Tax - 2.6k (6%)Landlord Gas Water Heating - 1.3k (2.9%)Landlord Gas+Oil Heating - 2.8k (6.5%)Landlord Other Utilities - 5.5k (12.6%)Insurance - 1.1k (2.6%)NOI - 11.8kLending costs - 8.6kAs is, cash on cash would be 5.5%Is (eventually) getting 10% cash-on-cash realistic here?  

I would check with a local HVAC (Hydronic) wholesaler or installer to see if you can still obtain them {be prepared for them to cost almost the same {probably more when you factor in labour} as replacing the entire unit.As for using PEX, I'm not certain if the heat-rated PEX (used for in-floor hydronic heating which runs around 140F) can be used for a boiler and radiators (which typically run around 180F).  

A hot water heater isn't a big deal, but an heat pump is, when you hadn't planned on it! 

I also pay for heated water for all 4 units and that gas bill is around $50/m.

What I am seeing is a lot more rehabs that are just carpet and paint and flippers leaving in old windows, heating systems which are on their last life. 

Heating bill and fuel type?

Lights on, tv on, heat or a.c. on.The other half of the time I get a call from someone who just came into town or I give them a drug screen in the morning to get into the refinery. 

Most calls i get is probably heating and plumbing.  

In Ontario (Ottawa) people with student rentals, do you include all utilities (heat / hydro / water) in the cost for students, or do the students pay the utilities on their own (split the costs between them)?

While most Net Zero buildings are energy efficient, perhaps even highly so in comparison to minimum building code requirements, there is nothing requiring them to anywhere new the efficiency level of a Passivhaus   .... this is why you will see odd things like warehouses with large rooftop solar installations being able to claim Net Zero.On the other hand, Passivhaus sets three metrics which a building must meet to be certified as Passivhaus compliant:  The building must have a heating and cooling demand of not more than 15 kWh/m2 (4,755 BTU/sq ft; 5.017 MJ/sq ft) per year  and a peak heat load of 10 W/m2.Total primary energy consumption (energy for heating, hot water and electricity) must not be more than 120 kWh/m2 (38,040 BTU/sq ft; 40.13 MJ/sq ft) per yearThe building must not leak more air than 0.6 times the house volume per hour (n50 ≤ 0.6 / hour) at 50 Pa (0.0073 psi) as tested by a blower door.Ironically, the research and pilot projects which fuelled what eventually became the Passivhaus standard was carried out in Saskatchewan as a National Research Council (NRC) project in the 1970s (the house is still in use today) and a similar project (Minnesota?)