I am replacing a gas furnace with a heat pump, looking for some advice from the community on sizing and a few other questions.

(I know this is a long post, but I tried to be as concise as possible, and still give all the pertinent details).

A summary of my questions are:

1)      Proper unit sizing (trust local HVAC contractor, or my Manual-J calculations, or neither)

2)      Zoning or not, (and bypass-duct use)

3)      True mini-split vs. Single Unit with zoning

4)      MERV-15 Media Filter or “regular” filter (Media would be in the attic, regular filter more accessible)

5)      Side question about Communicating Thermostats (worthwhile tech, or overly complex)

Pertinent Details:

Current situation: I’m running a 2-stage gas furnace on second stage only (51,000 BTU output); I do not currently have A/C.  The furnace is currently in the garage. The house is a split-level, and is located in Southern CA (Los Angeles); and is about 2100 S.F of living space.   The exterior is stucco, and there is currently no insulation (anywhere); but plan to have dense pack cellulose installed in the walls (which I think get you around R-15) and in the Attic (R-30+).  Windows are low-E.

Planning to install a heat pump to take care of both heating and A/C.

I am very limited on the location of the condenser due to property line offset requirements, so it therefore needs to be just outside of a ~45″ high masonry wall surrounding a deck (so the condenser needs to be shorter than that to remain out of sight).

I’ve gotten a few bids and only one looks like it going to make the cut.  The proposed bid includes installing a 4-ton Daikin Fit heat pump, replacing all the ductwork; and moving the air handler into the attic; with a few potential add-on’s:  Zoning (2-zones), and an “Air Quality” package (MERV-15 filter, and Aerus Airscubber).  I am thinking I would like to go with zoning (2-zones: upstairs and downstairs).

The only other option that I’ve found that fit my requirements, is a Daikin mini-split (MXS series); where I would use (2) ducted units (FDMQ), one serving downstairs (2 ton) and the other serving upstairs (1.5 ton), and a single wall unit (3/4 ton) for a large family room on the lowest level of the house.  The primary draw of this approach would be the potential to self-install (I consider myself fairly handy; but do not underestimate the effort involved, and other issues like warranty coverage etc.), assuming there is sufficient/any reason to go this route.

With that background here are my questions:

1) Unit Sizing with zoning:  The contractor proposed a 4-ton unit but did not run a Manual-J calculation.  I am assuming they sized it based on the size of the house and (hopefully) knowing what works in the area.  I ran a Manual-J calculation using an on-line tool (CoolCalc) and came up with a slightly lower number:  Overall cooling load of 38,000; and a total of ~43,000 BTU by summing up all rooms; I did a room-by-room calculation: (31,000 BTU for 1st floor, 13,000 BTU 2nd floor).  I asked the contractor about using a 3.5 ton system and they said they would sign up to that.  (I’ve read one article that a zoned system should be about a ½ ton bigger than the largest single zone, which would put it around 37,000 BTU).  I hesitate to “suggest” that they install a smaller unit and then have them use that against me should it end up being too small.   Question: What is the correct size heat pump; considering the overall output and desire for zoning?  and should I go with my Manual-J estimate and have them install a smaller unit?  (or maybe insist/pay them to perform their own Manual-J calculations and perhaps have them come up with the same conclusion?)

Zoning:   The basic question is to zone or not to zone?  In addition, they included a by-pass damper in their bid, for the zoned system.  I asked them about the by-pass since I’ve read it’s “bad”; but I can certainly appreciate why it would be needed.  Their response was it is needed to handle any additional flow if only the smaller (upstairs) zone is calling for cooling (or heat).  And that the unit is inverter-based and can reduce output to ~35% capacity (which would be 1.5 tons).  The zones are roughly split 70 (downstairs)/30 (upstairs) using the Manual-J calcs; or 60/40 using floor area.  They have proposed an electronic bypass damper; and a EWC UT3000 controller.  Question: should I do zoning, and if so is a by-pass damper really just “bad”, or are they okay/necessary if implemented correctly?  (not sure I can suggest dropping it if they want to use it; so trying to understand the implications)

Single system, vs. a Split System:  The proposed single-unit, zoned system may provide some versatility (for fine tuning) and additional capacity for worst-case days.  Whereas the “fixed” mini-split installation approach would be a “true” zoned system. Any advantages to a true mini-split system to a zoned ducted single unit installation?

Air Quality Options:  The optional Media filter (MERV-15) would be in the attic, so replacing it would involve more effort.  I assume the other option would be to have the filter installed at the return intake box, which would be accessed from the inside hallway ceiling (and would maybe be up to MERV-8-13, but likely only 1” thick?).  I suspect the added depth of the Media filter would produce less pressure drop, and would require changing less frequently  Question: should I go with a Media filter (with added hassle to replace), or a lesser filter that will likely get replaced more often (cause it’s easily and more accessible)?  And finally should I install an “Air Purifier”? does it provide a really benefit or is it snake oil; or perhaps a hold-out from the COVID-paranoia era?

 (Secondary question) Communicating Thermostat:  The Diakin Fit uses (exclusively) their Communicating Thermostat.  The net is fully on contradicting opinions on Communicating Equipment; the contractor indicates they put in lots, and haven’t had any issues.  Assuming I don’t go with a mini-split option, it kind of comes with the system, so it may be a moot point.  Question: what’s the verdict on communicating equipment?  Wave of the future, or proprietary headache?

I know that’s a long post, and appreciate anyone who reads through it.  Any words of advice would be greatly appreciated on any topic you care to comment about.  Wife wants the A/C in before the Summer heat arrives, so I need to make a decision soon.   Thanks in advance for your comments.
  -Scott