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Immersion heater with immersun

Hi. I have an array of solar pv panels which work great. ( with out immersun on in jan to March I got 160 gbp income from feed in tariff.

When I enable my immersun device which uses Excess generated power to heat the water via the immersion heater ( esp on less sunny days) it seems the water never reaches “hot” whereas it gets to “hot” with a short 30 min boost on the boiler.

So … my q is this : I think the immersun outputs a range of power levels depending on available power (not sure if it is throttling current or voltage levels or both). So what effect does this have on the immersion heater? I am wondering if it quite simply will use up all the generated energy and never get to hot as the immersion element itself is at a low temperature. In which case I could use all power forever and the water never signals”hot”.

Wondering if i might be more effective doing my 30 min boiler boost and then exporting excess energy to the grid on the feed in tariff. Or disable immersun unless a really sunny day when I get the highest power output.

Anyone got any thoughts or data on this and how immersion elements behave at different power levels?

I would say that is normal when it less sunny you heat your water up less… as it only diverts your excess power production. for me as produce almost all my hotwater from the sun, evacuated solar and power diversion… I installed a small electric 4.5 kw instant water heater after my hotwater tanks… that tops it up so if I have cloudy days and the water can only be heated to ( generally 40c) the instant water heater tops it up to 50c . and when the sun strong no top up happens as my hotwater will be at +70c

Assuming I am right and at low power there comes a point when the element stops increasing the water temp ( before it gets to the thermostat max), the sending further power is effectively wasted. Is anyone aware of a retrofitable device that detects when the water temperature stops increasing which could be used to turn off the immersion heater?

Welcome, Graeme to the OEM forum.

My understanding is the Immersun takes slices out of the mains power at very high frequency and feeds a smoothed version of that to the immersion heater. The proportion that gets diverted is continuously variable and the objective is to balance the power fed to the immersion so that the nett power flow at your grid connection is never export.

So after the filter, the immersion (because it’s constant resistance) sees both variable voltage and pro-rata variable current.

If there isn’t enough power being fed into the immersion heater to overcome the losses through the insulation and from usage, then the tank water will cool until there’s a balance. If there is a small excess at whatever temperature, then the water temperature will increase, and so will the losses, until there’s a balance again. If there’s enough excess power to take the temperature up to the thermostat setting, the Immersun will be deprived of its load and the energy will be exported. The Immersun will only import if you ‘boost’ manually.

Do you mean feed-in tariff or export tariff? You get the FIT payments depending on the energy you generate, whether or not you export it. If you don’t get FIT payments, you get an export tariff payment instead. Without knowing the value of that, it’s highly likely that you will be better off keeping as much generated energy as you can.

What I think would happen then is the tank water would cool at the natural rate. I think the tank losses will be proportional to the temperature difference (as most will be conduction and convection rather than radiation), so it might be more economical to not aim to maintain the water at the thermostat setting. Whether that’s acceptable is your decision.

I think the only way to determine this will be to run a series of tests, carefully measure and record all the powers, energies and temperatures and then work out the costs.

Thanks. Appreciate the comments. Agree ( feels to me there is room for some smart development for someone to produce a product that allows the user to tune the target water temperature to an optimal level. Maybe one day :/).

That said. ( I am new to FIT). I had totally missed the point that there are two tariffs!!! ( your post made me go check it out. :-). I had assumed feed in meant power actually exported. You are right that it doesn’t. Very good and given the rates reduces the urgency of the topic a bit. Many thanks. !!

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I might be missing something here, but my solar diverter just keeps taking the excess until the thermostat on the immersion kicks in (around 60Celsius) and then the solar diverter stops.

If something cools the water down (taps, shower, time) then the solar diverter kicks back in and does it’s job.

We’ve had a few days this year when it’s been all we needed. On less sunny days it does keep slowly warming the water up and fails to get to the peak.

As long as we’re putting energy into the immersion it’s adding energy to the tank. It’s not as though the water is heating up the element and pushing electricity back. I believe it’ll just heat up less quantity of water when less power is introduced because the water won’t move up and away from the element quickly.

BTW our thermostat on the immersion is set to 60, but our ASHP only targets 42 so it can stay efficient. We let the solar diverter run so high to catch as much energy as we can.

Now THAT would be a very neat trick if you’d got solar collectors as well as PV. :wink:

I too didn’t quite see what Graeme was getting at - I think there is (or was) a degree of misunderstanding - maybe influenced by overzealous marketing hype? I’d certaily agree with you that what you describe is exactly how I’d expect any diverter to work - maybe not relying on the cylinder 'stat if it had it’s own water temperature sensor (as MartinR’s design published here originally had).

I presume when ‘the diverter stops’, the box actually remains active but cannot divert anything because the 'stat has opened the load circuit.

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that is why I use instant hot water heater for topping up… i heat a heck of alot of water around 1000 liters to +70c when it sunny ( as it acts as heat bank for my house as well…) so if the sun only heats it to 40c on a cloudy day… it only tops up what i actually use. which might only be 20 liters in a day. but otherwise for 8 months out of the year i have hot water galore… and actually many days i have figure out ways to usefully use it . but in the dead of winter i use a heat pump like you that heats my storage heats to 45c an if it sunny bring it up to maybe 55 - 60c and if not enough the instant water heater tops it up to again to 50c

Yep, absolutely. The UI even says “Water hot” or some such.

That is a heck of a lot of water!

If you want to go nuts having an outdoor pool would quickly use it up. We have a second ASHP for the outdoor pool which might go up soonish (too decadent?) so then the outdoor pool ASHP and the solar diverter will both want the solar power and the diverter is too polite so it’ll lose out :slight_smile:

Another way to use oodles of water is to have teenage children. There’s a long lead-time on that approach though.

BTW Your setup sounds fab.

my 3 young “adult” kids just left the house to strike it out on their own ( 2 month before covid) – and had a swimming pool heated by solar as well… but no kids so now i have a huge amount of hot water … so currently it big long huge baths, filled to the brim for me and the wife…once in the morning then another one in evening for the most part. well no one complaining I guess other then wife when she forgets the water super hot on sunny days ( going to have to put in a mixing valve again my other one failed )

thank you i like my system it works well even though I have hiccups now and then…

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