Mini Splits are for Shoulder Season

My minisplit has really proven to be useful past month (april ) to allow me to cut off wood burning. Although I could run the wood boiler or my wood stove, it really is a lot more convenient to run the mini split on low to medium fan speed and cover the house's heating demand which normally is overnight and in the AM. My Defiant is in the basement and definitely takes a while to transfer heat to the rest of the house leading to overheating when I do use it. Cranking up the wood boiler and bringing up the storage tank to useful temperature would also lead to initial overheating. I expect if I had radiant instead of baseboard I could just run the storage at low temp but I don't see that upgrade happening anytime. I also notice that the heat pump definitely has more heat available as the outdoor temps rise. I don't keep written records, but many years I remember running wood on and off until early May even in warmer years so it has cut back my burning by at least a month and hope I can delay the start of winter heating at least a month or more.

I have made it over the hump with my electric bills. I have used roughly 2100 KW since Nov 1st and just about burned up my surplus on my power bill from solar. I am starting to bank power for next winter so now its a race to see if I can store up enough juice before winter to run another winter this way.

The current temptation is to replace a seven year old mini split AC unit with a new Hyper heat minisplit heat pump so I can zone the house further. NH has another rebate to cover 1/3 the cost.

I am heating a full dormer cape with only half the second floor finished. It is 20 years old but has 6" walls, high performance windows and double cellular blinds on most windows. The house is quite tight and is borderline for needing a HRV. The first floor is roughly 950 square feet, the second floor located above the room with the mini split is 400 square feet. The unit is Mitsubishi hyper heat rated at 12000 btus nominal. I don't have the model number at hand but it is a recent vintage high SEER unit. I haven't been through a cooling season and rarely have used AC except for the second floor office. It is wall mounted in my living room with an attached kitchen which is about 65% of the first floor space on the first floor. My bathroom is on the farthest corner of the 1st floor and is a Northwest corner, it is the coldest and at times I use an electric space heater intermittently as heat distribution is the poorest to this room.

The unit does work down to 10 below but its output and efficiency drops. In those conditions I expect my heat loss is higher than what the unit can put out and if I am home the wood boiler is online. The break even point where I can heat the entire house with the heat pump is around 35 degrees but I have to leave it running 24/7 as there is no thermal recovery available, I.E. if I turn the temp down for several hours it struggles to heat the house back up to setpoint at 35 degrees outside temps. I have noted before that the performance is better when its dry outside, add in snow or rain and the unit defrosts more often and puts out less heat. I do have a snow roof on top to shed snow and will probably leave it there for the summer. I will lose some performance in cooling as I mounted the unit on the sunny side of the building outside of the prevailing wind which is contrary to where a cooling optimized unit would be. This is decision that an owner will have to make, in order to optimize heating, the unit needs to be in snow free spot out of the prevailing wind preferably on the sunny side of the house.

The air distribution to the second floor is via an enclosed stairwell. I have to keep the setpoint on the units about 3 degrees above what I want the second floor desired setting (I.E. set the unit at 68 degrees to get 65 in the 2nd floor bedroom. I expect cutting a hole in the floor would help distribution.

I had considered a 18000 btu unit but decided I would try a one ton unit and then upgrade my office mini split AC to a newer 3/4 ton Hyper heat unit . I am still on the fence on this as I can not justify it on energy savings at this point since my power is "free".

One of the primary reasons for the unit was as much to have a source of affordable back up heat when I am on the road for work. Previously I would need to leave the oil boiler running. I still do set it quite low and run the heat pump to cover most of the load. Its a hot boiler so I basically get hit with about 1 GPD standby load until I get home and them turn off the boiler.

For a standard construction house I would expect a two 1 ton mini splits on the first floor on both ends of the house would be good with a 3/4 ton unit on the second floor. Keep in mind I am in northern NH with a much higher heating load than folks in southern NH or Mass.

The caveat to keep in mind is the air produced in cold weather is warm but not hot, at best the air leaving the unit my only be 80 deg F, those prone to drafts may object if they are in the air stream .