Title: Bay Area split-level: replace gas furnace with 4 mini-split heads, or keep central HVAC?

I’m considering removing the ducted gas furnace from my 1930s, ~1,600 sq ft split-level house in the San Francisco Bay Area and replacing it with mini-splits.

Our climate is pretty mild:

• Winter cold snaps: occasionally upper 30s°F overnight
• Summer heat waves: occasionally mid/high 90s°F
• We currently have no A/C

My main reason for considering mini-splits is zoning and comfort, not just efficiency.

The bedrooms are on different levels, and with the existing furnace it’s difficult to keep the downstairs primary bedroom comfortable without overheating the upstairs bedrooms, or vice versa.

Longer term I also:

• Want to reclaim some of the space currently occupied by the furnace and ductwork
• Expect to replace my aging roof and add solar after the roof replacement
• Will probably eventually replace the gas water heater as well

House layout

Main floor — open living area

• Living room: ~250 sq ft
• Dining room: ~90 sq ft
• Kitchen: ~100 sq ft
• Total connected area: roughly 440 sq ft
• No doors between living / dining / kitchen
• Open staircase connects this area to the second floor

Envelope is pretty typical for an older house:

• Exterior walls are uninsulated
• Floor is over an uninsulated crawlspace
• Some attic insulation above the living room
• Little or no attic insulation above the kitchen/dining room

I’m thinking of this entire area as one mini-split zone.

Second floor

Bedroom 1

• ~150 sq ft
• Two exterior walls
• Uninsulated walls
• Single-pane windows

Bedroom 2 / office / occasional guest room

• ~170 sq ft
• Two exterior walls
• Uninsulated walls
• One single-pane window
• Large dual-pane French door

There’s also:

• ~70 sq ft bathroom
• Bathroom opens into the hallway
• Hallway connects directly to the open staircase
• Attic has older insulation that I’d probably improve later

Bedroom doors are often closed, and Bedroom 2 is sometimes used for sleeping, so I don’t think one hallway head would give us the individual control we want.

Primary suite

This is a newer addition attached near the kitchen and sits slightly below the main floor because it’s on a slab.

• Primary bedroom: ~190 sq ft
• Primary bathroom: ~60 sq ft
• Bathroom pocket door is basically always open
• Walls are well insulated
• Concrete slab
• Dual-pane sliding door
• Skylight

I’d effectively treat the bedroom + bathroom as a ~250 sq ft zone.

Current mini-split idea

I’m thinking:

• 18k BTU — living / dining / kitchen
• 9k BTU — primary bedroom + bathroom
• 6k or 9k BTU — Bedroom 1
• 9k BTU — Bedroom 2 / office / guest room

Rather than putting all four heads on one condenser, I’m leaning toward two dual-zone outdoor units.

Possible pairing:

Condenser #1

• Primary bedroom
• Bedroom 1 upstairs

Condenser #2

• Living room
• Bedroom 2 / office upstairs

Those pairings also make sense physically because the rooms are relatively close to each other despite being on different levels.

Advantages I see with two condensers instead of one 4-zone:

• Some redundancy if one condenser fails
• Smaller outdoor units
• Better zoning
• Potentially better low-load operation
• I can potentially install the systems in stages

Equipment

I’m mainly considering:

• MRCOOL DIY 5th Gen
• Pioneer Quantum Ultra

MRCOOL appeals to me because of the precharged DIY line sets.

With Pioneer, I’d probably do much of the physical installation myself and hire an HVAC tech for:

• Pressure testing
• Vacuum/evacuation
• Refrigerant-side commissioning

Questions

I’d especially appreciate feedback on:

• Would you eliminate the ducted gas furnace entirely, or replace it with a ducted heat pump instead?
• Do 18k / 9k / 6–9k / 9k sound reasonable, or am I oversized?
• For people with mini-splits in every bedroom, do you still like that setup after living with it for several years?
• How annoying are the wall heads aesthetically or acoustically?
• How well do bathrooms without their own heads stay warm?
• Would you choose two dual-zone condensers or one 4-zone condenser?
• Has anyone regretted removing their ductwork altogether?
• If you switched from central heat to mini-splits, did the room-by-room comfort improvement justify it?
• Anything you wish you had known before making the switch?

