[Redmohawk]

AC the initial setup using water boxed unit is a temporary initial install .

Functioning hot water will be needed in under a week maximum. I have access to a free unit for initial testing hence the unit is pre chosen. I have access to a pair of 315L storage tanks , both can be retro fitted with conventional element type heaters Though I'd rather not. They may even have one already of smaller than conventional wattage for they're size .(both units are glassed steel solar storage tanks of known origin removed early in life due to converting to continuous gas heater).

Both tanks should have heater ports , plus they both have 5 access ports , initial inspection makes me believe both have copper heat extraction coils and have been designed to run a glycol mix in the tank (designed for steel flat plate collectors) With the tank mounted on the ground and a circulation pump.

I understand the 60C requirement, but that doesn't totally rule out 'low tech' compressors. If you looked at your heat pump as a pre-heater, you could then do the daily 60C bump-up with your heating source of choice. No need to go CO2 all the way... at least for some years hence, when scrap CO2 compressors come available.

I see the benefit in a pre heater , BUT I also hate waste, heating water to 60 deg c to only cool it down again for bathing seems a waste to me if I do not require 60 deg c for anything else. A previous system I installed with my father years ago does just this using a wet back off a slow combustion stove with electric backup. The extra heat in that case was used for hydronic heating through UHDPE pipe in a concrete slab under mud brick house.

Long term I wish to go 100% solar as the electricity grid here is 100% privately owned , a 200% increase in charges (both per Kw and connection fees) in the last 3 years has made electricity (and Gas is the same deal) non viable long term in my opinion. Off grid is the only way forward without great stuffing about . (even grid interactive solar is at great cost due to non existent (a pittance anyway) feed in rates. Add monthly extra connection charges for the right.

Current rates are sitting at 16c off peak , 22c shoulder ,and 32c peak ! with connection fees hovering around $54 a month on top. This will continue to increase significantly due to stupid governance in Aust.

Anyhow enough bitching about politicians. I did seriously look at evacuated tube solar water , As it is currently believed to be "The ideal off grid setup" By many over here. And I a very familiar with its function and setup (installed 3 units so far for others) It has strong merit for this area's solar aspect , but cost is high for an initial install. Gov rebates will drop the price to around the $6900 mark installed. BUT it must be a brand new system installed by a certified plumber. DIY is 100% out , and plumbers are not very keen to have an "Assistant" so to speak to reduce costs.

Also a 30 tube system would provide us with hotwater unboosted almost 100% of the year, boiling the water in summer often and being a little cool (due to inclement weather) maybe twice in winter max.

I could cover tubes to prevent boiling in summer and deal with a few cooler showers every year (the girls maybe not without some serious talks to me).

The initial $7000 is a little painful , and evac tube systems are not common enough to acquire damaged or second hand. And I'm less than keen to have to make a shading system so the tanks not ready to cook lobster by 11 am in summer. Also I can't use the waste heat , its just waste.

I Have the knowledge to make/modify and inverter very cheap to run a Aircon setup directly off PV panels, PV panels are a dime a dozen so to speak ATM with access to panels under a $1 a watt in the 200 watt size and above.

There is a rebate for solar PV , it requires the same qualified deal with a solar accredited electrician to do the install for a grid connect or independent system . But the initial costs for a small system are much lower (1.5kw system of my choice , of inverter and panel type) would be around the $1600 mark after rebates.

I can use the "waste electricity" to run the house, no a 1.5kw system would struggle to power the water heater in all times of the year besides peak output summer. But once the initial install is done I could "ADD" to the system without the fear of big brother looking over my shoulder and the extra cost of paying for a "Certified installer". The initial rebate is only cost effective for a smaller system install. But I can upgrade for similar $ per watt prices to the rebate price by DIY.

Once the system is upgraded (by adding banks of extra panels and parallel linked battery backed inverters) I can go off grid 100% at my pleasure.

The air-source-HX-in-a-water-box idea is OK for someone who wants to prove that it can be done. But for long time, dependable service, you should go whole hog and get or make a good heat exchanger. (Imagine you family members and member-ettes standing in front of you wanting to know why the thing that you call your exchanger-in-a-box, has become an ugly wad of green-slime, and will not allow them to take a bath!!). Since you are doing this for domestic water consumption, a plate type HX is the wrong choice. Some form of tube-in-tube or tube-in-shell is what you want, to avoid mineral build-up from rapidly killing your system.

Indeed the plan is to use the (OLD) in window unit as a proof of concept , with the backup of the water storage units having resistive load (just in case). The hope it to have the unit function 100% off the bat with the compressor, If the heat moved is effective and the heat sink is stable enough.

If after the initial trial is effective , I plan to implement a better more efficient setup. Using a more modern compressor, possibly a inverter based unit with variable pump drive. A "Coil in Coil" transfer to the heat storage would be a definite. As would another for heat sourcing, I think the copper cost to benefit of direct vapor in earth for a system large enough to extract regular large amounts of heat might be prohibitive.

I've worked as a plastic welder , welding HDPE pipe , this is very familiar and cheap for me. Large amounts of pressure copper tube in the ground over a wide area looks expensive. Drilling holes should be no issue to depths of 10m without issue. I have 1 ah of land to access for a heat bank.

I hope the "inverter system" may allow tailing off so to speak, so as target heat is achieved load sharing between the hot water unit and the other house loads can be maintained. Instead of a compressor on or off situation as temp is maintained . This should also reduce load on the end design when 100% off grid throughout the night. (no solar gain) though I have plans for a micro wind system to augment the solar. And another power generator of yet undisclosed design.