Please advise someone who just got central air conditioning.

[Le Ministre de l'au-delà]

The quick version of the question - I'm 'balancing' the newly installed forced air central AC. I am trying to get the third floor bedroom comfortably cool overnight without turning the lower two floors into a deep freeze. At present, I have the vents closed on the first and second floors, open on the third floor. The thermostat is on the first floor; there is a 'return' on each above-ground floor.

This setup did not produce the desired results. Last night, it was too hot to sleep comfortably on the third floor, while the ground floor with the thermostat was too cold. It was cooler outside the house than on the third floor, and pre-AC, I would simply have opened the windows and run a fan. I had the forced air fan switched to 'on' rather than 'auto' for a while, but I'd prefer not to have that constant noise and energy use. (Frankly, I'd prefer not to have the energy use of an AC unit at all, but I'm not the only one who gets a vote...)

I stopped running the AC in the evening at 25 C - the overnight was set to allow the temperature to rise as high as 28 C. By the end of the night, the thermostat was set for 24 C. We have a programmable thermostat that will allow for 4 different time/temperature settings for both weekdays and weekends.

Is there something counter-intuitive about AC that I should know about? Please advise me, you who are wise in the ways of AC.

[Vinyl Turnip]

Our condo has dual A/C units and separate upstairs/downstairs thermostats. Works quite well, but I presume not the quick fix you were looking for.

[troub]

Yeah, different "zones" would probably be best. It's also necessary for maximum comfort to make sure the attic/roof is properly ventilated so the rising hot air can actually leave the building.

[CookingWithGas]

We leave the fan set to "on" to continue to redistribute air throughout the house when the A/C is off, though you say you don't want to do that. It sounds to me like your installer used a cookie-cutter approach instead of one designed for your house.

Another solution which works a little better than just closing the room vents is to install baffles in the main ducts coming out of the system. You can install levers to close off an entire branch, though feasibility depends on whether they have a different branch going to each floor. This is not short-term as in today, but probably not that hard for a contractor to do.

[ZipperJJ]

Before my brother's baby was born, I helped him put an overhead fan in each of the 3rd floor rooms (one of them being the nursery). It's been almost a year, and I just asked him how it's helped and he says it helps a lot to have the air circulating up there. I suspect they have the fans set to "suck up" instead of "blow down."

[FoieGrasIsEvil]

Is your first floor a basement by chance? If so, that's definitely the wrong spot for your thermostat. If you've closed all the vents on the first two floors, I'm a little surprised that you're not getting a decent amount of cool air up there on the third floor. In a 3 story home, the thermostat should always be in the middle floor, right guys?

[sachertorte]

One thermostat for 3 floors simply isn't going to work. You need to send the cool air to where you need it, and the only way to know where you need it is to have multiple thermostats. I think there exist wireless thermostat+baffle systems that you can put into the duct work that will open and close based on additional thermostat readings.

Fans seem like a good bet too.

Also, have you checked that the air coming out on the top floor is actually cold? If the ducts are not well insulated, all the nice cold air can get warm before it reaches the exits.

[FoieGrasIsEvil]

One thermostat for 3 floors simply isn't going to work. You need to send the cool air to where you need it, and the only way to know where you need it is to have multiple thermostats. I think there exist wireless thermostat+baffle systems that you can put into the duct work that will open and close based on additional thermostat readings.

Fans seem like a good bet too.

Also, have you checked that the air coming out on the top floor is actually cold? If the ducts are not well insulated, all the nice cold air can get warm before it reaches the exits.

I think it can work if it's on the middle floor and you close all the first floor vents, and close the 3rd floor vents and reopen the first floor ones for winter heat. That's the situation I have.

[Dag Otto]

Is there something counter-intuitive about AC that I should know about? Please advise me, you who are wise in the ways of AC.


No, nothing counter intuitive - the fact is that the system you had installed is not ideal for a multi-level house. With one thermostat, you have a single zone system. The AC will try to satisfy the area where the thermostat is located. With three levels, a zone for each level would be ideal. One fix would be to relocate the thermostat, but then you'll have a similar problem - the bedroom will be ok but the first floor will be freezing.

Did the contractor discuss this with you before installing the system?

[Le Ministre de l'au-delà]

Oh, my goodness - such a flood of replies while I made lunch...

I should clarify a couple of things - The insulation, exhaust fans, duct work and HVAC calculations were all done as part of the renovations of this house, and I have no reason to suspect there is anything wrong with the installation of the AC unit, nor the design of the HVAC system of which it is a part. I'm asking advice about its operation because I've never lived anywhere that has AC, and I'm not sure how to rig it so that it does what I want it to do. I think my first mistake last night was in not letting the system cool the house enough before we went to bed, coupled with setting the overnight temperature too high.

I should mention, too, that the HVAC system's heating performance has been exactly as we'd hoped for. Of the two, the heat is much more important - people have lived in this part of Ontario for centuries without AC, but never without some source of heat for the winter.

And when the exterior temperature is in the low twenties, I'd much rather cool the house by opening the windows and shutting the HVAC off.

