80°F when set to 70°F @ 107°F outside: dual zone mitsubishi heat pumps

You don't have something crazy going on, like a supply duct in the wall behind the thermostat or air from a supply register hitting it?

Seen it before and it really messes with the system.

And no broken or seperated ducts in the attic?

I put Ukiah, which is the hottest of the local weather stations, into weather.com. Fifteen days above 100F for the month of July, 114F on July 6. Yet the design temperature is 97F there.

There is a reason that reporters use the word "historic" in news stories about the heat wave hitting California right now.

What type of air filters do you have?

You should run the system for a day with them removed and see what happens.

I have found a lot of duct problems in attics. Both new and old. Typically on the underside or backside where you can't see it and it's super difficult to seal. A 2"x20" gap will cost you a lot of money over 15yrs.

I disagree. Everyone is different. From skinny as twig to as heavy as a sumo wrestler. From those who wear a hoodie in the summer to those who can shovel a foot of snow in January in a short sleeve shirt(points to self).

I don't handle heat well unless I'm sitting, doing absolutely nothing on a tropical beach with a cold drink under a palmtree. But while I hate the cold I'm particularly adapt at tolerating it. And after a long day/night in an unconditioned building at work, when I walk in the front door I want to see penguins playing poker with Frosty in my living room. Period, end of story.

So your point is? Just because someone decided at some time that keeping a building at 70 or lower is unnecessary and put it in a design book, it means its not possible? Or that someone, regardless of how much money they're willing to spend is not going to get it? If a potential customer with a fat wallet calls you up and says I need a new AC, the one most important stipulation is that it MUST hold 70 even on a 120f day are you gonna say "nope, can't do it. This book here says you don't need that"?

if you want the space maintained at 75, really no advantage to setting the thermostat to 70. It will not speed up the cooling.

The structure is what dictates the load on the heating or cooling system at any point in time.

Maybe consider an energy audit including a blower door test. And an infrared scan of the structure

Www.dsireausa.org is where you look for incentives or utility weatherazation

The design temperature is the temperature that is hotter than 99% of the time in your location, so that would be the 1% column. DB stands for "dry bulb" and MCWB stands for "mean coincident wet bulb." Dry bulb temperature is what we think of normally as temperature. MCWB is the temperature that a thermometer reads when it is wrapped in wet cloth and subject to a stream of air. It is a measure of temperature and humidity. When water evaporates it absorbs heat from the environment and the area around is cooled. The more humid the air, the harder it is for evaporation to happen, so the higher the MCWB. The "coincident" in MCWB means that they measure the wet bulb at the same time as the dry bulb is at the 99% temperature. So basically it's the answer to the question, "when it gets real hot, what's the humidity like?" Humidity is important in the design of AC because the AC has to remove humidity as well as cool, and it takes cooling capacity to remove humidity.

A MCWB of 68 is really low, humidity isn't an issue where you are.

Your design temperature is 94F.

The point is that AC systems have to be designed for particular parameters, if they are undersized or oversized they don't work so well. Most houses go through many occupants in their lifespans, they're designed for what a typical occupant will find comfortable.

All this discussion about design conditions is fairly pointless—the numbers that were actually used are much more important, but I don't see them being all that important just yet. Can't feel much air moving??? That points at a performance issue, either a mechanical fault or installation deficiency, or perhaps a configuration issue?

Properly done, insulation is always a good idea. Mini splits don't work like a regular split—they're not always going to be pumping out 55° air when they're cooling. That said, it gets hot in Asia too & I can't imagine that mini splits couldn't keep it chilly indoors even in 100° weather.

"When set to 75F, my interior reaches 80F when it's 95F outside. "

Considering that your design temperature is 94F, this indicates a problem. Either the system is not operating as designed, or it wasn't designed properly. Literally the standard for a HVAC system is being able to maintain 75F when the outside temperature is at the design temperature. That standard comes from Manual J of the Air Conditioning Contractors of America. Where you are, California, that is literally the law, as California has passed legislation that all HVAC systems have to be installed in accordance with Manual J.

This isn't a legal advice forum, but at a certain point the expert you will need may be a lawyer.

The solution to your problem isn't setting the thermostat lower, and it isn't really adding insulation. The fact that the installer suggested those things rather than actual fixes is telling.

At this point you should be focused on diagnosing and documenting. Get an indoor/outdoor thermometer that has a logging capability, so you will have documentation that when the outdoor temperature is at the design temperature the system can't maintain 75F.

While legally it's not your problem, it's probably to your benefit at this point to get an accurate diagnosis as to whether the system is not working as designed and needs to be fixed, or whether it is working as designed and wasn't designed properly. If it's the former there may be an easy fix, but if it's the latter your lawyer is going to have to get busy. So you need to bring in an independent expert to do an assessment. When talking to that person, tell him what you just said, that when it's 95F you can't maintain 75F indoors. Don't tell him about what happens when it's 107F outside, that's not the issue, that's a distraction, it took us two pages to get to the bottom here.

Above I say that adding insulation isn't "really" the solution. If it turns out that the system you have is undersized, it may turn out that the most cost-effective way to mitigate that is to increase the insulation in the house. But if you come to that conclusion, that's the installer's problem: you paid him a not-insubstantial amount of money to install a system that works, and he didn't hold up his end of the bargain.