I hate to interrupt the ongoing political trolling but...
If an HVAC system was "appropriately" sized, what maximum duty cycle should be seen? In other words if my upstairs AC runs continuously -- that probably means it has a problem or is undersized. But then, if it was working correctly and was properly sized -- what duty cycle range would you expect? Thanks.
A perfectly sized A/C unit would run continuously on the hottest day in the design criteria to maintain the design inside temperature.
Depends on a lot of factors. Upstairs tends to be hotter for two reasons, heat rises, also it is directly under the roof and solar load.
What was the design criteria? Phoenix will have a different mas temperature than Concord NH. Was the appropriate temperature used? Most units are sized for a 20 degree drop from maximum expected.
On the hottest days of summer I'd expect it to run near constant, less so at night. Cooler days can easily get down below 50%.
Check to see what is running. It is possible the blower is not shutting off yet the condenser outside is cycling as expected.
What is the thermostat set for? What is the actual temperature?
Also, if it is 90 outside and the thermostat is set to 55, it is not going to shut off and it won't get down to 55 for another couple of months, depending on location.
Does it maintain the desired temperature on hot days? If it can't it's undersized.
It's ok, as long as you occassionally chime in so we know where you stand...
Air conditioning is a funny thing.
My AC can run continuously if the outside temp is 72 and my interior set point is 70 but the outset humidity is 65% and I want my house humidity to be 40%. In that situation my interior temp might be 71 and the air blowing out of the outside condensor unit is luke warm.
I'm pretty sure that the higher the temperature the air blowing out of your outside condensor unit, the harder your AC is working. It also seems that the lower the temp difference between outside and house air, the cooler will be their air blowing out the condensor unit.
I'd say that half the time (from May through to end of September) I use my AC to lower the humidity in my house vs bring the house air temperature down. I think that a proper or useful HVAC control system (ie wall thermostat) should give me the ability to set a set-point for humidity control to turn on and off my AC in addition to the thermostat to set set-point for temperature. I've never really looked, but I get the sense that no off-the-shelf residential thermostat allows for that.
A variable-output furnace is more clear-cut in terms of judging size / efficiency vs duty cycle I'd say.
So my take on your AC being sized correctly is to ask the question when your house air temp is not at your desired temperature set-point AND the air blowing out of your outside condensor unit is really really hot. In that case I'd say yes, your AC is probably undersized for that condition.
I don't understand. Are you saying your system has a humidity dial on the wall thermostat in addition to a temperature dial?
But what is a "reasonable" desired temperature? Is 20F less than outdoors considered "reasonable?" So I should be able to maintain
75F indoors at 100% duty cycle if the outdoor temp is 95F?
My system was installed 10 years ago. The compressor/condenser unit is Amana. The Honeywell thermostat has a humidity setting that allows the unit to run (over run) in order to satisfy the humidity setting. It allows the inside temp to go 2 degrees lower than the set point, if necessary, to satisfy the humidity setting. This had worked ok for me. But, if I were in a different climate, it may have been even better. I am in the western mountains of NC. The temp rarely gets above 85 or 86. But the humidity is high May through September. So this year I installed a whole house dehumidifier. I'm tired of buying the $300 units that only last 2 or 3 years. This one, Aprilaire, is made in Wisconsin and parts are available from Wisconsin. It seems to work very well, except that the furnace blower does run a lot more than before.
I used to keep the house at 72° F. Now I run a pair of dehumidifiers in my basement and am I'm comfortable at 76° F.
Re comfort, the most important step is to get the humidity down to the 25% level and then cool the air if needed.
Yes. Like Gfre said, it should be able to keep the house at a comfortable temp on the hottest day that you could expect. NYC area for example, that;s about 100F, so it should be able to keep the temp at say 76F when running
100% of the time. I'd want it sized a bit more than that, and typical installers are going to make sure they err on the side of it having a bit more capacity instead of less. The benefit to them is they won't have customer complaining, it would require changing eqpt to fix. For the customer there is benefit in that if the system has been off and the house is hot, it can cool it down faster. So, oversizing it a bit isn't bad, but if you oversize it a lot, then it won't run enough to even out temperatures in the house, it puts more stress on the eqpt with more starts and stops, etc.Another factor, you say the upstairs is the problem. I assume that means that downstairs the temp is OK? If there are two units and the one for downstairs isn't running, the temp there is higher, that will increase the load on the upstairs. Another common problem is that with one system for a two story house, it's common for installers to screw it up, eg not enough returns for upstairs, which makes it impossible to cool it properly. Without two systems, I think about a 3 deg temp diff is typical during the day, when it's hot.
