Central Air and Schrodinger’s Thermostat

I recently had central air installed at Casa Kedrosky. It’s a fat R2D2-ish looking Trane thing, with a new blower, etc.

The trouble is, the system blows prodigious, wind tunnel-esque amounts of cold air to two downstairs vents. It cools the downstairs so damn fast that you can nearly make ice cubes on the downstairs floor while the upstairs is still balmy.

I called the installers back and told them we needed to rebalance the air flow. Bringing the downstairs to 15 C just so I could get the upstairs to 25 C didn’t strike me as a good idea. Here’s the ensuing discussion:

Paul: So, how can we fix this? Is there some way to cut the air flow downstairs? It’s literally strong enough to part your hair.

Installer: We could turn down the blower. That would reduce the air flow.

Paul: But wouldn’t that cut air flow everywhere?

Installer: Yes.

Paul: Well, that would make it slower to cool the whole house, and the downstairs would still cool way faster than upstairs.

Installer: That’s true, but it wouldn’t blow so hard downstairs.

Paul: Right, but it’s not the blowing alone that’s the issue. It’s the big difference between upstairs airflow and downstairs airflow that makes it so much colder downstairs.

Installer: Okay. Well, my recommendation would be to move the thermostat. We could move it to the stairs from the kitchen.

Paul: Why would we do that?

Installer: Well, if the downstairs is cooling too much, moving the thermostat somewhere else would help bring that to the right temperature.

Paul: I don’t get that. Why don’t I just lower to whatever temperature with the thermostat here in the kitchen that gets me to the temperature I want elsewhere in the house? What difference does it make where the thermostat is, it’s all relative.

Installer: No, no. If you want a lower temperature upstairs, you should put the temperature sensor there.

Paul: No, that doesn’t make sense. If I put the sensor upstairs and lower it to 22 C, then that’s still going to lower the downstairs temperature to 15 C or so. That doesn’t change anything in terms of the unbalanced air flow.

Installer: We’re moving the thermostat though. It will pick up the upstairs temperature.

Paul: It doesn’t change anything. The furnace doesn’t change its behavior because of where we put the sensor. It doesn’t change anything because it is being observed; this isn’t Schrodinger’s thermostat.

Installer: What?

Paul: Never mind.


  1. a little more attention in high school math would go a long way in situations like this…

  2. Exactly why I like my window units, no ducts=less uncertainty. Isn’t central a/c what brought down the Soviet Union?

  3. Whatever you do, downstairs will be cooler. Doesn’t cold air fall, while warm air rises? 😉 No?

  4. Did this guy sign his name with an X?
    We had the same problem and resolved it with a a technician who knew what he was doing. He checked the ducts for leaks and found that we had huge leakage in the ducts going upstairs. He also put dampers in the ducts for the downstairs. Problem solved.

  5. lol pretty funny but what do you really expect from an installer :)
    Did you ever get this resolved? My wife and I just bought a 3 story townhouse in Florida and we thought it would be an issue, ie first floor being much cooler than the 3rd floor, but that turned out not to be the case.
    Assuming, however, it was going to be the case, we did a lot of research before buying and figured out that the cheapest solution is to actually cut off flow to the 1st floor by closing the vents a little. I know it sounds basic but not sure if you had tried it yet :)
    The next step was to add a fan in the duct(s) to actually get more air going to the 2nd and 3rd floor. We never got a price on it but we were told that would help by getting more air to where it is warmer.
    Good luck getting your house all the same temp.

  6. I’ve actually been saying this for a long time. There is no reason that modern central air systems don’t have electro-mechanically operated dampers that can close the flower to the rooms that cool faster. It wouldn’t take more than a pair of wire running to every room, hell it could be wireless. The dampers themselves would be trivial to implement. Its pretty asinine that this doesn’t exist yet.

