Both units are less than 1-year old.

Upstairs unit is 2.5 tons, downstairs unit is 4-tons.

Since I put the attic foil in the attic, I've been looking at other ways to control energy use (CFL bulbs in appropriate locations, be more dillegent about kids' turning off lights, etc.). Both my thermostats are programable and I've got them set to change depending on the time of day, etc (kids in school, at work, weekend, night, etc).

I looked at the energy use on the upstairs thermostat the last few days and it's running ~12 hours a day. The down stairs unit says 2-hours a day.

My upstairs has a connector bridge that is open to a 2-story entry and 2-story front room. So a lot of the AC simply falls down into the downstairs and the down stairs thermostat is near the bridge and gets the effect of that falling cold air. The upstairs thermostat is also near the bridge. The upstairs is set at 76° and it read 76° last night, but the unit was still running (it was plenty comfortable up there as well). Downstairs was also plenty comfortable.

Did I make a mistake and should I have put the larger unit on the upstairs system even though the square footage is smaller? Or do I need to play with the dampers and distribute the air more evenly. Or do I need a new thermostat?

Note that the downstairs thermostat also read 76° but that unit was not running. That thermostat is brand new.

[This message has been edited by agracer (edited 6/6/2011 8:18a).]

Cold air sinks. Hot air rises. It is a trade off on your systems between summer and winter. Just make the adjustments.

I think you are fighting a losing battle that is pretty common in many households. My house is the exact same way. Downstairs unit can keep up just fine, but the upstairs unit in the heat we've been having struggles just to stay at 76*.

We also have the programmable thermostats and we normally kept the upstairs one between 69* to 74* while we are there. While we are gone, we bump it to 67*-76* but the damn thing still has to run wide open just to keep up. I'm going to do some modifications to my attice (thermostat controlled exhaust and attic foil....just haven't gotten around to it yet) and that should help some, but I expect it will still run more. I think it's the simple fact that the 2nd floor acts like a huge layer of insulation for the first...no way to beat it, it just is what it is.

Something that may make it read inaccurately though is if you have the thermostat in the hallway, but there are no diffusers nearby b/c they are all in the rooms. The rooms might be at a good temp, but that hallway is a little warmer by a couple degrees, especially if the doors are closed. Also, if you have a lot of windows upstairs, you are gaining heat there too.

Check your ceiling vents too. You might have to make sure they're open or you might need to replace them too. Also, keep your fans on upstairs.

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I think you are fighting a losing battle that is pretty common in many households. My house is the exact same way.

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That's why I'm wondering if I should not have installed the larger unit on the upstairs system. It's basically cooling 2/3rds of the house.

First, I'd check the temp difference from the supply at the filter vs the temp at the registers. It's a good time to invest in one of those point and shoot temp guns, I got this one and it's great. Compare the differential with your downstairs unit. But no, there's no need to have to do a 4-ton upstairs, something is either wrong with the unit itself, the T-stat (or location of it), or the air balancing.

http://www.amazon.com/gp/product/B000TM7HXC

Agracer - did you get advice to size your units that way? Assuming comparable living area, I would have always assumed the upstairs unit needs to be larger because it has to deal with the roof+attic space. The downstairs has the benefit of the stable earth temperatures and a cooled upstairs.

My home has a 5 ton upstairs unit and a 3 ton downstairs. The lower floor has a bit less living area (the home is on a hill). The upper floor bears the brunt of afternoon heat. Hence the larger unit (replaced last year with a 2 stage unit).

Sounds like the thermostat may be in a bad location which is getting too much influence from the open design. I would tend to agree that it sounds like it is running too much.

As said, check the temps coming out of the supplies. There are a couple ways to look at it, but basically you want a differential of 14 - 20 degrees between supply and return. It is possible that the upstairs unit is low on freon, and is doing a little but the unit is mainly circulating air from downstairs. Checking the temps will help determine that.

If it is a thermostat location issue, you can move the wire, or there are a couple different thermostat option out now that offer remote sensors that work with the thermostat in the base location. They give you much more flexibility for 200 - 300 dollars.

Kevin Weiss
http://www.AbsoluteInspections.net

Has a heat load calculation been done on the home?

If not, start there. I'd be glad to do one for you if you're in the Houston area.

It doesn't have to be super duper scientific, just some actual numbers put down and run. I own a copy of WrightSoft Right-Suite if you're curious as to which one I use.

You'll find that most AC systems are installed backwards (need larger tonnage up than down for the previously mentioned reasons).