What's going on in the attic? Properly ventilated and lots of insulation? IF that's not right, fixing it, adding insulation could be the best money spent. There should be adequate exit and entry for air, eg ridge vent and soffit vents. Make sure the soffit vents aren't blocked with insulation.
The biggest problem with oversizing a unit is it won't get the humidity out and you end up with a clammy house.
Reasonable is whatever is acceptable to you. You can turn your house into a refrigerator if you want to go to the expense. Personally, I don't have AC so if it's 95 outside it's close to 95 inside.
That sounds like our bedroom. We have a 3/4 ton mini split in around
250 sq/ft that is in addition to the central air. My wife wants it 68 at night. It makes the stat.
I just have to wait a while for nature to take its course. We had three
96 degree days last August. For two of them the overnight temperature was 53. The other one only made it down to 61. If I turn the window fan on when I go to bed I usually turn it off sometime during the night.
Well, I think that is part of the problem. The upstairs gets hotter and so the upstairs AC runs far more than the downstairs AC, however the downstairs AC is definitely a larger unit. The downstairs of the house does have more square-footage but there is an open staircase so I'm sure some warm air flows upstairs. This makes me wonder if the upstairs AC should be the larger unit.
The upstairs AC unit also suffers from the disadvantage of the ductwork being in the unconditioned crawlspace attic.
I've seen exactly that problem here in new construction, large McMansions. You'd think by now they could correctly size these, there are programs they are supposed to use where you enter a lot of detailed info, like type of construction, insulation, number/size of windows, which direction they face, etc. But I think many of them, maybe most just wing it.
Having the AC in the attic is a disaster too. They use those flex ducts that only have maybe 1.5' or so of insulation, they get hung with straps or whatever that eventually crushes them. Would be a good idea to check up there for any of that, for leaks around connections, etc. It's possible the system isn't charged correctly too, you could have it checked. It's possible they could run larger ducts or add returns etc, without a lot of work or eqpt change. It's also a good idea to have a pan under the AC so that if the condensate drain gets plugged for either the furnace or AC the water will go in there and to have a float switch in there that will cut off the eqpt if it fills with water. They have battery water alarms for ~$10 that you could put in there too.
One quick rough check you can do on the AC is measure the temp delta between the air coming in and going out. About 18F or so is normal, but it also depends on the blower speed, humidity, etc. If you're seeing 10F, then something is likely hosed.
If not already insulated, wrap it. Consider adding more if it is insulated already.
For a proper diagnosis, use a thermometer in the duct to measure air at the evaporator and at the end of the duct. You can see how much loss there is.
Attic installation is the norm here and you are right, flex duct sucks but you can make it somewhat better if you blow in insulation around them. It is also standard practice for good HVAC guys to "mastic" all of the joints. (Sealed with a goo that gets firm but not brittle/hard) They slather it on pretty well. The other thing is keep the runs straight and not squeezed in any way. The best solution is to lay
1x12s down and run the ducts on them across the truss chords so you don't have a roller coaster for the air to go over. Running with the truss is OK if it is flat. Best is to build plenum boxes everywhere the ducts make transitions or 90 degree turns. I knew a guy doing his own that just used flex to sleeve round metal duct that he assembled in the attic and a box at every turn. He said it allowed him to use a smaller blower to get the same air flow and the payback was less than one season. The aux drain pan is code here.
No, the actual HVAC here is in a closet. I've seen the kind you are talking about (a neighbor had one) where the whole unit is up in the attic. That seems like a bad idea. But even with the AC in a closet they still run the ductwork upwards into the attic crawlspace where you end up with the same hot/cold problems. One problem I have is just moving around up there in the attic. The insulation is deep enough that I can't see where it is safe to step. I should probably build some elevated 2x4 walkways or something. Then perhaps I could build flimsy troughs around ducts and fill them with more of the insulation. Need some cool weather first. Or I am also pondering adding a bypass vent that would simply dump the AC output into the largest upstairs room, without going through the attic ductwork.
You could certainly add some output to a room or two if it's feasible. I'd first carefully measure the temps in various areas. If they are all about the same, then making one area significantly better will just make the other areas worse. You could check around, get some recommendations for good HVAC guys and get a couple over for evaluation, starting with seeing if the existing system is operating correctly. If it is and it's just insufficient capacity, the only real solution is to put in a new system. Depending on how old and what the eff rating of the existing one is, that might not be a bad idea. If it's running a lot, then it's more important how eff it is. Switching out the 25 year old AC here for 14 SEER, peak electric bills in summer went from $300+. So if it's an older system, you could have a reasonable payback, plus comfort. If it's just one room or two rooms that are a problem, then either a window unit or mini-split could be an option.
to $150, maybe less.
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