The unit is about 15yo. It hasn't been working properly for about 12 months and we've been needing to electric boost it most mornings (especially through winter). I finally pulled my finger out of my arse and got a plumber to have a gander at it, as the $700/quarter electrical bills we're getting tiresome.
From the symptoms I gave them their first step was a flush and recharge. It failed the flush part. It's simply rusted out.
15 year lifespan is not too shabby. I should be able to knock about 30% off the power bill with a new unit. And we'll get nearly a grand back in gov rebates.
It's just a bit of dosh to fork out straight after Christmas.