You'll likely already understand that a destrat pump is used to circulate water in a HW tank to try to ensure that the temperature of water is uniform across the entire volume within the tank. Without a destrat it is possible that the temperature of the water in the tank is distibuted such that there will be a hot 'layer' at the top and cooler layers lower down. Depending on the temperature of the water it may be that a particular 'strata' of water sits at or around 35 degrees C which is the optimum for promoting Legionella growth.

The destrat pump should ensure that the entire volume of water sits outside of the danger zone as far as temperature is concerned thus helping to ensure that Legionella cannot easily thrive in the system.

L8 is the appropriate ACOP but I'm not clear if destrat pumps are explicitly mentioned within it.

Occasionally a post comes along that is a little gem. Well done.

Destrat pumps are mentioned in l8

However does anyone know of a standalone controller for this.

Tony et al,

Destrat pumps may be required if there is a likelihood of the lower regions of the calorifier sitting at 35-40 degrees, which is ideal for the proliferation of legionella. You could also consider routing your secondary return back into the calorifier lower down or reduce the size of the calorifier so the demand is always causing the stat to raise the temp to above 60 more often. Lots of hospitals keep the calorifiers running at above 65 degs all day long so may not need a de-strat pump, some schools do the same, although you'll need TMV's at the outlets. If you go for a de-strat pump then you should only run it of an hour a day, usually at times of low demand so as to ensure that the bugs are killed. Any de-strat pump would also need a timer so it can be set to run for the hour. Most Building Management Systems can happily accommodate this, but other wise, a simple plug in (stand alone) timer could be used.
Horses for courses!
Holmezy

No, it isn't.

Primary circulation is a pump that circulates the water between the heat source (typically a gas-fired boiler, but could be anything) and the coil in the calorifier (not water cylinder). It runs whenever there is demand for heat from the calorifier.' This is the primary circuit and the water in it never gets to any tap.

A secondary circulation pump circulates water from the calorifier (hot water cylinder), round the pipes of the building and back to the bottom of the calorifier. This is needed where there's a long pipe run, and you don't want a long wait at the tap before the hot water comes out. It ensures there's 'fresh'' hot water near each tap all the time. You would normally run this pump full time, but you might shut it off when the building is unoccupied. This pump is pumping water that eventually will come out a tap.

A destrat pump pumps from the top of the calorifier straight back to the bottom. It doesn't send water anywhere. Its sole purpose is to mix up the water in the tank. You only need to do this every so often. Running for an hour a day is common, and it only needs a simple time-switch - could be a plug-in one, but for a permanent installation more likely just to be a domestic hot-water system timeswitch.

If you have a secondary circulation pump, you possibly/probably don't need a destratification pump, because the water is probably mixed by the circulation pump, but that doesn't mean they are the same thing, and neither is a primary circulation pump.

You can, of course, call it whatever you like. However, a destratification pump is not a primary pump. It's also not the pump you are describing.

A destratification pump should NOT operate continuously. See for example paragraph 158 in the Legionnaires ACOP (where it is called a shunt pump):

"...Arrangements should therefore be made to heat the whole water content of the calorifier, including that at the base, to a temperature of 60°C for one hour each day. This period needs to coincide with the operation of boiler plant (or other calorifier heat source) and is usually arranged during a period of low demand eg during the early hours of the morning. A shunt pump to move hot water from the top of the calorifier to the base is one way of achieving this, however, it should not be used continuously except for about one hour each day (see above). In all cases the operation of the pump should be controlled by a time clock."

Figure 6 shows it to you.

The pump you are talking about is a secondary circulation pump. It does a different job, as I described. It may also destratify the calorifier (depending on configuration of pipework), but that is not its purpose. That is, the pump you are describing is the pump in Figure 5 in the ACOP. A destratification pump is the pump in Figure 6. Neither is a primary pump.

As noted, you can call it what you like, but as I understand it this discussion is about what is normally called a destratification pump, which is not the pump you are talking about.

oh,,,,i forgot to add,,,,,

destrat pumps should run for an hour a day after the calorifier has reached 60 degs!

Most destrat pumps are 'controlled' with either a simple mechanical or electronic timer. It is quite rare to see anything more complicated than that.

If you really, really feel that some kind of closed-loop system is needed then any BMS (Building Management System) controls engineer could set-up something more complex but it would seem to me (and most others it would appear based on the prevalence of simple timers in this application) that the risks can be adequately managed without adding expense/complexity.