Multizone climate control

[dscline]

making sure that the system is distributing the heat/cooling evenly is a one time process that will ensure your getting the best the equipment can deliver . . .

Unfortunately, it rarely works out that well on two story houses with one system. Especially when dealing with high ceilinged areas that span both floors. Too much room for stratification. It's usually impossible to balance such that temperature is even everywhere in both winter and summer.

Personally, I think dual systems are well worth the price in two story homes. Or zoning IF you have a system with ductwork sized to handle it.

 

[quadmasta]

I went up in the attic last night and rooted around. Right off the air handler, there's a big rectangular duct and then there are other flex ducts off of that to feed the rooms. Here's where I think a major problem is. On the top of the rectangular portion there's a really big duct that comes off and goes to a duct box that's probably about 2x2x2 and then has 5 6" ducts that split off and feed some room downstairs. This big duct is at the exact same part of the duct as almost all of the upstairs ducts. Seeing as it's bigger it will have less resistance than the smaller ducts and would get more air. Is this a safe assumption?

I have no dampers anywhere in the system. Last night I partially closed a lot of the registers downstairs to see if it made a difference upstairs but it didn't really do much at all and made a lot of noise.

 

[dscline]

Right off the air handler, there's a big rectangular duct and then there are other flex ducts off of that to feed the rooms.

That is not going to help your situation. If flex duct is used all the way from the furnace/air handler to the diffusers, that's going to introduce a fair amount of pressure drop. Flex duct is not smooth on the inside, and is therefore fairly resistant to airflow. It's best used as a replacement for elbows where it's difficult to get an elbow to line up well, but it's often used as the primary duct because it's easy.

Seeing as it's bigger it will have less resistance than the smaller ducts and would get more air. Is this a safe assumption?

Well, not exactly. The resistance is going to be comprised of the resistance along the entire path. The resistance after the box, where individual 6" duct goes from the "box" to the individual diffusers is going to dominate the total resistance for the run. Just because the duct between the "box" and the initial plenum off the air handler is larger doesn't mean it has less resistance. It's also handling more air (the sum of all the smaller runs downstream of it). It's the velocity of the air that's important.

I have no dampers anywhere in the system.

That does not surprise me. In residential systems, I think most HVAC contractors consider the dampers in the diffusers to be the balancing devices, even though technically that's not what they are supposed to be.

Last night I partially closed a lot of the registers downstairs to see if it made a difference upstairs but it didn't really do much at all and made a lot of noise.

Then that tells you what to expect from zoning dampers. The only difference is: they'd be automated, and would likely make less noise since they most likely wouldn't be right at the outlets. But from a temperature balancing perspective, closing off the diffusers, or having a zone damper that closes, is going to provide the same result. If the former didn't help, neither will the latter.

As I mentioned earlier, it's really hard to get a good balance in a two story home with one system. If you don't have access to make significant changes to the duct system, you probably won't get really good results without splitting the system up, or adding spot cooling from something like minisplits where needed.

 

[quadmasta]

Please tell me how I can fix the problem. I have access to pretty much all of the ductwork upstairs and most of the ductwork downstairs. Even if it's multiple hundreds of dollars it'll be cheaper than my A/C compressor running every 3 minutes.

 

[dscline]

I have access to pretty much all of the ductwork upstairs and most of the ductwork downstairs. Even if it's multiple hundreds of dollars it'll be cheaper than my A/C compressor running every 3 minutes.

I'm sorry, I've not been paying close enough attention... there are two of you posting with problems in the thread, and I've kind of lumped you together. bfisher indicated he didn't have access to his ductwork... now that I realize that you do, your situation may not be quite so grim.

First, we need to identify exactly what your problem is. Do you have areas that are consistently under-aired, or do you have problems with load "shifting", where sun, interior heat generating items, and stratification can cause the "hot spots" to move around? For example, if the problem were simply that your bonus room was too hot in the summer, and a little cool in the winter, then it's just not getting enough air. I would increase the size and/or number of runs to that room. BUT, if that room is hot in the summer, and in the winter it's still comfortable to too warm, then a zoning system may help to deal with the "shifting" loads.

