duct sizing for changeover to zoned HVAC

[cybersprings]

Hello,

I have a 2000 sq ft ranch in GA. with a 3 1/2 ton conventional furnace/ac. I am zoning the system into 6 zones
3 bedrooms each on their own zone
1 bedroom/small bath zone
1 large bath/closet zone.

There is nothing wrong with my current system other than I like it very cold at night in the MBR. I am spending a fortune in electricity to TRY to cool the ~300 sq ft MBR to 66 by also cooling the other 1700 sq ft most of which has 12 ft ceilings. Also it does not cool very quickly. On hot days at 11pm the temp is still @74 trying to get to 66. The thermostat was never set above 72, but it usually only holds 74 until sometime after midnight when it gets below and actually gets to 66 before morning.

I know that zones as small as 1 bedroom (~120 sq feet) is not the ideal set up, but I want to give each room control over the temp. I am setting up a bypass damper with a temperature sensor at exit.

My questions are:

1. Should I increase the size of the ducts to the smaller bedrooms from 6" to 7" or 8" so that, if they are the only zone calling for air, that I am not forcing too much to bypass?
2. The bedroom/small bath zone is fed by a 5" and a 4". Should I increase the size of those to larger ducts for the same reason?
3. The MBR will almost always be the only zone calling for air at night during the summer. It is fed by 2 7" ducts. Can I/should I increase those to 2 8"?

What is the down side of increasing these ducts?

I would not ask for designing duct sizes for a house; I am hoping that asking about tweeking an already designed/installed system is not out of bounds.

Thanks for your time,

Paul

[beenthere]

You should see if you can by a warranty for your compressor that your going to knock out.


If you want to zone your house the way you stated, you need to use a modulating zone system.

The system your proposing will short cycle the compressor, and kill it.

[cybersprings]

Originally posted by beenthere

If you want to zone your house the way you stated, you need to use a modulating zone system.

The system your proposing will short cycle the compressor, and kill it.

I assume that you mean more than just modulating dampers? Are you talking variable air flow in order to cause the one room to cool much slower and have the compressor run longer? Is that available in residential controllers?


The thermostats that I am going to use have short cycle protection built in. Would something like that, along with modulated dampers rather than 2 position dampers do the trick or do I need something more?


Thanks again for your time.

Paul


[Edited by cybersprings on 07-11-2006 at 10:00 PM]

[Advanced Response]

Cyber,

Contact a Contractor that is familiar with modulating zoning and 2-stage equipment..

I would reccommend a Carrier/Bryant Infinity/Evolution Zoning with Infinity/Evolution 2-speed Condensing unit.. Even with a good Zoning system with 2-stage a/c you still must be careful about how small each zone is..

A must is:
Find a qualified Contractor familiar zoning!!!

Goodluck
J

What you are planning right now is a disaster without proper planning and equipment.. Also 66 degrees WOW!!

[Advanced Response]

Cyber,

Just a thought to think about.. Have you thought about a Mini-split or a window unit for the masterbedroom.. Talk to a contractor about any other possibilities..Much cheaper and may suit your needs better than zoning..

Goodluck
J

[cybersprings]

ok, you guys have sufficiently scared me.

How about 3 zones?

zone 1 MBR 300 sq feet
zone 2 3 bedroom ~400 sq feet
zone 3 Living area ~ 1500 sq feet

I am investigating the mini-split since I had never heard of them and they seem intriquing and may fit the bill.

Thanks,

Paul

[beenthere]

The mini split will be more cost effective operational wise for what you want in temp control for your master bedroom.

The modulating system I was talking about doesn't require a VS blower.

But would require some one familiar with them, or had factory tech support if it was the first one they were installing, not a DIY project.

[mjk_na]

Yes, a split unit would cater for an extra cooling to your MBR. The central system will cater for normal operating conditions for the rest of the house.

More cost effective.

[cybersprings]

thanks for the advice.

Paul