Minisplit selection anxiety
I know.
The mini-split selection has been discussed and written about over and over here at GBA. But reading it all still causes some stress. I don’t want to screw this one up!
The situation: 1750 sf PGH is under construction in zone 6B. The design temperature for our area is 6/89 degrees. A good manual j indicates a 17600 btu/h heating load. Looking at Mitsubishi hyperheat models MUZ-FS12NAH or maybe MUZ-FS15NAH. Looking at NEEP’s site the max heat at 5 degrees is 19360 btu/h for the 15k model, a couple of thousand over our load but at -13 degrees 3k under our load. Sure, this might be for a shorter time, the house loses heat slowly.
Going for the 12k model I will be 3k under at 5 degrees and 6k under at -13 degrees so that model is out. Stepping up to an 18k model gives some ridiculous numbers way over the calculated load at any temperature.
To make things worse I read here at GBA that manual j is “padded” with 10-20% to play it safe. Should I take that into the decision?
So the question is – should I focus on the capacity at 5 degrees and keep it at or above the load of the building?
Sorry to bring this up again….
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IMHO: If it were my house, I would focus on 47 degree COP
it's going to be running there most of the time I expect.
would there be an issue with serving peak load with $50 of strip heaters?
In HVAC Design, we look for the Hottest/Coldest Hours in a year. Depending on the budget, this method can be compromised to value engineer a project. In your case, the ME MUZ-FS15NAH is a fine choice. According to the submittal (mylinkdrive.com), the rated conditions of the "H2i plus performance offers 100% heating capacity at -5° and 70% to 81% heating capacity at -13° F"
The system is variable. So, the indoor unit may have the potential capacity, but the three smart sensors in the IDU will send feedback to the logic board in the ODU to fine-tune and adjust the refrigerant delivery. The IDU will respond with a given blower motor speed. In other words, it is really hard to oversize a ductless system. It is possible but not a concern.
Keep in mind that the AHRI-rated condition is limited by a maximum of 25 FT refrigerant line length. So, if you were to install the refrigerant pipes to the maximum limit, the BTU capacity would decrease. Also, altitude would decrease capacity. It's about 4% loss in capacity above 2000 FT sea level. More as you climb approximately 2k foot increments.
Mitsubishi Electric has engineering publications to discover a projected BTU in a given condition and site location, or you can use the free Diamond System Builder software. Your greatest X factor in proper installation. If you can install the outdoor unit in an alcove, garage, or dog house, protected from the Sun, wind, snow, and rain, the ODU will provide better performance.
And remember, Energy Efficiency is the Enemy of Comfort. Do you really care how much energy you are using when it is cold inside your home?
"In HVAC Design, we look for the Hottest/Coldest Hour in a year."
Really? I thought you looked at the hottest/coldest 1% of hours in a year. At the 1% threshold, 86 hours are still colder.
If you are truly looking at the hottest/coldest HOUR, you are using a 0.01% design temp rather than a 1% design temp.
Never heard of that before...
Typos do kill, don't they? Hours. Please forgive me. I'll be good, I promise :)
I know it is sacrilege but I feel having the HP be slightly undersized and allowing resistant heat to kick in for a few hours a year is not that bad. Yes, it will cost when the strips are making heat. When it is that cold you COP will be more like 2.5 or so not the 4 or 6 people expect. The smaller unit will get much more run time at max speed where it makes the warmest air = better comfort and the smaller unit will have a lower min speed so it will cycle off much less often when the COP is more like 4-6.
Be sure to avoid systems mutable indoor heads connected to the same outdoor unit.
Get the resistance strip heat option because sooner or later the system will go down without the strips it is a full-blown emergency and need service now! With the strips it is simply just more expensive to stay warm and you can easily wait until Monday in comfort.
Walta
Strip heat is a COP of 1. IMO, if a given heat pump system is at a COP of 2.5 on the design day, that's still great. In the case of Hyper Heat, the manufacturer's submittal data and performance charts "guarantee" the capacity. If you trust the manufacturer and trust your design load, then auxiliary or emergency heat would prove unnecessary.
I've witnessed a Hyper-Heat system deliver 139F air at outdoor temps of 9 degrees. Just a couple days before, it was the coldest temperature in the country and the occupanant did not notice or feel any discomfort.
The min modulation between all those is not all that different. I would go for the one that can carry your place. The 15 and 18 are pretty much the same unit with different software (you can tell this when you check the weight of the units), generally the "detuned" version is a better deal as you get better efficiency since it has the larger coils.
My only $0.02. 6B might be less of an issue, but if you need cooling and humidity removal go for a ducted unit so each bedroom can get some airflow. Cooling with a single wall mount can work but only with the doors open.
Sounds like it will be the Mitsu 15 for us. Our summers are dry and the cooling needs are not a big concern. Our design is quite open and we live by the open door policy.
In addition I'm adding a 1500w kickspace heater on timer in each bathroom for comfort. I think we will do OK.
I agree, I'm sure you'll be fine.
you might consider a wall mount resistance heater in that bathroom that can double duty as a towel warmer. Many are nicer looking and warm towels are amazing...
Reply to @walta100
Sounds like a good idea to go with a smaller unit and let some ancillary source provide the extra heat when it's needed. Adding a strip heater to a mini-split appears to be a challenge. Not an option for many models and is difficult to implement.
Being new construction consider avoiding the wall warts and going with a concealed duct mini split in a dropped hallway ceiling.
Walta
Wall-mounted ductless units are the least expensive of all the various styles. And they are the most effective method of exchanging thermal energy ( Mass Flow) with this technology. Calling them wall warts is funny. The question stands: what exactly are they laughing at? Cheers!
First it appears you have the right questions in mind, so worry less.
Second, depending on the layout of your house, does it make sense to have two smaller units?
Not a multi head unit, two separate units might cost only slightly more in an equipment sense.
Code wants heat in every room, but that leads to bad split systems.
I used to love the little 100 buck wall heater in the bathroom of my last house. Cost little to run and you could crank it up while sitting on the pot and get hot air on your bare knees.
They are 150 bucks now
https://www.homedepot.com/p/Cadet-120-volt-1-500-watt-Com-Pak-In-wall-Fan-forced-Electric-Heater-in-White-with-Thermostat-CSC151TW/100569364