Converting Dual-Zone central AC to multizone


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Old 10-03-13, 04:45 AM
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Converting Dual-Zone central AC to multizone

I am currently purchasing a 2 story house that has about 3000sqft. It has one central AC Unit with one upstairs zone and one downstairs zone. I would like to swap out the current thermostats with insteon ones and add extra upstairs thermostats in master bedroom, home office, and perhaps one in each bedroom. I would then add electrically controlled induct dampeners to each of the ducts feeding the master bedroom, home office, and bedrooms that would be controlled by the new thermostats. Basically splitting the home upstairs into 4 or 5 zones. The downstairs would remain one zone since it is an open floor plan. I will be the only one in the house, and would rather not cool rooms I am not in most of the time, our during the night. This is in Florida so I will pretty much be cooling the house 9 to 10 months of the year. Why do I need such a big house? Well it is in a great location that I would like to raise a family in.

From different forums and talking to a friend in the AC business, the pros seem to indicate this would be a bad idea. I am a huge "do it yourself" guy. And I know sometimes the pros are just wrong. It may be a lack of experience or just bad or outdated info passed down from their teachers. In this case I think they maybe wrong. I would love if any one could provide me with a solid logical reason this is a bad idea. The reasons I have heard is that the A.C. systems are designed to handle a certain amount of airflow and by restricting it, it will cause some "issues". I have yet to hear what issues beyond "it will not run efficiently" could occur. This house is only 7 years old and has return ducts in each room. So unlike an older house that could have closed doors and develop rooms with air pressure differences that could tax the AC, this should not be possible in this house. I believe all the return vents will keep the air pressure constant through out the house. In my judgement, even if I restricted the ac all the way down to a single duct/vent, the air is always going to make it back to the main air handler's return vent. The only other bad outcome I can envision is that the single duct is so small that it can not vent the air fast enough that it tries blow up the duct system like a balloon until something pops or rips. I could be wrong, but I would think the air coming out of a single duct would be blowing things off shelves before that could happen.

I will absolutely agree duct work has to be designed correctly to provide adequate air flow to each room. But as I see it, each room that stops getting air, just raises the amount of air coming out the rest of the vents. By chance if the A.C. comes on to only cool the master bedroom at night, and all the other dampeners are closed, all that focused cold air is just going to cool the bedroom mega fast.

Please tell me if I am right or wrong, and point out any false assumptions I maybe making.

James
 
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Old 10-03-13, 07:09 AM
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Bad idea!! each zone will have to have all the air to what ever room are you will have to have some place to dump the extra air. lets say you have a 5ton unit which id guess is what you have you will have 2000cfm of air going to that one room if its the only one calling for heat/ac, are you will need to dump the air in a basement are some place. Two systems would be much better one for each floor. They will give you lower cost and better comfort. If you want to control each room you will need to go to a ductless split system.
 
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Old 10-03-13, 07:20 AM
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oh 2000cfm of air will need a 19 duct on supply and 21 on return If your home has 6in duct to your rooms thats good for 95 CFM
 
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Old 10-03-13, 02:46 PM
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Hi Airman, I would seriously consider a mini split system. But since the A.C. works, I have no intention of replacing it. I currently don't know what size any if the ducts for supply or return are. What is the proven worse that could happen? Not just supposition. If I put a pressure nozzle on a hose, all that happens is the water "shoots" further. But air can even be compressed unlike water, so that should be even better. If anyone has a 3000ft house, return vents, and a barometer, perhaps you could close all your vents and doors except a vent in one room. I bet that even though all 2000c.f.m of air is going in to the one room for the few minutes it would take to cool down, that the pressure reading is practically the same as other rooms. I think no matter how much air from a 5 ton unit or lower, the air pressure and air flow is going to even out faster then it could push air into the room, because of the return vents. Like putting a small hole in a balloon. You are not going to be able to blow fast enough to inflate the balloon. Maybe a huge commercial unit could blow fast enough to "pop" the ductwork, but a residential unit....I just don't see it happening. Anytime with real world stories of people closing all the vents and the actual problems and damage it caused? I could even put pressure sensors in the ducts that would reopen the dampeners if that was an issue. But that still seems like overkill at this point.
 
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Old 10-04-13, 05:31 AM
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real world problems would be burned up compressor, fan motor, frozen coils, water damage. I can tell you it will not take long to kill a system with what you have in mind.
 
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Old 10-04-13, 01:01 PM
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Please heed what the Airman is telling you. Zoning of most residential forced air systems is a bad idea, sometimes a VERY bad idea. Microzoning is ALWAYS a bad idea.
 
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Old 10-05-13, 02:12 AM
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I already know the pros think it is a bad idea. I am trying to find out exactly why it would be a bad idea. Airman's predicted outcomes are:

1. Frozen Coils

I can definitely see that as possible problem. if the air is restricted and "backed up" and not moving across the coils fast enough, this is bound to happen. In the worst case scenario where all dampeners are closed except to one vent in a small bedroom, the question is: Can the coils even freeze up in the very small time it takes to cool that room? I would say no. BUT if the door is open to that room it may take longer to cool and could possible freeze up. Do modern systems have any sensor protection to prevent frozen coils? If not, I could try to add sensor to turn of the system if the coils start to freeze.

