Off-Grid in Australia - Yes it does get kinda cold.

jumper said:

If you're off grid how do you drive your pumps? I'd go photovoltaic with high temperature storage. Year round. Gleaning and stacking wood only works when there's nothing better to do. Is the LPG delivered? BTW LPG differs from propane around here.

For this house there is also a grid-ready solar PV electric power system with batteries with diesel gen backup, it would be sized to suit the heating system loads (as well as A/C and house loads). On $ these are competitive with running the power poles in on many farms now in Australia, even allowing for 2x battery replacements and a set of panels for a target 40year life. Self contained PV systems can often be more reliable than the rural grid in some areas.
I hadn’t thought of oversizing the PV system and using electric resistance for the heat source, I’ll look into it, it would simplify the system. Flickering flames and a warm kitchen cookstove has other appeals for us though.

So rev 3 went in a different direction for the backup supply and also incorporated a living room wood stove with 20kW heat exchanger to allow flexibility and heat capture for whichever wood appliance has been fired.

I recognised that the house is an off-grid installation and will have a PV system with batteries and diesel generator backup for electricity supply with no option to feed excess PV generation back into the grid. An abundance of free energy on Sunny days.

Since the PV array will be oversized on sunny days to provide useful battery charging on less than ideal days I tried to see if I could incorporate that into the heating system. There is a bit to like with this since it eliminates the LPG boiler, as well as the evacuated tube solar thermal system. Along with that disappears:
- The extra capital and maintenance costs of LPG boiler and EV tube system.
- The LPG boiler corrosion issues I was wrestling with.
- The summer overheating issue and possibility of a heat dump
- Several components eliminated.

The DHW priority is still achieved with the full available use of the electric system (PV, batteries or Diesel Gen) to keep the top of the tank hot enough for DHW.

The tradeoff is losing the option to run the heating system on LPG if we were ill or too lazy to burn wood and it was persistently cloudy. Though realistically such LPG use would send us broke quickly anyway. The DHW priority element needs to be sized for the flat battery case, i.e. diesel generator capacity less house loads, say 3kW to DHW leaving 4kW available for house loads and battery charging using a 7kW backup generator. No real heating use would be possible there, but this would be a rare scenario.

I think if the storage tank is a repurposed pressure vessel, I could also set it up either open vented to meet the code requirements with the option to switch to closed with PRV if the storage tank is designed large enough to prevent boiling. If a genuine muppet were to leave the stove doors open and overfire both stoves for many hours with a starting tank temp of 95degC and no heat demand then the PRV could be sized to ensure the vessel is not pressurized excessively, would not be code approved but possible.

As far as the ceiling mounted tank goes, its partly a packaging thing to fit it into the house. I was also considering the potential to connect two stoves to one tank where the stoves are some distance apart, not sharing a common wall with a mechanical room. In order to have continuously rising & falling thermosiphon piping the piping & tank would need to be in the roof space to achieve two heat sources into one tank. If the tank was at grade the piping would traverse the living space to get to the tank.
The elevated tank also helps for NPSH considerations on the circulators in a open vented system. I could redesign the layout to incorporate a shared wall plant room, but I like this layout.

@hot_rod We're a 30 something growing family, sourcing and burning wood is standard fare for a farm in our area, comes with the territory. Cooking on the woodstove would be optional since a summer LPG stove would be installed also.
The labour free options for us are Air-Air heat pump A/C, which would be the button on the wall solution. A zoned system that does the main living area ~100m2 and provides A/C in summer would be doable off-grid, but a full house solution pushes the PV-battery feasibility on heating, which is typically an evening/night time load, so the diesel generator would auto for heating if we wanted to let the fires go out and do it with A/C.

Most places with grid power run ducted A/C for heating & cooling and supplement the heating with a dry wood heater. A/C is doable off-grid in summer, if you don't need to run it thru the night which is typical of our climate. Nearest city, Orange NSW.

On sizing;
My very rough calc of steady state heat loss is between 18-23kW (61,416-78,476BTU/hr) on a -7degC day for the full house 245m2. dependent on chosen glazing & insulation spec. Still need to confirm that heating load. The bulk of the loss is in the main living area, which would also receive the substantial radiant heat from the wood appliances when they're running. The typical use for the hydronics would be to keep the bedrooms & bathrooms warm, which are remote from the wood heaters, by rads and towel rails. The approx load for those areas is about 9kW (30,708 BTU/hr) steady state heat loss.

For vessels, thin wall insulated tanks are readily available in 315, 400, and 500L sizes. Thick wall vessels like anhydrous bullets can be had in 800L. Air receivers and LPG tanks can be found in 980, 1000, 1200L sizes, bigger than that starts to push the limit on cost and weight bearing. The tank would be the only modified component if I went with a thick wall vessel, otherwise would all be commercially available equipment. Mods to the vessel could be done to the vessel code by a coded welder (I do a bit of this in another life)

Hopefully no closet bombs here, the stoves are commercially available, supplied with heating jackets by the vendor. www.wiseliving.com.au is the locally made products, there are also imported options (Esse, Rayburn etc.). They are all available dry or wet.

I did have a look at using an LPG combi prior to rev1, there is limited options in Aus since hydronic is uncommon, so the sizing is often bumped up to cover the most number of applications with one or two locally certified products. Either a 24 or 37kW (81,888 or 126,244BTU/hr) mod-con model would be available from reputable vendors, quite expensive, and probably oversized though they turndown say 5:1.
The other downside is the DHW firing cycle is triggered by a flow switch, so unless I had some control logic interrupting the call for DHW, the combi would short cycle every time a tap is opened even with preheated DHW available from the storage system. That interrupting control may need to modify the combi since the DHW controls are onboard the combi. Some models may be able to receive a digital input.

There are a few more simple & cheaper capital options in the mix too, involving either higher running cost and/or a bigger PV-battery system or not heating the bedrooms & bathrooms which is normal around here. The wood appliances are going to stay in the design even if they were dry, just 'cos we like a wood fire and a warm stove ticking over in the kitchen. So this exercise is to stretch the brain and see if we can make a useful integrated system. Cost/benefit analysis comes next which may also torpedo the idea, but lets see.

jumper said:

Photovoltaic produces higher temperatures than solar thermal unless you use concentrating collectors. Higher temperature storage is more useful and smaller. Photovoltaic can also be used for A/C.

Thanks @jumper are you suggesting to consider more electric resistance heating using PV power rather than heat pump?