Hello Your heat pump air-to-air sounds like the best options. If you dont need to use it for water the COP is quite high, usually COP-3-4. The COP means a ratio of what you get out vs what you put in. The COP of 3 is 3 times the heaat of what you put in. There are a number, some that get a COP of around 5 using source of 30F. Others list between 3-4 quite generally. Carrier is a maker of one type which has a COP of 3 and is a small size. When the source (outdoor temp) is low the COP goes down as does the output. (Below freezing especially). Its worth having an oil or wood or gas heater to go with the system as back up. Heat pumps that can heat water have to work harder as the water is warmer than the air used in buildings, so they use expensive CO2 heat pumps which will set you back about $5K easily. Therefore I would suggest keeping a boiler system, outdoor boiler. An indoor stove needs fresh air which is not ideal for heat pump system which works ideally with heat recovery on the ventilation system (extract heat from waste air at 90% or so efficiency). A neat approach may be to operate an air -to-air heat pump, aim for as high a COP as possible, but install a gas boiler if you have this. The smaller boilers, condensing systems, are surprisingly cheap. If you install a waste water heat recovery system which is just aa heat exchanger on you waste water from your shower, you can half the energy you need and thereby get by with a smaller boiler and consume only about 4-5kWh/day for shower water. A condensing boiler works most efficiently when working with water heating rather than central heating, and plus, the typical condensing system now heats so quickly that you dont need a hot water tank, reducing loses. You would still have at least 15KW available for heating the home. This can be done by putting in a heating loop into the air ducting that is shared with the air-pump. This could be a compact fan assisted system. There would be expense in heating controls for all the ducting. However, if you cannot put in ducting, then you can install a water radiator central heating system. To work with a heat pump, you can go for the super-efficient CO2 based systems that are emerging but expensive. The efficiency can be raised if the heat is needed at a low temperature, so requiring either under-floor heating or a 'fan assisted radiator' which extract more heat and can return the water to the heat pump much cooler than normal radiators, and require a lower temperature, raising COP. I would go with: Waste water heat exchanger ( I guess $500 est with capital cost $200-$300 plus basic installation if incorporated with a boiler installation and pipework anyway) -this will reduce the energy requirement of the condensing boiler, potentially allowing a smaller system. Condensing boiler - 15-25kW output, 90% efficient, capital cost $500 ebay Air-to-Air heat pump (in loft) - COP 3-5 (later figure seen in an Austrian GSHP but should work with airsource unless outside air is usually very cold?) cost? controls? Fan assisted radiator in ducting for backup (run off boiler)? Ventilation heat recovery

Much has been written on your very problem and it can get quite complex. I'll just be general on the topic but I think you'll get the idea.....At minimum, a building should have 4 complete air exchanges per day to be healthy. Public buildings are required to have air exchange systems installed in my area. This of course means you will also have to condition that air which is counter-productive to high efficiency equipment. Older homes that leaked air didn't have the poor air quality that newer leak tight homes have. Think about it, just about every material now days is either from oil based products or, covered with them. Oil evaporates. When you clean the inside of the windshield on your car, that brown stuff that everyone calls nicotine is actually an oil film from the plastics in the car, evaporating. That's why dash boards in an older car eventually have cracks in them, the seats also. They are dried out i.e. the oil has evaporated from them. In your home, the carpeting, the sealant on hardwood floors, stain on furniture and so on, all these things add up to a poor air quality. Then there is dust (mostly dead skin cells) and no matter what you do, there is a certain amount of mold in the air. So everyone is breathing that stuff. Some are allergic to some of these things or at minimum, irritated. Also, when the heat comes on and dries the air, it also dries the mucous in the nasal cavities. This mucous is the body's first line defense mechanism to capture airborne germs and irritants. So in an air tight environment where the air gets stale, people just keep passing airborne germs and bacteria around, generally new ones they bring home from school or, you bring home from work. These things appear to be contributing to higher instances of asthma reactions also. So from the monetary side, you save money on utilities in air tight homes with efficient equipment but if you are sensitive to the stale air, you lose that money savings in medical expenses, even if only cough syrup, aspirin, Kleenex or prescriptions and doctor visits. You can minimize some of the problems by exchanging with fresh air, proper filters, proper relative humidity and of course hygiene.

Sizing can be less of an issue when you have multiple units. Because each unit is controlled independently, provided of course the unit itself is not over sized for the area in which it serves resulting in excessive short cycling. In your case 3 ton plus 2 ton isn't necessarily equal to 5 tons of cooling... unless you drive those thermostats really low. By having two systems with total capacity of 5 tons (which, most of the time this would be oversized for a 1700 - 1800 sq ft structure) has a few advantages. 1. You have the ability to use one system only during low heat days or set the opposing unit higher. The unit set higher only runs if load conditions warrant.... meaning if it's not hot, the thermostat controlling the unit does not call... the unit can not run without a call... 2. Utility bills can be much less... because you rarely use both systems at the same time. You run the bedroom system when you sleep, the other when you're awake. If you have a home office in one the bedrooms you may need to rethink things a bit. 3. Extra capacity for when it's hot out. When it's hot you have the capacity to cool the home. (even while entertaining guests --- additional people add heat) Remember heat waves come, heat waves go... a so call 'properly sized' AC system is essentially properly sized roughly 10% of the time... especially if that system is a single speed AC system. Alot of people I come across like to sleep cool. So for that reason I doubt I would change the configuration of the space. Once you realize what you have, the choices you make are mostly budget type concerns. Essentially a set up like this runs like a 2 stage system. This kind of subject is above the pay grade of many... choose your contractor wisely. I service the Katy, Texas area.

@ranchtastic The existing HVAC does not work at all, and the zoning is so awful that even if it worked it is extremely uncomfortable. If I can remediate this issue for under $15K I just do not want a big item that might steer a potential buyer away. In a similar way, I have 40 year old carpets that look toxic. Ideally I want to get those replaced with a decent light colored wood because I do not want someone looking at the carpet and thinking that they would never want to live a day in that condition. The floors I can do under $10K.