Maybe it is a thermostat that can be replaced as opposed to a new heater.

I’m looking for a pump (for forced hot water heating system) that would not let return water flow whenever thermostat is set to “low”.

I have lowered the thermostat to 70 degrees and it is running all the time.

The key is managed properly, you need to make sure windows are not open during winter season and if tenants do air out their units they turn off their thermostats.  

Treat it like you would your own house if managing yourself yearly inspection :- furnace/boiler - clean out burners (do in Fall to make sure heat is on in winter)- clean dryer vents- caulk bathtubs- change out thermostat/CO2 detector batteries- clean out gutters in Fall- while on the roof looking at gutters, take a look at roof condition - fix early- vacuum and clean utility room near any furnaces and water heaters (keeps dirt/dust out of systems, can extend life)- rinse water heater screens (can fill with dust and won't stay lit)- drain water heaters yearly - prevents sediment buildup and will extend life- change furnace filters (more often than yearly)- if in cold environment, turn water supply to exterior off before winter- take a look under sinks to check for any slow leakThis shouldn't take much longer than an hour or so per unit per year unless you find something unexpected, and if you do find something then good job catching it early. 

They could put a small table next to the door by the thermostat. 

All that being said, I have seen situations where tenants complained about ovens not cooking properly and it turned out to be a bad thermostat.

Examples include (USB outlets, updated digital thermostat, updated light fixtures/faucets,etc)Any ideas?

I pay for gas heat at one of my 4-unit properties, and I can live with that because I can control the thermostat, but ceding control of electric would be a red flag for me.

Does the HVAC have a thermostat?