One of the contractors who has bid on replacing my a/c and furnace tells me he can use a smaller a/c unit because of using zoning with automated dampers. He has specified a 3 T unit when the gain is about 40,000 BTU/H. The gain was calculated by doing a manual J take off. We talked about the difference in the gain and the unit he sized and he tells me he would use a 3.5 T unit if we were not using zoning. I don't buy it. It seems to me that he needs to balance the gain with an equal a/c output; zoning will just distribute the cooling capability more evenly through the house. I have a 3 T unit now and it can barely keep up on hot days. He also tells me that using a system with R410a allows him to specify a smaller unit. Again, it seems to me that a/c output is output no matter how you get there. Any comments about this would help end my confusion.

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Originally posted by jrminator
One of the contractors who has bid on replacing my a/c and furnace tells me he can use a smaller a/c unit because of using zoning with automated dampers. He has specified a 3 T unit when the gain is about 40,000 BTU/H. The gain was calculated by doing a manual J take off. We talked about the difference in the gain and the unit he sized and he tells me he would use a 3.5 T unit if we were not using zoning. I don't buy it. It seems to me that he needs to balance the gain with an equal a/c output; zoning will just distribute the cooling capability more evenly through the house. I have a 3 T unit now and it can barely keep up on hot days. He also tells me that using a system with R410a allows him to specify a smaller unit. Again, it seems to me that a/c output is output no matter how you get there. Any comments about this would help end my confusion.

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Thanks for your input. I live in a 2000 sf 2 story house in Atlanta, Ga. It gets uncomfortably hot upstairs most of the summer. We would like the upstairs and downstairs rooms to be a uniform comfortable temperature all day and all night long. With my 25 year old equipment there is at least a 5 degree temp difference between upstairs and downstairs. The contractor suggested zoning to even out this temperature variation. However, since someone is home most of the time we won't want to set the thermostats at different temps, since both zones will be used at the same time. So in my situation I don't see how zoning could justify a smaller a/c unit than the load calcs would indicate (house will be zoned upstairs/downstairs and peak load should occur at the same time for both). But, that's why I'm asking for advice. I have been reading posts here for the last week or two and I value your expertise. Any add'l comments on this would be appreciated.

With the thermostat on one level that will be the control point and it could care less about the upstairs. With zoning you'll have two thermostats this will level out the temperatures between floors even if both are set at the same temperature.

On the hottest days it SHOULD be running near 100%

I understand that zoning will even out the temp differences...thats why we are considering doing it. I just don't see how zoning will result in a smaller a/c unit. Load calcs show I need a 3.5 T (40,000 BTU/H) unit. This particular contractor wants to put in a 3.0 and says zoning allows a smaller unit. The only way I can see his arguement as making any sense is if I keep one zone at a temp higher than the design temp and I don't want to do that. None of the other contractors who quoted have specified a 3.0 T unit. Most of the rest call for 3.5 T. One called for 4.0 T. Thanks for your help.

A good zone system and good zone installer CAN reduce the size of your system. And reduce your utility bills as well.
I installed a 5 ton system on a 80,000 BTU load. Worked great untill they had a large party in the middle of the day in the summer. Give the installer all the details and it could be the best system you could ever have.
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"I'm offended by political jokes. Too often they get elected." Henny Youngman

I'm not a HVAC professional, so take the following with the grain of salt. Having said that,

If the zoning controller is smart enough, it will be able to serve one room at a time to keep up with the demand, or balance the air distribution in such a way that the overall temperature will be just stlightly higher than the setpoint so you don't even notice. You'd be surprised what you don't notice - the temperature swings, as the A/C cycles, can reach 4-6F.

The perceived inability of your system tp do the job definitely has to do with the lack of balance - I was thinking the same thing before I started to catch up on HVAC, then it turned out that my unit was in fact oversized. It is not possible to reach the balance no matter how oversized the unit is - as johnl45 rightfully pointed out, you have a single thermostat, and it doesn't care. In fact, the more oversized it is, the less balance you have.

Another consideration is that you have a two-level - similar to what I have. To give you an example, before the zoning system was installed, the temperature difference was reaching 14F - how's that for a change? And, all the time one of the rooms (in the basement, facing north) had the register permanently closed all year round - it never needed extra conditioning.

I'd take a look at the insulation - just replacing the single pane windows and doors may be enough to bring the requirements 1 ton down, and that would be a better investment than a bigger A/C, and less expensive. Get the load calc done before you do anything else.

The only thing is - zoning systems are notorious for lack of information about them, and the usual tactics are to hard sell them on you in hope that you don't understand a thing and just go with the flow. I ended up with my own, cost me about $500, but this is not for everyone - it's rather an ongoing hobby than a solution, and R&D cost is not included

Pros, correct me where I'm wrong.

