[tguvot]

actually i'll advise you to take a look at temperature of your walls. both inside and outside. they have a bunch of thermal mass. during the summer they effectively heat the air inside the house. unless you manage to cool walls somewhat (lets say during night), hvac will have to really fight to keep temperature down. I live in bay area and I installed whole house fan which is typically runs through the night at summer. For most of the cooling season it sufficiently cools air and walls in order to require only a couple hours of hvac a day.

additional point, you wrote that you have current system installed 15 years ago. back than design temperatures (i.e. how hvac sized based on outdoor temperature) were different. recent updates sometime bring those temperatures up by 10F or so, what can result in extra half a ton - ton of hvac.

you could retain company to make a proper hvac sizing (called manual j calculation). hvac contracts from my expirience won't do it.

[criddell]

Our walls are good but our windows are a weak point.

During summer, the night temperature drops to the low 90’s here. It’s A/C all day, every day, for at least 3 months.

Any idea why the HVAC guy and just about every source online insists that A/C can only give you a temperature delta of 20F? I know it can’t be true, but that claim is everywhere.

[sokoloff]

There's a lot of poor engineering fundamentals among some HVAC salespeople and some websites as well as some sloppy writing.

The delta-T that is correctly cited as being "around a max of 20°F" is the delta in temperature across the evaporator (IOW between the return duct and the supply duct [aka the goes-inta and the goes-outta]). It's not the delta-T between the outside air and inside air.

With good enough insulation and/or enough cooling, you can get whatever delta-T you want as is obviously the case with refrigerators and freezers.

[tguvot]

>The delta-T that is correctly cited as being "around a max of 20°F" is the delta in temperature across the evaporator (IOW between the return duct and the supply duct [aka the goes-inta and the goes-outta]).

well said. i have temp sensor on supply and return and I graph them and delta, to make sure that hvac works within proper parameters. otherwise during heat waves you start thinking that hvac maybe out of shape

[criddell]

So in places like Arizona where the outside temperature can reach 120F and the inside temperature has to be 82F (what landlords are required to provide to tenants), are they just using bigger units than they would in some place where the temperature rarely breaks 95? The refrigerants are the same?

[sokoloff]

Same refrigerants work. Getting a large decrease in temp vs outside is a matter of insulation, air sealing, roof/wall color, and sizing of units/ducts.

[criddell]

Thanks for the information.

One more question- what about commercial spaces where they will have a door open and be flooding the store with ice cold air. It sure feels like more than a 20 degree difference.

[sokoloff]

It's still just a 20°F difference from the vent that the AC sucks air from to the supply the AC blows out of.

That has (essentially) nothing to with the relationship between the indoor and outdoor temp.

For simplicity, imagine a balmy 110°F outside temp, an 80°F inside temp. That's 30°F from inside to outside [and will feel refreshing], but probably results from a supply temp [the coldest air coming right out of the vent] at 68°F, a return temp [the air being drawn into the AC duct] of 88°F.

The key 20°F delta-T is the measure between the 88°F and the 68°F (both temperatures inside the conditioned [in other words, "indoor"] space). All of the other temperatures can be all over the place. Even if the outside temp was 120°F, provided the unit was sized large enough or the insulation good enough, the indoor air might average 80°F and keep the same 88°F/68°F [20°F delta-T].

As a more extreme example, your freezer probably works on a delta-T of ~10°F. That's the difference between the air going into the evap coil area and the outlet. If your indoor kitchen temperature is 80°F, your freezer still keeps things frozen even though its delta-T is only 10°F. You don't take the 10°F from the 80°F. You take the 10° from the 30-ish°F and get a warm temp of 30°F and a cold temp of 20°F in the freezer.

[tguvot]

>Our walls are good but our windows are a weak point.

are you sure ? at summer I have walls that are 170F outside (at 2pm) and 80-90F inside. Also, window will pass through heat only during the day. Wall will keep heating your inside air through the night as well. Get some cheap thermal camera - it's very educational.

You should also check your ductwork/registers. To see if there are any tears/openings/etc. Also, in case you are using flex duct, if it's not tensioned properly, sagging, etc - there will be a lot of air velocity losses that reduce overall cooling experience.

>Any idea why the HVAC guy and just about every source online insists that A/C can only give you a temperature delta of 20F? I know it can’t be true, but that claim is everywhere.

Something around refrigerant properties, mechanics and physics, etc. I also spent some time in this rabbit hole