From my experience, having a warmer upstairs in a 2-story house is simply a fact - unless you're willing to make significant modifications. If you have 1 AC unit cooling your entire house then an upstairs thermostat really isn't going to help your situation. You'd have to set it much lower, maybe 66, in order to dramatically affect your upstairs temp. In your case it's always going to be cooler downstairs since heat rises. Leaving your blower fan on 24/7 will probably help some but it will never totally equal things out. There are a couple legitimate options to pursue;

1) Get a massive attic fan to help cool off your attic. On a hot day your attic might be in the 140-150 degree range, which certainly isn't helping your upstairs temp. The key here is to get a seriously big fan that will move a lot of air because its very difficult to cool down an attic. You also need to have sufficient ventilation or else the fan will be ineffective.

2) If you get another thermostat for upstairs, you would need to have the systems zoned, so that when the upstairs one does kick on, the vents for the downstairs are switched off and the cool air only goes upstairs. This is certainly an option but will involve a decent amount of cash.

3) I knew one person who created a 'custom' circulation system in their 2-story house. It was very crude, but basically they bought a few fans and installed them in the floor of the 2nd floor to blow cool air upstairs. I think they had 6 different points where they did it in between the floor joists. Like I said, not professional but it was effective.

Depending on where the vents and the returns are, it may help to close some of the vents on the first floor. That will mean more of the cold air is blown out through the second floor vents, which could help balance it.

BTW, the constant running of the fan also works, but that usually means that less humidity is removed. This is a challenge pretty much no matter what--the only half-decent (but expensive) solution is to have a separate system for the second floor.

The Honeywell Vision Pro IAQ thermostat has a circulate position. We use it to circulate air between our main level and walkout basement when not heating or cooling. It works.

Circulate runs the fan for about 20 minutes an hour. It does not run it right after a cooling cycle, so that the condensate has time to drain off the coil before it can rehumidify the house.

There is nothing adjustable about circulate. You get Honeywell's opinion of when the fan should run, period. On our system, the fan runs very slowly on circulate. It is hard to tell it's on. We have a variable speed blower, which I am sure makes a difference.

As far as I know the IAQ can take up to four thermostats, but does not require four. I believe you can set the unit to average them. You should verify this part, as I haven't looked it up recently.

Thanks, guys, but the system is actually fairly well-balanced. It's just that it often doesn't get warm enough downstairs (due to all of the shade) to trigger the AC to turn on. Once it does turn on, the upstairs and downstairs are comfortably within a degree or two of eachother.

Although I'd like a zoned system, that seems to be more of a luxury than a necessity. The house is 'zoned' north-south (manual damper boxes) rather than upstairs-downstairs, and it would require ripping out the basement ceiling to reroute everything.

I was thinking that a multi-sensor thermostat, which bases it's decisions on the average temperature (e.g. average of upstairs and downstairs) might be a much, much, cheaper solution.

Ok - have you tried closing the downstairs vents and setting the stat a few degrees colder?

Why do you care about the upstairs when you aren't home or does it not cool down later?

Obviously, heat rises. Upstairs will always be warmer than downstairs unless you force the a/c to run upstairs more or circulate the air better (which is pretty hard).

It sounds like you can't change the dampers on the downstairs relative to the upstiars? If that is true, then having a seperate sensor will help but only slightly. Let's say the difference is 7 degrees - 80 up, 73 down. And you set your stat to turn on at 78 degrees - if it is taking an average, it won't run, right? If you set it to 75, it will. But how is that different from just setting the downstairs to 70? The sensor won't change the 7 degree difference.

My point is that you must damp down airflow to the downstairs or circulate air better between floors, or all the sensors you add won't change the 7 degree reality. Now - can you shade the upstairs better? - that might help.

As far as zoning being a luxury, I always like to point out that in NC it is code for new construction - 2 systems or 2 zones on a 2 story house.

Thinking out loud.

During the cooling season only use the newly installed upstairs thermostat set at 75 degrees say.

During heating season only use the original downstairs thermostat set at 66 degrees say.

^^^^^

Bingo!!!! My thoughts exactly.Most "remote" thermostats enables you to use one or the other...And lets say the one you buy doesn't,whatever thermostat is set higher or lower should overide the other

[I'm replying to these somewhat out of order -- thanks for all the great suggestions and brain storming!]

(1) Thanks creekside. I'll take a better look at the IAQ. I know that some of the Honeywell's, due to their parallel-serial wiring and funky logic, need to have the sensors installed in squares (for lack of a better term), such a 1, or 4, or 9, or 16, etc.

(2) saltidawg & qdwag: The AC thermostat upstairs and heat thermostat downstairs sounds like a great idea and is one that I have been previously been kicking around. I was hoping there was a dual sensor system/single thermostat system, as I was kinda afraid of having some strange scenario where both kicked in at the same time. I guess what I would like, if one exists, is a thermostat that kicks in the AC whenever either sensor (OR) exceeds a setpoint.

