Multisplit temperature control per zone
I am considering a multisplit heatpump system for our 3000 s.f. two-story renovation located in Chicago, Zone 5. I like the idea of a multisplit system for its high efficiency and zone control. I would like to provide 1 unit per bedroom using ducted interior units that supply conditioned air to each bedroom.
Because even the smallest interior units will be overkill for these 150-200 s.f. bedrooms, I plan on ducting some of the air from each unit into common spaces while keeping the temperature control unit in the bedroom. I was told that in high performance (near passive) houses like this, i can’t really control the temperature in each bedroom like this. I find this a little hard to believe. If the unit is targeting a certain temperature in each zone, why can’t I get this level of temperature control?
Does anybody have real life experience with a solution like this?
Thanks!
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If you are really at near-passivehouse levels there's no way you would ever need or want to have one unit per bedroom, dumping the excess heat into the common area, since that would overheat the common area. If you put the majority of the flow into the bedroom it will just cycle on/off, and never modulate.
Also, a multi-split that can handle 4-5 heads/cassettes has a minimum compressor load at which it will operate, and that min-load may even be comparable to your whole house design heat load. By having multiple heads it forces the compressor to be many times the size needed to heat the place, which cuts into both efficiency and comfort (not to mention, the installed cost.)
A better solution is to have one mini-duct cassette unit serving 2-4 bedrooms based on the calculated heat loads, with the flows designed & adjusted for temperature balancing between the rooms, just as with any non-modulating ducted air heating solutions. The individual occupants can then micro-adjust the flow to suit their preferences by tweaking the registers.
Thanks for that insight Dana. I will try to find a solution based on your suggestion. Am I correct to assume that I CAN get true zoning in a near-passivehouse as long as the total capacity of my indoor units is matched to the total load of the home? i think my initial layout would have provided about 30-50% more capacity from indoor units vs house load and result in excess cycling.
Alok,
Frankly, perfect control of indoor temperatures, room by room, can be difficult with a ductless minisplit system (or even a ducted minisplit system). Not impossible, but difficult.
If you need perfect alignment between your thermostat temperature and your indoor air temperature in each room, you might be happier with electric resistance baseboard heat.
Even when a heating system is perfectly matched to load, it is way oversized for shoulder seasons, and even for a typical day. So when you say 30-50% oversized it's actually worse than that sounds.
Oversizing mini-splits isn't the same as oversizing a cast iron boiler or a 2-stage gas furnace. They modulate, with very substantial turn down ratios. But those turn down ratios are not infinite.
The 3/4-ton Mitsubishi FH09NA can cut back to as little as 1600 BTU/hr. The 3/4 ton in that series can pull back to 2800 BTU/hr, and the 1.25 ton unit can drop to 5150 BTU/hr. But they can also deliver 10,900 BTU/hr, 13,600 BTU/hr, and 18,000 BTU/hr respectively. If the thing is 50% oversized for the load running at it's MAXIMUM capacity at 0F (the 99% outside design temp at Chicago's Midway Airport) it would still modulating MOST of the time. But if it's even 30% oversized for the 0F load at it's MINIMUM speed, it literally never modulates, only cycles on/off.
The Fujitsu _ _ RLS3H series can all drop to 3100 BTU/hr, even the 1.25 ton 15RLS3H (which can also deliver 16,000 BTU/hr @ -15F.) The 3/4 tonner can deliver nearly 16,000 BTU/hr @ 0F. With this series it can be 100% oversized for the load at max capacity and still modulate most of the time.
But even at mid-modulation at 0F any 9000' head is going to be oversized for the load of an individual bedroom, even in a code-min house, since it can deliver the full heat load at or near it's minimum modulation, and would be cycling on/off at the average winter temp. In a "...near passive..." house it's going to be INSANELY oversized. The heat load of a 200' bedroom in a code min house is tyically less than 2500BTU/hr @ 0F, and in a high-R house it's often in the 1000BTU/hr range or less, and it would be cycling on/off even during design conditions.