I’d be especially interested in experiences from people with older Bay Area / coastal California houses, since our heating and cooling loads are fairly modest compared with most of the country.

I have an older (build in 1930s) 1600 sqft 2-level/split-level house in San Francisco Bay Area, CA where our winters and summers are relatively mild -- Upper 30s for occasional winter cold snaps at nights and mid-upper 90s during occasional heatwaves. We dont have an A/C.

Our electricity is expensive, but eventually i plan to replace my aging roof and install solar.

Because our home is two-stories/split-level, its impossible to maintain preferred temperature level in the downstairs bedroom and upstairs bedroom at the same time and i have been eyeing mini-splits for individual room climate control for a while.

Lastly i have some plans for space currently occupied by gas furnace, ducts and soon after the gas water heater.

House Details:

Main floor

• Living room: ~250 sq ft
• Dining room: ~90 sq ft
• Kitchen: ~100 sq ft
• Living/dining/kitchen are connected without doors, so I’d treat them as one ~440 sq ft zone.
• There’s also an open staircase from this area to the second floor.
• Original exterior walls are uninsulated.
• Living/dining/kitchen floors are over an uninsulated crawlspace.
• There’s some attic insulation over the living room, but little/no insulation above the kitchen and dining room.

Second floor

• Bedroom 1: ~150 sq ft
• Bedroom 2 / office: ~170 sq ft -- also used as guest bedroom
• Both have two uninsulated exterior walls.
• Some single-pane windows, although Bedroom 2 also has a large dual-pane French door.
• Attic above is insulated, but the insulation is old and could be improved.
• Upstairs bathroom: ~70 sq ft, located between the bedrooms and opening into the hallway/open staircase.

Primary suite

• This is a newer addition with well-insulated walls and a concrete slab. build off kitchen and a little lower than the main floor due to slab.
• Primary bedroom: ~191 sq ft
• Primary bathroom: ~58 sq ft and the door is basically always open (pocket door)
• Dual-pane sliding door to the yard and skylight.

My current thought is four indoor heads:

• First floor Living/dining/kitchen: ~18k BTU
• Primary bedroom/bath: ~9k BTU
• Second floor Bedroom 1: ~6k or 9K BTU
• Second floor Bedroom 2/office: ~9k BTU

Rather than one large 4-zone condenser, I’m leaning toward two dual-zone outdoor units for redundancy and better zoning:

1. Primary bedroom + Bedroom 1 (2nd floor Bedroom1 is closer to Primary Suite addition)
2. Living room + Bedroom 2/office (2nd Floor Bedroom 2/office is closer to living room).

I’m considering MRCOOL DIY 5th Gen because I’m comfortable doing a lot of the installation myself, but I’m also looking at Pioneer and would potentially hire an HVAC tech to do the refrigerant-side work.

A few things I’d especially like opinions on:

• For a mild Bay Area climate, would you replace the ducted furnace entirely with mini-splits, or keep/replace the central ducted system?
• Does the proposed 9k/6k/9k/18k sizing sound reasonable, or am I oversizing?
• How annoying is having individual wall heads in every bedroom long-term?
• For people who switched from ducted heat to mini-splits, did comfort improve or get worse?
• Would you favor two smaller multi-zone condensers over one large 4-zone condenser?
• How would you handle the bathrooms? I’m assuming they’d get incidental heat plus possibly small electric supplemental heaters.
• If you were starting over, would you still choose mini-splits over a central ducted heat pump?

I’m particularly interested in experiences from people with older Bay Area / coastal California homes where heating loads are fairly low.