Minisplit sizing for cooling and auxillary heat.

Voodoo90

Hello,

We have a 3100sqft house on three levels including a basement. The houses primary heat source is an AWHP with radiant in floor heating on the main and top floor and a low temp radiator in a bedroom downstairs. The family room still has old baseboards, which are ok in keeping it at 15°C as it is only used as a gym.

We are getting mini splits for, primarily for cooling as we heat with the AWHP.

Design heating load for the house is 36000 BTU.

The proposed design is a 30k Mitsubishi hyper heat outdoor unit with a 9k head for the family room, 12k for the main level and 15k low static for the top floor supplying 4 bedrooms.

The top floor has roughly the same area to heat as the main floor and the basement is just half a floor.

This would be 36k and is over capacity. The usage scenario though, would be in the summer cooling on the main and top floor and maybe dehumidification in the basement family room and in the winter heating only for the family room. Maybe some auxiliary heat in the winter when necessary.

Does that sound sufficient? Our contractor says it will be fine with our usage profile.

It assume it would still be able to heat the entire house to a certain point in a worst case scenario, where the AWHP has a problem or similar?

What do you think?

EBEBRATT-Ed

The heads can't produce more than the condenser puts out. I think you will be ok. I am guess this is Canada. Cooling load is probably half of the heating load

pecmsg

Avoid oversizing inverter systems, humidity removal drops quickly as they throttle down.

Kaos

If mostly for cooling, you don't need a hyper heat unit which will saves you a bit.

If you are going to use that 9k for heat most of the season, it won't work well running on its own. Issue is that it is too small for the outdoor unit and will need to bypass refrigarnt through the other zones. This reduces COP (way bellow nameplate) and might cause overheat on the off zones. A better setup would be a on-to-on for the 9k and two zones multi for the rest. The 9k would be hyper heat, and the two zone regular.

Voodoo90

Splitting it into a single and a 2 zone id a good point. I will look into that. In that case we might just upsize the low static to 18k for the 4 bedrooms and use the 30k outdoor for the 2 zones and the 9k single for downstairs. Or is 15k for those 600sqfr split into 4 bedrooms enough?

Kaos

There is no just thing as is "15k enough to 600sqft". If you have a man J, it will tell you what you need. The upper floor does tend to need more cooling since heat rises so a bit oversized doesn't hurt. Still watch what you pick as dehumidification will suffer if oversized.

Voodoo90

The calculation of our contractor says it is enough.

Eventually we are putting the basement one on its own outdoor units and the main and top-level one on the multi unit. They will both be hyper heat units as the normal ones are not eligible for the rebate. That way the entire hyper heat system will come down to 8000 CAD.

pecmsg

sounds more like that’s what he’s proposing. Asked to see the calculations for the basement unit.

Voodoo90

Heating load of the basement bedroom is 5500 BTU and the family room is 8000 BTU.

We are pretty much at the end with the design. I just need to make a decision on the basement.

A 20k unit with 2 9k heads or 2 single 9k units, one for each room. The single units can modulate way lower than the 20k, especially since both units would not run evenly at the same time usually. The 9k go down to 2kish each and the 20k says 11k is the minimum.

So probably 2 single 9k are better than the 20k in this case. I was even debating if a 6k single unit is sufficient for the bedroom.

pecmsg

In heating they reduce capacity by diverting refrigerant flow to the unused head. If too large that head will overheat.

5500 BTU's for a basement?

Where is this structure located?

How many Sq Ft?

Are the walls insulated?

Voodoo90

The bedroom in the basement has 170sqft, walls are insulated in Nova Scotia.