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--vt

Poorly installed a zone system can be very loud. You can't dump 5 tons worth of air through 2 tons worth of duct without lots of noise.
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"I'm offended by political jokes. Too often they get elected." Henny Youngman

Make sure the contractor you use is reputable. Don't go with the best price, unless he's the best guy. Have him explain why the R-410A system allows him to use a smaller unit.

I've installed zoning systems. As a matter of fact, many of the commercial buildings we do, depend on zoning to get by with smaller equipment. One OLD example of zoning for 2 story homes is one system for upstsairs and another system for downstairs.

For example, in your situation, with that big temp diff between upstairs and down, without zoning, to keep the upstairs at the temp you desire, the downstairs must get a few degrees colder. That's wasted cooling capacity that could be directed upstairs. A 2 story house is ideal for zoning, which, in my opinion, without knowing ALL the facts, will probably result in being able to use a smaller unit. If you use a reputable dealer, he will "guarantee" that his system will provide a certain temp inside, with a certain temp outside.

If you doubt the smaller unit will work for you, have him put it in writing, that he will install the next size larger unit, of equal ratings, at no cost to you, (or for the price 'difference' between the two systems) if the system does not perform as described, within 10 days upon finding it does not perform.


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The more I learn, the more I realize I've got more to learn.

Ask your Contractor about the static pressure build up in the duct work when you zone off your 1st level. It could lead to alot of noise up to Duct Failure. Be Careful. Can you afford 2 Systems? That type of zoning works great here in S. FL. By the way if your Contractor say's Uh!!! about the duct work and static pressure, get rid of him. Good luck

Kim, you said it, and you also can't dump 5 ton worth of cooling into 2 ton worth of duct and expect the system to operate properly. Most zone systems I see are poorly designed. Had a 4 ton with 3 zones and 2 of them fed with 8x14 duct. Now if only one zone is calling, what happens to your humidity removal, temperature drop, superheat, and heat exchangers and limit in the heat mode? Not to forget the noise this will cause by the extra static pressure.
If there was a portioning zone system that monitered temp rise and drop, static pressure and adjusted modulating dampers to direct the airflow in portions rather than an on/off damper system this might be a good thing. All a bypass damper does is recirculate the same air back through the blower time and again with some of the return air mixed in. Seems counterproductive to me.
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Rottendog

At some point in most peoples lives they come to the understanding that some things are best left as is.

My Arzel zoning system is so quiet, I haven't noticed any change in noise since it was installed (even when only one of the 4 zones is calling for air). I have a 2 ton unit for the downstairs and a 3.5T second and third floors (only the upstairs system was zoned [4 zones]).

I would highly recommend zoning to anyone who has temp diffences such as hot side / cold side of house and upstairs/downstairs. I don't even cool or heat the 3rd floor of my house if I'm not using it, but it does cool down very quickly when I call for cooling. I had four estimates; price is roughly $1K per zone. A little cheaper if you don't use Arzel, but I have read many posts about noise and maintenance issues with Honeywell / Trol-a-temp.

RDog's last sentence about the bypass damper recirculating air can be a positive. For instance, I'm cooling the 3rd floor right now by simply sliding the fan switch to the ON position. I may just be pulling cooler air from the other zones or if one of the other zones is calling for air conditioning, I will be pulling air that's being dumped from the bypass damper (room has cooled 5 degrees in 20 mins). When I leave this floor, I will turn the switch to OFF and not be concerned with temperature since I won't be using it.

JR, here's an idea for you. Since you never mentioned your existing equipment was not working, why not have the zoning done? Then you'll know if a 3T unit will work. My gut tells me you'll be fine since that's what you have now and you make do with it anyway. Who knows, you might get another 3 to 5 years out of your existing system especially if the zoning is done correctly and you don't run the unit nonstop (for instance, at night you don't need to cool downstairs so you will be cooling less area/volume). When your system is officially "broken" you can replace the AC unit and enjoy the added comfort of having a zoned system.

JR, think about it a little while, ask lots of questions and interview contractors who are experienced with zoning in your area before making a decision.

Just two more cents: Instead of buying a Lexus ES300, I bought it's lower priced cousin, the Toyota Camry XLE. The difference in price was the cost of having my house zoned. I know the car won't appreciate in value, but as for the house.....

Jrminator, Your upstairs load will be higher during peak times as the ceiling of the first floor will generate very little load due to basically no temperature difference between first and second floors. Therefore, the unit can be smaller because you have reduced the load to the lower level.

