I recently asked for recommendations for multi-zone heating in a small home (intended for college students). The responses overwhelmingly recommended multi-zone mini split systems. I fully agree, my question here is about how to implement this. If you disagree, please direct comments to the thread below.

Efficient whole house heating on a budget. Electric baseboard?.

Here is the situation. I've plugged the home into a few online calculators which estimate heating and cooling load requirements between 30,000 and 32,000 BTU's. Obviously these calculators are making the assumption of free airflow. The rough layout of the house is something like this

              Floor 1                     Floor2

          -/ -------------            ----------------
          |  |           |            |              |
          |       LR     |            |      BR2     |
          |  |___________|            |_/ ___________|  
          |Ξ |           |            |Ξ  |    BR3   |
          |Ξ \    BR1    |            |Ξ  /__________|
          |Ξ |___________|            |Ξ  /   BATH   |
          |Ξ         | BR|            |Ξ  |__________|
          |          /___|            |/ _|          |
          |     KIT      |            |      BR4     |
          |              /            |              |
          |______________|            |______________| 

There is a basement as well, open. As long as I can keep it above 50 to protect the pipes, I'm not too concerned.

The smallest evaporator I have seen for these systems is 9000BTU. Way to large for a bedroom, but four of them is also borderline too large for the entire home. Unless of course, I can get proper airflow. One of the things I have considered is a series of vents between various rooms, such as between BR3 and the bathroom or BR4 and the kitchen. Something else I considered is open transom's above all the doors.

Any thoughts or better ideas? I worry about the rear of the first floor not getting enough heat. I guess a 5 zone system is an option as well.

1 Answer 1


9000 btu is not two large for a bedroom, I have a 12000, and 8000 on 1 split. For 2 bedrooms, Mitsubishi makes 6000 btu so smaller units are out there for small bedrooms.

With a mini split used for both heat and cooling I would go larger based on experience. A mini split with an inverter controlled compressor will only run as hard as is needed. If the unit is two small it will go into defrost more often and during that time the temp drops so I think a bit oversized actually helps for heating. I like a zone in each bedroom and depending on size 1 for the kitchen dining area and one for the family room. In my case it was cheaper for multiple compressors, allowing shorter line sets and feeders for power, so that should be investigated also.

I think your sizing chart is off a bit, I currently have a small 3 Bd Rm house and live in climate zone 2. If I turn all the rooms on at 68f and do not run my pellet stove my units can barely keep up on a cold day low 30’s. the house is well insulated, floors walls and ceiling. Double Pane windows and all. My systems total 48000 btu but the summer it can really freeze us out if we turn it down and that is with temps +100

Oops almost forgot how to heat without an inside unit in a room. In my bathrooms we leave the doors open, when the bathroom is in use the exhaust fan is on pulling conditioned air into the bathrooms. If I do not have a inside unit to a room it is usually a room that is open next to a room that is conditioned. Like kitchen/dining area where I try and have the air flow directed to the other room. In the case of not having a spare bedroom having its own inside unit we normally leave that door open and used a fan to blow air into it. Today each BdRm on my home has its own unit. I purchased compressors that were inverter controlled with the max taps I would need and added the other 2 rooms later allowing me to stagger some of the cost and not have to purchase a new compressor when I added the new zones. Hope this helps.

  • That's actually a huge help. I was considering adding a 4th to the kitchen, but perhaps I would need to up the size of the two on the lower floor to 12000, and get a larger condenser to match. That would give me 24000 on the lower floor and 27000 on the upper floor.
    – mreff555
    Feb 20, 2020 at 18:05
  • If you have a well insulated house that should be enough, my place is a ranch style so it is spread out , with multi story more heat on the lower floor for winter and more cooling on the upper floor for summer was the setup in ducted days. With splits each room controls itself so as long as it is large enough the more heat/cool is automatically accounted for and your total inside unit size can be larger than the compressor allowing for the seasonal shift.
    – Ed Beal
    Feb 20, 2020 at 19:08

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