Workarounds for single-zone HVAC? Second thermostat? Adjust vents?

I have a two story brick home built in 1970 with a relatively new HVAC system blowing through original ductwork. In the summer its too hot upstairs and too cold downstairs. I know that the full solution is to rework the ducts and introduce a second zone. This is not practical at this time.

I can often get the upstairs temps to be comfortable (mid-70's) by simply forcing the fan on and closing vents in unoccupied rooms. When I do this the temperatures downstairs will drop to the mid-60's. At night this actually makes the entire house too cold to be comfortable. There is also the added cost of having the blower run all the time.

If it matters I work from home. I am here all day so summer comfort is important.

Is there an intermediate fix I can make until I have saved enough to go to a two-zone system?

Can I install a second thermostat? A smarter thermostat?

Are there any vent opening/closing tricks I can try?

• What about moving the thermostat upstairs (with the downstairs vents closed as you suggest)? This way the upstairs will be the comfortable mid-70's, and the downstairs should be cooled by the cold air moving down (which should prevent it from becoming frosty). – Tester101 Jul 6 '11 at 16:55
• I really wish someone would make, and builders would install, a simple tube with a couple thermostats at each end and a reversible fan that would run from the ceiling of the upstairs to the floor of the downstairs. Configure it to run whenever the temp difference is too large, either sucking up cool air or pushing down warm air. – BMitch Jul 6 '11 at 19:32

There are a few things you can do which can help:

1) Adjust the dampers in the duct work. Partially close dampers in the ducts going to the first floor and fully open the ones to the second. Take some time to mark them so you know what goes where. It will probably take a few tries to get them set right. That's usually the first thing an HVAC guy will do for this situation.

2) Seal off the leaks in your duct work. Go around and find leaks and seal them with the aluminum backed tape. It's real sticky and has a peal off backing. Oddly enough, duct tape always seems to come loose on my duct work. Go figure. There are services which will come out and blow a smokey substance through your system to locate leaks and seal them.

3) This can be controversial, but you could cover your low returns in the summer and high returns in the winter. The theory is that this will pull more warm air out in the summer and more cold air out in the winter. There are magnetic covers you can get for this purpose like this:

But you may have better luck with stronger magnetic sheets and custom cut them yourself.

I'm not a fan of closing registers, but that's probably one of the simplest things to do.

There are systems that can be installed which will automatically adjust dampers in your duct work based on thermostats placed throughout the house, but they're pricey.

• Why is covering the returns controversial? Unproven? Varying effect for each house? Dangerous? Same question for covering registers, what are the pros and cons? – Freiheit Jul 6 '11 at 18:40
• I agree, closing the returns is controversial. :) You want to have as much air coming into the HVAC for efficiency, and all your doing is stopping it from taking cool air out of the downstairs during the summer. Start with the dampers, close unused rooms, and use the covers on the leaky vents, not the returns. – BMitch Jul 6 '11 at 19:24
• And duct tape is not meant for ducts, despite it's name. The adhesive isn't designed for the temperature swings, particularly the heat. – BMitch Jul 6 '11 at 19:26
• Well, what I've heard (take this for what it's worth) is that if you cover returns but not vents, you'll increase the air pressure in the room, thereby forcing the "conditioned" air into unsealed room crevices and possibly out of the house. I've also heard people say covering vents and returns make your unit work harder. As far as whether it works, I don't really know. There's too many variables in the equation. Maybe MythBusters has the answer. – billoreid Jul 6 '11 at 19:31
• @B Mitch - Ah Ha! So there IS something duct tape can't fix. I knew it! :) – billoreid Jul 6 '11 at 19:36
• You could replace your current thermostat with programmable model, which would allow you to schedule the temperature/fan settings. You could set the thermostat's schedule to back off the temperature/fan settings at night, but turn them back to something more agressive during the day. This would resolve the "too cold at night" problem. Check the specs and wiring diagrams on the themostat you are considering to make sure it is compatible with your HVAC system, and it supports the necessary programming modes.

• Try to keep the blinds shut in the areas of the house that are too hot, and keep the blinds open in the areas of the house that are cool. Blocking solar heat gain in rooms that the thermostat doesn't "know" about will help even the temperature out. However, if you shut the blinds near the themostat, you'll just magnify the problem. Consider investing in energy-efficient blinds and window coatings.

• (GENERAL SUGGESTION FOR MOST HOMES) Look at beefing up the insulation in your attic (helps with cooling and heating). Maybe consider adding a radiant barrier on top of the attic insulation to reflect some heat away.

• (UGLY AND INEFFICIENT, BUT EFFECTIVE) Consider buying 1 or 2 small inexpensive window-mount AC units to provide supplemental cooling in the hottest rooms, until you have a more permanent solution. Once you have a proper zoned system, you can then store the window-mount AC units and keep them handy in case your Central Air craps out during a heat wave. This isn't very energy efficient, and the AC units will probably be louder than the central air, but you will be able to provide cooling directly to the offending rooms.

Also, keep in mind that a "zoned" system may not be as expensive as you think. You don't necessarily need two central A/C units. Depending on your duct layout, you may not have to re-work the ducts much to add a second zone. You may just need a pair of motorized dampers, a second themostat, and a zone control unit. Still expensive ($1000-$2000), but maybe less than you were expecting to spend.

• +1 for the window units. That's the solution I have for now until I can retrofit some dampers (or do a complete overhaul). – James Van Huis Jul 6 '11 at 20:31