I live in Melbourne, Australia, in an 1880s brick house typical to this area, which we bought two years ago. The house has hydronic heating with a gas boiler, and at purchase time the old boiler was shot and needed replacement. We did the replacement last year, and for cost and simplicity reasons just stuck with a new gas combi boiler. I looked at air-to-water heat pumps that would just plug into the existing system, and switching to air-to-air mini-splits that would have to be installed in each room (we currently have one in the living area, mainly used for cooling in summer). Although I would like to get off gas completely, I couldn't justify the high up front cost of these solutions at the time.

Now I'm thinking about the future, and what I would replace this boiler with when it's time to get off gas completely. It's just an academic question at the moment, but I am wondering what others would suggest. The house doesn't have ducts, and adding them would be complex, so any kind of ducted central system is probably out. Mini-splits in each room would be feasible and may be the way to go, ideally if I can control them from a central point. But the hydronic heat is nice for comfort, and I prefer it to fans and hot air blowing around the place. However my impression is that air to water heat pumps struggle to match the efficiency and performance of air to air ones for home heating. I suppose the other upside to air to air is that they can provide cooling in all rooms as well, something that we don't have today... I also note that there are more exotic things, like the Tepeo ZEB boiler. Maybe something like that will be a better option.

Thanks in advance for any thoughts!

You're right about the Tepeo unit, it is specifically meant for time-of-use electricity tariffs and/or people with PV, and it integrates/times heating cycles for best cost efficiency.

I am definitely planning to add PV as soon as I can, but I'm not sure how much I'll be able to put in. Hipped roof on the house with two ridges facing north, and then a garage I can use, but with a tree in between garage and house that will shade some panels at different times of the day. I'm not sure I could fit more than about 6 kW north-facing. Also our PV generation isn't reconciled in the way you describe - we just get a flat feed-in tariff all year round, and that is slowly shrinking as the deployment of solar increases.

Now that I think about it, a central air-to-air heat pump could be feasible, but the ducting would have to all be in the attic with vents into the ceilings - the house has no crawl space, so ducts and vents under the floor are not possible.

It would just be a bit of a shame to have to remove the whole hydronic system just because there isn't an electric heat solution that suits it. But maybe that is just the future reality...

Out of interest, are air-to-water heat pumps easily available in Australia?

I had heard that the biggest heat pump market in the world is for air-to-air heatpumps (aka aircon). But that there is a much smaller market for air-to-water, for those Northern countries that are heavily invested in water-based central heating and don't use air conditioning. So prices and availability of air-to-water are not yet as good as air-to-air. But is that true? What's it like in Melbourne?

I posted a bit about that last year after attempting to get an a-w ashp. It turned out that the radiator size was not that big an issue because

1) the new generation of R32 ashps now have high efficiency at the same highish flow temperatures (~60⁰) as a condensing boiler, they have been designed to be drop-in boiler replacements.

2) the optional cost to replace our radiators was a relatively small component (10-20%) of the total cost of the heat pump.

3) edit: and same as Peter, many of our existing radiators were already oversized, either because insulation has reduced loads, or because original boiler installer had bumped up the radiator sizes due to vague wet-finger sizing methods.


In the end we didn't go ahead because the local installers here were swamped with demand and so had given up on customer service. Waiting to see if the installer market here catches up with demand.

Thanks for all the additional replies! Responding to a couple of comments/questions:

You CAN get air-to-water heat pumps here, for example these are sold by the company that installed my gas boiler: https://www.hydronic.com.au/products/heat-pump/ But the boiler they installed is a 30 Kw unit, whereas these HPs are 14-16 Kw maximum. That would seem to suggest that I would need two of them to handle the heating load, and that's consistent with what I have read anecdotally from people who have installed them. Just one of these 14 Kw units sells for around $18k AUD, compared to just under $10k AUD that I paid for the gas boiler. And compared to around $6k AUD to purchase 4x mini split units I would need to just switch over to air-to-air units. At these prices, it's hard to see why anyone would install an air-to-water system. I could still easily afford to switch to a stand-alone heat pump hot water heater at those prices.

Today, Melbourne doesn't have too many really hot days in the year, and so our solid brick house hard up against the houses on either side doesn't tend to heat up that much in the summer. It takes a string of 3+ hot days before it starts to get uncomfortable inside. That's how we are able to make do without AC in any of the bedrooms. But if we assume that average temperatures are only going to get hotter over my lifetime, then both price and flexibility (heat and cool) seem to suggest that the mini-splits will be the way to go.

@WillInAberdeen thanks for your comment! Actually when I said at first that I have a combi boiler, that's not right - I have a condensing boiler, but my hot water heater is separate. So in light of your comment, I'm surprised my current unit is a 30 Kw capacity. The next size down is 18 Kw, so maybe they felt that was just under the size needed. At the time the house had no insulation in the attic, but they mentioned that their head load calculation assumed at least R4, so performance would be worse until I insulated it. I have since put R6 insulation throughout the attic/loft, which has made a noticeable difference. Glad I did that before this winter's sky-high gas prices landed.

So, maybe in theory I could get away with just one heat pump boiler of around 16 Kw. Today that would still cost 3x the price of the 4 additional mini-splits I could use instead. Also, if I understand right, the heat pump boiler requires a buffer water tank that may have to be indoors. I doubt I could find space for something like that - my regular water heater is outdoors.

But you're right, time will tell as the market shifts over the next few years.