And yes... a poor T-stat location can affect things greatly.

What filter size is the return upstairs?

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Agracer - did you get advice to size your units that way?

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Not really. The originals were installed this way and the installer double checked the sizing and basically swapped the units out. The upstairs is smaller, but open to the down stairs (stairs and balcony x 2). All the installers I called did the same thing on the estimates. They just did a simple calculation and load but I don't think any accounted for the balconies/open 1st floor plan.

I think it's the t-stats location. The downstairs t-stat is near the stairs and below the balconies so it gets a lot of that cold air falling from upstairs. It's basically at the entry to my MB. The MB is 3° warmer than the entry alcove where the t-stat is located and that has always been an issue.

Another factor is that old downstairs unit did not have dampers but the new unit does and the installer of the new unit reset the air so a little more is pushed to the S & W side of the house (where all the sun is) and where the house was consistantly warm (see MB note above). He did say if it was an issue after a few weeks to call him and he'd adjust them again (which I should probably do).

He also told me to buy new rigisters b/c the builder grade units are crap and restrict flow (he even told me to get them at Home Depot and DIY so he wasn't just trying to add $$ to his bill).

The air coming off both units seems pretty cold and but I will double check it with my thermometer (have a small digital thermometer that reads consistently thru the house and matches what the thermostats read as I move around the house).

Unfortunately, many HVAC contractors have no idea how to calculate heat loads or even size refrigerant lines properly.

Upstairs will have a much larger heat load due to the roof/attic.

The attic will add some load, but again the open floor plan is what's putting it into flux.

Next winter, I'm going to want the big furnace downstairs, not up. Of course that load is short lived compared to AC loads in Houston.

Yes those old builder's grade registers are junk, restrictive and bulky. That was one of my first upgrades and probably cost close to $150 for both stories but was well worth it.

Here is a link to that thermostat with multiple sensors. Not sure how well it works, had a client send it to me.

http://www.prothermostats.com/product.php?p=proliphix_nt20e&product=110243

There is also one out there that my FIL has that has a single remote sensor. He really likes it. Just google remote sensor HVAC thermostat.

Kevin Weiss
http://www.AbsoluteInspections.net

What type of vent covers did they recommend?

You are way over thinking this. Due to hot air rising and cold air sinking, you will always have an issue with this. The key here is to put your upstairs t-stat on a few degrees warmer than the first floor.

I have built many 2 and 3 story townhomes and this is the way it is. In the winter, you reverse them.

My current house has the upstairs set at 78 and down at 73. And both are comfortable. My up may be a tad warm, but we are rarely up there.

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You are correct. I said this in the first comment above. It is not rocket science.

I don't disagree completely with over thinking it a little - but 12 hours? That seems like it is running excessively. My upstairs thermostat is not particularly well placed, my wife's office is up there and she works from home, and I don't think ours runs that much up there.

Given the things I have seen in new homes with HVAC design, I do not think it is a stretch to at least investigate. Keeping in mind that the open design will make if less than perfect by design.

Most of the winter we can turn off the unit upstairs and just keep the heat on downstairs. In the summer, with the help of some ceiing fans, we can just about use the upstairs cooling unit alone and turn off the downstairs. There are exceptions of course.

I do agree that alot depends on the layout of the home.

I already set back the upstairs thermostat to run a little 'warmer'. It said 79° yesterday evening and was running b/c it was set at 77°, but the 2-bedrooms on the SW side were ~77° and the rooms on the NE said were at ~75°. So it was plenty comfortable in the living/bedroom areas, just the hallway/bridge was warm (which is relative b/c it didn't feel warm).

I played with the dampers a bit to push more air to the SW bedrooms (away from the NE gameroom and bedroom which is always cold/comfortable).

I have the downstairs fan running full time now. It has 2 return registers, one downstairs by the alcove (right below the downstairs thermostat) and one in the game room overlooking the 2-story front room (pulling in air over the 2-story downstairs room).

One other factor, the attic stairs are right above the upstairs thermostat and I'm sure that is contributing to the issue.

As others have said you have to have a temp difference between the two floors.

At my house it is 3 degrees so if the downstairs it at 75 then upstairs is at 78. Yours may vary.

I measured the air temperatures last night coming off the registers. Air in is ~79° and coming out was ~65°.

Looking online there does not seem to be any consensus if this is a good way to check the system.

