Brivis installer, Hampton Park.

The Wombat is the straight swap. It shares its footprint with the older Brivis and Braemar units, so the existing ducts, controller wiring and return-air grille all stay where they are and the job is usually done in a day. It's a 3-star heater, which is the right call for a house that's being sold, rented out, or heated a few weeks a year. If you're staying and running it all winter, a Starpro burns less gas doing the same work, and we'll show you both numbers.

Same in-cupboard footprint as the Wombat, with a better fan and a longer service life behind it. What usually decides it is noise: when the heater cupboard backs onto a bedroom or the hallway you walk down at 6am, that's the difference you actually notice. Fit it and forget about it for 15 years.

Starpro 4/5-star is where the gas bill starts to move. Same in-cupboard footprint as the Wombat so the retrofit stays clean, and enough of an efficiency gain that a family running the heater every day through a Melbourne winter feels it on the bill. The 6-star adds a modulating burner on top of that, which is worth having in some houses and not in others, and we'll tell you which yours is.

The modulating burner is what you're buying here. Instead of firing flat out and shutting off, it varies its output to match what the rooms are asking for, so there's less temperature swing and less gas burnt getting there. It earns its money in a house that heats a lot of hours, and it's the most efficient gas ducted heater Brivis builds.