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101 posts

Master Geek

  #1093363 21-Jul-2014 23:13
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Thanks for the reply

The current fireplace is just a small Masport Minos2, tiny you could call it. 
The wetback you can get for it is 1.5kW apparently, but looks to be the most simple of loops.
We were going to custom make the wetpack ourselves by braising some copper tubing parts and fitting them to the default position on the fireplace.
I might contact that company, but most likely we will end up just making it ourselves. Dad has done a lot of plumbing in his time, so we should be able to sort something out.

Yes the fire is tiny and totally undersized for what we want to do, however we plan to just start with 1 radiator and see how it fairs up, and then next year most likely supplement it with a boiler, or replace it entirely that way, and just use the fire for the lounge itself.
Still a lot to decide, but taking it step by step, and planning it out so we dont waste anything or much at all, so if we upgrade it can be ported over without much effort. Might get a bigger fire place, or get one of that companies hot water burners (if they suit the area we live) and dedicate a bunch to hot water, and a small amount to local heat.

Doing more and more research today, we decided the wetback is going to syphon heat a 25L hot water cylinder in the roof space above it, and have that vent out to the roof like normal as the safety. Going to cut open the copper cylinder and install a coil of copper tubing as a heat exchanger, and this is what will feed the radiator system. No risk of bursts of hot water coming through the system and it will be an even temperature output. We will then close it all back up again, and the hot water cylinder will then be an insulated tank, purely to transfer the heat from one closed system to the other, with heat minimal loss. Have a small ball cock on that to make up any lost water in the system if that ever happens. The radiator system will then feed off the heat from the heat transfer system, and be pumped around the radiators with the inline hot water pump. Still have the in line expansion tank to take up the expansion of water when you heat it etc, but it shouldnt need a safety valve since the water will never be able to boil due to not being directly connected to the fire place.

Still got more planning to do, such as the diameter of tubing used for the wetback, and up to feed the hot water cylinder, and then the diameter of the tube for the heat exchanger. Since there is only going to be 1 radiator to start with, maybe 2, it should hopefully cope ok. I dont think the output will be huge, but it will deffinitely be better than nothing. We shall see anyway. then when we upgrade to a boiler, it will just be retrofitted to either be both, or just the boiler.

The little hot water cylinder has a 2Kw element in it, so if we need to boost the system to help it get started, we can - but I wouldn't imagine it would be too efficient, but again we shall see.

All in the name of experimenting.

Got most parts I need, minus the plastic tubing and insulation to run to the radiators (if anyone knows a good supplier that sells this for a good price, please do tell. Most likely looking for 10mm tubing), and then the copper tubing to make the heat exchanger, wetback etc.

Got the hot water cylinder, which has a thermostat and element obviously.
Got the pump
Got the radiators 

So far, the spend is <$600 delivered.

More replies welcome, always happy to get more input to sway the course of events.


929 posts

Ultimate Geek
Inactive user

  #1093366 21-Jul-2014 23:30
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Yeah that sound about right the minos 2 puts out 14kw of heat and the wetback from memory does not sit in the fireplace rather attaches to the back of the fire box

To get a tad more heat performance out of it putting it at the back inside the fire box and copy the design of the metro LTD wetbacks LINK (at your own risk).

I would suggest a low pressure open loop system rather than having a heat exchanger its just too inefficient from my experience.

101 posts

Master Geek

  #1093370 21-Jul-2014 23:57
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Stan: Yeah that sound about right the minos 2 puts out 14kw of heat and the wetback from memory does not sit in the fireplace rather attaches to the back of the fire box

To get a tad more heat performance out of it putting it at the back inside the fire box and copy the design of the metro LTD wetbacks LINK (at your own risk).

I would suggest a low pressure open loop system rather than having a heat exchanger its just too inefficient from my experience.

I havent taken the back off the Minos2 yet, but I was very much under the impression it did go into the fire box... but maybe it doesnt.
The plan was for it to go into the fire box however.

