A thermostat is a switch that turns a furnace or A/C on and off to maintain a set temperature. If the heater is rated at 50,000 btus that should be more than sufficient, unless maybe they leave the windows open for fresh air or perhaps the delivery system is insufficient.

What is a 50,000 BTU thermostat? A TS doesn't care how many BTUs it's controlling.

How about make and model number?

V

Boy, talk about dueling posts.

We live in an older, but insulated, 1600 square foot house (with equal size basement) in the northern suburbs of Washington, DC. Our furnace is 80,000 BTU, 90% efficient.

It's plenty of heat, but looks like the Cozy is just a wall heater in a single location, with no distribution through the apartment. Your tenants must be roasting in one room and freezing in the rest.

Here is a link that might be useful: Cozy furnace

weatherize your house first, then worry about the heat. You might want to gtet the heater inspected for proper oper.

I lived a block off the beach in L.A. about 20 years ago, in a similar sized, unrenovated apartment. I can't remember whether there was even any heating in the apt; neither do I remember feeling particularly cold in the apt (and I grew up in Hawaii). But definitely the coast can get the coastal fog and low clouds that don't burn off til later in the day, so it can be quite cool outside in winter.

I suspect the 50K BTU's would be adequate, except that it is mostly not circulating, maybe mostly rising to the high ceiling and being lost through the roof. Poor weatherizing also may make it drafty towards the outer walls, feeling more uncomfortable than actually being so cold. In addition to sealing the doors, walls, windows, maybe a ceiling fan run in reverse can get the heat off the ceiling and push it back down towards the outer walls, evening out the temperature.