We'll know more from the Temp Regs? 

The utilities are split for the units except for common electric (landlord pays), and heating (one heating system for both units, set at a locked temp).

But if they are zoned correctly it shouldn't be too much, relatively speaking, to add two more systems or even check into newer minisplit heat pumps that work down to negative temps that don't require complicated new pipes/duct work through the walls. 

Craigslist short term and temp rentals.

We get cold weather here (I'm in the mountains after all) but we are far enough south that it rarely goes below freezing for more than just the overnight, as average day time temps are usually in the low 50's. 4.

The valves apparently allow you to increase the temp of the heater, with the mixing valve adding cold water to lower the temp to a safe level.

The night time temps had dipped to the low to mid-40s.

As for the cause, maybe they screwed the stuff down too tight when they installed it, so it warps when the temp changes.

So...If you have been a property owner for long enough in the colder climates its inevitable that the call comes in during the sub freezing temps. 

The ones we have in Tempe are just blowing up