de stratifying DHW tank

[openspaceman]

As you lot have put me off a heat pump DHW tank and we are fast approaching the time when I will have excess electricity I am thinking about options to increase my hot water capacity cheaply.

Current vented tank is a slimline 100 litre one with a 36" immersion and 18" thermostat so probably only get about 30 litres up to 50C before it cuts out. So a means to destratify it seems to be the way to go except the special pumps with ss impellers are 400 quid and I only need to circulate a few litres a minute. A little diaphragm pump should do it but where to find one that is plastic and can stand >50C?

The other thought is the coil currently heated by the ancient gas boiler could be preheated by a heat pump and I was wondering about an A2A split with a plate heat exchanger in place of another fan coil unit.

Any thoughts?

[Stinsy]

Have you considered a Willis Heater?

It is basically an external IH that heats by thermosyphon. This is the default in Ireland and is seen as "better" than a traditional IH because it heats the top down all the way to the bottom of the tank. Cheap a chips and only a bit of plumbing (just make sure you get it the right way up).

[openspaceman]

That was quick, I'll have a search.

[AGT]

£160 pump in Screwfix for DHW

[MrPablo]

Interesting idea with the Willis heater, I like the top down heating approach.
Would this still work with an unvented cylinder?

[openspaceman]

£160 pump in Screwfix for DHW

Normal CH circulating pumps are far too powerful, also they expect to run in corrosion inhibited water whereas the destratifcation pumps are running in constantly changing fresh water so need better quality impellers.

Stinsy''s suggestion looks good, the unit can be TEEd in th the cold water feed at the bottom of the tank and then into the hot water outlet. I suppose as the hot water rises it is replaced from the bottom of the tank and the hot water then goes in the top to replace it and thus no non return valves needed.

[AGT]

I never said it was for CH, it’s for DHW.

[openspaceman]

I never said it was for CH, it’s for DHW.

Sorry I see what you mean now, was it the little 60W Grundfos? Yes that would have done for what I originally asked but the external immersion is cheap and easy so that's where I'm heading for now.

[Stinsy]

Interesting idea with the Willis heater, I like the top down heating approach.
Would this still work with an unvented cylinder?

Yep.

[Stinsy]

Stinsy''s suggestion looks good, the unit can be TEEd in th the cold water feed at the bottom of the tank and then into the hot water outlet. I suppose as the hot water rises it is replaced from the bottom of the tank and the hot water then goes in the top to replace it and thus no non return valves needed.

Exactly. No pumps, valves, or any other complexity required. Just a resistive heater, a few feet of copper pipe, and physics!