I get that AC systems usually are limited to a temp differential of about 15-25 degrees. My system seems to be about 15 degrees (old, low SEER, builder special).

So, my expectation now is that, best case, I will only get air at about 85 degrees coming out of my vents if the outside temp is 100 degrees.

How do I get the air to be cooler - say 70 degrees? How do freezer trucks manage it? How do commercial buildings (seem to) manage it? Other than residential houses, it seems like these commercial or mobile systems seem to cool to a larger differential.

When my old AC system pops smoke, what do I need to do to get cooler air w/ the next system beyond that 15-25 degree differential? Seems like this is agnostic to AC size (3 ton vs 6 ton) but is it?

I have solar which is well past the break even point so additional power costs is not too concerning (my annual true-up is only about $100).

The workload of the system cooling the smaller space in the truck, vs your larger space and the insulation of your home compared to the truck.

In other words, you don't have a system that is able to handle equal heat rejection.

Large buildings remove greater amount of humidity and almost always are continuous running, vs cycling of the home untits.

And home units are designed to operate at the minimum of removing humidity and lower temp to be "comfortable/livable" over a specific range.

And in home systems, once the unit ability vs outside temp is overwhelmed, it has lost the battle.

And other than a system that can handle the cooling you want, there is little to do expect throw money on insulation and sealing the home.

Adding shade bearing trees and waiting for them to start producing shades...

I would consider a mini-split or two if it can support your cooling needs.

You might get by with a couple small units of around a grand each, and if you can do your own installation (or someone can assist), you can add supplemental to areas most benefit, and the "upside" is they will help the Central unit keep up with high demand on hot days.

Thanks! Don't understand all of it (ie - I don't know what heat rejection is but I think I get the conceptual gist - a home is not the same as commercial, the ambient conditions nor the systems applied.

I could consider a mini-split as 90% of our time is spent is 2 or 3 rooms. But wouldn't the mini-split be subject to the same differential limitations? Or how would it work together w/ the central AC? Is the differential for a mini-split different because the central brings down the nominal temp to start?

Yes, Minisplit will have similar issues, but you are increasing your ability to add cooling capacity, so that permits you to cool more than you can now.

Second, mini splits are more often more efficient use, especially when limiting operating to certain rooms.

The difference is you have more total cooling going on, and the mini-splits doing most of your "comfort cooling" in small spaces you occupy.
Overall, your home HVAC may actually operate less as a result and/or more efficiently, because it is not working at a point where it has "lost control" to cool any more.

HVAC can become "overwhelmed" on the hottest days, and they cannot get rid of all the heat they have absorbed from the inside and dump it to the outside. Adding cooling ability helps that.

HVAC in extreme heat may "short cycle" and it often runs at a point where it cannot remove heat from inside and transfer it outside enough, that it's ability to cool is diminished.
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Similar to a car's cooling system overheating in really hot day, the system can only remove so much heat, and once the conditions that effect how much heat is being produced, overwhelm the abilities of the cooling system to cool, it can not longer maintain the cooling desired.

Heat rejection is just the description of the refrigerant "transferring" the heat it has absorbed in the evaporator (inside the hose), when it goes through the condenser.(outside unit).

Have you had your freon level checked lately. Our 8 year old system got noticeably colder after a few pounds of missing refrigerant was added this summer.

And, shouldn’t the temperature delta be calculated between the incoming air temp of the return vents rather than outside air? The house’s rooms at those vents don’t usually get to 100 even when it’s that hot outside, do they?

Thanks sigmonkey, that helps. I like the mini-splits used in Japan (panasonic, mitsubishi, toshiba). They always seemed to work well and quickly. Sounds like it warrants more consideration for my house. I'll include it in the AC purchase / install quote when the time comes.

Final dumb question: for the mini-split, any guidelines how to size it? Just based on the room size? Differential temps? Central AC size? Anything would be okay - just cost?

quote:

Originally posted by tsmccull:
Have you had your freon level checked lately. Our 8 year old system got noticeably colder after a few pounds of missing refrigerant was added this summer.

And, shouldn’t the temperature delta be calculated between the incoming air temp of the return vents rather than outside air? The house’s rooms at those vents don’t usually get to 100 even when it’s that hot outside, do they?

I haven't checked it recently but when I had it recharged several years ago (7 years ago?), the differential was about 15 degrees. It's about the same now. Now, I think the differential is different depending on outside temp (ie - 15 degrees if outside is 100, 20 degrees if outside is 80; this is not certain) but I think the system is behaving as after the recharge.

I don't know about the deltas. Perhaps there are two, one between outside and in, and one between output and return. But I believe the outside temp is important (at least that's the impression I got from the past).

How old are your windows? I added extra insulation in the attic and it made a hugh difference. Closing curtains during the day helps as well,

quote:

I get that AC systems usually are limited to a temp differential of about 15-25 degrees. My system seems to be about 15 degrees (old, low SEER, builder special).

So, my expectation now is that, best case, I will only get air at about 85 degrees coming out of my vents if the outside temp is 100 degrees.

That differential is supposed to be between the air temp coming in through your returns, and the air blowing out of your vents.

