Fitting a Willis Immersion heater

With the NRV I was hoping to prevent the feed from becoming warm as this is likely prolonging the stat staying open. Right now we know the willis's stat can't measure temps at the top accurately so I've set it low to account for this but then there is no excess solar being generated the flow through the willis stops and heat moves downwards. Then I have the problem that this water is warm enough to open the stat so when excess solar returns the stat stays open due to the reverse flow of the hot water.

No noticeable change in temperature with the short-circuiting at the top of the tank but I can feel it on the feed to the willis that will be warm to the touch and after a few seconds of running a hot tap it cools right down the feed from the willis its self says hot/warm even when it's not heating but I haven't checked say after a shower when its not heating.

I'm not sure but it's staying warm enough to keep the stat open and the feed to the willis is becoming warm despite the bottom of the tank being cold. The willis now needs to drop to below 40c before it will start heating again and I think I observed the following today. The temp dropped enough in the willis to close the stat and it started heating with the excess solar but as the water in the willis and feed from the cylinder was still warm it quickly got to 40c and the stat opened before circulation could draw enough cold water from the bottom of the cylinder to start the cycle properly. I think I may have to go with a pipe stat rather than relying on the one in the willis. I'll monitor it for a few days though before changing anything.

Definitely needs tweaking / a pipe stat

I've added two more temperature probes yesterday. The willis cutout at 4.2kwh today into a mostly cold/empty tank. tanktemp1 is at the middle of the tank and tanktemp2 the bottom with the willis temp being taken at the top. The dip at 14:30 was me running off some hot water to attempt to jumpstart circulation it worked briefly but there was too much latent heat and the stat opened again. I double-checked and the stat on the willis is approx 48c its hard to be precise as it's upside down with difficult access! This probably explains climbing above 70c.

It was a crap day for comparisons plus I'd use the gas to heat water this am. I've noticed this a few times now where the willis fills with cold water when hot water is drawn off(shower) its short circuiting but not notably so. It then comes back up to the same temp as the tank without any power applied this has to be reverse circulation e.g. hot water from the tank is flowing to the willis do you think is this expected?

Sorry sure Red is the temp at the top of the willis, Orange is the temp at about the middle of the hot tank and Blue is the temp a couple of inches from the bottom.

To further complicate matters I just discovered the stat on the cylinder immersion had failed! No matter what temp I set it to I could get it to heat and I've swapped it now with a 7" spare I've had for years but what are the chances.

I can try just need to figure a way to attach to the pipework.

Yes this gets quite warm/hot once the willis shuts off - no solar excess or the stat opens.

The willis joins the vent at 15cm above the top of the tank and the vent is coming off the very top of the cylinder. I've added insulation to the 3/4" coming from the wills and fitted the pipe stat here just above the top of the willis on the return piping set to 75c(I tested manually and it was cutting out too quickly when set to 65c and just when convection started to get going) and reset the willis internal stat to 60c.

I didn't fit the stat to the tank as the willis will create very hot water if not controlled more directly. Todays been a bit rubbish for testing but when low levels of excess <1.5kw are available the convection can keep a nice constant flow through the willis as opposed to when it goes higher and starts tripping the stat.

Yes the isolating valve is fully open and that's my next thought to see if the diverter can limit its output, I don't think it can as there's nothing obvious in the config but there is an option to buy a relay board which will run a destrat pump when the immersion stat trips so that could be interesting! Also an interesting graphic below I've overlayed the export the green line on top of the temps. You can see the export jump when the willis stat opens and then disappear once it closes. Its not perfect but not bad and will hopefully perform well on marginal days when the excess is less.

The pipe stat is doing the switching at about 75c, the wills stat is set to 60c with its previous performance, I'd expect that to actually switch out at 85-90c. The scale on the right is the kw export. I messed it up it should be showing 0-3000 and its relevant only for the green line.

Yes I agree, the export is occurring when the stat opens and is 2.5-2.8kw close to the max input of the willis so it's safe to say the solar was providing this level of power to the willis when the stat opened and it can't dump the heat into the cylinder fast enough. Very good point re the ball valve I have it on the 1/2" supply-side so it could well be restricting flow.

I agree re the lack of other reports on success or otherwise I wonder if it's because other users may have a battery and car soaking up excess too but it is very quiet out there regarding them!

Not a great day for the Solar but the tank soaked up all of the excess with 5kwh dumped into it. On the point of the isolation valve restricting flow I had a look at one, I found in the shed today and this type reduces the diameter by about 40% so I think I'll replace that over the next couple of weeks too.

I've also added a CT to measure exactly what the diverter is supplying to the immersion and willis. KW supplied by the diverter (Eddi) is the blue line below, Yellow the temp at the pipe stat above the willis and red the temp at the middle of the hot tank.