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Mitsubishi Tri-Zone Mini Split Heat Pump System

First timer needs a little advice

I'm considering replacing my current forced air oil heat & AC on the second floor of my 1920 farm house with a Tri-Zone Mini Split Heat Pump System. Planned replacement is due to basement remodel require move of current duck work, planned partial second floor addition as well as HVAC contractor indicating current supply & return duckwork to 2nd floor not large enough to provide correct heat/cool. Current 2nd floor is only around 600 sf (3 bedrooms & bath) - planned addition will add another 500sf. Current oil heat works ok for 2nd floor (not great but ok) but AC does poor job during hot summer months - I'm forced to install a window AC. I have also added around 20 inches of blown insulation in the attic as well as installing ventulation in the attic. The forced air oil heat & current AC will continue to be used for basement and 1st floor.

My concerns is whether or not a Mini Split Heat Pump will be able to supply 60to 70 degree heat when the outside temp could get down to between 10 - 30 degrees on average during the coldest winter months in Southern MD. I've read that the Mini Split Heat Pump System only works down to around 35 degrees.

Any poster/readers from the Washington DC or Maryland area that have the Mini Split Heat Pump system install - what do you recommend - how does your system work?

Re: Mitsubishi Tri-Zone Mini Split Heat Pump System

i am hopefull someone will answer as well. i am in baltimore md and am planning a zoned split system for primary heat and cool for a 800 sq ft addition.......

Re: Mitsubishi Tri-Zone Mini Split Heat Pump System

You asked for someone with a mini-split in your area, and I think you may have to wait a while for comments. I will offer mine based to (formerly) having a 1-zone system in Calif.

First, my guess is that your proposed 3-zone mini-split will cost about $6,000 to $8,000 installed by a HVAC contractor. With that budget in mine and electric heat, it is not a good first choice.
So, the 2nd expansion will eliminate attic space, and won't accomodate new duct work, registers, return, and evaporator/blower unit, etc. ?? Typically, the 2nd zone furnance goes in the attic, I'm told, in new & old construction. It seems so obvious to do it on the 2nd flor rather than pull it from the first floor (and also looks like you have a cape cod layout where the expansion comes from unfinished attic space.

I am also under the impression that mini-split heat won't work well below 38 degrees as it is pulling heat from outside air (it adds no heat on its own). It is a technical spec of the unit so the Mitsubishi docs should be very clear about suitability when outside air is below a specific temperature, and it is a deal killer if heat transfer drops off (simply because there's not a lot of heat BTU's in 25 degree air).

When operating at specs, each air handlers (3 for 3 zone) should pull 9,000 to 12,000 BTU's from the outside into the rooms. It should be OK to keep 3 rooms comfortable, but it is VERY dependent on how efficient it can pull heat from very cold outside air (a big if). Also, I think it will run for many hours so the electric meter is going to spin a lot (cha ching) -- you will trying to keep a differential of 40 degrees when the outside is 30.

You then have heat eveness problems; 3 air handlers will blast hot air from 3 point sources like window A/C's, and you have to compare that to 1-2 registers per room with central blown air systems. A 4th room and common areas won't be heated much by a 3-zone mini-split, and the same for cooling.

Re: Mitsubishi Tri-Zone Mini Split Heat Pump System

To first answer your question yes it will work. Now I will elaborate. I have a 1920's farm house in PA with a rubble stone foundation and prior to installing split units I had the opportunity to have a tour of the inner workings of the largest pipe organ in the world. This is the Wanamaker organ in Philadelphia. The organ chambers must be climate controlled so they were using split units with the condensing units housed in another room near by. After spending a summer with ac units in the windows of the farm house and everyone complaining about the heat and a oil furnace on the way out this made me think of split units in each room. After observing the basement temperature between 50 and 65 degrees year round. I thought of the Wanamaker Organ and the compressors in a separate room not outside. My basement was damp so I used a radon fan just to vent the basement to the outside creating air circulation. I first installed a condensing unit in the basement and could not believe how good it worked and the units fan helped circulate the basement air making it more comfortable also. Everyone is now happy with a unit in each room and they now can control there own temperature. Cost, the electric bill increased about $100 a month but I had no oil bill that was $2400 for a year at $2 a gallon. Okay one might argue the room units are ugly well now they have ceiling units and units that are housed behind a picture. The bottom line is everyone is comfortable all year round and you can still open the windows on a summer night. Good luck on the project and please post back.

Re: Mitsubishi Tri-Zone Mini Split Heat Pump System

I am using a inverter type mini split heat pump today in the Albany NY area, at 0 degrees F. It does not work well at this temp, but is functioning. The output BTU's and the air temp leaving the unit are not high. Consider that this is a great solution to about 20 degrees F. It also seems to be very efficient even at these lower temps. Far ahead of resistive heating, perhaps at least 2:1 at the very worst. The air leaving the inside unit is not very warm, reaching 75-83 max or so at an outside temp of 5 degrees. The operating data is part of the manufacturers specifications, and some units have good test data showing the low temp operation expectations. You have selected a quality unit, mine is a no-name, so you should expect more reliable results. In a high humidity area, the outside unit will ice up, requiring a defrost cycle, some times frequently, every 20 minutes, depending on conditions. This will result in no heat for the cycle and possibly shorter life span and a bit higher costs. Consider the winter humidity ranges before installing these as primary heat in the winter. Also assume that you will get half the BTU rating at the worst outside temp and make sure that you have some form of backup heat. You don't want to oversize the units as that effects the summer operation and the efficiency and installed cost.

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