Need advice: What type of HVAC system & capacity to install, 3-bedroom house

NYNorm

These are the options I am considering, based on contractor suggestions:

HEATING BOILER:
Retain existing boiler and add new gas conversion head (non-HE)
Existing Heating System installed Sept. 2010: Oil fired boiler, hot water boiler:
Peerless WBV-03 (not HE) boiler, 3 zones ,Honeywell 4006 Aquastat, Taco Relay, (3) Taco 007 circulators
or,
HEATING BOILER - install HE on demand boiler (possibly in combination with a mini-split heat pump system):
Three contractors recommended the following three systems, each has a $2,000. rebate from utility company:
HTP HE model UFT 140 Wall mounted boiler (140 KBtu, 95% AFUE) (recommend by: CENTA Plumbing)
Alpine ALP 150 (150 KBtu, 95% AFUE) (recommend by: (Glenn) Davis Plumbing)
Navien NCB-210 (100 KBtu, 95% AFUE) (recommend by: Yost & Campbell Heating & AC)
Navien is a combination unit including domestic hot water

HOT WATER:
Superstore Indirect fired storage tank model SSU-60 (60 gallon, lifetime warranty) (CENTA Plumbing) ($300. rebate)
HE On-Demand unit: Bosch / Rheem or A.O. Smith / Bradford White (models & capacity not specified)(Davis)
(but some told me will be inadequate for our usage) ($500 rebate)
(Existing: Hot Water is a 60-gallon, gas standard Rheem hot water heater (at least 20 years old, time to replace!)
Standard replacement (non-HE) standalone hot water heater ($200. rebate)

and/or
Mini-Split Heat Pump system, three units
plus, convert existing oil fired boiler head to gas (non-HE)

Note: gas service is adequate in the area. Utility company has issued written approval to proceed. Rebates for HE boiler will be approx. $3-4,000. Rebates for mini-split system will be approx. $400. per unit x 3 = 1,200. plus federal income tax credit ($300. - not sure if total or per unit): therefore, total mini-split rebate: $1,500 - 2,100. If both systems: $4,500 - 6,100.

Current Usage:
One kitchen, one full bathroom shower in tub, one powder room, three sinks total; clothes washer, dishwasher

Proposed Usage: Conversion to two-family. Two or three kitchens, Three full bathrooms, each with shower, toilet and sink; 2 clothes washers, 2 clothes dryers, 2 dishwashers or three dishwashers (one likely a mini)

Usage: I frequently have large family groups stay over, 15-20 people, where I add air beds in the basement and living room. I can expect all three showers going simultaneously for hour in the morning while others are cooking.

Location: New York City suburb
Existing house and layout / size / heat & AC distribution:
House: 1950 Three bedroom, semi-attached, colonial, with hot water radiators and hot water baseboards (newer sections)
First floor: 588 SF (original area) Living / Dining room combo, kitchen, sink & (no shower)
AC: Main area: single 14.1 KBtu Sears Kenmore window AC (2015), one ceiling fan, Heat: hot water radiators
First floor: 231 SF Rear extension to kitchen added with hot water baseboard heating
Laundry: Currently have a washer + gas dryer in the first floor extension. Kitchen: gas range with electric oven
AC: Receives some (inadequate) airflow from AC in main area of house
Second floor: 630 SF, 3 bedrooms, 1 full bathroom (tub with shower)
AC: Bedroom: three window ACs between 5 & 7.5 KBtu, (5 to 10 years old), 2 ceiling fans, Heat: hot water radiators
Finished basement 630 SF with hot water baseboard heating, portable 12-14 KBtu dehumidifier / AC
Total above grade: 1,450 SF. Total including basement: 2,080 SF

I would like advice on which system / equipment is most practical for each of three scenarios:

Scenario I: Keep the overall house size and configuration as-is, but convert from 1.5 to 3.0 bathrooms (three showers).
Convert to HE or standard gas conversion heat?
Install with mini-split heat pumps and gas conversion for supplemental heat (HE or Standard)?
Indirect or HE Domestic hot water system
Scenario II: Enlarge to three full bathrooms as above (enlarge main floor powder room, plus add full bathroom in basement.)
plus, Add second kitchen either in basement or second floor as part of conversion to two-family house,
including a second washer, dryer and dishwasher in the second kitchen
Scenario III: Same as Scenario II above, plus adding a third kitchen (second floor), enlarged rear extension
and additional attic bedroom. The added areas would have hot water baseboard and/or mini split heat pump
Total size above grade: 1,780 SF. Total including basement: 2,710 SF
Scenario III includes: 3 kitchens, 3 dishwashers, 2 clothes washers/dryers, 3 full bathrooms with showers
Additional heat in attic:

I am thinking of doing these in stages as above, but want to have adequate heat and AC systems. I am assuming if using mini-split heat pumps I can use three to start and add an additional one or two later.

Also, this confuses me:
Utility approval calls for an upgraded meter bar, which the contractors all mentioned would be needed.
The contractors all said I would be required to upgrade ALL gas plumbing in the house from 1/2" to 3/4" or 1 1/4" at a cost of thousands of dollars. The utility Con Edison showed me their specs book and said that would absolutely NOT be required. Who is right? Is it possible there is a NYC building code that Con Edison doesn't know about?

thanks so much. I know this is a welter of information, but I imagine more makes it easier to recommend correctly.

Jamie Hall

First, a note -- we don't talk price here...

Having said that. The very first thing to do is to determine the heating loads for the various zones. Without that information, you really won't be able to size any of the equipment properly.

With regard to the mini-splits, they work very well, and people have been happy with them -- in the shoulder seasons and for air conditioning. You will have to have full capacity hot water heat, though, since they lose efficiency pretty fast when it gets down in the design temperature range -- if the work at all -- and that's exactly when you need the most heat.

And last -- others will comment, I'm sure, on the various heat and hot water sources -- there are numerous code issues involved in converting a residence to a two family. You may or may not want to be the general and architect wrestling with those issues...