80°F when set to 70°F @ 107°F outside: dual zone mitsubishi heat pumps
Our newly installed dual zone mitsubishi heat pumps are unable to maintain 70°F interior temps for our 2500 sq ft 1980's home in northern CA:
- 30K Mitsubishi MXZ-2C30NA2 outdoor unit
- 36K Mitsubishi SUZ36NA2 outdoor unit
- 36K SVZ-KP36NA air handler
- 24k SVZ-KP24NA air handler
For example, our house warmed to 80°F with both thermostats at 70°F when it reached 107°F yesterday. The company said the problem was that we only had 10" of insulation and said another $3500 to increase this to 16" insulation was the answer.
I'm skeptical that something else is wrong. The vents show a 20°F difference with the inlet air which they say "means the coolant is correct and system is operating as expected."
However was just in AZ where a friend's 15 year old HVAC was pumping out cool air so much that I had change spots on his sofa to avoid the blast. With this system, I have to put my hand up to the vent to barely feel air flow.
Recommendations?
Comments
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5 1/2-Ton is more then enough for 2500SqFt with little insulation.
The company said what? It was like that before they installed it. I call BS!
can’t troubleshoot over the internet but I see troubles coming.Get a Mitisubishi Diamond dealer to review ALL work done and make recommendations.
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Has anyone one done a heat load analysis? How tight is the home?
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First, "your house needs more insulation" is a BS excuse, they're supposed to install a system for the house you have, not the one they wished you had.
I can think of three possibilities:
- Your system is working as designed and the design is flawed, it's undersized for your actual heating load.
- Your system is working as designed and the conditions you are experiencing are outside what the design anticipated.
- Your system is not working as designed.
I want to talk about #2 for just a bit. When a system is designed, they calculate using an outdoor temperature and indoor temperature. At this site:
you can find the outside design temperature for your county. The process for calculating the cooling load is called "Manual J," it's specified by the Air Conditioning Contractors of America trade group, and it uses 75F for the interior design temperature. So if the design temperature for your county is 102F outside and your system was designed for 75F inside, if it's 107F outside and 80F inside your system is basically working as designed. I say "basically" because it's more complicated than that.
I agree with others who say that five tons of cooling is a lot for a 2500 SF house, that's 500 square feet per ton. I would look into the air delivery. To deliver that much cooling the 36,000 BTU unit has to be pushing about 1350 cubic feet per minute (CFM) of air, and the 24,000 BTU unit has to be pushing 900 CFM. That's a lot. It may be as simple as the fan on the indoor units is not on a high enough setting. Or it may be that the fan isn't powerful enough for the ducting that you have, the ducted minisplits tend to have less powerful fans than the older single-stage units they replace.
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High speed setting max. external static pressure is .80 in. wc. That's grills, filters, ducts, registers and door under cuts. Standard air flow duct design has a hard time staying under .80. I've seen high merv filters kill performance with these fairly low static air handlers. Confirm the duct design can flow the rated CFM at low static pressures. Measure the total static pressure once it is confirmed that it's on the high setting and confirm there's some room for the filter to load up without going way above .80 In wc. Problems tend to be: return grill size too small and style is restrictive , filter area too small and merv too high, then under sized duct and registers. Wood floor registers don't flow air well at all.
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As others have said, to expect your systems to maintain 70* inside when it’s 107* outside is totally unrealistic.
You need to find the outside design temperature for your locale and expect at that temperature the system is correctly sized and operating properly if it maintains 75* inside. As the outdoor temperature goes above that design temperature, the indoor temperature will rise accordingly.
In my county, the design temperature is only 91*, but we use 95*. That means at 95* outside, the indoor will be 75*. If it gets to 100*, the indoor will be 80*.
IDK of anywhere in the country (except Death Valley) that the outdoor design temp would be 107*.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
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5 tons of cooling should be plenty unless the house is a sieve or you have tons of glass. I would expect an airflow/ducting issue.
Some things you can check.
- Is the outdoor unit running non stop when it can't keep up?
- Have you noticed any ice or frost on the AHU coils?
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I thought 5 tons equals 60,000 btus?
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36K SVZ-KP36NA air handler24k SVZ-KP24NA air handler
24+36=60
There is 66K of outdoor units but they can't put out more cooling than the air handlers can deliver.
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I thought the same thing, but I looked at the county list and there are five counties in California where the design temp is over 107F:
Imperiral
Inyo
Kern
Riverside
San Bernadino
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This is where I'm leaning. The house is from the 80's. Imagine this scenario: the old AC was three tons upstairs, two downstairs, always worked fine. So they replace with the same. But the new system is a ducted minisplit, can't handle as much static pressure, can't deliver the air through the ductwork that exists.
