Heat Pump system design for maximizing efficiency
I've been reading this forum for sometime trying to educate myself and have finally decided to post as I need some advice to make some equipment choices.
I live in the mountains of Santa Cruz, CA mountains where winters are fairly mild, although we are about 5-10 degrees colder than the beach area. I have a single story ranch house with two wings. The front is larger, has vaulted ceilings and is being completely remodeled with good insulation, sealing, and double pane windows. The back has the bedrooms, is smaller, but has single pane glass, no wall insulation, and although we are sealing where we can it is still somewhat leaky.
I did a manual J using HVAC-Calc and came up with a heating load of approx 27,000 for the front and 24,000 for the back (design temp 33 F), which were almost identical to independently derived numbers that my contractor produced.
We are on expensive propane and planning to go solar in the future, so I'm planning to put in two (2) separate heat pumps centrally located over each area to minimize the duct runs and losses, and have separate zones since the heating dynamics are quite different. I am not very concerned with cooling in this location, although we do use A/C a few weeks out of the year
I have a few questions on equipment strategy as follows:
1) Manual J - I've been told by multiple people that man J oversizes the requirements by about 25-30%. Is this true? Should I consider the back of the house load to be 18K (instead of 24K) to avoid oversizing the equipment?
2) Sizing - I am talking with several local contractors and have recommendations for 1.5 ton, 2 ton, and 2.5 ton units for each area. How should I use man J to determine the right size heat pump that maximizes efficiency? If my adjusted manual J (based on Q1) is 18K do I need a 1.5 ton unit, or a unit capable of producing 18K at the 33 degree design temp?
3) Single vs two speed compressor - Theoretically it seems to make sense to go with a two speed compressor, but my understanding is that would imply different air flows for the different compressor speeds. In practice does that cause problems with the air mixing differently in low vs. high modes? Are there issues with getting the refigerant charge correct since theoretically it can only be optimized for one speed? Am I better off with a single speed compressor and fixed air flow speed design?
4) Duct / grill optimization - If I used a two speed compressor, I would expect it to operate in low 80-90% of the time. So, would I optimize duct design for low air flow mode at the expense of higher static pressure when it switches to high mode?
5) Air flow - for this area which is fairly dry one recommendation is for an air flow of 500 CFM per ton, which I understand is a bit higher than norm. The ducts and grills are being designed to produce 500 ft per minute at the supply. Does this sound right?
I realize this is a lot of questions, but I've been thinking about this for quite some time and now need to make some decisions. I would appreicate any feedback you can provide.