Zoning overlap, and dampers on return ducts

**garrettcook** · 01-10-2009, 03:27 PM

Hello,

My wife and I are thinking about building a home some time in the future. I'm an engineer and I like the added complexity

and efficiency of zoned HVAC units. I've done quite a bit of research on zoning but I've got a couple of questions that I have not been able to find out answers to:

1. Zoning overlap
Is it possible in a multi-zone environment to create an area that is served by more than one zone, but for which the ductwork uses shared air register(s)? Here is the scenario I'm thinking of...I'm designing an upstairs to our home that would have four smallish bedrooms, a bonus room, and two baths. Primary HVAC activity for the bedrooms is going to be at night while sleeping or getting ready for school or whatnot, primary bonus room activity during the day. The bathrooms are to be shared amongst all the upstairs rooms. I am wanting the bathroom vents to be active when either the bedrooms or bonus room is calling for heat/AC, but yet not be overheated/overcooled. My thought is that really small ducts (to restrict the extra airflow) to the bathrooms could merge before reaching the air register. Is anything like this ever done?

2. Dampers in the air return
When designing zoned systems, are dampers installed on the return side of the furnace? I'm wanting for only the returns in an occupied zone to be pulling in air.

Thank you in advance for your help!

Garrett

**beenthere** · 01-10-2009, 04:30 PM

Can't merge ducts like that.
The bathrooms will have to be on one zone or the other.

Doubtfull you actually have enough return in all zones, that one zone or another won't need to pull some return from another area.

If you do have enough return in all zones.
Then the returns in the zones not calling won't pull any air in.

**Big** · 01-10-2009, 04:41 PM

two stats control one zone ,there is no control. Put 2 steering wheel in a car ,who's driving . Used another system of the add on .

**t527ed** · 01-10-2009, 05:04 PM

#1 is easy, thermostat controlling a damper only with no power to bring the system on.

if either zone is calling it will be fed. if it gets to warm or cold the damper will close.

#2 no dampers in returns.

**beenthere** · 01-10-2009, 05:11 PM

Originally Posted by t527ed

#1 is easy, thermostat controlling a damper only with no power to bring the system on.

if either zone is calling it will be fed. if it gets to warm or cold the damper will close.

#2 no dampers in returns.

Wouldn't that be 2 dampers.
1 for each zone its tapped into.

**garrettcook** · 01-10-2009, 05:20 PM

Originally Posted by beenthere

Wouldn't that be 2 dampers.
1 for each zone its tapped into.

If the ductwork is merged, then maybe it would just be one damper. In your earlier post you stated that ductwork can't be merged; is that a code requirement, best practice, or just unusual to do?

Garrett

**beenthere** · 01-10-2009, 05:24 PM

Air from one main supply would feed into the other.
Most likely during heat/cool calls while the bathroom doors are closed, and the exhaust fan isn't running.

The 30 minutes while make up is being put on.

**garrettcook** · 01-10-2009, 05:28 PM

Originally Posted by t527ed

#1 is easy, thermostat controlling a damper only with no power to bring the system on.

if either zone is calling it will be fed. if it gets to warm or cold the damper will close.

I like that idea! Is there a zoned HVAC system available that allows a remote sensor to control a damper but not bring the system? Or would it have to be done separately?

Garrett

**skippedover** · 01-10-2009, 05:48 PM

In the system suggested, you can use whatever zone control system you want on the main zones. For the bathroom you would need to have a damper that is not included with calling the equipment on. Some systems you can easily create a damper or zone that is "Non-Voting", meaning it can only operate as a high/low temperature limiter but cannot initiate a call for heating/cooling. However on some systems all dampers are 'Voting' and cannot be "Non-Voting". If you need that type of system, then a separate damper with its own thermostat and transformer would do the job.

I also disagree on the balancing dampers for returns. The amount of return air from a room should be exactly equal to the supply air going in. If it isn't equal, then the room is under either positive or negative pressure whenever the zone is operating. That by necessity leads to either exfiltration of infiltration of air to the outdoors. Flow hoods are designed to read airflow in both directions for this very reason. So if you want to install the most energy efficient system, I'd recommend installing balancing dampers on the returns. As for zone dampers, I think that would probably be overkill, though in the purest sense, I suppose you could put them in to totally isolate the zone when it's not calling. That's a good, thought provoking question. Thanks for asking.

**garrettcook** · 01-10-2009, 06:15 PM

Originally Posted by skippedover

...However on some systems all dampers are 'Voting' and cannot be "Non-Voting". If you need that type of system, then a separate damper with its own thermostat and transformer would do the job.

I also disagree on the balancing dampers for returns. The amount of return air from a room should be exactly equal to the supply air going in. If it isn't equal, then the room is under either positive or negative pressure whenever the zone is operating. ... So if you want to install the most energy efficient system, I'd recommend installing balancing dampers on the returns. As for zone dampers, I think that would probably be overkill, though in the purest sense, I suppose you could put them in to totally isolate the zone when it's not calling. That's a good, thought provoking question. Thanks for asking.

If the returns were sized appropriately, would there be any need for balancing dampers? (These are different than the zone dampers I was talking about, right?) My thinking on the zone dampers was that if the zones were totally isolated (all the doors closed, etc) then the return air should equal supply, as you explained above. But in situations where doors were opened then it seems like any return might be pulling in air, since there is no effective barrier between the returns.

Also, are thermostats made that are just for these "non-voting" zones? I wasn't able to find any online. I have in mind just a simple two-wire deal that has a low and high temperature limit on it.

Garrett

**t527ed** · 01-10-2009, 08:02 PM

Originally Posted by beenthere

Wouldn't that be 2 dampers.
1 for each zone its tapped into.

one extra trunk off the plenum before any of the zone dampers..

or if bathrooms are close to the unit you could do it with a couple small round dampers.

**beenthere** · 01-11-2009, 03:04 AM

Originally Posted by t527ed

one extra trunk off the plenum before any of the zone dampers..

or if bathrooms are close to the unit you could do it with a couple small round dampers.

Ok, didn't think of that.

**cliebold** · 01-11-2009, 10:38 AM

I have a three zone upstairs damper system that has 3 rooms, each with their own thermostats and 2 bathrooms without controls. I have the bathrooms ducted from the main supply before any dampers. This allows the bathrooms to recieve heat or a/c whenever any of the three rooms are calling for conditioned air. Mine has not resulted in any over cooling or heating of these zones and when not being used the bathroom doors are usually open and the air will leave those rooms and mix with the hallway air.

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