Willis solarsyphon

[panmure]

Does anyone have experience of using a Willis Solarsyphon instead on installing a new twin coil hot water tank?

[kevymoss]

Yes I use them all the time as an installer.
Great job

[JohnS]

Kevymoss

Have you ever installed one onto a Megaflo cylinder?

John

[wookey]

You can get the same effect for a lot less money using a standard plate heat exchanger.

It'd be nice to have a side-by-side comparison with the the Willis Solarsiphon (which does have a significantly larger heat-exchanger area), but I  get a 20 - 30C temp rise through the PHE (between bottom and top of tank), which I believe is similar to what the Willis device achieves. (good day today, with 90l of water ending up over 55C (stratified from 69-56C). (It started the day from 23-28C)) Just had a nice hot bath :-)

[mick]

Wookey,

How is your set-up arranged - I recollect some photos but not a diagram.

With ref to my knowledge of the Willis, does your side loop start at the bottom of the cylinder and go higher than the top to tee into the DHW feed? If so, how high above? Or does it go into the cylinder side just below the top?

How high up the side loop is you HE.

Where do you have the lower cylinder sensor S2? I ask because if it is in the lower cylinder the pump will run but the modest heat in the panel will not go anywhere except round and round the solar loop - it would seem better to have it on or next to the HE so the T diff is between that and the panel - I think (this is where I am told it should go on the Willis - but that has a small 'tank' around the HE).

What are your controller settings - particularly DT O and DT F (pump on/off).

Finally - how does it perform in marginal sun, particularly in winter when the panel needs to get to a higher temp than the top of the cylinder before the thermosiphon starts to work?

Thanks,

Mick.

[wookey]

I did a diag about a year ago, but it's not much like what I currently have :-) so here's the current state of play:



So yes mine is plumbed much like the willis - from the cold feed at the bottom to tee into the DHW feed. Only about 12cm above the top: http://wookware.org/pics/solar/037-IMG_2549.JPG.html
I never really understood why the solarsiphon recomendations suggsts teeing it in quite high. Do you? Anti reverse siphoning?

The bottom of the HE is mounted level with the cold feed/oulet. i.e. as low as it can go without a dip.

I've got sensors all up the cylinder - every 20cm, but the ones I am comparing against for pump control are the bottom of the tank and the top of the tank.

The control algorithm is:
if tanktop > 80: pump off (overheat)
if panel > tankbottom+20 or panel > tank_top+4: pump on
if panel < tankbottom+10: pump off

This is pretty random, but the idea is that if the panel is hot enough to heat the tank top, or is between 10 and 20C higher than bottom of tank, then pump. This has worked well over the summer - it may not be optimal for days when things don't really get very hot. How well it works in winter remains to be seen.

You can see what it did today here: http://aleph1.co.uk/munin/wookware.org/control.wookware.org.html

There are some interesting effects there, some of which I don't understand, but discussing the minutiae of such things deserves it's own thread I think :-)

[mick]

Wookey,

All brilliant stuff this - but the sun (and work) beckons so I'll have to respond later.

Agree re:own thread - perhaps this explanatory diag could go there as well - the two threads will separate after a while.

One question - what controller do you have (should I know that?)?.

This is really sterling and very useful work Wookey - I rarely award them but another applaud to you (making you higher than me!).

Mick.

[martin]

As the Willis Solasyphon is a Navitron stock item, any information that anyone has on their use will be very useful for those tempted to use one - so please, feel free to bung it into the thread - any tips and wrinkles that you have!

[wookey]

Yes, I didn't mean to hijack this thread - just pointing out that for the DIY-inclined (and innovative installers) there is an alternative approximately equivalent approach.

Actual details/pros/cons on real solarsiphon installations would be very interesting.

[martin]

I'm all for it! If either makes solar more affordable/possible for more people, then it's all to the good!

[Brandon]

having tried them in a couple of instalations, all i can say is that I am n'er convinved or impressed.

Ulster uni found themm to be fine in independant tests, but it is all down to the available heat in the panels.  Some days panels will not rise above 40-50C regardless of there being no heat taken from them. A syphon therefore that has to displace water at the top of your DHW cylinder will be ineffectual if there is any usable heat in the DHW already.
Conversley, that same heat entering a coil at the bottom of a cylinder through a decent coil will more often than not contribute to the energy in the cylinder.

I have recently removed one from one of our test setups, and replaced it with a PHE, coming from the injection tee that the willis was previously conected to, and returning into the 15mm sensor pocket half way up the (sh*te clear skies approved) "solar" coil (OSO cylinder- my advise wouod be to not use one of these). It is working better than the willis everdid.  My next alteration is to fit a statifying tube to the PHE outlet, in order that the hot from the solar can enter the cylinder at the point that it will be of most use, ie as high up as physics will allow it.

[Ivan]

Brandon's experience is what I was expecting out of an external heatexchanger, although I have heard a lot of opinnions to the contrary. I'd expect them to work much better with vacuum tubes than flat plates. I'd also expect them to work better if the 'bottom of tank' sensor is fitted to the external heat exchanger rather than the cylinder - to me it's the only logical way to connect it, but I think often people don't do this.

Wookey, Have you tested the effect of putting sensor on tank vs on your PHE?

[wookey]

Not yet. Obviously I don't actually move the sensors, I just tell the software to use a different one. I'm not sure why it would make much difference, though: the bottom of the tank sensor is generally a very similar temp to the bottom of the PHE.

The 'correct' algorithm is (perhaps) to average the temps up the column of the tank, then turn the pump on if the panel is some hotter than that (because then the thermosiphon should circulate in the correct direction). I haven't been doing anything that sophisticated. My current algorithm is in the PHE control thread.

Originally my 'bottom' sensor was 20cm up from the bottom - now it is the same height as the PHE bottom - didn;t seem to make that much difference in practice.

[Flamethrower_]

I have just fitted two Willis solar syhons to  2 existing megaflow cylinders in the north of Scotland.

1. A 175 litre Megaflow connected to a 20 58mm collector.

2. A 125 litre Megafow connected to a 20 47mm collector.

So far the results look pretty good with customers saying that they are impressed with amount of solar hot water.

As Ivan says the cylinder sensor I have placed inside the insulation jacket of the solar syphon as this is the only sensible place to put it to allow the temperature differential controller to operate.

One other consideration is to fit an injection tee to the cold feed between the pressure control valve and the cylinder on the Megaflow otherwise all you will draw off is cold water, as it will short circuit the system (I know because I found out the hard way and had to reposition the valve!) 

Rob

[JohnS]

Rob,

Thanks.  It is helpful to know they can work together.  Are there any issues with the air bubble if the cylinder gets hotter than what there thermostat would turn it off at?

Megaflo technical department told me not to use one but I think they were (a) ignorant of how it works and (b) wanting me to buy a new Megaflo with a solar coil.

John