Vinyl Turnip Well, yes, I was rather hoping for a response like 'No, you'd think closing off the 2nd & 3rd floor ducts would do the trick, but actually because of (something I'd never thought of) you need to open them and all will be well.

troub Yes, covered on both counts. We totally removed the interior walls of this house and insulated to R24, and the third floor roof is vented properly. Pre-AC, the interior of our house was consistently cooler than the outside until late afternoons of sunny, 30+C days, particularly on the ground floor. The third floor is a challenge because it's higher than the neighbouring houses, therefore receiving a good 16+ hours of sunlight on cloudless days.

CookingWithGas I'll experiment with the fan switched to 'on' - last night was our first night with the new rig, and I couldn't hear whether the AC was kicking in or not. It could be that I need to suck it up and deal with the fan.

ZipperJJ We've had ceiling fans in previous houses without AC, and that may be a longer term solution...

FoieGrasIsEvil No, the first floor is the first floor above the foundation. I have not mentioned the basement in all this, and the thermostat is not in the basement.

sachertorte Yes, the air coming out of the vents on the third floor is cold air.

Dag Otto Yes, as mentioned above, the unit complies with the HVAC design that we had to get back in 2007. I'm just not at all experienced in its operation, not having ever been a fan of AC.

[Surly Chick]

I have no advice but can I come over and enjoy your central air?

[Moonshiner]

Like Dag Otto said, you need a zoned system. Depending on how the ducts are run you may be able to retrofit dampers and add thermostats to accomplish this.

If you're lucky.

[Le Ministre de l'au-delà]

I have no advice but can I come over and enjoy your central air?

But of course! I'm more accustomed to offering our guests wine, whisky or song, but I'm sure we can scrounge up some central air...

We can be easily found - there is a sign on our door that says "Dopers Welcome" in the ancient tongue. Speak friend and enter.

Just don't disturb the waters in the front...

[Acsenray]

I'm asking advice about its operation because I've never lived anywhere that has AC, and I'm not sure how to rig it so that it does what I want it to do.

There's no real trick to using central a.c. With a single zone system like you have, only one of your three floors is going to be at exactly the right temperature. The other floors are going to be either too hot or too cold.

If it were me, I'd set it so the floor I slept on stayed comfortable for the night and forget the other two floors until the morning when I had to go downstairs.

[AuntiePam]

Before my brother's baby was born, I helped him put an overhead fan in each of the 3rd floor rooms (one of them being the nursery). It's been almost a year, and I just asked him how it's helped and he says it helps a lot to have the air circulating up there. I suspect they have the fans set to "suck up" instead of "blow down."

I've wondered about this. Which setting is which? Our fans have a switch to change the direction of the fans -- clockwise or counterclockwise. Will one direction suck up and another blow down?

[Acsenray]

I've wondered about this. Which setting is which? Our fans have a switch to change the direction of the fans -- clockwise or counterclockwise. Will one direction suck up and another blow down?

Yes. Which is which depends on which way your blades are angled ...

Do you feel the direct blow in one direction and not in the other?

[Zsofia]

I've always been told that closing vents makes the whole thing less efficient. Not sure if that's true, or true for every installation.

You definitely have to give it time to cool the place down, though. My thermostat is programmed to start cooling at 5, assuming I'll be home around 6. The summers we have, though, it often doesn't make it.

[t-bonham@scc.net]

and I have no reason to suspect there is anything wrong with the installation of the AC unit, nor the design of the HVAC system of which it is a part. I'm asking advice about its operation because I've never lived anywhere that has AC, and I'm not sure how to rig it so that it does what I want it to do.Actually, there was something wrong -- the installers failed to explain to you how to operate it. You should call them up and ask them these questions. They installed it; they will want you to be satisfied with their work.

Personally, I agree that during a big renovation like this, they should have done a multi-zone system. Even if it costs a bit more to install, it will be much more efficient over the years. Even more so for heating than AC (and in Canada, you will have a lot more days of heating).

[ZipperJJ]

I've wondered about this. Which setting is which? Our fans have a switch to change the direction of the fans -- clockwise or counterclockwise. Will one direction suck up and another blow down?

You have to look at the tilt of the blades. If you want it to blow down, you make it spin in the direction of the higher side of the blade. If you want it to suck up, spin in the direction of the lower side of the blade.

The best way to tell is to flip the switch and stand under it, perhaps with a tissue, and see exactly what ends up happening! :)

[Yeticus Rex]

Maybe see if somebody that has knowledge of Zone Dampers (http://en.wikipedia.org/wiki/Damper_%28flow%29) could modify your current system?

[Dag Otto]

I've always been told that closing vents makes the whole thing less efficient. Not sure if that's true, or true for every installation.



As you close off vents, you increase the staic pressure the system is working at. While you will probably get more air on the third floor where you want it, the total volume of air and the power used by the fan will also change. If you had the fan curve for this fan, you could plot out where the fan is operating at. But the problem that might (might!) occur is that the airflow across the evaporator coil may drop too low.

How this works is unique to each specific installation.

[Snnipe 70E]

Your big problem is not supply air but return air.