2. Water Damage (guess Airman means the condensation building up in or on the ducts.)

Would the cold air getting "stuck" in the ducts, make condensation form? This may be one of the harder issues to conquer. Where do you think I might see water damage? Would it be from inside of the ducts, or outside dripping from the surrounding duct insulation? The only solution I can think of is under the next part.

3. Fan motor (I am assuming you mean the air handler blower)

If the air is backed up and the fan is fighting against a higher pressure environment. It would of course wear out faster. The question is: If the air is restricted to one vent, how much does the air pressure increase at the fans position or above it in the duct? I guess I could easily test this before trying to Microzone the upstairs. Just need to seal off all but one vent and use a barometer to measure the change in air pressure. If it is indeed a problem, maybe I could have the system open up the current entire downstairs zone anytime pressure goes up.

4. Compressor

I am totally unsure about how additional air pressure or colder air in the duct work would stress the compressor. Please elaborate.

With all these concerns, I am very curious to how these issues are handled with the current upstairs/downstairs zoning that is already in place.

Separate topic: I am investigating adding a Heat Recover Unit - HRU (sometimes called a superdeheater) to my AC and Hot water system. It takes the waste heat from the Freon and uses it to make free hot water. It is also more efficient to transfer heat to the water then to the hot Florida air, making the AC system more efficient. What is your opinions on the ones on the market? Any in particular you would suggest?
 
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Old 10-10-13, 01:47 AM
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After Airman's warnings, I was able to find a lot more information on zoning AC systems. It seems most problems are rectified with a bypass pressure release duct. Since the house I am buying already has 2 zones, I am severely hoping that it already has a bypass pressure release duct.

Here are some great links that helped me.

For a complete DIY-Zoning solution for dirt cheap try:
https://sites.google.com/a/homeclimatecontrol.com/www/
Here he explains the excess pressure release options in detail:
Technical FAQ: HVAC: Excess Static Pressure Relief - Home Climate Control . Forum: https://groups.google.com/forum/#!fo...limate-control

Good zoning info & meantions need for a Barometric Pressure Relief Bypass Duct.: Electronic Smart Home Improvement for DIYs

Honeywell's own Zoning Manual:
https://customer.honeywell.com/resou...0s/70-2321.pdf

Maytag video on zoning:
Maytag iQ Zone | Home Zoning - YouTube

There appears to be plenty of zone controller manufacturers. The zone controllers appear to be designed specifically to counter the issue's that may occur.
 
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Old 10-11-13, 12:23 AM
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I am continuing down the rabbit hole of information. I found that while installing a bypass duct that feeds back to the return can eliminate the pressure problem, it can also decrease efficiency. So much so that a California CEC law that goes into effect next year, bans bypass ducts. This confused the hell out of me at first. I wondered how taking cold air back in to the system could be bad. In fact I read somewhere that it was described as "turbocharging" the cool air even more. But it turns out it lowers the temperature of the coil. The colder the coil gets the more inefficiently it works, until it gets so cold it freezes up. Doh!

So what is the solution to the excess pressure? From what I can find, it is to allow the zone dampers in the closed zones to "bleed off" the access pressure. This can be done with zone controller that can control the dampers with varying degrees of closure. Or with dampers that open by themselves for access pressure, like the Jackson Systems' BZD.

Another option is to create one or more "dump zones" that you don't mind access cooling or heating going to. My first thought is to the main living space since it is the biggest area/zone and thus any excess air would have a smaller effect. My second is to a uncooled or unheated area like a garage or basement (all though aren't really basements in Florida). The garage would probably violate code since it could allow carbon monoxide to enter the house through the ducts.

A Couple more links that helped me:

http://jacksonsystems.com/themes/jac...stemseBook.pdf

Navigating the Twilight Zone: The Hidden Flaw in a Zoned Duct System
The comments were very useful as well.
 
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Old 10-11-13, 01:06 AM
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Now you are beginning to see why so many professionals frown upon zoning for residential systems. The two biggest reasons are they have complex controls (read, expensive) and they cause inefficiencies in operation. The complex controls can, and often do, fail in a manner that can cause the evaporator coil to freeze placing a strain on the compressor shortening its life or in the heating mode cause the heat exchanger to overheat from a lack of airflow. The smaller the zones (microzoning) the more prevalent these problems. Few homes are so large as to really benefit from zoning for any kind of economical gain so it comes back to zoning for comfort.

With a properly designed and installed system (sadly, not all that common) a home CAN be comfortable and energy efficient without all the extra expense and bother of zoning. Of course if you live in a five thousand square foot mansion zoning becomes more advisable but then it is quite doubtful that you would have a single forced air furnace or single stage cooling system.
 
 

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