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 Post subject: Indirect thermosiphon help please
PostPosted: Tue Mar 29, 2016 12:40 pm 
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Since last June, I've been helping a girlfriend with an off-grid house she's building.

I designed and installed the solar system and because of the limitations of solar ( especially in winter), I recommended that she use a closed thermosiphon system for her heating and hot water. I will do basic plumbing for us but not for somebody else, so I told her she had to use a qualified plumber to install the system.

The system is indirect thermosiphon using a wood burner with a back boiler as one of the heat sources. The tank has two heat exchangers as solar water heating will be added later this year.

She found a plumber who unfortunately wasn't familiar with non-pumped systems and he visited our house a couple of times to examine our water systems and seemed confident that he understood the principles. He started the work just before I left for Australia and when he filled the system and lit the woodstove to test it, the water in the expansion tank becomes hot but the indirect tank water isn't heating.

The house is almost ready to move in to and my friend needs to move out of her rented accommodation at the end of March. So, in the meantime, she has asked a couple of other plumbers to look at the system but none of them have given a definitive answer of what to do to make it work as it should.

I went round to have a look at the system yesterday and the plumber has connected the woodburner to the lower heat exchanger. (Normally for the solar heated water). That shouldn't affect the heating of the exchanger in the tank as there's plenty of height for the thermosiphon to work but it's worrying.

He forgot to include a purge tap at the lowest point in the system.

He has put a thermostat on the radiator (dump load) in the bathroom behind the woodburner.

He's also added a pressure relief valve and another valve to the flow side (See photo) of the heat exchanger. As I understand it, the flow and return pipes should have a clear interrupted run to help the heated water move easily and I don't understand why he felt this was necessary as he's connected an (Open) expansion vessel to the flow pipe. Is this restricting the flow and the cause of the expansion tank water heating I wonder ?

Image

The original plumber is too busy to help solve the problem, so I said we need an old timer familiar with this sort of system to help get it right. :D

Any ideas please ?

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 Post subject: Re: Indirect thermosiphon help please
PostPosted: Tue Mar 29, 2016 8:37 pm 
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Thanks for the call Peake, 'yer a wee darlin'. :ymhug:

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 Post subject: Re: Indirect thermosiphon help please
PostPosted: Thu Mar 31, 2016 2:16 pm 
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Thermosiphon, what a bloody 'orribal name, thought up by somebody who does not understand plumbing no-doubt, it should be called Gravity circulation, because it relies on the difference in the density of water (or to be exact, the specific gravity) between the flow and return, for circulation, see chart below

Specific Gravity - SG

Temperature
oF oC
40 4.4 1.000 Water at its highest density
50 10.0 0.999
60 15.6 0.999
70 21.1 0.998
80 26.7 0.996
90 32.2 0.995
100 37.8 0.993
120 48.9 0.989
140 60.0 0.983
160 71.1 0.977
180 82.0 0.970

A few other things about water, all at 4°c, and standard atmospheric pressure
1 litre of water weighs 1 kilogram
1 cubic metre of water weighs 1000 kgs = just about 220 Imperial gallons
1 Imperial gallon (not the yanky crap stuff) weighs 10 Lbs
1 cubic foot of water weighs 62.5 Lbs = 6.25 Imp gallons

You also have to take into account the frictional resistance of the pipes and fittings as well

Horizontal pipes should be kept as short as possible, and with a slight rise (turn of the bubble) to the highest end

Now to turn to the photo and your question

He forgot to include a purge tap at the lowest point in the system.

Not that important, fit a drain off on the boiler

He has put a thermostat on the radiator (dump load) in the bathroom behind the woodburner.

A heat dump rad should be unvalved, remove the thermostatic rad valve

From the photo, it looks as if he has set the job in hand as a pressurised system

For a Gravity system to work the cold feed from the expansion tank, should connect to the return by the boiler, and the expansion should be taken off the flow close to the cylinder connection

The cold feed can be in 16 mm tube, and the expansion should be in 20 or 22 mm tube, with the expansion pipe taken above the water level in the expansion tank, before the turn over of the expansion pipe, be 45 mm for each metre hight from the center line of the boiler to the center line of the pipe coil in the cylinder, plus 150 mm

One thing that has just struck me, and that is the pipe coil in the cylinder could be restricting the flow of the water

I would be inclined to go with the set up as it is, but without the expansion tank, and fit a low wattage circulator, wired to a thermostatic switch on the flow, to turn the circulator on when the temperature in the flow pipe reaches say 45°c, together with a combined hot water dump/cold fill safety valve

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 Post subject: Re: Indirect thermosiphon help please
PostPosted: Sun Apr 10, 2016 10:32 am 
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Just to let you know how we got on with the, "Let's try the cheapest and easiest solution first" strategy.

We teed off the copper feed at the highest point leading to the water tank into a 14mm pipe (Much smaller than the plumber had used) horizontally (Rather than vertically as the plumber had done) and this small resistance in the pipes has meant that the water in the expansion tank is no longer heating up.

We also added a purge to the lowest part of the closed system.

The water in the tank is heating beautifully !

Thanks Peake. :)

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