home AC in an old multistory
 

We have a 2 story house (~1900sf) built in '67. It's got a 3-4 yo outdoor unit and the indoor part is less than a year old.

Because of the 2 story deal, the upstairs is usually 5-6 degrees warmer than the downstairs which really sucks. Right now it's about 72* down and 77-78* up. Logically, I thought that closing the downstairs vents would force the cold air out of the upstairs vents and that should improved things, but I've read that closing too many vents can cause problems too. Having a 6 degree diff between upstairs and downstairs means that to get the upstairs comfy I have to set the AC pretty low. I think that causes the AC to run too long which I think can also cause problems.

Does anyone know what I can do to maximize the efficiency of my AC, make us more comfy, and maybe help out the Elec bill?

I had wondered if raising the temp of the thermostat would actually help, but I haven't wanted to try it for fear of how hot it could get upstairs.

We need new windows. Our existing windows are Al framed crap from '67. Some of the upstairs windows rattle. In the summer I can stand 3' from some of the windows and feel the heat on the side of me that's facing the windows. The problem is that the cost of new windows is ridiculous. I'm decently handy around the house, so I'm considering putting at least the downstairs windows in myself. I've thought about buying something like just the kitchen window to put one in and see how it goes.

Re: home AC in an old multistory
 

Closing too many vents can reduce flow across the coil, reduce efficiency a bit, and worst case cause the coil to freeze over. However, I think you can get away with closing the vents partially.


If the system is designed and installed properly, there is little consequence to running the system for extended periods of time. The most immediate effect will be a higher electricity bill.


I do not own or work on HVAC systems in two story homes, so I might not have the right answer. The first thing you can do it close the downstairs vents slightly. That'll only give you minor benefit. One thing you can do is install booster fans in the upstairs ducts. When the A/C turns on, the booster fans will turn on and increase flow to the upstairs rooms, supplying more cold air upstairs.

Close doors to prevent cold air from going downstairs.

The ideal solution is a second A/C unit servicing the upstairs portion only.


New, energy efficient windows will definitely help. I've installed replacement windows. If you can find windows to fit well, it is not a bad job, but it can become a nightmare DIY job.

Thanks for the info.

Since we just had the compressor/outdoor unit replaced last summer/fall, does anyone know of a good way to check to make sure it's charged and running correctly?

You need gauges, easiest thing to do is stick a thermometer in the vent and see what temps you get. You should probably see 30-40 degrees cooler in vent than in the room. As far as "fixing" the upstairs downstairs temp issue, run the fan on "On" rather than "auto" it will help to equalize the temps, between cycling of the a/c.

30-40 degrees difference? OK, I'll get a couple of thermometers to check things out.

Old rule of thumb I've heard was 20 degrees difference between your return air vent and supply vent. In practice, I usually see a 16-18 degree drop. A 30-40 degree drop might put evaporator temps at or near freezing.

If you don't have gauges, there is no sure way to know the charge is proper.

One thing you can do to really help the system is make sure the filter is changed frequently. Also, flush out the outside condenser coil with water. Getting rid of dirt and grass helps the condenser work better, which means you stay cooler and use less electricity. Don't spray the coil from the outside. Remove the fan and spray from the inside-out. This assumes you have a condenser with fins as opposed to the spine-fins (looks like tinsel).

One simple option is to set your fan to run continuously and then let the compressor cycle on/off. This should help even out the temps a bit.

Is your attic well insulated? Do you have some sort of ventilation in the attic space? Thats usually a good return on investment if its not already done.

We do have spray insulation in the attic. The ducts in the attic are insulated. The roof has ridge vents and soffit vents.

I just put new filters in.

My 2 story in North Carolina has duel zone. One for downstairs and one for upstairs. Both heat and A/C.

In the winter we rarely run the upstairs and let the heat rise from downstairs.

In the summer we set the upstairs a little cooler than down same deal cool air dropping.

Is it possible to add a second zone upstairs?

An attic fan could help. It draws the stagnet air out of the attic.

Install a ceiling fan in the upstairs area.

Check if you have return air ducts on the second floor.My house is 90 years old and I don't have return air ducts on the second floor.

New windows would help.

I have two return ducts, both 16x25. One at the top of the stairs and one directly below it in the middle of the house.

I don't know about converting to dual zone, especially since we have pretty much all new stuff. It sounds expensive, but I've thought for years that we needed separate controls/system for up and down.

Ideally, I'd love to have a multizone system w/ a sensor in each bedroom and a central thermostat downstairs as well as active valves that could control the flow and temp to several areas of the house, but I don't see that happening. I don't think we plan on being in this house that long either.

Could always get a couple of window units. They are cheap and actually work better than central air as you don't have all the ductwork robbing the cool air as it travels upstairs.

Well, it seems that my AC is minimally operational. The upstairs vents read -- return air 78* and vent 72*, but the downstairs read 72*/72*. We lived here for 8 years with AC that would get the house how I wanted it. Then the outdoor unit went out. We got it replaced and it ran ok for a couple of years, but then the indoor portion got dirty and was REALLY old with lots of bent fins, so Oct of last year we had it replaced. Unfortunately something's now not right, but I suspect it hasn't been for a few months, but it's been cool enough that I couldn't tell for sure. Now that it's beginning to get warm, I can tell. At least it happened now and not in July or Aug.

I'll call the AC folks tomorrow. What a pain in the azz. I'm about ready to take a couple of HVAC classes at the local community college so I can at least know exactly what needs to be done and make sure it's done right if I don't do it myself.