In my case I've got 2600 sq.ft. of conditioned space. Load calcs come out to a little over 38KBUT/H. The system will have 4 zones, two down stairs and two up and will have a 3-ton unit must likely two speed. The heating side has about a 36KBTU/H loss.

I've run the numbers looking at the loads in each zone. When you look at the time of day each zone will be getting most of the heat you then start to understand why it will work.

So in the end of the day the logical side of my brain knows that it will be fine but the emotional side keep turning on warning bells. It's a concept that make you go nuts if you don't trust the numbers and turn off the emotions and that is the hard part.

So jrminator trust that if the numbers are right it will work very well. Just make sure it's a good zoning system. From what I've read here carrier/bryant is one of the best but they do cost more. Find out what your contractor would use and post here. I'm sure the members will give you some good advice about the equipment.

And my thanks again to some of you here that have helped me understand this with our off line chats.

Can a zoned system result in a an AC unit that is smaller than the manual J load take off says it should be if I want all of the zones at the same temp at the same time. Manual J assumes that all loads happen at the same time which is essentially true for my house. When I say smaller AC unit I mean smaller than the calculated load, not smaller than what I already have. If the calculated gain is 3.5 T how can you use less than 3.5 T? It seems to me that if the load is 3.5 T you've got to supply 3.5 T no matter how you divide it with zones if all of the load happens at the same time. Thanks !

jrminator:

There was this long and complicated joke about answer that is absolutely exact, but absolutely useless... Here's one of those:

You will not find the answer to your question until you

- install the zoning system
- install the data acquisition system that records the temperature in all the zones at least, all the rooms at most, and creates temperature charts.

Order doesn't really matter, in fact, it would be a good idea to take a look at the charts first.

Then, you will know if

- all of your house needs conditioning at the same time (I doubt so)
- existing unit is working at the limit (and smaller will not suffice)

Not before.

Can it be done? Yes. Take look at what I did: http://freehold.crocodile.org:8080/image_archive/2002-04-18/

Can you do it? If you want to go through all the hassle yourself.

Can you make someone do it for you? Not likely, I haven't heard about such a thing offered as a service.

So, is this answer exact? Hope so.
Is it useless? Absolutely.

You have to take a leap of faith here and go with your feelings.

Just my $0.02, my house now has a 4 ton unit, every bypassing contractor tries to sell me 5, load calc shows 3 in existing configuration with single pane windows and no ceiling insulation, and 2 after that's fixed.

Go figure.
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--vt

From what I understand "YES" and in fact everything I have read states that you should undersize a little. Why? Because not all rooms will load the same way during the day. Some will take more and some less. Zoning take advantage of that by directing more air to the hotter zones. Also, from everything I have read and others have stated here Manual J is 20 to 30% oversized in the first place.

As one person stated here you can have a problem if you have a BIG party on a HOT day you might not have enough to cool down the whole house unless you shutdown some zones.

Again, don't take my word for it but there are others here that can fill you in much better than I.

quote:

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Originally posted by jrminator
Can a zoned system result in a an AC unit that is smaller than the manual J load take off says it should be if I want all of the zones at the same temp at the same time. Manual J assumes that all loads happen at the same time which is essentially true for my house. When I say smaller AC unit I mean smaller than the calculated load, not smaller than what I already have. If the calculated gain is 3.5 T how can you use less than 3.5 T? It seems to me that if the load is 3.5 T you've got to supply 3.5 T no matter how you divide it with zones if all of the load happens at the same time. Thanks !

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quote:

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Originally posted by jrminator
Can a zoned system result in a an AC unit that is smaller than the manual J load take off says it should be if I want all of the zones at the same temp at the same time. Manual J assumes that all loads happen at the same time which is essentially true for my house. When I say smaller AC unit I mean smaller than the calculated load, not smaller than what I already have. If the calculated gain is 3.5 T how can you use less than 3.5 T? It seems to me that if the load is 3.5 T you've got to supply 3.5 T no matter how you divide it with zones if all of the load happens at the same time. Thanks !

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TGravlin:

And to your point about the smallest zone calling, the best answer to that is a combination of modulating dampers and variable speed multistage unit.

jrminator, you will quickly find out that your options are in fact unlimited, all depends on how much you are willing to invest (it's not just about money, time is involved too) into the solution of your problem.
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--vt

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Originally posted by vt
TGravlin:

And to your point about the smallest zone calling, the best answer to that is a combination of modulating dampers and variable speed multistage unit.

jrminator, you will quickly find out that your options are in fact unlimited, all depends on how much you are willing to invest (it's not just about money, time is involved too) into the solution of your problem.

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