Which models are y'all using out there on HN, and for what square footage/location? I'm in SF and have a gas heater, but a 115 year old house with only 4 vents

Which models are y'all using out there on HN, and for what square footage/location? I'm in SF and have a gas heater, but a 115 year old house with only 4 vents (none in bedrooms), and it's been cold. I'd like to replace, maybe DIY, but not sure.