  7. Paul – your first problem is that you were mentioning temperatures in celsius. What AC Tech is going to take you seriously at that point? I’m surprised that you don’t have dampers in the vents now, but an easy way to see if that would help is to close your vents on 1st floor (and cover them with a towel or something), then see if you can achieve some balance. If not, then you probably don’t have enought vents (or volume within the existing vents) to make it work anyway. One caution – if you are totally restricting airflow to the first floor, you can put a strain on the blower itself if it’s not powerful enough. I upped the HP on mine a couple years ago and it really helps. Once you do find a solution, run the fan all the time. The “Auto” setting will just reset your problem every few hours (see tech’s comments about placement of thermostat).

  8. Shut all the downstairs vents completely.

  9. Mike O'Connor says:

    The Solution: Split ductless A/C. All the benefits of central air, but can also be controlled on a room by room basis as well. Also much more efficent.
    Of course, you would have to rip out the central air units that were just installed….

  10. lol- Schrodinger’s thermostat
    until you look at the thermostat, you’ll be both hot and cold.

  11. The problem with heating and cooling multi-story houses is that cold air falls and warm air rises. Here are some possible solutions:
    1. Put the air conditioner on the roof. Of course, central air is “central” because it is usually tied into the blower and vents used for the furnace, therefore you have to move your furnace to the roof. Unfortunately, you will just have an equivalent problem when you are heating!
    2. Have a separate blower, vent system and thermostat for each floor in the house. For example in a three story house, the output might be very high to the top floor vent system, lower on the main floor vent system and might simply be off in the lowest floor’s vent system. Actually, I don’t know why they don’t build houses and heating/cooling systems this way anyway.
    3. Move to a bungalow without a basement.

  12. Andy Nelson says:

    It is hard to balance using the dampers that are attached to the vents because of the immediate static pressure regain on the room side of the vent. Effectively, the lower pressure in the room will suck the air out of the vent even if the vent damper is 90% closed. Manual balancing dampers about ten feet upstream of the vents will have approximately equal static pressure on both sides of the damper so that the damper behavior is more linear, i.e. closing it half way cuts the airflow almost half way. The relationship between damper or valve position and flow is called alpha. Close to one is good. The problem with electro-mechanical control dampers for residences suggested by ddn is that for the feedback system to be stable, alpha has to be close to one. Such dampers cost hundreds of dollars each.
    For a short term fix, cover the back of the lower floor vents with aluminum foil. I wouldn’t worry about increasing fan horsepower but I would worry about shifting the intersection point of the load curve and the fan curve to the left. This can lead to unstable operation, vibration, and additional stress on bearings. But you can easily diagnose this yourself by listening to the fan and motor before and after, and if it still sounds smooth after, then you will be OK. Most residential fans have steeply backward curving blades so they can take a lot of variation in load. Switching the auto-on-off switch to on will help to equalize the temperature. It doesn’t cost much to run the fan all of the time compared to other solutions, and the motor is designed for it. Ceiling fans on the upper floor will help to circulate air and equalize temperature. Ceiling fans are cheap to install and run, and if installed in bedrooms can eliminate the need for air conditioning at night unless it is really hot out.
    The best long term fix would be to increase the capacity of return air lines from the upper floor. I believe that the cold air is short circuiting from the lower floor back through the air handler and back out to the lower floor. Less resistance for upper floor air to get back to the air handler will help the upper floor temperature.
    I learned some of this stuff during my days as a consulting engineer, but most of it I learned trying to make modern air conditioning work in our 85 year old house.
    For a reminder of the recent past when air conditioning was not common, watch Body Heat with Kathleen Turner and William Hurt.

  13. That is why in the south we have two units, one for upstairs, one for downstairs. Works great in the summer and winter.

  14. Schrodinger? No. Maybe if the temp changed based on whether or not you observed the thermostat. :-)
    Clueless? Yes. Simply clueless.

  15. Jeremy
    I was exercising metaphoric license… :-)

  16. All new houses in Atlanta have a different unit for every floor. My house was built in the 1950s and is thus freezing downstairs and sweltering upstairs.