Looking at that manual, that is very intersting. havent seen that design of wetback before... they claim it doesnt affect the performance of the fire.
"All LTD Metros can be fitted with an optional 3kW or 4kW
wetback, designed to give maximum output with minimal
effect on the operation of the fire"

Do you have any more detail on these wetbacks? what is their construction? copper tubing or something else?

Regarding the low pressure open loop system... can you explain more of what you are recommending?
So you wouldnt use the hot water cylinder and have a heat exchanger in there, and have a higher pressure system going to the radiators? You would operate with the same water that goes through the wetback, as what goes through the radiators? Low pressure, as in < 5PSI?

Still maybe get the fire to syphon heat the hot water cylinders 25L of water, and then low pressure pump that same water through the radiators?

Please detail as much as possible - really appreciate the input


101 posts

Master Geek

  #1093380 22-Jul-2014 00:34
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Ah I see now, did more reading, it is a cast wetback. Very interesting.
Could weld up a steel one and treat it and that would probably have a similar affect.

Revised with your idea to be open loop, still using the tank, with a vent to the roof, but water still pumped around.
This idea will mean the hot water cylinder doesn't need to be cut open, and only needs the normal 2 ports + 1 vent. 
Means no copper required for the heat exchanger as its all the same system, but we now have 1 open loop system rather than 2 closed loop systems, which would be safer too.
Water pumped into the radiators and returns via the fire into the tank.
Tank is then the expansion chamber, so a separate expansion vessel isn't required anymore either.
Saves some money and should work better too I think

Open Loop IDea

Thinking this is getting somewhere now...


822 posts

Ultimate Geek

Lifetime subscriber

  #1093405 22-Jul-2014 07:11
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Our house is heated by a very similar setup. We have a Kent fire with wetback, heats a hot water tank (also electric/solar heated) that feeds fan forced, thermostat controlled wall mounted radiators.
Works well.
We have a small commercial business, also wood heated.
A stainless steel wood fired boiler (made by Marshall Heaters in Tauranga) feeds a large header tank through heat exchangers.
The tank feeds a ring main in the main building - a backpacker's hostel - which has 8 wall mounted heaters.
They're Myson brand, all from Trademe.
Again wall mounted, thermo controlled, fan forced, on separate loops so they can be shut off completely In Summer, or used to heat rooms individually.
The Marshall unit came with a diesel burner backup installed, which has only been run in testing as was never required.
We do have a LPG backup with heat exchanger, and will be adding solar to this system as well.
This all works for us as we have endless good quality firewood, and our LPG is supplied by Rockgas, part of Contact, our electricity provider, at a discounted rate.
The quality of firewood is important.
If we burn too much pine the tar produced gums up the heat exchanger surfaces in the Marshall firebox, and the copper wetback pipes in the Kent, as well as building up in the flue (potential fire hazard) particularly when the fire is banked and choked overnight.
We burn hardwoods overnight and radiata, macrocarpa, etc when we have a good burn happening.
Good luck with your project!

2200 posts

Uber Geek

  #1093408 22-Jul-2014 07:15
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You need to make the radiators and wetback separate loops. If you have the fire going and not the pump for any reason you will boil the water in the wetback.

The wetback needs to be able to thermosyphon.

Location: Dunedin


101 posts

Master Geek

  #1093487 22-Jul-2014 09:21
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The pump will be on a controller which senses when the fire is on, and there will be a temperature probe in the pipe by the fire, along with an alarm. 
That is the plan.

I could put in a link down from the tank to the fire so it could thermosyphon, and have it on a solenoid valve, so if the pump is off, the value is open, and vice versa. For a 'just in case' situation.

Fair point.

Having them on separate loops then goes back to the old idea, where you have to have some medium to transfer from one to the other.... not sure if I want to go back to that.
Will do some more thinking.

2200 posts

Uber Geek

  #1093496 22-Jul-2014 09:27
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Why do you have to transfer via a medium? 

The intake for the radiators is from the tank, you gain nothing by having the radiator output go to the wetback then the tank rather than having separate lines.

