Fitting a Willis Immersion heater

I have (only) replaced 3 or 4 dual immersions for relations/neighbours and none had the thermostat pocket longer than the short (sink) element, ie a 24"/11" ins dual immersion would have a 11" thermostat and a 11" pocket so stick a bit of wire down the existing one to confirm that it will take a 18" stat before purchasing one. Even if you can fit a 18" it will bring its own problems, none, if you are heating a whole cold cylinder (same with 11" stat) but if the whole cylinder is at 65C then if say 1/2 contents are only used, the stat will cut in and you might get very hot hot water in the top 50L or so as the stat is measuring the average temperature over the full 18" length, the same applies to the 11" but not to the same degree, of course the main problem with the 11" is that it doesn't cut in until most of the cylinder vol is used. Interesting to see how it performs if it fits.

No less than any other plumbing fitting. Don't forget that any open system could still have a large static head of water so the Willis would as a minimum need to withstand the same pressure as a grade 1 copper hot water cylinder (80 ft head) which is over 3 bar ~ 50psi.

There is nothing I know of in a pressurised system that is likely to damage a Willis heater particularly as a pressurised system is unlikely to be working at anything over 2 bar.

That could be for several reasons. Might be to do with the potential for over heating and allowing a build up of pressure to be released and also because its often installed at the top of the tank into the expansion feed pipe. If you can deal with a pressure build up because there is no water flowing over the element and the thermostat fails then I think you have covered any potential issues.

Its being used as a in line (pumped) heater in the above application, its role in heating a unvented cylinder is different, unvented cylinders have a TPRV (temperature and pressure relief valve) mounted on the cylinder and also have a expansion relief valve on the cold feed to the cylinder. The question is, can the Willis Heater be considered part of the unvented cylinder as long as no isol valves on the heater's flow&return piping?.

imo - As low as possible but bottom of Willis 100mm above the bottom the HWC.

Somewhere I have seen the standard installation diagram

with the min 100 mm shown between the bottom of the Willis and the HWC.

Too high and I don't see it working at all?

I can't imagine the cost of fitting a Willis heater would be any different? Although the wiring of a destratification pump might be interesting - when does it go on and off and what switches it?

The Willis Heater is going to cost around €70 a pump would be a similar cost. For both installs you need to connect to the bottom of the tank so the whole tank may need to be drained down and refilled. The ball valve on the inlet diagram for the Willis is so you only need to drain part way down if you change anything later. The pump will normally have isolation valves on the inlet and outlet fittings. Both need a connection to the bottom and top of the tank. Both need an electrical connection. Therefore I'd assume very similar installation costs.

The Eddi obviously simplifies the electrical connection on the pump.

My bad there, I already got the Relay Board for the temperature sensors so that's where the pump would go.

Are we not allowed to ask about referrals for plumbers in this thread btw?

Thanks. So if anyone knows someone who could install a Willis on my pressurised system, give me a shout please.