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Could be, but those units are the air handler style mini splits sized equal to what the existing air handlers put out right? the air handler style mini splits aren't as finnicky as the "low static" or "medium static" little fan coil type units. The brand we sell just gets installed the same way as an equal size air handler would.
Silly question but has anyone checked the filters?
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Mini duct work is much less forgiving. If they want .5 DO NOT exceed it.
almost impossible to install a ducted mini on existing ducts!0 -
I believe those 2 models are rated at .3 Static Pressure.
not going to get those #’s with old ducts.1 -
Wow that is quite a bit different than the brand I sell! thats in the range of our little fan coil low static units.
edit: the spec sheet says the 3-ton unit at least has a selectable external static pressure setting, 0.3 0.5 and 0.8, with each static pressure setting having 3 fan speeds. so has the external static pressure been properly set on these? I'm guessing they left it at factory settings
"Selectable external static pressure: 0.30, 0.50 and 0.80 in.WG with 3 fan speeds at each static setting"
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Factory is .5 in wc. Pushing to .80 might see a performance gain but if the air can't move at the rate the blower wants to spin it either stall/slips or the ecm backs off.
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Maybe there's a service app that will show you the load on the compressor with all the temp readings. At the least you should be able to get a compressor frequency reading from the control board while it's running trying it's best to hit 70F IAT while it's 80F. That would give some good info. If the compressors is not maxed out it's likely an airflow issue but I agree that it may be running as expected at 107F. I think the spec. at rated capacity is 95F OAT.
Edit: duct work in an unconditioned attic that is not well vented can see huge temperature differences between the air in the duct and what's in the attic space. Hopefully it's R8 ducting. This type of heat gain can reduce a chunk of capacity when you need it most. A tight the duct system is also critical with all metal insulated. Here we have third party testers who do a test for leakage before a permit is signed of on.
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Thanks for all the great feedback. I'll start answering from the top:
@pecmsg Great idea to contact a Mitisubishi Diamond dealer. I found this site but holiday now so will contact on Monday:
@PC7060 No heat load analysis was done and unsure how tight our home is. Will see who Mitsubishi recommends to come investigate.
@DCContrarian Our home is in Sonoma county in CA which shows 96° in the temperature reference guide. However Sonoma spans a huge range from coastal CA to inland where temperatures vary considerably.
@Ironman We specifically asked the HVAC company to give us extra capacity though as we expect temperatures to rise over next many years. It is over 105° in our area for 10+ days every year. Everyone that has seen our system said it should be overpowered for our house.
At present even on max, you have to hold you hand close to the vent to feel any air coming through though. Would be great to know CFM of our system. How would we measure that?
@EBEBRATT-Ed The outdoor unit is running non-stop and putting out heat. I haven't noticed icing on either air handler.
@Teemok Great technical advice there. Seems we need to have the house tested for those numbers. The house had not prior AC system, only heater. We paid to install completely new ducts in the ceiling as prior system was in basement, but this one is in attic as no room for the huge systems they installed. Also enlarged the old air return plus installed 2nd air return for zone 2.
@GGross This is a new system. Not yet one year old. It had this cooling issue when installed but only had a couple hot days to complain before colder weather settled in. This was first hot weather of the year. They looked at the filters and said everything was clean.
@pecmsg All new ducts installed in attic for this new HVAC system.
@GGross Thanks for the images and info on static pressure setting. Is the "selectable external static pressure settings" something that I can easily view as a homeowner?
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So your design temperature is 96F and it's 107F outside? I'm surprised the guy is answering the phone at all.
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We can only hope that our A/C guy is more knowledgable than you about the area. That design temperature is for Cotati, 25 miles to the south of us and covered in fog most of the summer due to a gap in the mountains along the coast. We on the other hand have two mountain ranges between us and the coast so get none of that fog.
It's currently 84F in Cotati and 105F at our house.
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I'm looking at your response to @Ironman , where you say you have to hold your hand close to the vent to feel any air coming through.
Um. No. If that thing is running at full song, even if there isn't exactly a gale force breeze, there should be a very good and clear flow of air. Three possibilities: far too much restriction in the duct work — or somewhere there is an actual obstruction involved — or you have a significant air leak of some kind.
Time for some serios investigation of the air flow…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Do you have fan speed settings on the thermostat? Probably? If so, make sure you don't have it set to a low speed.
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What temperature do you get at night when it's cooler outside?
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BTW, I would also point out that the ratings on these imported units are drastically different than the way american systems are rated. For example, your 36k unit's actual ratings are 33,400 BTUh at 80°F indoor temp and 95° outdoor temp. Not only that, the SHR (sensible heat ratio) is .84 and you live in a hot humid area.
It is entirely possible your equipment is undersized.
