Scott3:

 

To answer the subject line of thread "Best home heating system?", it's hydronic underfloor heating, fed by a heat pump.

But it is more suitable for a new build, and if you want cooling, this likely needs to be tacked with a different solution.

 

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For OP's retrofit needs, we went through a similar exercise a year back.


Our house is ~30 years old, and not partially well designed / built thermally. Pressing need for the upgrade came from the need to cool our upstairs office and bedroom as it bakes in summer and I now spend a lot of time working from home (bedroom was previously cooled by a noisey portable AC unit). Lounge has a ~9 kW heat pump, which due to the layout of the house, means a decent amount of warmth end up upstairs, so winter heating was less pressing.

 

There was a wide variance of pricing, and these numbers are a year old but to give some idea:


Covering the 3x upstairs rooms from the vendor I ultimately chose:

 

• 8 kW Mitsubishi ducted system with zonal controls: $12,408
• ~6-7 kW (there is some error in the quote) Mitsubishi multi split outdoor unit with 3x AP25 indoor units: $9073.50
• 3x AP25 mini splits: $7417.50

Initially none of the quotes I had were for the 3x minsplits, Asked for that quote last minute and was glad I did.


Ultimately went the 3x mini spits for the upstairs rooms. Key decision factors:

 

• Lower cost
• Slightly higher efficiency than ducted system. (ducts run through the roofspace and are only insulated to about R0.9, so some heat/cool is lost into the roof space).
• Higher efficiency than multispit (and greater peak power if we crank them all at once, but as it happens they are abundant powerful) 
• Greater efficiency / less short cycling with only one room on. our general use pattern is air-conditioning the office during the day and one bedroom during the night. Both ducted and multi-spit unit would be oversized for running only a single room.
• Ability to run one unit on heating while other's are cooling (my spouse is strange with temperature, so this gets a lot of use).

Also got an AP25 for the one bedroom downstairs, so we have 4x matching units.

 

Downside of the lots this approach is that, (when combined with the existing lounge heat pump outdoor unit), we now have a total of 5 outdoor units around our house -given they layout of our house this is not an issue, but could be for others.

 

Also need to pay attention to wiring, installers like to save money by tapping into the back of existing power outlets for the smaller heatpump's. I paid some money, not included in the above numbers to do some extra wiring to avoid this.


On the Mitsubishi AP20 - Gave this a lot of consideration for my child room (the least roasting of the three upstairs rooms in summer) - confident the size would have been enough, but ultimately didn't go this route:

 

• It's main selling point is the more compact indoor unit, something which was not important for our application
• No built in wifi, and if you add it as an option, it errods all of the $150 - $200 cost saving vs the AP25. (I wanted wifi control at the time, but actually have found manual control is fine and never set it up).
• Slightly nosier although at 18 dBA vs 21 dBA on the quietest mode, both units are extremely quiet
• Slightly Less efficient EER / COP: 4.35 / 4.17 vs EER / COP: 5.00 / 4.78.
• I was fairly confident the study, downstairs bedroom, and our bedroom needed the output of the AP25, and it seemed neater to have 4x matching units.

I prior poster mentioned that the AP20 does have better turn down (minimum output), and this is a valid advantage.

There was fairly massive variation in prices between suppliers. Happy air was the the one we picked.

Note our installs were timed for when we already had a key wall open, and we had to cut the jib of another wall to fit the drain pipe of two other air'cons. Repair of this was done by other trades we had on site, so not counted in the estimates above.



Should note we have underfloor heating (resistance) in our bathrooms so don't need further heating in them.

 

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OP mentioned gas.

Gas is in a sunset period in NZ.

LPG (bottled gas) is already more expensive than using a resistance heater on my power prices.

Natural gas (piped gas) based on a random retailer I picked is $2/day + 14.147 c/kWh incl GST. My power is currently 17c/ kWh, and going up to about 20c / kWh, so even ignoring that daily fee, I am much better off to use power in a COP 3 or 4 heat pump than to burn gas in a 90% efficient gas heater...

Personally I would not invest in any more gas appliances at this point, and replace the gas hot water with electric (and disconnect the gas) when the appliance reaches it's end of life.

Should note that gas fired ducted central heating, can supply a much lower volume of much hotter air than heat pumps, which can provide a less drafty experience. also can heat water hot enough to run radiators etc. Heat pumps get less efficient heating stuff really hot, so are more suited to underfloor heating or heating large volumes of air.

 

Thanks for this comprehensive reply, @Scott3. Sounds like you really did your homework! I'm reasonably confident that the solution I've settled on should fit our needs pretty well, and think I've get a decent deal. You're right that the AP15 & AP20 don't have Wi-Fi (although apparently it can be added via a dongle) but I don't think it would be needed for the smaller rooms. It only adds around $85 on the larger units so for that, it's worth having.

I ruled out gas after doing some research and, in fact, as and when the gas boiler needs replacing, I'll probably change to electric as I hae paying the standing charge just for hot water.

Thanks again.