80°F when set to 70°F @ 107°F outside: dual zone mitsubishi heat pumps
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Fog is our natural coolant here. It burns off in the mornings and the evenings can be very hot and dry. It is typical for the effects of the fog to return as the sun sets giving us good cooling at night. Areas vary to the degree that they are effected by this cycle. The intensity of the fogs varies and on some rare days, when the wind is right, there is no fog at all and we are baked with heat from land to the north,east or south and we have warm nights. Keeping a typical house at 70-75F is done all the time.
I'm going to fill you all in on a key bit of evidence. "Lots of west facing glass." Hopefully I'll have some other relevant details next week when I see the problem in person. A green house isn't an easy thing to cool.
Evaporative cooling has been an effective tactic for countering heat here but change is real and swamp cooling has its limits and drawbacks.
We have a related problem in other areas near the bay with homes that have high mass radiant. They are chilled in the fog all night and the floor mass thermal flywheel gets going. In the morning the fog suddenly retreats and the OAT rises rapidly and huge south and eastern glass walls get low angle direct sun and the home cooks. There's not much simple controls can do to counter it. ODR is way too slow. Predicting the ten minute window when the fog vanishes isn't easy. Motorized curtains, shades and high capacity problem targeted a/c helps. Lots of money and energy invested to get rid of energy you just burnt so that free energy doesn't drive you from the beautiful view home.
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In 26 states it's the law that systems have to be installed in accordance with Manual J, and Manual J says indoor temperature of 75F. So that's who's to tell them otherwise.
Now, I realize that you can pretty much get any result you want out of a Manual J by putting your thumb on the scale.
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"I'm going to fill you all in on a key bit of evidence. "Lots of west facing glass." Hopefully I'll have some other relevant details next week when I see the problem in person. A green house isn't an easy thing to cool."
This is a situation where I typically explore the possibility of parallel evaporator coils. Sunrooms, high glazing areas, ect. And often has to be addressed separately from the rest of the building.
Good luck 👍
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So much talk about "well if this home owner wants to burn through $$$" then let him do so. My calculations don't show a significant "waste of $". Rather I see that rather I see this simply based on what I've already paid:
- My 36K handler cost only $800 more than the 24K handler
- My 36btu outdoor unit cost only $600 more than the 30btu handler
So roughly speaking, a larger system of say 42K and 42btu should only have cost another $2K or so. That's only 5% of what I paid. Even if it were twice that at $4K, an increase in cost of 10% is a small price to have my system scalable into the increasingly hot coming years, rather than running full speed for much of the summer and requiring early replacement.
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And since this ideally professional Forum is subjecting my polite questions into personal attacks, I'll simply say that I doubt I could visit any of your houses and have the temperature at 85F with your AC running. If so, that's likely why none of your friends want you installing an HVAC into their own.
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The last missing piece of the puzzle is what the system that was replaced looked like. I'm still betting that they replaced the existing system with something of the same nominal capacity, not realizing that the ducting requirement for a ducted minisplit would be more demanding.
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You deserve to get want you want. And I might add, I would be fuming if I was in your position. There is no way I would put up with the house being above 75 degrees.
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I don't think the folks here intend any ill will to you. Off hand I don't think you should have to buy a larger unit, I'm not convinced that is the problem. It is difficult to figure that out remotely though. Buying a larger unit will only work if the duct system can handle it, and It seems more and more likely that the duct system is not capable of delivering what you already have installed. You currently have 5 tons of cooling total and the system can't maintain 75 on a 95 degree day, and the house is not terribly large at 2500 sqft. If you have a ton of windows it's possible you need more, but it still seems unlikely. Of the two options you had at the start of the post (add insulation, or larger ODU) It seems the ODU is a guess, the insulation will help but we can't be sure it would fix the issue. We really need to know if the duct system is capable of delivering what you already have installed and that is a difficult thing to do remotely. A manual J load calculation would solve the question of whether or not you have enough cooling from your ODU currently, duct design would tell if the system will deliver what you have.
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As to the effectiveness of adding insulation in the attic:
It has 10" now and the proposal is to increase that to 16". So let's say R40 to R64. The house is 2500 SF, two levels, so let's say 1250 SF of attic. On a hot day let's say it's 150F in the attic and 75F inside.
With R40 and a 75F temperature difference, 1250 square feet will transmit 2343 BTU/hr. With R64, it will transmit 1465 BTU/hr. A difference of 878 BTU/hr.
If the house was engineered for five tons at 94F out/ 75F in (which I don't believe it was, but bear with me) that's 3157 BTU per degree of difference. Reducing the heating load by 878 BTU/hr will increase the difference by about a quarter of a degree.
If you feel any of the assumptions above are unrealistic, insert your own. I don't think it changes the big picture.
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The problem, @DCContrarian , isn't that it can't maintain 70 on 95 day — it's that it can't maintain 70 on a 107 degree day. Almost exactly half again the load. Interestingly, it appears that it can maintain 80 on that 107 day… which is the same delta T as 70 on a 95 day… and delta T is the name of the game.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
OP has commented at some point in the thread that on a 95 degree day with thermostat set to 75 the house will be 80 degrees and not cool to 75. They aren't trying to actually maintain 70 inside, the thermostat was set there at the recommendation of the contractor to solve the issue of the system not maintaining temp (spoilers, it didn't work).
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We'll see what the cause ends up being, but my suspicion is that not that the compressor or air handlers don't have enough capacity, it's that the ductwork can't deliver enough air. If that's the case, it doesn't matter what the capacity of the air handler or compressor is. You could double both and you'd still get the same amount of cooling.
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From the OP: "When set to 75F, my interior reaches 80F when it's 95F outside. "
The 70F thermostat setting and the 107F outside temperatures were red herrings. That sentence sums up the entire problem.
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I haven't seen It and I now know enough about it that I think it's best for me to stay in my lane. Just because I have many years of HVAC experience and i love to trouble shoot doesn't mean I have the standing to be a mediator in this kind of situation. My license is for hydronics only. It sounds like a real shame. He's was, for a moment, the customer we want but I think the puzzle was too hard for the contractor to solve. It might be a sizing, design or install skill issue. Someone will need to figure out which one or combo it is. I hope it can be resolved gracefully.
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One thing you could check is the air temp in and out of your air handlers. If the house where the return grill is is 75 degrees you should have 75 at the return going in the air handler. If you have 75 at the return grill and 80 at the ahu you could have leaking ducts.
Your supply air usually is about 20 degrees colder than the return temp. That is a ball park. If your ductwork is undersized you would have a wider TD say 75 return and say 50 supply instead of 55.
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