MXZ-SM36NAMHZ oversized 4 bedroom system short cycling, looking for opinions and suggestions
snappysalmon
| Posted in General Questions on
Facts:
Climate Zone 5: Boston
4 relatively small bedrooms, 100-140 sqft each. Partial wall/attic insulation in 1930s colonial.
Outdoor unit: MXZ-SM36NAMHZ
Indoor units: MUZ-FS06NA X 4
(First floor heated by a MUZ-FH18NAH single head unit installed 2 years ago)
Symptoms: Short cycling in heat mode: 8-9 cycles per hour, about 5 minutes on 5 minutes off in outdoor temperatures ranging 30F-50F (lower temperatures not experienced yet), data attached
A diamond contractor just finished installing a second-floor-only heat pump system in my house. I had gotten 5-7 quotes to install the 4 bedroom system (either ducted or ductless), each bedroom is relatively small at 100-140sqft. Quotes varied between Samsung DVM Eco 36K, LG Red 30K, 2x MXZ-2C20s, Mitsubishi PUZ-HA24NHA ducted, and a couple quotes for the Mitsubishi MXZ-SM36NAMHZ.
Knowing I have 4 small bedrooms, I asked each contractor questions regarding sizing and short cycling, all of them referenced the usual variable speed technology. The Samsung DVM Eco contractor mentioned the superior 7000BTU minimum capacity modulation and was by far the most knowledgeable but was about $2K more expensive, and 10 instead of 12 year warranty. https://ashp.neep.org/#!/product/31921/7/25000///0
The SM36 ductless hyperheat quote was the best price, and NEEP seemed to show good modulation capacity with the newer Mitsubishi smart multi technology https://ashp.neep.org/#!/product/57008/7/25000///0
17F: 8500BTU; 5F: 6687BTU; -13F: 2716BTU. I figured at 17F ~8500BTU the system would hum along pretty steadily, with a solid COP of 3.72. Although it said 47F: 22500BTU, I was hoping it was some sort of measurement error and not a big deal.
The workmanship was pretty good, they followed all the right installation techniques and overnight nitrogen tests. After a week of power monitoring and comparison with my FH18 single head downstairs unit, a couple of symptoms and concerns from the SM36:
1. ~8 (!) outdoor unit cycles per hour, each lasting 5 minutes. (see attached graph for data taken at ~30F). Indoor unit cycles more infrequently, and each fan cycle lasts no more than ~15-20 seconds, which feels like excessive short cycling.
2. Presumably due to the short cycling, a lot of creaking noises that are distracting when sleeping.
3. Individual rooms tend to overshoot the setpoint, presumably because the refrigerant is circulating and the fan has the low-speed mode when the system is not active.
I’m seeking advice on the situation and available options to improve the existing set up (MHK2 thermostat? Change in settings?), or suggestions of how to reasonably push back on the contractor to change units (refit with PUZ-NH24 ducted system; install 4 separate MUZ-FS06NA or NAH ductless units). Thanks in advance for all the input.
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Replies
I’m sorry to hear this, that’s ginormous. I’d expect something around 12kbtu, ducted, so you’re > 3x oversized. The modulation on multi-splits is awful, often not even 50% if sized perfectly, which obviously this is not. You may as well start from scratch and try to get a ducted unit if you can swing it/they’re feeling generous. Frankly, electric baseboard and window AC is better than this system at probably 90% cheaper.
Ohh boy. That beast puts out almost 4 tons of heat, this is well beyond ludicrous oversizing for four bedrooms. You would need something that big for heating an entire 3500sqft new build.
Neep information isn't always accurate, the engineering information on the SM series is pretty limited so hard to guess if the turndown is fine. By the sounds of it, definitely not 8500BTU.
At least they could have installed the non-hyper heat version. Would not have solved your oversizing issue though but at least would have saved some money plus a much smaller unit.
