Multizone climate control

[quadmasta]

<edit> IVB </edit>suggested I sign up and post this question here. I just bought a new house and the temperatures range dramatically in the house. The foyer is 2-story and the living room is 2-story. When you walk up the stairs, the foyer and the living room are open on the second floor so there's kind of a catwalk.

The house is 1908 square feet and I've got a single 3 ton A/C unit with the furnace in the attic. There are 4 registers upstairs and 5 downstairs. There are also 3 registers that are above ceiling level of the first floor in the 2-story areas. For whatever reason the HVAC guys only put two returns in the entire house. One is in the master bedroom and it looks like it's a 6" duct. The other is upstairs on the catwalk nearer one of the upstairs bedroom and it's got a very large duct on it. The grille is 2'x3' but I'm unsure of the duct size.

The bonus room is over the garage and has attic crawl access on either side of the room. The bedroom nearest the return upstairs has a single attic crawl access door. 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.

I was thinking about possibly implementing a multizone climate control to help my issues as well as heat and cool more effeciently.

I looked online and found several electrically closed dampers of various sizes. I assume these would hook to some sort of central control panel and then thermostats would have to be set up in each zone.

Am I correct in assuming this? Will it be cost efficient to do this? I live in Georgia and it was 96 today and it's still got 2 months to get hotter. I think the problem will only compound itself further as it gets hotter.

Thanks in advance for the help and mods you can move this if I posted in the wrong section. I found this to be the most relevant forum.

 

[dscline]

It may help, but it won't solve a poorly balanced and/or designed system. It sounds as if you barely have enough ducting as it is for that size unit... adding in dampers that are going to close off some of the runs in an attempt to balance out the temperature is only going to hurt your airflow even further, cutting down on the total capacity. Honestly, I'd look at supplementing what you have with another system or a minisplit or two in the worst areas, rather than dumping money into a zoning system that will be working with an already crippled system.

 

[quadmasta]

My HVAC buddy said that the ducting adds up to the tonnage of the unit but it's distributed oddly. He also said the return vents still allow for positive pressure in the house(which is desired).

dscline: What's a minisplit?

 

[dscline]

Nine 6" runs may be sufficient for a 3 ton system, it's at the low end of the acceptable range. If you start closing off zones to try and cool other areas better, your overall airflow will go down.

What's a minisplit?

One example There are several manufacturers, Mitsubishi is a fairly popular one.

 

[quadmasta]

So it's a wall-mounted unit similar to what would normally be a window-mounted unit but the parts are distributed?

Would running a bigger duct to this room and splitting it off to two registers help?

 

[pete]

first and foremost, you need to be sure your existing system is balanced and the thermostat is properly located . . . you also need to be sure that return air can get back to the unit from each room . . .

return air . . .

make sure that your not just dumping cold air into a room, the warm air needs a way out . . . a seperate return duct is best, but louvered or 'undercut' doors are more common . . .

to balance your system . . .

. . . check and see if there are manual dampers at each branch take-off to each diffuser on the supply duct . . . if not, you need to add them . . . (dampers at the diffusers will work somewhat, but make alot of noise)

. . . get a handful of instant read thermometers and put one in each room . . . then adjust the dampers so each room is the same temp with the system running . . .

thermostat location . . .

somewhere in the return air path is best, but be sure it's not in 'airshot' of one of the supply diffusers . . .


Pete C

I love my country, but fear my government.

 

[dscline]

So it's a wall-mounted unit similar to what would normally be a window-mounted unit but the parts are distributed?

Basically, yes. Or a normal split system like you already have, without ductwork. Just depends on how you look at it. :lol:

Would running a bigger duct to this room and splitting it off to two registers help?

Yes, if you have access to the ductwork, improving airflow to the areas is the best thing to do. THEN once you have capability for more airflow than you need, you can add zoning if necessary.

 

[bfisher]

Rather than adding dampers to reduce airflow in the areas with too much cold air - what about adding duct fans to increase airflow to the areas that aren't cooling enough? I haven't done this (yet), but it seems to me to be the more efficient solution...

 

[pete]

Rather than adding dampers to reduce airflow in the areas with too much cold air - what about adding duct fans to increase airflow to the areas that aren't cooling enough? I haven't done this (yet), but it seems to me to be the more efficient solution...

how could adding fans be more efficient ? ? ? if an area is too cold, then that air would best be used in an area that is not cold enough . . . by using a damper to reduce the airflow to one diffuser, that air will end up going out a different diffuser (one with less restriction) . . . you don't 'lose' that air . . .

. . . a properly balanced system is the most efficient use of the equipment you have . . . and adding airflow doesn't add cooling . . .

they're not usually 'handy' , but if you look at a duct design manual I can guarantee that dampers are recommend at all terminal branch runs, so you have control over each individual diffuser . . .

Pete C

(20+ years in HVAC ductwork layout, design, and occasionally, installation)

 

[bfisher]

Obviously you have more experience than I do - but in my mind...

- if room A gets enough air that it's too cold
- room B does not get enough air so it's warm
- the system is "properly designed" so it's a max efficiency now

wouldn't boosting airflow into room B reduce some airflow into room A? Wouldn't B cool faster since it's now getting more air? Wouldn't A still cool since it's still getting airflow (more than if A damper was closed, but less since B is drawing more of the air)?

I'm far from an expert - but this makes sense to me.

