Electronic Dampers on Return Air?

[Daltex]

I was considering the possibility of combining two units into one. The upstairs is on it's own system as is the lower level. I have been toying with the idea of getting a single unit that is smaller than the combined load required for both and setting back the stat in the upstairs during the day and the downstairs at night.

I stumbled across a flaw in my initial design as the supply air would be dumping in the downstairs during the heat of the day but the return upstairs would still be drawing in air that is 10* hotter than the downstairs air.

Can dampers be installed on returns so that they open only during a call to that zone? I guess I wouldn't be able to set the upflow AH on a plenum box as a always closed damper would need to be ducted.

Upstairs Load:
Total Gain- 42,859
Sensible- 38,582
Latent- 4,277
Loss- 54,573

Downstairs Load:
Total Gain- 40,747
Sensible- 36,460
Latent- 4,287
Loss- 58,087

[21degrees]

Why would you ever waste money on Installing dampers on return air. Their is no need for it. Call a professional and get him to design a system for both floors by zoning supply only.

[Daltex]

Why would you ever waste money on Installing dampers on return air. Their is no need for it. Call a professional and get him to design a system for both floors by zoning supply only.

I thought the return air temp had an effect on efficiency (pretty sure it does). You're saying a return air of 80* cools just as much at the same efficiency as 73* return temp.

By zoning I can cool the bedroom at night (upstairs zone) and have the empty downstairs set back 10*. If the downstairs 10* warmer air is being drawn into the return, it doesn't seem that would be efficient but I'm no pro.

[21degrees]

I never said It does not have affect on efficiency. But last time I check cool air is more dense than warm air meaning that cool air will end up in basement sooner than later and putting dampers on return is waisting money. I would never do it on my house so why would I recommend to you.

[dash]

Many zoning system can "slave" a second damper for that reurn,so it opens and closes with the supply damper.

If your design was perfect,theory the supply would be returned by the return in the same zone.However it will pull air from the path of least resistance.If the zones are separated by a closed door(s),you probably don't need to worry about a return damper,if not and you are planning a large temperature difference between zones ,it may be worthwhile.

[beenthere]

I think you might be better off leaving the returns pull what they need to pull for the unit.

At night time, when the second floor is calling. Some of the return will come from the first floor. But that will aid in recoery in the morning, because it means that the first floors humidity won't be as high because some of its air has been dehumidified through out the night.
Same with the second floor during the day.

It also means you don't have to try and increase the return size for each floor.
As I recall, your returns aren't the greatest at the moment.

[Daltex]

Thanks for the replies. I am going to replace the returns however I wind up getting it done. I had forgotten about the path of least resistance. I think this solves the return concern but I still need to see if a 5 ton will cover both zones.

As always, Thanks!

[dash]

Thanks for the replies. I am going to replace the returns however I wind up getting it done. I had forgotten about the path of least resistance. I think this solves the return concern but I still need to see if a 5 ton will cover both zones.

As always, Thanks!

Each return needs to have equal resistance to have a chance of returning air from only the zone that's getting supply air,or zones need to be separated by a door.Not saying it needs to be,just addressing your desire to keep the return coming only from the zone that is calling.

We have had the ocassion to install return dampers,in an open floor plan home,due to the owner wanting to hold specific set back temps.,with an open floor plan,it was the only way.

[S_Helton]

One negative that I can think of by going from 2 systems to 1 system with a damper system, is breakdowns. If you have 2 systems and one has a problem, then at least you have 1 working system until repairs can be made. With 1 system, if it goes down you have no heat/air.

I never recommend downsizing a system just because of zoning. Your unit still needs to be able to take care of the whole load of the house. Otherwise you will have parts of the home that may suffer from lack of capacity.

[beenthere]

I never recommend downsizing a system just because of zoning. Your unit still needs to be able to take care of the whole load of the house. Otherwise you will have parts of the home that may suffer from lack of capacity.

His plan is to never have both zones at the same temp.
He wants to leave the unoccupied floor rise 10° above teh occupied floor.

EG: Second floor 74 while first floor is 84. And first floor 72 while second is 82.

[S_Helton]

So, when he sells the house and the new owner has a different lifestyle? Or if his life style changes? He can control the house the same way with 2 systems.

[beenthere]

He already has 2 systems.

[dash]

Can't plan for future owners.

Homeowner wants 72 degrees,we design for it,next one wants 78,system is oversized at 78,doesn't dehumidify well.Not much you can o.