with a PV divert to run the immersion up to its max (even if the thermostat is working, the tank could reach 70C regularly) you must fit a blending valve to the hot output of the DHW tank to avoid potential scalding at the taps.

Posted by: @iancalderbank

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with a PV divert to run the immersion up to its max (even if the thermostat is working, the tank could reach 70C regularly) you must fit a blending valve to the hot output of the DHW tank to avoid potential scalding at the taps.

im sorry but im struggling to see the difference between the immersion being heated by solar harvesting or by standard electricity supply, surely it only heats to the level the thermostat is set to and then it shouldnt go any higher. 70C is way in excess of what it should be set to anyway, i set mine to under 65C and even that could scald an elderly or young person. i have thermostatic mixers on the shower though not admittedly on the bath, but then again, there are rarely old or young people using the bath. 

I don’t dispute your suggestion that a blending valve (thermostatic valve) is a valuable safety addition but i am also surprised that when an immersion thermostat fails, why it doesn't fail safe and prevent water being heated.

@misterb 

Until last years November freeze, we had solar thermal connected to one of our DHW tank inputs.  Sadly an ST panel bust, and I decided to give the other away.  Anyway, every year in summer the tank would get silly hot (80C), and trip the immersion.  There's a tiny little button to reset the safety trip on it - in addition to the adjustable trip it has.

Don't think our immersion is anything special.  We use it every week to boost up to 65C.  Otherwise the tank gets to 50C with the gshp on Go elec.  Could perhaps be a bit lower, but Mrs RobL smiles every time she sees 50, while 40C is cause for words.

Immersion heater built-in thermostats are not that accurate. Mine is set to 65degC and cuts out when used with a solar PV diverter. However, the tank temperature indicates 54degC. There's quite a large stratification of temperature in the cylinder. Locally it is hot around and above the immersion, but much cooler elsewhere. That questions how effective the immersion is for Legionella protection. I've found the cylinder contents heat much more thoroughly and up to 55degC with the ASHP. The indicated temperature stays warmer for longer, indicating a much more even and thorough heating than using the immersion. If you consider the surface area of a 3kW immersion element vs the surface area of the heating coil, much more even and consistent heating would be expected with the coil. Heating tanks with hot outlet at the top and cold fill at the bottom are designed with intentional thermal stratification, to avoid cold water inlet cooling the hot water outlet down as much as possible. That's good for your hot water supply, but bad for Legionella protection.

@misterb, that sounds crazy and dangerous. What I find intriguing about solar diverters is that, as long as there is electricity to divert, they will happily continue doing so until instructed to stop.

Regarding our immersion heater on the HW tank, once you remove the protective cap, you'll find a dial that can be set between 1 and 5. Setting it to 5 is risky because the tank temperature exceeds 75°C, making the water scalding when you open the hot water tap. On sunny days, we often choose 4, which maintains the temperature at about 60°C, allowing the heat pump to perform a Legionnaires cycle purge. For most of the year, we keep it at 3, ensuring that the immersion heater doesn't allow the water to heat above 50°C. This is acceptable because our heat pump heats the water to 44°C, and the solar diverter further raises the temperature.

@editor 

thanks, i just have settings in degrees and have set them to just over 60C 

re my solar diverter, i also have PV and a 14.3kw storage battery - the PV system is independant of the storage battery as it was installed under the FiT scheme, which means i also have two inverters! but it all works together. i have installed the CT clamp for the solar diverter after the CT clamp for the batteries, which means that any excess solar not being used by the house will charge the battery, then when thats full the excess solar will divert to to the hot water cylinder, when the thermostat kicks in and no longer requires heat then it just passes to the grid. 

i do have two immersion heaters in my hot water tank and my solar diverter has two 'outs;, i have set the top immersion as the primary and the lower immersion as the secondary. 

at least i know what the danger signs are should something similar happen, excessively hot water from either tap! i am also into the habit of turning the hot water off when we are away for longer than a day or two, though more of a cost saving exercise than a safety one! 

the solar diverter is a recent purchase to try to squeeze as much free stuff as i can from the PV, it may yet be removed after 12 months if it doesnt behave as i hope !