Every room upstairs is mostly comfortable with the exception of the bonus room but the temperatures in the rooms throughout the house are dramatically different and that makes it uncomfortable.

If you can detail which rooms are uncomfortable, when, and where they are, it will be easier to understand the problem. We first need to distinguish between "some rooms are consistently under aired", and "some rooms have too much air at some times, and too little at other times". The former is a balancing issue, the latter is a need for zoning.

 

[quadmasta]

I'm afraid I don't have enough historic data to answer your question completely. As it sits, the bonus room is WAY too uncomfortable and is probably 4-6 degrees hotter than the other rooms upstairs. Should I run another 6" duct to the room or run a 12 or 14 and then split 2 runs off of that larger duct?

Also, should I add a return in this room?

 

[dscline]

Personally, I wouldn't spend TOO much money until I had a better understanding of what the problem was. If the bonus room is the bulk of your problem, I would certainly just work on adding airflow to that room first. I would simply add another run/register to that room, and see what that does. That would be pretty cheap, and in the mean time you can see what the effects are. You may find that things become much more tolerable, and if that room gets too warm in heating season, you can just shut off one of the vents in winter. As far as a return, just keep in mind that however much air you dump in that room has to easily be able to get back out. If there is no door, or the door is rarely closed, I wouldn't worry about it. If the door is frequently closed, then you should add a return, or ensure that there is sufficient space under the door (or other paths) for the air to get out.

 

[quadmasta]

Do you have AIM? I'd like to talk to you a bit more.

 

[pete]

just jumping in for a quick question (or two), . . . is your bonus room fully insulated and sheetrocked ? ? ? does it have an non-typical heat load we wouldn't be aware of ? ? ? (alot of marginally insulated ceiling without any attic is non-typical, even worse with a dark roof)

also, 3 tons of cooling could be iffy in your area for 1900 sq ft ( I have a 3 ton unit for ~2000 sq ft, but in NY) ? ? ? was the bonus room considered in the original design (sizing of the A/C units) ? ? ? does your "HVAC buddy" (with local knowledge) think this is enough ? ? ?

. . . trying to fix HVAC issues in a text only forum can be dicey, a visual inspection is a bitch to do without . . . but I think your getting some very valuable input, it's just hard to fully understand the environment your working in . . .

Pete C

 

[quadmasta]

The bonus room is fully insulated. The sides with the attic crawlspaces are insulated with R19 and the cieling is insulated with blown cellulose to R19 as well.

My HVAC buddy and my home inspector both said the 3 ton unit was within spec for this size home and this area.

What would you like pictures of?

 

[dscline]

I agree with Pete... a live inspection by a qualified contractor/engineer would be wise. That being said, since your unit is in your attic, and presumably reasonably close to the bonus room, an you can put an extra run in for not much fuss and very little money. So not too much to lose with just trying that out.

I don't use aim... not a big chatter. But feel free to use the email link if you have any specific questions. But I'll qualify that upfront.... I am NOT a contractor, nor do I do duct design. I do applications support for a large HVAC manufacturer, but for the commercial division. So I'm not exactly the kind of guy you should be talking to, but I'll help where I can.

 

[quadmasta]

I added a damper at the main HVAC trunk that comes off of the furnace to the 12" run that supplies downstairs and that fixed the heat issue in the bonus room. Now there's some extremely long runs that aren't on that trunk that feed the opposite side of the house that have poor air flow.

Would it do good to put a damper on all of the runs that aren't off of the big 12" pipe and balance all the runs?

I was looking on Smarthome.com today and I saw that a 4-zone controller was only around $250 and the dampers are between $50 and $150 each depending on size. I guess the expense comes in rezoning the ductwork and the thermostats.

I read the other multi-zone thread and saw that there are communicating controllers and communicating thermostats but I didn't see on Smarthome where either would be easily wired to a home automation computer. What did I miss?

Thanks in advance!