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95 posts

Master Geek
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# 251480 27-Jun-2019 09:23
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We're considering putting an offer in on a (totally) off-grid property. Has a 5kw solar system with batteries. Currently the hot water is heated by a wetback from an AGA style range in the kitchen. The hot water can also be diverted (from the wetback, not the cylinder) to six radiators around the house. Obvious drawback of this is the range has to be running year round for hot water to be available.


I'm trying to figure what modifications (and at what cost) could be made to ensure hot water all year round even when the range isn't in use.


If we purchase we'd almost definitely have bottled LPG installed to add a gas oven/hob for year round cooking.


This from the vendor re the cylinder:


"The Hot Water cylinder is a 300 Litre Baxi complete with coil, supplied with connections to Wood Fire, a separate hot water pump and piping for the 6 hot water radiators.  There is wiring to the hot water cupboard ready to wire the cylinder for electrical power if desired.  There should also be connections ready for solar hot water piping system to be installed if desired, this was an option we were considering for future, especially for summer."

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1447 posts

Uber Geek
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  # 2265648 27-Jun-2019 10:11
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Gas hotwater is cheap to install, assuming you have a water pump already, probably be about $1500 to buy and a few hundred to get installed.  The wetback and radiators would be a seperate system .. nice in winter time.

95 posts

Master Geek
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  # 2265654 27-Jun-2019 10:21
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I understand the hot water cylinder has a dual coil. Would I be correct thinking that (instead of a totally separate gas hot wanter on demand system) we could add a gas boiler (Luna Duo-Tec for example) to the second coil. This would cycle on and off as required to keep the tank at temp but become redundant in the winter if the range was heating the water via the second coil?


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550 posts

Ultimate Geek
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  # 2265663 27-Jun-2019 10:45
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"There should also be connections ready for solar hot water piping system"

This will be refering to pluming points to install piping (and a circulation pump) to connect to a roof mounted thermal solar panel (vacuum tube type is the best). Doing this would give the lowest running costs, and should complement the solid fuel burner well. Power usage of circulation pump should be pritty small, and not have a big impact on your running costs.

That said, you could tap those points to inject any heat source you like. Kits are available to use one of the common continuous flow gas water haters with tank, which could be an option. If you have a water cooled backup generator, you could also have that plumbed (via a heat exchanger, to add heat to the system) 

Couple of things to watch out for:


  • Legionnaires disease. Pritty much you need to ensure your cylinder is heated to 60 deg C, at least once a day, to kill of the bug. Hard to reliably achieve if you are solely reliant on uncontrolled heat sources.
  • Overheating: A solar panel (or wetback) can boil the hot water cylinder. Some are set up to dump hot water, (so it is replaced with cold to prevent boiling). Not ideal if you are on a limited tank supply of water. My grandparents put a mat on half of their solar hot water panel during summer to reduce it's output (and take it off in other seasons).

If you are feeling your inner geek, the latest thing to do is to install direct DC solar to your hot water cylinder, with dedicated PV panels. Saves the cost of an inverted,  can be switched to prevent overheating, and avoids issues with heat losses from piping etc. Would require serious research.

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Uber Geek
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  # 2265681 27-Jun-2019 11:25
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What sort of Hot water load are we talking about?


if its a typical shower/bath/handbasins + kitchen use then I would seriously just look at installing an on demand gas water heater (rinnai etc) and plumb it into the current hot water pipe leaving the HWC with a valve. 


In Summer ( when the AGA is not heating the system) isolate the HWC and run the hot water on the Gas heater,


In Winter, switch it the other way and use the wetback heated cylinder),



95 posts

Master Geek
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  # 2265683 27-Jun-2019 11:32
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Good question. Two adults and two kids (currently 12 and 14). House has a bathroom
with bath and separate shower and an en-suite with shower.

Presumably cheaper to run gas in demand than constantly heating the tank? Although would heating the tank via a gas boiler give the option to run the radiators when the wetback isn’t operational?

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Uber Geek
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  # 2265691 27-Jun-2019 11:44
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Presumably cheaper to run gas in demand than constantly heating the tank? Although would heating the tank via a gas boiler give the option to run the radiators when the wetback isn’t operational?


A gas boiler to heat the tank is an option ( but this is different  from an on the wall ondemand water heater most people have)


If you were going to go that route , it would be better to just install a gas HWC... 

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Uber Geek
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  # 2265701 27-Jun-2019 12:13
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On demand gas water heater. Rinnai make a system that is designed to integrate with solar hot water cylinders. Where if the cylinder is hot enough, the gas heater does nothing. Otherwise the gas heater then heats the water to 70deg (hot enough to instantly kill any legionella bacteria) Then mixed down to 55deg. Benefit is that if the cylinder is only warm, you still use less gas. As the gas heater is not heating stone cold water. And no need to maintain a min temp in the cylinder.