  17. Jon Smirl says:

    If you only have one zone, switch the fan for the system to ‘always on’ instead of ‘auto’. It will move the air from the lower floors to the upper ones. This doesn’t work with multi-zone systems.
    I’m in agreement with a previous poster, why are HVAC systems still in the stone age? The technology to do per room zoning has been available for years. Haven’t the HVAC vendors heard of microcontrollers and wireless home automation?
    Fund a green startup to make home HVAC more efficient. There are tremendous opportunities for energy savings available and you can make deals for subsidies from the power companies.

  18. THIS thermostat goes to 11….. it’s like one “colder” (louder) than other ones……

  19. Maybe my earlier message was lost in my attempt at humor. Window units with remotes in the bedrooms even WITH central a/c are not a luxury, they give control to the occupants and freedom to the householder.
    Anything less is abusive to your guests, children or whomever sleeps over. It’s not like window units are expensive. But then I too had softer opinions on the matter before I moved from the coast to the desert.

  20. In the desert, why not use evaporative coolers? They work fine and use way less energy.

  21. And, just to be clear, my clueless comment was directed at the other guy on the phone–not you. :-)

  22. >>> In the desert, why not use evaporative coolers? They work fine and use way less energy.
    Evaporative coolers do not work fine, they raise the humidity (which is why they are called “swamp coolers”), they are useless when the humidity is high, and I don’t pinch pennies when it comes to personal comfort.
    Actually there is a swamp cooler installed on the roof of this building that I do not use. I have five window a/c units. The window units provide much better local control since many of the vents are hard to reach. Window units do have drawbacks, but they are fewer and less annoying than those of central a/c.

  23. Schrodinger’s a dingus who got lucky with a unique last name – would his “thought experiment” (my apologies to Einstein for placing him in such company) have been as famous if it had been deemed “Smith’s cat?” You’re actually onto something though, and are in luck with your own unique surname: you should rename this “Kedrosky’s Thermostat,” take some air temperature differential measurements at the vents (in Celsius of course) and publish your findings.
    Also, by A/C repairman’s logic, why don’t you do us all a favor and mount your thermostat OUTSIDE your house – that way, it won’t be hot out there anymore and we can all stop using our air conditioners.

  24. you could always try this solution -)

  25. Rafael Montoya says:

    Bottom line: either the A/C guy do not want to work more or he just realized he made a mistake with your install. I suggest you consult with your better business bureau to get some commitment from the installer.
    I have been to houses, old and new, who have near perfect Central Air systems, where not only you get the proper temp and humidity all over the place, but also you do not hear or feel it. They are not high tech or have fancy accesories, they are usually straightforward.
    But many places have crummy systems, similar to your installation. These lousy systems are usually a consecuence of someone cutting corners, not by cutting cost but by doing the installation the simpler/lazier way.
    Also the installer should have prevented you about the consecuences of installing your new and more powerful A/C in your house´s older vents and ducts. With the info you are posting it appears you will have to reengineer your ducts and vents to accomodate the new Trane R2/D2.

  26. The comments from Andi are misinformed to say the least.
    Window AC units are very inefficient and have to be noisy since they site the compressor right there in the room. Unless you are deaf, like a box in your window and get your electricity for free, I don’t see them as a good solution.
    As somebody has pointed out, there are multi-zone mini-split systems which can now cool up to four zones and have no ducting. They are very efficient but rather expensive per ton ( I would estimate about $1,000 per tonne ). Fujistsu make efficient ones. The latest technology involves the use of an inverter to drive a variable speed compressor. They allow you to do things like not cool your bedrooms during the day. These units are 16 to 21 SEER which is very efficient.
    The other alternative would be to install dual systems. I also suppose that what should be designed in is a powered air return duct system to suck air back down from the upstairs to the location of the air handler intake. It would seem to be common sense to such the warm air out of the upstairs and move it down to the other side of the room where the air handler is.