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.
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.
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.
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.
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.
…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.
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.