Based on what I've found at the manu. web site, my system has a rated capacity of 47,000 BTU/H (sensible 34,800). We kow the air is humid so using Sensible for äT;

CFM*1.08*äT = BTU/H

1600 * 1.08 * äT = 34,800
äT = 20°

so I'm losing ~6° in the walls/duct/leaks attic (although I just put in Attic Foil and it's defintely cooler up there but still same as outside so ~95°F).

Obviously a lot of other factors come into play such as humidity levels (which we know are hight in Houston), leaks, accuracy of thermometer - which could be of 1 or 2°. Also the 1.08 factor will change based on conditions.

I don't have capacities for the upstairs unit, but I was only getting ~ 12°dT off that system.

To do it right I'd really need to check wet/dry bulb temperature AT the coil, not registers to verify it's working correctly.

Since I modified the upstairs temps, the unit ran a little less yesterday. One thing I did realize is that I was having the thermostat drop a few degrees at 4:00 PM b/c this is when the kids got home from school. Well, I didn't alter anything for summer so basically the unit was trying to drop temps during the hottest part of the day. In addition, that thermostat has been screwy for awhile now as it would sometimes kick on the unit, it would run for 5m, then shut off (and the themostat reading would match the thermostat setting - which means it should not be running). I probably should replace that thermostat.

The downstairs system was just installed, and they came back a week later to verify everything was working correctly and checked the system charge, etc. and said the reading matched from the 1st set up so there were no leaks. The installer did a good job and was up front/seemed honest from the get go so I have no reason to doubt them.

[This message has been edited by agracer (edited 6/10/2011 8:28a).]

Agracer,

A 14 deg delta T is low. I would have the refrigerant levels checked. If you are low on refrigerant your unit will cycle on more to try to keep up. I would do this before moving the stat. Due to higher heat loads the upstairs unit will typically run longer.

You say you added the radiant barrier, but did you add insulation in the attic. Ideally you want R-30 to R-40 in the attic. Also I would look into a blower test for your ductwork. This will help you located leaks in your ductwork. Leaks may be contributing to your low delta T.

UPDATE: I need to be more careful.

I was having doubts about this temp things so I rechecked them. I checked them again and the downstaris was 61° and the upstairs was 61.4° I had to ask myself how I got so far off.

I realized when I checked them the first time, the condensor outside had probably just cycled and the just the fan was on so I was getting residual cooling from a coil that wasn't doing anything. When I put the thermometer at the face of the outlet it was on, but while letting it sit there for 15m or so, I likely cycled off.

So the dT are more like 16-17° for both units.

[This message has been edited by agracer (edited 6/13/2011 5:56p).]

That's where a point-and-shoot temp gun comes in handy. You can also check the area around your attic stairs and see how much of a hotspot it is.

Point and shoot only measure surface temps and must be used correctly. Besides I have a thermometer that works fine.

If anyone cares, I did some readings this week since I was home recovering and found the thermostat on the upstairs unit temperature (the air temp at the thermostat) was consistently 1-2° higher than the bedrooms 5' down the hall. Even the hallway area between the rooms was cooler.

I'm going to move the thermostat between the bedrooms and see what happens. I'm betting it will run less often and the downstairs will kick on a little more to compensate. It won't necessarily reduce the energy usage, but it will even out the house temperatures a bit and keep my bedroom a bit cooler.

Amazing what a little rain and cloud cover will do though. Yesterday, it ran for 4-hours...compared to 12 for most of the hot days we've had.

agracer,

Rather than moving the thermostat, you might look into a thermostat with a remote sensor or multiple remote sensors. They are not expensive and will give you flexibility down the road to play with the system control. I know people that have them and are very happy with them.

I've had this problem with my past two homes. The upstairs had an open area which opened onto the downstairs area. For some reason, builders love putting the main return and thermostat here.

In my current home, I installed a Honeywell remote thermostat (where the remote is battery powered). I put the remote in a bedroom away from the open game room.

I also keep the upstairs 1 to 2 degrees warmer than downstairs during the summer to encourage the downstairs unit to run.

-- Lee

but I'd be putting the remote sensor 5-feet away.

Moving it will take me all of 20-minutes. I know where all the wires are in the attic and have enough to move it a few feet down the hall. There are not switches or any outlets that would be in the way.

If that does not help, then the remote t-stat might be an option. But I'm pretty confident this will help with controlling the house temps and help me control it better.

Advantage of the wireless remote unit is you can move the thermostat anywhere you wish, whenever you wish.

-- Lee