Consider putting extra PV panels on your solar system. Then adding a PV diverter to use excess electricity to heat the cylinder. Morning star Make a PWM solar controller that can be used as a shunt regulator. Benefit is that you have your full panel capacity available for battery charging. And the excess summer capacity will then give you hot water. (system would be: panels, then MPPT regulator (adjusted to overcharge the batteries) , then battery bank, Then Shunt regulator (which draws off power as needed to stop the batteries from getting overcharged) then the Cylinder element.

And consider using the coolant from your backup generator if you have one for supplementary water heating as already mentioned above.

High efficiency gas boilers struggle with NZ LPG. So dont bother trying to modify the radiator system to use LPG.

Get Rinnai Energy Saver Flued gas heaters installed if you want quick heating for when you cant justify lighting the fire.


261 posts

Ultimate Geek
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  # 2265719 27-Jun-2019 12:50
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I have on grid solar 4.9 kw (installed 2013)


Good location and sunshine hours.  Last year replaced 180 litre low pressure hot water cylinder to 275 litres and fitted a Paladin PV diverter to heat hot water.  Previously used a timer in the circuit to direct power to hotwater.  Modest house, all electric with heat pump, family of five, works out great.  Annually we produce more kwh of power (export), than we use. 


In your case what ever the controller you use needs to work in with the battery charging system, and for winter days you have the fire and wetback to heat the hot water and all PV can go to battery. 


A hot water controller should be less than $1000 installed.  You should not need additional solar panels depending on use, situation, batteries. 


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  # 2265721 27-Jun-2019 12:55
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If you use gas to heat water, you'll need to factor in the per KWh cost of bottled LPG.


Bottled LPG is typically $100-120 per 45 Kg, plus $10 per month bottle rental.


45 Kg is about 600 KWh. (2200 MJ)


An Infinity 26 Litres/minute max water heater produces about 5 to 45 KW (15-200 MJ) of heat when running, depending on flow.


You'll need to work out your own hot water consumption. I suspect the figures will be discouraging...

990 posts

Ultimate Geek
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  # 2266009 27-Jun-2019 23:53
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You can reduce your hot water requirements for showers, see and .



Solar thermal makes more sense where it's warm enough that you don't require antifreeze. Solar ready hot water cylinders designed for antifreeze contain a coil instead of directly pumping water through.



The Kingspan solar tubes have a heat control valve inside at the end of the tubes to help prevent excessive temperatures being distributed to the system.

95 posts

Master Geek
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  # 2266053 28-Jun-2019 08:16
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Thanks so much for all the comments - heaps of useful information and I'm coming to a better understanding (I think).


The optimum solution which has been suggested to me seems to be:


- Excess solar diverted to hot water heater in cylinder when batteries are full (see how below),


- Wetback heats the water in winter,


- Gas califont installed in series with the cylinder to top up temperatures in spring/autumn (when excess solar isn't enough) or if the wetback isn't lit.


Has the advantage of maximising the solar output but giving a fall back to ensure we have continuous hot water with (expensive) LPG only used as a back-up.


From what I now understand, Paladin and other devices work by monitoring the voltage being sent to the grid, and cause this to be diverted to hot water heating. However since we don't have a grid connection, Paladin and other devices won't work 'off the shelf' in this scenario. However it may be possible for Paladin to modify a unit to achieve this, or alternatively someone mentioned there are charger/inverters that can handle this, so upgrading the charger/inverter could be an option.




Would love to hear thoughts on this as a solution!

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Uber Geek
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  # 2267394 30-Jun-2019 17:50
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I was just thinking about that morningstar controller idea above. 


We have used this with wind based systems where they need to be constantly connected to the batteries but when the batteries are full, the excess electricity needs to be burned immediately rather than storing in the batteries. So they simply monitor the voltage of the batteries and switch on a "dump load" output. 


They dont work so well with mppt solar systems where they only allow the batteries to get about 80% charged on solar then it triggers the diversion as they expect wind to do the 80-100%. 


I could be wrong and there might be a way to make it work. 


If thats the case then I would definitely be looking into a way to run a standard cylinder off of the excess solar power. 




Otherwise I am thinking a solar tube system with an LPG top-up califont after it. 
I am usually against gas but since its not occupied all-year-round you could probably do it with a standard 9kg BBQ size cylinder instead of the large ones that get rented. 


The thing I am trying to wrap my head around is the overheat relief for the solar part of the system and the pumps. It would be good if it was possible to send this back into the radiators. 



Ray Taylor
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