[Whatarethese]

Sounds like some pretty poor planning. Modern heat pumps including the ones that I'm familiar with work down to around -5F. They aren't very efficient obviously at that low temp but mine also has a resistive backup that fires up if needed.

[kccqzy]

OP said "for 11 years" in their post. So I assume they have a unit that's at least 11 years old. Not really comparable with a modern unit.

[torpid]

This building opened 11 years ago and I've been a tenant since then. The HVAC is 2013. Each floor has ~20 apartments and each floor connects to a rooftop unit. The hallways are forced air and stay toasty, it's just the apartments that are on mini splits.