Central Air Struggling

[JoeSpareBedroom]

Just after I moved into my (used) home, I had the AC unit cleaned & checked over by a company I've been happy with for 22 years. It got a clean bill of health. This was in September, though, so there was no really good way to watch it do its thing for any length of time. I'm finding that on a very hot day (92 outside today), the system runs almost constantly to keep the house at 76. If I set it at 77, it cycles on for an hour, off for two, and the house is comfortable. I don't NEED that extra degree, but I'm still curious: For a given house, and a particular AC unit, is there usually a point where it just won't do more? Or, does this mean that the previous owners didn't have the right unit installed?

By the way, the attic is vented & insulated perfectly.

[m Ransley]

It could be low on freon or your coil could be dirty, it worked last year you say, I would call a pro. Can you get to see your air handler coil I would start there. You say your attic is insulated, but to what standard or R value, accepted codes are outdated with presint energy prices.

[Edwin Pawlowski]

Yes. In most cases, the design is for a 20 degree temperature drop fron the outside temperature. If that was the case, you should be good to about 72 degrees. OTOH, it may have been designed for a 15 degree drop.

Or, does this mean that the previous owners didn't

Depends on your definition of "right". There may be some problem that is starting to show, such as a refrigerant leak. filter getting blocked, dirty condenser coil, etc. To go from a 2 hour cycle to running near constant for only 1 degree does not seem right. It may pay you to have a tech check it out now under heavier loads.

[JimL]

Sound like your system is designed perfectly.

The downside is that it can't quite maintain on the hottest days.

The upside is that it keeps the house dry and comfortable with minimum energy usage.

[JoeSpareBedroom]

Well, it's actually comfortable at 77-78, especially since I work at home, so I'm sitting at a desk all day. If you're not up & moving around, 70 can feel pretty chilly. I supplement the situation with a slow fan in the office, too, so as the temp creeps up (before the AC comes on), it doesn't feel stagnant.

I'm just curious, though.

[PipeDown]

I agree with the others that it is probably properly designed for a little less performance than you would like on the very hottest days. You can probably squeeze more out of it by improving insulation or shading high heat gain areas of the house. Basically the same things you do to save heat.

Shading the A/C unit helps, putting silver mylar (emergency blankets from the camping store) in the windows to reflect sunlight are a couple inexpensive things you can do quickly and are reversable if they also turn out to be cosmetically undesireable.

[JoeSpareBedroom]

Fortunately, the AC unit's in the shade for the entire day after about 10:00 AM. I've never understood people who say this isn't a factor.

[trader4]

I agree with Edwin that it sounds like something isn't right. The same system running under conditons where it can maintain 77 with 1 hour on,

2 hours off, shouldn't go to running continuously when it's set to 76. But on the other hand, for a system to maintain 77 with it 92 outside and only run 1/3 of the time sounds like the system at that point must be running OK, as it's not much of a duty cycle.

[JoeSpareBedroom]

I'm sure it helps that the house has 3 foot eaves. No sun directly into the windows until maybe 5:30.

[Gideon]

JoeSpareBedroom wrote: Fortunately, the AC unit's in the shade for the entire day after about 10:00 AM. I've never understood people who say this isn't a factor.

===========

There is considerable debate on the topic and there have been some rather well done, well controlled studies.

Obviously, shade vs direct sun is a factor. The important question is: "How much of a factor?"

Compute the hourly air flow through the compressor unit on a very hot day when the unit may be running continually. Better yet, measure the mean temperature of the air flowing into the unit over the course of the day. I believe that the studies have shown that there often is very little difference in that average air temperature and the average air temp for a unit in the shade. I would guess that the more the unit cycles on-off, the more important sun vs shade would be.

Personally, I'm moving my compressor from the very sunny east side of the house to the very well shaded north side of the house when I replace the system. Primarily because it will be further from my patio, but also because being in the shade certainly must help some small amount. :)

Gideon

[Jeff]

A simple check. An AC system should have about a 20 degree drop across the evaporator coil. Just measure the temperature difference of the incoming and outgoing air of the unit. If it is much different something is wrong. Since it seldom gets above 100 where I live, the delta T maybe less if the outside air is approaching 120 as it is now in the west. Good to do this for your furnace too in the winter. That way you can tell if there is a problem quickly.

[JimL]

I added a small high eer window unit to save money and liked it so much, I mounted it in the wall of the room where I spend most of my time. Huge savings and Huge comfort factor.

[JoeSpareBedroom]

What's "eer"?

[JoeSpareBedroom]

Measure incoming where? Inside the AC unit's cabinet?

[barry]

m Ransley wrote

And ... insulation is (relatively) useless without good sealing. Else it just adds a bit of turbulence to the flow-through. Potential inlets and outlets should all be sealed. Around doors is where I'd look first; note that near a low point of the house with cooled interior, the flow might be out.

J

[Jeff]

Measure at a vent in the house where the air is entering to return to the AC unit. Measure outlet air at the nearest outlet to the AC. For example on my system inlet 78, outlet 59, outside temperature 90.

[Kathy]

What is the thermostat set at?

[JimL]

SEER is used on Central Units and EER is used on window units, but they measure 'Energy Efficiency Ratio'. Lowes has some units that are 10.8 EER and use less electricity for the same amount of cooling as a 9.8 EER unit.

[JoeSpareBedroom]

Thanks!

[udarrell]

Good advice Ransley. We used to say and think that 15-F below the outside temperature was okay.

It could be that below 77-F sensible temperature that you have an air infiltration rate with a latent-load (humidity heatload) that consumes a lot more than the average 30% latent capacity of the evaporator. The "Combination of Both Sensible & Latent loads," could cause the sensible air temperature to NOT drop below the shut-off when set at 76-F.

The higher the humidity infiltration load the higher percentage of the E-Coils sensible capacity is lost to the latent workload! You may have some negative pressure rooms drawing in these two heatload sources.

It would appear to me that humidity infiltration could be the factor that is stalling the sensible temperature drop under that specific set of conditions.

There are far too many factors to cover here, that could be causing your system to only handle the heatload at that temperature & humidity load levels. Under a specific set of conditions all units will reach a point where they will stall at a specific TH setting.