If it's 100 degrees outside, and 80 degrees inside your home, the air coming out of the vents should be 65 degrees(or so). True, that only goes so far, but if your home is properly insulated and you're taking other measures, there's no reason you can't make your home cooler than a 15 degree differential between inside and out.

my last house was old. i had to use a dehumidifier in the basement (where the ac/furnace is) and it helped the ac work better.

not sure if that was it, but it helped me.

The system works by "removing heat", not "cooling air".

The hotter the outside air is, the less difference (delta) there is for the refrigerant to "work with".

Just like you, if you are very hot, and you take a shower in cool water, you will "reject heat" into the water as it flows, the greater the delta between your body temp and the cooler water, the more efficient the heat removal.

Sane you, same hot, now run a hot water shower, you are not going to cool off at the same rate, even though the same amount of water is flowing over you for the same amount of time, because the delta is too close and the efficiency is lost on removing the heat from your body.

The refrigerant needs to be compressed to a higher pressure the hotter it is outside to provide the same "set temp" inside.

The higher ambient temp, the harder it is for the refrigerant to both absorb (greater heat) inside, and transfer it outside. The refrigerant is in effect like the "hot water" and is no longer efficient.

It's logarithmic, and once a system is near it's limit, a small degree increased temp is a greater degree in ability to keep the temp in the house under control.

Basically, as the ambient temp goes up, the HVAC slows down, the harder it works, the less it produces useful work.

quote:

Originally posted by konata88:
I get that AC systems usually are limited to a temp differential of about 15-25 degrees. My system seems to be about 15 degrees (old, low SEER, builder special).

So, my expectation now is that, best case, I will only get air at about 85 degrees coming out of my vents if the outside temp is 100 degrees.

How do I get the air to be cooler - say 70 degrees? How do freezer trucks manage it? How do commercial buildings (seem to) manage it? Other than residential houses, it seems like these commercial or mobile systems seem to cool to a larger differential.

When my old AC system pops smoke, what do I need to do to get cooler air w/ the next system beyond that 15-25 degree differential? Seems like this is agnostic to AC size (3 ton vs 6 ton) but is it?

Beyond the HVAC system itself, things like your insulation, window efficiency, and leakage/sealing all factor into it as well.

You can throw a fancy new HVAC unit at it, but if your windows are old, your walls and ceilings have skimpy insulation, and your house leaks air like a sieve, etc... it still may not get you what you want.

My last house struggled to keep the inside 20 degrees below the outside temp during the heat of the summer. But my new 2020 construction house easily stays at my preferred 69-70 during the summer, even while it's 100+ degrees outside. Without overtaxing my HVAC unit, or spiking my electric bill dramatically. All thanks to modern efficient construction/insulation.

Yea but I'm kind of stuck unless I want to invest heavily in the house (not preferred for several reasons at this point). The windows I think are decent - low E, double paned, inert gas and I close the south and west side blinds in the afternoon.

But the house insulation sucks - minimum required by builders which is just stupid. The builders for this area could either make some additional margin and/or attract more buyers for the same labor. It's not completely bad - the house is 25 years old so whatever the standards were at the time. Still, of any upgrade if offered, I would have upped the R rating of the insulation in the walls and attic. Nothing I can do now though (w/o substantial expense).

I have concerns w/ AC limitations and overtaxing it. But I'm okay w/ the electricity bill. Even if it's 10x today, it's still less than $1000 annually (less than what I used to pay before solar).

Shade trees would have been nice if I did it 20 years ago. Maybe even 10 years ago. But I'll be gone by the time trees are useful if I planted them now. Besides, I think it's not allowed with the new home insurance requirements.

Sigmonkey - again, thanks for all the education. Really helpful and appreciated.

I don’t think your numbers are accurate. We have no problem keeping our house at 75 degrees even when it’s 115 outside. I think you need Freon or something else is wrong.

Yeah, I kind of agree with that. Last May we replaced our inside coil unit. We had a slow leak and it was coming from there. With that and a recharge of the entire system our vents are blowing cold air between 56-58 degrees all summer. We had had a lot of days well over 100 here.

quote:

Originally posted by konata88:

quote:

Originally posted by tsmccull:
Have you had your freon level checked lately. Our 8 year old system got noticeably colder after a few pounds of missing refrigerant was added this summer.

I haven't checked it recently but when I had it recharged several years ago (7 years ago?)

Does "several years" also mean that you haven't had your system checked regularly?

I also suspect that your freon levels are low

quote:

But the house insulation sucks - minimum required by builders which is just stupid. The builders for this area could either make some additional margin and/or attract more buyers for the same labor. It's not completely bad - the house is 25 years old so whatever the standards were at the time. Still, of any upgrade if offered, I would have upped the R rating of the insulation in the walls and attic. Nothing I can do now though (w/o substantial expense).

It isn't that expensive to up the insulation values in an existing home. I would definitely look into what it would cost

I do not, for one minute, regret having additional insulation blown in to the attic and adding a second air return at the far end of the house when we bought our new Heat pump and air handler.

Ignoring the science for a bit:

Did your A/C ever keep your house comfortable? If so, then figure out what changed and fix it.

My family has never had a house whose A/C couldn’t keep the house 75 degrees in 100 degree weather.

Not sure where you are getting that you can only have a 15 degree difference between outside and inside?

My Lennox system can easily make the house 68 if I want with it being 100 outside. We’ve had a couple of 100 degree days. I choose to keep mine at 72.