Not to hijack this thread, but I'm looking at my own situation as well. My current house has 3 floors - and 3 dampers (and has been "properly designed and installed"). The problem I have is that my top floor still doesn't get enough airflow when the bottom 2 dampers are closed that it can't keep up with the hot summer sun (in summer, we usually get 90+ degrees outside). The air out of the vents is very cold - but there is not enough of it. Meanwhile my lower 2 floors are very cool. Why wouldn't adding duct fans to my topfloor to boost the airflow help??

 

[dscline]

. . . by using a damper to reduce the airflow to one diffuser, that air will end up going out a different diffuser (one with less restriction) . . . you don't 'lose' that air . . .

Well, technically, you do reduce the total airflow of the system. If room A is getting more air than room B, that's because the ductwork to room B has more resistance than the ductwork to room A. Closing off the ductwork to room A reduces total airflow because some of the outlets have been closed off, forcing all of the air to try to get through the remaining (open) paths. Since those paths were the more resistant paths to start with, you can have a very noticeable effect on total airflow. Residential systems don't have a lot of static capability as it is.

. . . a properly balanced system is the most efficient use of the equipment you have . . . and adding airflow doesn't add cooling . . .

I agree that having a properly balanced system is important, but for clarification, airflow DOES impact cooling, just not at a 1:1 ratio. Reducing airflow causes the saturated suction temperature, reducing the capacity of the unit. It's the same reason they tell you to change your filters frequently to keep the unit cooling efficiently: a dirty filter reduces airflow, reducing capacity. Looking at some random correction factors shows changing airflow 12.5% typically changes system capacity 2% (at least for the 12.5% above and below nominal CFM). Reduce airflow too much, and you risk icing up your coil. Increase it too much, and you reduce latent cooling to the point where humidity gets out of control.

they're not usually 'handy' , but if you look at a duct design manual I can guarantee that dampers are recommend at all terminal branch runs, so you have control over each individual diffuser . . .

Yes, balancing dampers are supposed to be used, but I haven't seen many actually installed in residential use. Don't know if it's shoddy southern work, or just lousy houses that I've lived in. :lol: IF the OP has access to all of his duct runs, he should certainly see if balancing dampers are there, and if the ones that serve the undercooled areas are fully open.

Depending on the furnace or air handler he has, he MAY have one that is sized such that it has more blower capability than it needs for three tons of air. It might be set on a lower speed tap. If so, a higher motor speed could offset the additional restriction of dampers in the system. Regardless, he can easily see how HIS system will react to zoning it by simply closing off the vents in the rooms that have more than enough cooling, and see how that affects the rooms that need more, and whether or not that significantly impacts his ability to cool the entire house. Much more can be learned by that than debating the theory of it here.

 

[pete]

. . . a properlly designed and installed system wouldn't need booster fans . . . and would heat/cool evenly . . .

. . . many residential systems are never 'balanced' (as I describe above) . . . this leads to inefficiencies . . . almost all of the commercial jobs my (boss'es) company does are required to be balanced (actual air flow is measured at each diffuser) . . .

. . . the adjustments made to the dampers are usually very small, they are only re-directing the air to other diffusers . . . you would never fully close a damper, never even half way (in a properly designed system). . it's more like throttling . . .

wouldn't boosting airflow into room B reduce some airflow into room A? Wouldn't B cool faster since it's now getting more air? Wouldn't A still cool since it's still getting airflow (more than if A damper was closed, but less since B is drawing more of the air)?

while this would work (and makes sense logically), why would you want to run another fan (with added complexity, controls etc.) when you could just adjust a manual damper once and be done with it . . .

in specific reference to your system . . . with the bottom two zone dampers closed (ie: all air/cooling going to the one top zone) you should have plenty of air flow . . . unless the duct is undersized (this would make a lot of 'air' noise). . . if this is the case, adding booster fans will not help . . . also make sure that the air has a return path . . . otherwise airflow will suffer . . . ie: the blower can only overcome so much resistance . . .

Pete C

 

[pete]

Well, technically, you do reduce the total airflow of the system. If room A is getting more air than room B, that's because the ductwork to room B has more resistance than the ductwork to room A.

or duct A is bigger than duct B, with the same resistance . . .

Closing off the ductwork to room A reduces total airflow because some of the outlets have been closed off, forcing all of the air to try to get through the remaining (open) paths.

I don't recommend fully closing the damper, only making minor adjustments . . .

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 . . .

Pete C

 

[bfisher]

in specific reference to your system . . . with the bottom two zone dampers closed (ie: all air/cooling going to the one top zone) you should have plenty of air flow . . . unless the duct is undersized (this would make a lot of 'air' noise). . . if this is the case, adding booster fans will not help . . . also make sure that the air has a return path . . . otherwise airflow will suffer . . . ie: the blower can only overcome so much resistance . . .

I don't think the duct is undersized (I could be wrong) since I don't hear a lot of wind noise. I did check the return path and found a problem (which I have fixed and it improved airflow, but it's still not enough). The air is nice and cold - it's just not enough.

Unfortunately both the ducts and the return path have a couple right angles in them (which I assume is a big part of my problem). And there is no way to remedy this situation.

I've been considering adding a duct fan since we've moved in (still haven't done it since it's a major project - I have no easy access to the ducts) to help move more air around and help the situation.

 

[pete]

if you have a multi speed blower you may be able to get it to push more air . . . it may be running on a slower speed when just the top zone is calling . . .

Pete C