I have had a 3Kw manufacturers original wetback installed in a Metro wood stove ( the small model ). Previously it had a booster wetback installed which included a pulse valve, but we got sick of the banging and crashing that went on when the fire was lit. Now, I get no thermosiphon with the new one at all, and no hot water in the cylinder...or so it appears. After a short period of running the fire I get hot water in both the up and down piping from the wetback ( the cylinder is in the roof about 3 metres above the stove, so you would expect thermo siphoning to not be a problem ).

The cylincer is small ( about 135 litres )...and it has a tempering valve installed. Is it possible that the temering valves is causing hot water to flow down to the wetback, or is it simply that both connections to the wetback are rising??  Are tempering valves adjustable?

I have called the plumber back but I would love to know whats going on myself before he gets round to coming> Appreciater any thoughts!!!

Murray



Linkback: https://www.plumbers.nz/q-and-a-wetbacks/61/wetback-problem/1463/

Hi and thanks for your reply. Sorry its taken me a while to get back to you...

The piping from the cylinder down to the wetback is the same as was installed for the 3Kw " Pulse Valve " model I took out. Its 20mm but it is smaller than the tubing ends that stick out of the new 3kw wetback, which are 25mm.

The plumber had to do some squashing ( not a professional plumbing term, but thats what he did ), to get it to where he could weld one to the other. Could this reduction in pipe sizes be the cause of the problem?

I crawled into the roof space to check: the tempering valve is connected between the top of cylinder outlet ( H ) and the cold water supply ( C ) and outputs to the hot water taps in the house. I guess this is to cool the water if it gets too hot at the top of the cylinder. So, no its not in the wetback side of things. My apologies for throwing that one in.

Yes the wetback up-pipe goes to the top of the cylinder where it is open vented through a standpipe on the roof, which is lagged and sleeved, and is about 3 m high. The cold water down to the wetback comes directly out of the cylinder a few inches above the base of the cylinder, which is my experience of where it should come from.

The wierd thing is that when you light the fire from cold, you get  gradual heating of the riser side, while the down pipe stays cold, or cool anyway...then after fifteen or twenty minutes there is usually a succession of gurgles and burps, and some rattling of the whole set-up, after which it settles down but both pipes appear to be rising. Eventually there is very little difference between the riser and the down-pipe as far as temperature are concerned, but there is little or no boosting of the water in the cylinder when you go to take a shower.

I would love to get this sorted out....so any input would be gladly received. I have installed a couple of systems before, myself, and they have worked fine. One included a temperature controlled pump to do the circulating as there was a distance of several metres horizontally and with no rise to the cylinder. But this one is a straightforward setup and should work without issues.

Is it possible that I should instal a non return in the downpipe...to replace the pulse valve that was in the existing system before I decided to change the wetback?  ( And wish I hadnt!! )...I was told that its bad plumbing practice to instal any kind of restricting valve in a wetback system, in case it fails.

Cheers

Murray

Very good diagram , The tempering valve is not the problem what confuses me straight away is that your flow is running into your open vent above the body of water in the tank (should be about 300mm above the return, as the hot water has no more space to rise it cant contribute to the circulation) This explains why they used a pulse valve. Also part of the problem is that your solar is on the same circuit this is bound to disrupt the natural circulation.

The greater the vertical height of a circulation system , the greater will be the circulating pressure. The greater the difference in temperature between the "hotter" rising flow and the "cooler" falling return, the greater the circulation pressure. Also the simpler and shorter the circulation system the smaller the loss of circulating pressure due to friction.  The amount of bends you have are too many and not swept enough and having a 20mm pipe does not help the cause at all. Sorry but looking at this I think without reconfiguring the system thermal circulation is impossible. If you don't llike the noise of the pulse valve I would suggest a thermostat triggering a pump, I know it defeats the purpose of a wetback using an electrical solution also this needs to be installed correctly as it can push water through the vent if not done properly. If it was my place I would redo the wetback or put up with the pulse valve. Hope this helps some what.. 

hi guys/Murray/Plumber, just to add my two cents worth, your diagram pipework looks like the wetback would be heating the solar panel and by-passing the h/w cylinder which by the way looks far,far too small to need a wetback and solar system to heat, if it worked correctly it would quickly relieve water out of the exhaust and continuously also why do you need two vents? what do you think Plumber? cheers

I think all you really need is an anti-siphon loop in the return pipe down by the boiler - return pipe only

the cooler more dense & heavier water volume in the loop would force the heated water up and through the flow pipe towards the hwc creating the ciculation you require

obviously not as pretty as std pipework or perhaps a pulse valve - but wont be as noisey either

what is the design of the wetback like? are the ports over/under as drawn or are they in fact side by side?

Hi,

Happily to relate that the insertion of a NZ made Pulse Flow Monitoring valve has cured the problem; no more banging and crashing, just a steady almost silent circulation and lots of hot water!! We left the circulating pipework at 20mm and the hotwater riser still goes to the top of the cylinder. Thanks to those who contributed, especially Plumber.
Just goes to show that sites like this one can be very useful in cutting through alot of the bull**** and getting to the core of the problem.

Murray

Hi

Sure, no problem. They are made in a brass housing by a small industry in Kaeo, New Zealand. The contact is Merv Foley, Mobile: 021 882542 or A/H 09 4050591. Email pulse.flow@xtra.co.nz. He has a website at www.pulseflow.co.nz.

They have a 5 year warranty, well constructed, and come with details on proper installation.

Cheers

Murray