First off, 4 stories sounds like a big vertical head for residential mini-split equipment just from my casual notice of those requirements. I would check that out first. I am testing a whole-house mini-split installation right now (Mitsubishi, 7 in, 3 out). I will let you know in a year or so if I like it ;-) On paper they are more efficient due to inherent efficiency and the inherent zoned nature. We have been having pretty hot weather and with my wife out of town this week, I have been pretty much been cooling the bedroom and the kitchen with some intermittent cooling in the utility room where the exercise machine lives. Note that I am not much of a sit and watch TV type. You are really a leg-up over lots of others on this decision as you have experienced mini-splits. One thing to think about is that, IIRC, most of them don't have back-up heat so you will not have any heat while they are defrosting. (Maybe none of them have electric resistance back-up, but I think I ran across at least one brand that does.) You also have to pay close attention to cold weather performance for the mid-Atlantic region. I know that with Mitsubishi's current residential equipment, the 1:1 systems have better low temp performance than the multi-splits. They are more efficient too. Even if they do have back-up heat, you can kill the efficiency pretty fast if you need back-up very often. Keep in mind that there are lots of options to the wall-hung indoor units. They cost more, but there are units that can be dropped into ceilings and others that sit on the floor. There are also short, ducted units that would solve that big room problem by splitting the supply and return. (You could also install more than one indoor unit/room.) Condensate needs to be dealt with using pumps. I am not an HVAC or building pro, so take my observations with a grain of salt. After replacing a ducted system with mini-splits, and talking to my install guys a lot, here are some observations and conclusions. Installing a mini-split through outside walls, as is it usually promoted, is pretty quick and straightforward (read fast and cheap) compared to installing a new duct system in an existing house with no central AC. My install was done through interior walls running the lines/cables under the raised house and out to the side which was much more complicated. If I were doing this over and I had a good duct system within a house envelope, I would stick with it. The trouble is, what is a good duct system? More often than not, they are outside of the living space. In addition, they are considered good when they leak 5% of the air and they only get worse as they age. My system was in the attic with parts of the duct system in the attic and partly in a furred-down space over the hallway. The worst part was one duct that started at the AH in the attic, dropped down under the house in a chase and ran for about 30 feet before it reached the first floor register in an addition with a flat roof (thus no attic). It then reduced in size and ran another 25 feet under the house. That part might have been mini-splitted even if the rest of the house retained a ducted system. By your description, your duct system was not installed very well, but if the design is sound, it could be revamped less expensively than installing minis. In general, I think that ducted systems are trickier to design and install than mini-splits so there is more tolerance for less than careful and knowledgeable installers with mini-splits. Due to their ability to handle variable loads, choosing the right size is not as critical either. (I had 4 tons cooling plus a gas furnace before, now I have about 106% of the cooling and 60% of the heating. The heating was way oversized.) My system will handle cooling fine with the house the way it is, but I should not have trouble after I make improvements to limit infiltration and add insulation. My installer is knowledgeable, smart, very workmanlike, and experienced (he has taught the trade for 30 years). He would have installed new ducts and a two-speed system for a slightly lower price than the chosen mini-split equipment. I doubt that he would do it for the same price if doing it on a similar home going through interior walls. He did not have much mini-split experience and brought in a collaborator that does, on this project because he was looking forward to a learning experience. I don't think that he lost his shirt, but I don't think he made as much as he usually does.

Differing opinions on mini split heatpumps but most would agree that Mitsubishi makes excellent equipment although it tends to be pricier than some other Japanese brands. Other Japanese brands...Fujitsu, Sharp, Panasonic and Daikin come to mind. Probably even more important than brand is making sure it will be installed by a reputable HVAC company familiar with that brand and that has been in business for many years and will be there in the future to assist in honoring any warranty issues and service calls. I'm surprised that your garage doesn't get significantly warmer or colder than the temperatures you reported. Would think when temps go down to the low 20's your garage wouldn't be as high as 60 degree. Is your garage door insulated? As inverter type mini splits can throttle back on their output I'd be inclined to purchase a 24,000 btu model that should easily handle your large garage when temps reach high 90's or low 20's outside. If in the dead of winter your garage really doesn't get below 60 degrees and in the heat of summer not above mid 80's you could probably get by with an 15,000 btu to 18,000 btu model.

Hello, Would like to know how many BTUs (for 1 wallmount system)would I need for two floors of 690 sqfeet (each floor). The Unit will be on the 2nd floor in the middle between 2 bedrooms and a washroom (over washroom door facing master bedroom). It will be a dual split system. ------------------------------------- Also would like to know which is a safe reliable cost efficient brand, example of wall mounted (split systems) air conditions I've heard of: Fujitsu (they give 10 years warranty parts and labour) Mitsubishi (6 yrs parts and labour however been making them longer) Daikin (6 yrs parts and labour) Igloo York Carrier Grey All about the same price... ----------------------------------- Finally, is it better to get a dual system (with heat pump), from my understanding they are almost the same price and the advantage is that it has a variable BTU which may save some $$. Since the heat pump will be at the higher level then it wont heat as much however it is coming at about the same price. Then again more electronics in a system more chances of malfunctioning sooner? Thank you, Stelios

As you know, resistance electric heat is much more expensive than heat pump. Almost surely, natural gas-fueled heat is significantly less expensive than heat pump. My experience with high-wall mini split heat pumps (seven of them in a seven-room home) is that they are great ceiling heaters. Your experience might be better with slab. I have a raised home with virtually no insulation under the floors. (Coupling the home to the earth can work better since the temperature is more moderate than air.) In my house, the air temperature gets very polarized when the fan speed slows as the units approach set temperature. Ceiling fans improve the situation. With mini splits, you have no heat during defrost cycles. (I honestly don't miss it, but some people might. Note that sinceI have three outdoor units not all in defrost at the same time.) A bunch of mini splits are going to increase maintenance and probably repair costs compared to a central system. Just think about it, multiple filters to get up on a ladder and clean, multiple blower motors, control boards, vane motors and such that potentially go South and need to be replaced,.... Mine need a lot of laborious cleaning to get the crud out that accumulates on the blower cage, in the coils, in the drip tray,.... You have multiple drain lines that can clog and there is no overflow safety. Putting heat on your slab, as is, will heat your slab and the earth beneath very effectively. The earth is an infinite sink for bleeding off heat or cool. Heat transfer by conduction through the concrete is much more effective than by convection into the living space. You need to maximize heat transfer to the living space and insulate below it. Radiant heat, especially on a slab, is not responsive. It is the least responsive compared to radiators/convectors (middle) or forced air (fastest). You don't just turn up the t-stat and have warm floors a few minutes later. What type of heat do you have now and why are you abandoning it? In other words, how are your floors, and everything else being heated now? What is your climate like?