Location: Dunedin


101 posts

Master Geek

  #1093497 22-Jul-2014 09:30
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It means 2 lines into the tank, which is a standard tank, compared to having to have 4 lines like you are suggesting.

Just trying to work with what I have.

Not gaining anything, just not having to open the cylinder and add 2 more pipes to it.

2200 posts

Uber Geek

  #1093500 22-Jul-2014 09:37
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Seems to me you'd be adding more things to go wrong, with little to no benefit. If you're using an old cylinder, perhaps you can find one with a wetback connection already included.

Location: Dunedin


929 posts

Ultimate Geek
Inactive user

  #1093565 22-Jul-2014 11:22
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Not quite what I was meaning by open loop I will see if I can do a diagram tonight on how I would have set I up.

101 posts

Master Geek

  #1093626 22-Jul-2014 12:35
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OK suggestions welcome, thank you

2158 posts

Uber Geek

Lifetime subscriber

  #1093694 22-Jul-2014 13:41
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andrewNZ: Seems to me you'd be adding more things to go wrong, with little to no benefit. If you're using an old cylinder, perhaps you can find one with a wetback connection already included.


The loops for the wetback and radiators should be separate and the tank is the easiest place to have them meet: the loops would be logically separate making any control far less complicated; also allowing the thermosiphon to protect the wetback.

It's not much different than a solar system with separate collector and tank. Except the collector will be under the tank and the outlet now has a return instead of going to a tap. The pressures are different so the pump moves to the radiator loop.

I'm not sure if I've misunderstood what you're planning to do here. You talked about using a ball cock which means an open tank but if so then the overflow is to a safety tray with drain under the tank rather than using a vent to the roof.
Your diagram doesn't show the water inlet or any valves which are worth having - giving you the ability to manually isolate the tank at each inlet. In this case on the radiator outlet too.

101 posts

Master Geek

  #1093750 22-Jul-2014 15:24
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Confusion may be due to the ideas changing virtually every post as ideas come into play. Sorry about that.

Ball cock idea was to top up the system as it runs out of water, for whatever reason. Same thing applies to this, if the water level in the tank starts to reduce for whatever reason, then a ballcock was a way to automatically introduce more water into the system, and since it has an open vent it doesn't have to be pressurised. Yes the tank will have a tray and drain out the wall or whatever if there is a leak.

I agree that having the wetback syphon into the tank is good, however I am actually preferring the idea of pumping the water through the wetback instead...
From the reading I have been doing and the talking to 2 plumbers now, their experience is pumping through the wetback would work well in this case. The tank isn't right on the fire, it will be maybe 5m away, which from what I read is pretty much a no go in terms of efficiency for syphoning.

It has the safety return so if the pump fails the flow switch will open the valve and the wetback will be able to syphon. If the pump is running, then the value is closed and it pumps the water around the system at the specified rate.
It may be simplier to do it just syphon for the wetback and leave the radiator system pumped, however I think this is a better solution. The water will heat up evenly throughout the system, and not relying on the syphon effect to heat the tank.

Here is the latest diagram which I have shown to the plumbers and I am quite happy with, however I am still open to ideas.

Idea 2

The controller will either control the pump and the value separately, or the flow switch will control the value directly. The Value is normally open, so in the event of a power outage, the system is open so it can auto syphon. Got a few temp probes around for the monitoring system as I will put this into a LCD touch display, and display the temps in the system. Might even have a flow sensor as well as a flow switch, so I can see the volume of water and adjust the pump as required to get the optimal.

I'm a bit of a geek.. what can I say.

If anyone has concerns with the above, please do share - still very open to ideas. Very much a think outside the box type person, but obviously if something works well and works safely, then obviously I wouldn't just change it for the hell of it.

Input welcome


4503 posts

Uber Geek

  #1093752 22-Jul-2014 15:32
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Are you still going to have individual tempering valves and a cold water feed to each radiator for indiviual temperature control,
or will you temper the whole circuit from one point"

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