bypass Dampers and Dump Zones

We wanted to touch on a little more technical side of the zoning setup.
Some HVAC installers say “You can’t truly set up a unitary ducted HVAC system,” like the ones the majority of us have set up in our homes.
Remember, zoning is for homes that have two thermostats. One upstairs and one downstairs. They typically allow for one HVAC system to heat or cool one zone or the other, but not the whole house at one time.
Trane and Carrier have some really 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, we’d say only about 7% of the market is actually buying this high-end equipment. They really are advanced technology compared to the traditional zoning equipment North Americans are using in their homes today.
But we’re 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 thermostats. And those two stats talk to the main zone board at the furnace or air handler. That main zone board then tells the air handler when to come on, either in 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.
We did an 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.
That large of a system is capable of producing 1000 to 1200 CFM. That smaller 800 sq ft zone cools the bedrooms and bathrooms upstairs as well as the laundry room.
But 1000 to 1200 CFM is way too big for 800 sq ft.
So, what do we do with the extra air? We bleed it off to another area of the house.
Now, we have 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.
  • We can create a bypass dump zone in another portion of the house.
  • Or bypass the air to the other zone through dampers that are set up properly for this.

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

In our 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 the air in the return plenum
to 80 to 90 degrees. On and on this goes, until the system has superheated the return air so high, that the limit switch turns off the burners because the supply air is too hot.
And that’s pretty hot because those high limits usually shut the burners off at 165 to 200 degrees.
What does that mean the return air got to? 125 to 160 degrees!
We 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 until that air gets so cool that the evaporator coil eventually freezes and blocks the airflow, which causes 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.
We’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.
We’ll admit, that was pretty scary installing that one. Trusting those ceiling joists to hold us up as we were cutting into that 20×20 can was a little intimidating.
What we learned from those jobs was 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.

We 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 Red Deer Heating and AC Team 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 Honewell AR Dampers that meter the correct amount the installer decides.
Referring back to the house that has 1150 and 800 sq ft zones. If the smaller zone is calling for cooling, the other 400 CFM is redirected to the bigger zone. This way, it won’t be dumped into one single room but will be distributed evenly throughout the larger zone through several registers.
Great thing is, this air won’t over cool or overheats that unused zone, and at the same time allows the system’s static pressure to be regulated at a closer level to manufacturer’s specs, which will extend the life of the system.
Ductless systems are becoming more and more popular. They’re great for zoning individual rooms one at a time. But 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, and some will just hack it in, and others who try and do it right.
If you’re an HVAC technician, let us know about how you like to set up bypasses and dump zones to get rid of the extra air zoning creates. We’re sure it’s a little different all around the world, and we’d love to hear about it!
And for the homeowners out there – give us a call to set up your zoned HVAC system properly.

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