Immersun diverter

Immersun diverter has a fault (ERROR 11 CPU CRL CHECK SUM WRONG)
This was reported 14/7/22 to them, but still no response.
Anyone else had this problem please?
Any suggestions or help welcome

What make / model is it? Is it an Eddi unit?

Hello again
The immersun diverter now seems to have failed and has caused electrics to trip (having shown this fault several times over past months)
The company have not followed up my enquiry. I now hear from another supplier that immersun are not recommended.
Also that an ASHP would be more efficient at providing hot water (via immersion heater) than a diverter - any comments?
I can send a photo of the unit if you wish. It’s not an Eddi unit as far as i know
Thanks for any advice

I cannot comment on the state of your Immersun, being an Eddi man myself, but I can comment on the ASHP relative to a solar diverter and immersion.

A solar diverter can operate diverting anything from 800W to 3kW from your solar production to an immersion heater. This is very good at using your PV on site, which is by far the best way to use it. Solar diverters cost from £200 to £500.

An ASHP will use anything from 1kW to 9kW or more to heat your hot water and cannot operate below that. If you regularly have under a kW “spare” but rarely more, an ASHP is not a good “PV buffer”. An ASHP will cost from £6000 to £15000 including installation.

Clearly an ASHP requires a much higher budget but at the moment it might qualify for a government grant of £5000.

The other side of the argument is that an immersion heater has a COP of 1 (1kW consumed gives 1kW of heat) whereas a heat pump has a COP of 3 or more (1kW consumed gives 3kW or more of heat) so in the long term a heat pump should pay for itself.

I have had an Eddi for some time and have just had a heat pump installed. I have connected a relay on the Eddi to the heat pump’s “low tariff” switch, so if there is over 1kW of solar spare it tells the heat pump to do whatever it is programmed to do during cheap rate electricity (boost heating, if needed, and top up the hot water). If there is less than a kW of spare solar the immersion gets the power. As I have only just done this it is too early to comment on whether there is any point.
If the thermostats and timers demand it the heat pump will otherwise act in the normal way.

No, an immersion heater is always the same efficiency. It is by linking heating to spare PV that improvements in solar utilisation are achieved. The efficiency of the heat source (immersion heater) remains the same.

This is a very high budget topic and only relevant to a proportion of Carbon Coop members.

Hi Phoebe - I suspect the quick answer is buy an Eddi - any qualified electrician locally should be able to install it. Heat Water With Solar | Solar Power Diverter | myenergi UK

I am in a similar dilema about ASHP - I and 3kW solar, an immersion tank and gas heating at present. However you might want to consider getting a Mixergy tank - this has the solar divertor built in and is very clever - costs about £1600 - £2000. I’ve not got one as I’m in a new build (and it seems a bit daft getting rid of a new tank) and also I also have a Eddi, which apparently stops it being as smart as if it was just on its own.

Now might be the time to look at ASHP but take your time. I have been switching between getting one or a Tepeo - I’ll be honest, if Tepeo had installers in the North Of England, I would have bough one by now - just waiting!

Thank you Tim for this response and explanation.
The issue for me may be whether to replace the diverter. So I gather an Eddi would be preferable to an immersun?
Have others found immersun unreliable?
Having invested a lot of money in solar PV, diverter, battery and AHSP, I would like them to work well,saving carbon etc
Thanks again Phoebe


I have heard good things about iBoost too but have no experience.

If you foresee getting an EV and charger that can also take clues from PV production then I recommend Eddi. It is designed to work in tandem with Zappi in such situations. They can be controlled from an app where, amongst other settings, you can specify your priority for charging/hot water. Eddi can also control an additional heater or other device once the water is hot.


You can certainly have a Tepeo and Eddi working together. Basically a Tepeo is a heat store for either hot water, heating or both. It has a COP of 1 against heat pumps’ 3 or more. Tepeo comes into its own if you have an overnight cheap rate less than 1/COP, so using my example figures your cheap rate needs to be one third of the day rate or less to make financial sense and (here’s the really unpredictable bit) stay that way for the life of the system. You would need a large solar farm on your roof to rely on solar power only for your heating.
Mixergy and Eddi do not mix. Remove the Eddi if you go for the tank.

Thanks everyone
I’ll see what my solar installers - NPS - have to say and offer
So all this info is most helpful, but i would be relying on them somewhat
And i hope this will help others when considering their options - hearing from the pioneers, the experienced and knowledgeable …
Thanks again