Fixed Temperature Thermostats

[Robert White]

My son and I own two apartment buildings - a fourplex and a 5-unit.  In the fourplex, the tenants pay their own utilities, so I don't care how warm they keep their units.  But in the 5-unit, I pay utilities.

The thermostats (there are two boilers and two thermostats) are in two separate units.  I know that one of the tenants keeps the heat way higher than others in the building would prefer.  Obviously, that costs me money, too.

I'm wondering if anyone knows of a source for a fixed temperature thermostat.  I'm looking for a sensor that (for example) turns the heat on at 69 degrees and shuts it off at 71 degrees.  No knobs or adjustments where the temperature can be adjusted by the tenant.

If you know of such a device, I'd love to have some info on the source.