My only suggestion is to replace with a right sized ducted (probably 1 to 1.5 ton) unit that feeds all the bedrooms. The 4 zone multi feeding 4 tiny bedrooms will never run efficiently or quietly.
Might try replacing the existing wallmounts SM specific indoor units. These have integral EEV and might run better. That is a big Might.
Could you clarify which wall mounts are SM-specific? I'm not familiar with those.
Yes knowing what I now know, I'm also thinking optimal solution would be a ducted 1-1.5 ton unit, which would mean removing and replacing almost everything.
Thinking along the lines of least rework, what about 4x single MUZ-FS06 outdoor units paired with the current indoor units? Not sure yet if I physically have enough space for it, but it should help enormously with modulation.
Take a look at this tech note: http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10009/Application%20Note%201036%20ME%20-%20Applying%20MXZ-C%20Multi-Zone%20Systems%20-%2020190110.pdf and share it with your contractor.
Mitsu's specs also confirm that the unit's min output at 47F is 22,500BTU/H
http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_SUBMITTAL_MXZ-SM36NAMHZ_en.pdf
It's so oversized I can't imagine it'll ever work right. Did any of the contractors do manual-J load calcs?
The wall mounts are the PKFY series:
https://cdn.agilitycms.com/mesca/city-multi_indoor_units_section_booklet.5.24.22.pdf
With these you can go above the 4 unit max with a branch box, so you could get your main floor also onto the multi split. Whether this will make it run better is a big unknown, currently that FH18 is probably producing most of your heat for the whole house which means the SM36 is sill way oversized for the place.
To get this running efficiently and with better comfort, I think your only real option though is to get rid of the SM36. The four MUZ-FS06 can work and there have been people that have installed pretty much that on this site, you can try searching and contact the posters. No need for the hyper heat MUZ as well.
Thanks so much for all the help. This helped me have a follow up conversation with the installer and Mitsubishi sales rep to propose some solutions, including city multi VRF systems with more precise refrigerant control. I haven't figured out cost and coverage, but what do you all think between the three potential options? I want to get it right and avoid further rework for my sake and the installer's.
1) Keep SM36, but replace my downstairs FH18 head in order to hook a PKFY P18 to the SM36, with upstairs bedrooms 4X PKFY P04 (4k btu). Not clear if PKFY is compatible with SM36 or can utilize the VRF benefits.
2) Push to change out the SM36 for PUMY 3 ton city multi VRF outdoor condenser, then change out all indoor heads as reference earlier. NEEP specs for PUMY are slightly better in turndown, do they offer a better coordination with the PKFY indoor heads?
3) Leave the FH18 downstairs, replace 4 bedrooms upstairs with 12K attic-ducted single zone system.
I think 3 is best. The ductless heads are compromises even if they’re slightly better sized. Ducted gives you quieter operation and you can actually get meaningful filtration out of it.
Except for the attic bit, I would be leaning for option 3 as well. This is guaranteed to work.
Mitsubishi did publish some more info on the SM including part load tables:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/DB_M-P0855_MXZ-SM_NAM(HZ)-U1_JA22.pdf
Looking at P76, the turndown is surprisingly good, so if properly set up and configured reconfigured PKFY setup (option 1) should work. My only issue is why you are not getting this amount of turndown with the existing setup. The unit should would be still oversized as is but it should be running closer to 500W not 2.5kW.
This might be worth a call with Mitsubishi engineering department that knows about these VRF units.
Bit late to the party here but agree that the unit was oversized. Nonetheless, if things have not already been addressed, have the installer confirm that SW2-5 dip switch is set to "On" on the outdoor unit to ensure that it knows it's talking to a branch box. There can be weird behavior with indoor units, performance, and modulation if this is not set. I don't think this switch was in place before the Smart Multi outdoor units were introduced. I had to correct this on my installation (SM36NAMHZ with a ducted KP30NA and ductless GL06 connected by a 3-port branch box) but things are definitely working better since I corrected it. Time will tell if it was the root cause of my unhappiness with the system, but it's worth it to confirm.