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I should add that our friend's heat pump is 20 years old and keeps their house freezing when it's 115F outside in Phoenix. Our struggles to maintain 80F at 100F outside.
At our friends, we can feel the air blowing from across the room. In our house, you need to hold your hand up-to the vent to feel any air. The HVAC tech said "these new variable speed systems can't push air as well as the older single speed systems". So wondering if the fan speeds are at the lower 0.30 external static pressure instead of 0.80.
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The system comes set at .5 ESP
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OP,
can you post a picture of the thermostat / room controller, with any covers open, so we see the icons on the screen,
confirm the fan is set to high, not auto,
and if you could, a picture in the attic or basement of the ductwork, or flex duct?
known to beat dead horses1 -
The newly installed R8 ducting was inspected and no kinks or holes were found. We have the fan set at high when above 90F outside to ensure max throughput, otherwise we leave on Auto.
Here's a photo of the Kumo Cloud thermostat from today: 95F outside and set to 70F but can't maintain that so at 73F.
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nothing wrong with 20* drop from ambient.
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I think I may have an inkling… where, exactly, in northern California? It really does make a difference, but I still think I may have a thought…
What is your relative humidity? The clue here is your comment on your friend in Phoenix — where the relative humidity is very low. If your relative humidity is higher, which I suspect may be the case (basically anywhere west of the Sierras), even if your relative humidity is as low as 50%, an air conditioner is going to really struggle to get the exhaust air below 84 — which is the dewpoint. Your friend in Phoenix likely has a relative humidity of around 20 percent, and an air conditioner would have no trouble getting down to 60.
Yes, it is possible to have an air conditioning system cool below the dewpoint — but it takes a lot more cooling capacity to do it, and I suspect that if those numbers are anything like plausible, your problem is simply that you don't have enough capacity.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
@Jamie Hall Too little capacity is the conclusion that I'm reaching as well. I live in Healdsburg, CA where we have relatively high humidity, as the Accuweather link below shows.
I already paid my HVAC company $45K for the current system and now they want $10K to insulate my attic. Actually $3.5K if they simply blow new insulation over my 40 year old insulation, but if that was really the problem, I was opting to pay another $6.5K to remove the old insulation which is actually crumbling to the touch. Didn't want to be breathing that dust in the future, so was willing to pay for removal.
Given the system described in my initial post, what's the recommendation then? Is it possible to just upgrade my condensors and use the existing air handlers? What would be the recommended next step up for my outdoor condensors?
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I looked at a weather summary for Healdsburg. July 14 is the hottest day of the year on average, with a high temperature of 85F. It doesn't sound like 96F is off-base for your design temperature.
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If a contractor's response to your problem is to try and upsell you rather than solve it, run away.
You don't have to tell us their name, but I'd be curious if you could google them and find out who owns them. There is a trend in the HVAC business of private equity firms buying up long-established family-run HVAC companies. They then seek a return on their investment by "optimizing" operations, which means squeezing more out of both the customers and the employees. A few of the biggest firms around me are now owned by private-equity and they are awful.
This sounds exactly like what a private equity-owned company would do.
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Be that as it may, You will never convince somebody they are comfortable if the thermostat reads 80° and they want it at 70°.
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Hence my earlier comment that I was surprised they answered the phone at all when the design temp is 96F and it's 107F out.
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I put zip code 95448 into the sizing tool at NEEP.org. It gave me the choice of five weather stations, with their cooling design temperatures:
Buchanon Field — 82F
Napa County Airport — 87F
Oakland International Airport — 79F
Sonoma County Airport — 91F
Ukiah Municipal Airport — 97F
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I believe Ukiah hit 114°F in the current heat wave. I live in Northern California. Those design temperatures are absurd.
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Bburd0 -
Additional insulation is always a good investment.
Having said that, it is independent of the HVAC and you should go out for multiple competitive quotes.
Be sure to include sealing around penetration between conditioned space (wire, smoke detectors, lights , HVAC, etc.)
Regarding removing old insulation, can you post a picture? If you have modern insulation like cellulose I'd leave and blow in move over the top.
As a side note: HeatingHelp website rules state you cannot list contractor costs. Your cost are pretty general but just wanted to let you know.
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I think — much as I hat to say iit — you really need to find someone, probably not a contractor, who can actually analyse the real loads in your area, including humidity, and size the equipment based on that.
You probably can use the existing air handlers, with new bigger coils, but you may need a larger compressor.
I regret to say I don't know anyone in your area who can specifically do that, but @Alan (California Radiant) Forbes might know of someone.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Expectation of an indoor temp @70 degrees for cooling is ridiculous.
With proper humidity which you should have if the equipment is correctly sized you will be comfortable at 74-78 degrees
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