If you dump a lot of cold air on the third floor the cold air will be at the bottom of the room and the hot air will rise to the top, Cold air will drift down to 2nd floor and the 2nd floor's hot air will rise to the third floor.

I have a two story house that I added AC to and that is my problem. I need to get the hot air off the top of 2nd floor returned to the AC unit and cooled. In my case it will be a major change to increase the return air from the 2nd floor.

If I leave the fan in auto my second floor will be over 10 degrees F hoter than first. But by running the fan on hand I can get the two levels within 2 to 4 degrees of each other over time.

[Le Ministre de l'au-delà]

Tonight's experiment has been to leave the 3rd floor bedroom/bathroom doors closed while the AC has been running - the third floor return is about 2 metres off the floor in the bedroom. Results so far are quite promising - it seems to have effectively trapped a fair amount of cold air in the room. I'll let you know if it works out...

t-bonham Yes, I agree, although it would have been better if I had fully explained to the installers exactly what it was I had in mind. I'm now back from vacation and will be bugging them for further instructions over the course of the next week.

[LSLGuy]

You definitely bought the wrong thing if you have a 3-story house and a single zone HVAC system. The problems you are having now will be mirror-imaged in winter. Since you just did a total renovation I'm mystified how any HVAC contractor with the brains to get a license would have agreed to install a single-zone system.

Here are some prior threads on topic.

http://boards.straightdope.com/sdmb/showthread.php?t=530348
http://boards.straightdope.com/sdmb/showthread.php?t=471015
http://boards.straightdope.com/sdmb/showthread.php?t=349312

I posted to those; there are several other threads on topic as well. The key thing, and I cannot stress this enough, is to close off stairwells to the degree possible. Anything you're doing with 6 inch by 12 inch vent louvers is totally overwhelmed by having a 3 foot by 8 foot hole connecting two floors.

An additional thing to consider. You will NOT suceed in cooling the 3rd floor by 10+ degrees F in just a couple hours. So if you're expecting to turn on the AC at around dinner & have the room comfortable at bed time, that's unrealistic even for a properly designed and installed multi-zone system.

Try this: Get your floor-to-floor flows minimized as described in those threads. Then maximize the flow to the top floor. And set the thermostat to what you think will be a decent value, and leave it alone for 24 hours. No raising or lowering during the day. Then evaluate the upstairs at bedtime. If too hot or cold, adjust the thermostat by that number of degrees and wait 24 hours. Repeat as needed to get to a 3rd floor temp that suits your sleeping.

Once you know what setting gives you the right answer in the 3rd floor for a steady state, THEN you can start doing things like turning the stat up during the day and down again a few hours before bed. Note I said "few", not "couple". Particularly if the top floor has lots of windows to soak up that sun, I'd bet 4-5 hours is about right. Getting this right will also take 2-4 days of experiments. Use the stat's time-of-day feature to be consistent.

Once you finally know what works to make the bedroom right, then you can start playing with altering the balance between the rooms & floors to try to avoid a deep freeze on the ground floor.


You can also expect to need to reverse the air balance settings each Spring & Fall. In Fall you'll be closing vents on the top floor & opening them on the ground floor. And setting any ceiling fans you may have to blow upwards. In Spring it'll be the opposite; air vents open on top floor, closed on bottom floor, and fans blow downwards.

But I'm gonna bet that when all is said and done, the best you're gonna get from a 1-zone system in a 3-story house is 1 hot floor, one medium floor, and one cold floor. And that'll be true year round. You can get a better result than you have today, but yuo're not gonna work miracles with a bad installation. Which, sad to say, is exactly what you have.

[Le Ministre de l'au-delà]

...Except we've lived in the house since Dec. of 2008 and there is nothing wrong with the heating system. All ducts and interior doors open, Away and Overnight temperature has been set to 18 C, Wake Up and Return temperature set to 21 C. It has worked well, and given satisfaction in terms of comfort and efficiency.

Last night's experiment worked quite well - the third floor bedroom/bathroom doors were closed when the AC kicked in at 5 PM, and the third floor cooled nicely. I also opened the vents in the kids' rooms on the second floor - they're visiting Grandma and Grandpa at the moment, but their rooms will probably require some cooling as well.

It could well be that my expectations of AC are unrealistic - I view it as an occasional luxury to take a couple of degrees off when the outside temperature is 30+ C. The fact that I need to take a sweater to a film or a shopping mall drives me insane.

Yesterday, our interior temperature held at 26 C while the thermostat was set to kick in at 28 C - it never came on. Outside temperature was 31 C. 5 PM the 'return' setting started to cool the house to 25 C, and other than running the system fan overnight, it was a comfortable evening. It would be a good idea to get a thermometer for the third floor so I can start keeping track of what works best.

And of course, if the outside temperature drops to 25 C overnight or lower, I'll shut the thing off and open the windows.

[Acsenray]

It could well be that my expectations of AC are unrealistic - I view it as an occasional luxury to take a couple of degrees off when the outside temperature is 30+ C. The fact that I need to take a sweater to a film or a shopping mall drives me insane.

Perhaps your area has a low-humidity climate. Around here, AC is mandatory around the clock in the summer to keep the damn humidity at a comfortable level if nothing else.