(3) David Cary: the problem isn't just when I'm not home (but that's the most extreme example), it also can happen on the weekends, but to a lesser extent. On the weekends the thermostat is set lower, so it is more likely to kick in downstairs, but the upstairs still has some time to get a little toasty (but not as bad as when it is on a higher setting.
Also, turning the thermostat down a little won't always help. For example, on an overcast 78 degree day, the thermostat (set to 73) will kick in when both the upstairs and downstairs are roughly the same temperature (74). But, on a bright sunny day, the upstairs may hit 80 before the downstairs hits 74. Closing the dampers won't help, as the system is nicely balanced at night, and even during those bright sunny days, once the system kicks in, the downstairs never really becomes too cold or the upstairs too hot.

mattpete,

My suggestion was meant to suggest only using one thermostat and ONLY ONE temperature sensor at a time. Use the upstairs sensor ONLY during cooling season and the downstairs sensor ONLY during heating season.

Forget about averaging two and forget about the higher or lower. Just connect ONE temperature sensor each season.

I looked at my IAQ documentation. You can install a single remote temperature sensor or 4, 9, 16 or more of them. The sensors are actually just thermistors, and there is a single remote input.

By series-paralleling them, you can average the remote thermistor values and make them electrically the same as a single unit. That's why the series of "squares." Each square is a series-parallel combination equivalent to a single thermistor.

You can set the IAQ up to display either the internal temperature measured at the main thermostat, the temperature at the remote thermostat (or "square," if you have more than one), or the average of the two.

Splitting hairs a bit, it says "display" the average. I assume that also means uses the average to control the system, but I don't see any definite clarification one way or the other.

What salti said...

I still think zoning is the only reasonable solution. If you run your A/C based off a stat placed upstairs, your downstairs is going to be noticably cooler than your upstairs. If you think that is a good solution you might as well keep your downstairs stat and just turn it down several degrees.

Zoning requires either a dump zone or a bypass. If an entire floor is bypassed, the humidity won't be properly removed from the air as it will be cooled too fast, and it will feel cold AND damp. The air handler will also probably have more capacity than the duct work to a single floor can handle.

Where are you going to put a dump zone? In the area not calling for cold? That defeats the purpose of zoning the house.

The OP stated in his first post that leaving the fan running evens the temperatures between the floors. The only thing left to do is solve the rehumidification issue. That can be taken care by installing a thermostat with a circulate function.

Sometimes, the simplest solutions are the best.

My upstairs is hotter than my downstairs. I installed ceiling fans upstairs in the bedrooms and they make a huge difference. Just a suggestion!

you won't be happy until it is zoned...either with a separate system up or a motorized damper system w/ a stat down and a stat up. Not that hard. If you don't have room for any type of subplenum, then you could install individual motorized dampers on each duct leaving the supply plenum (this only works if the duct doesn't wye to different areas of the house). Make sure you have a modulating bypass back to the return. The more even the zones the better. Older furnaces w/ poor insulation will sweat from the bypass air, so this could kill the deal.

I'd love to be able to zone the place, but that's not going to be real easy. There are two trunks coming off of the basement AC, one going to the north side and one going to the south side of the house. Off of those two trunks, lines branch off to individual rooms, whether they be upstairs or downstairs. The upstairs (2nd floor) lines go vertically through the central walls to registers in the floor (and horizontally to floor registers on the first floor). It's not like previous houses I've lived in where a trunk goes up to the attic to feed ceiling vents in the top floor.

So, to zone this house would require ripping out the ceiling in every room in the finished basement (and later repairing the ceiling), removing the north and south trunk lines and replacing them with a north trunk for the 1st floor, a north trunk for the 2nd floor, and separate south trunks for the 1st and 2nd floor. And that's before adding any remote controlled dampers, new thermostats, and remote sensors. While that would be ideal, I can't imagine what it would cost (maybe I'm over estimating it in my head, but ballpark figures would be welcome).

I did discover this week that the attic fan isn't working, so I'm going to have an electrician come over and check it out. Once I get that fixed, that should take some heat load off of the 2nd floor (I have plenty of soffit vents but the only vent near the peak is a mushroom-style attic fan). I still think that a Honeywell thermostat with a circ function might be enough to stir things up between the floors and even out the temperature a bit when the AC isn't going (and cause the AC to kick in sooner as the downstairs warms up a bit due to the mixed in upstairs air).

Like I mentioned earlier, once the AC kicks in, the differential between the upstairs and downstairs isn't too bad. Nonetheless, I'm also thinking about hiring someone to add return air vents to each of the upstairs bedrooms. There is currently a single return in the hallway, and undercuts are used to return air from the bedrooms to the hallway. I was thinking that adding air returns to the bedrooms not only might improve airflow, but would pulling the cool air higher up (if that makes sense).

Attic fans can depressurize the house and makes things worse. I would be careful with that.

Can you add a ridge vent? It shouldn't be a very difficult job for someone who knows what he is doing.

I have hip roof, so adding a ridge vent might be a little more complicated.