Also to add, on the original Grant wiring there was a separate tank stat which is in the pocket at the top of the cylinder. I have disconnected this and am just relying on the thermostat inside the immersion heater. Was that a mistake?

Posted by: @prunus

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Also to add, on the original Grant wiring there was a separate tank stat which is in the pocket at the top of the cylinder. I have disconnected this and am just relying on the thermostat inside the immersion heater. Was that a mistake?

Not sure which sensor you are referring to. 

When the heat pump heats the DHW (ie normal DHW heating not the legionella cycle) it relies on the tank sensor (NOT the one associated with the immersion) to tell it what the temperature of the water is.  This is normally in a pocket about half way down and must not be removed.

Here is the full setup (I really should tidy that wiring...):

Disconnected tank stat at the top pocket, thermistor going to the ASHP in the middle pocket, immersion in the bottom pocket. The buffer is below that out of shot.

Tank stat: 

Grant's wiring for this is as if it was a classic gas boiler: boiler programmer -> tank stat -> immersion. There is additionally a single DIN timer and contactor for immersion boost. AIUI the tank stat is used for setting the normal ASHP target DHW temperature (eg 45 as it's currently showing) where the Grant wiring centre then shuts off hot water mode once the tank stat opens. I believe the boost timer overrides the tank stat and heats the hot water using only the thermostat in the immersion, to get it up to 60+C for legionella. Because this immersion timer has no temperature sensing the whole legionella cycle is done open-loop by immersion alone (ie it's just run for a fixed time, without preheating by ASHP and just hoping nobody ran a bath in the middle).

I have disconnected the whole Grant wiring centre and replaced it with controls from the ASHP, ie the ASHP does it's own legionella cycle and then it's own DHW boost output kicks in the immersion once it has achieved maximum temperature, according to the thermistor. That means the ASHP just powers the immersion until 60C is sensed. The thermostat on the immersion wants to be set higher (say 65) so it will reach 60C and be shut off by the ASHP before the immersion shuts itself off.

But perhaps I misunderstood something about this particular immersion? The instructions for those Thermowatts I can find are clear as mud.

Posted by: @prunus

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Grant's wiring for this...

like almost everything else in Grant's original wiring schemes, should be ignored - as you have

Posted by: @prunus

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I have disconnected the whole Grant wiring centre and replaced it with controls from the ASHP, ie the ASHP does it's own legionella cycle and then it's own DHW boost output kicks in the immersion once it has achieved maximum temperature, according to the thermistor. That means the ASHP just powers the immersion until 60C is sensed. The thermostat on the immersion wants to be set higher (say 65) so it will reach 60C and be shut off by the ASHP before the immersion shuts itself off.

Very sensible!

Posted by: @prunus

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But perhaps I misunderstood something about this particular immersion? The instructions for those Thermowatts I can find are clear as mud.

Do you know what the 'tank stat' did before you disconnected it?

Posted by: @prunus

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I have a Grant Aerona3 13kW and I've wired it so the ASHP controls the legionella cycle (disconnecting the spurious Grant timer box they install). The way this works is the ASHP runs up the hot water to the maximum temperature it can manage, then hands over to the immersion heater for the last few degrees to get it to 60C. On the standard setup it has 2 hours to do that - there's a service manual parameter to extend it to 3 hours, which I've done. After 3 hours the water is only at 50C and the ASHP flashes 'LC' meaning legionella cycle failed. Next week comes around and it does legionella again and the same happens... every week.

Does it actually ever enable the immersion.  If not it may be that 'backup heater' has not been enabled in the installer menu, or if it has its parameters set up wrongly.  

Alternatively it may be that the immersion simply doesn't go far enough down the tank so it heats the top third (above the Grant temperature sensor) then turns itself off before the the bit down to the Grant sensor gets heated.  This may conceivably be worse than not running legionella at all!  My Vaillant tank is like that, the immersion heater really only heats the top bit of the tank and the sensor barely notices it.  In this case however its doesn't matter much because the Vaillant does the legionella cycle natively without invoking the immersion heater, and gets it to 69C.

I think we're talking at cross purposes here. The above test is with the ASHP completely turned off. The setup is mains -> power meter -> contactor -> immersion, and the contactor is turned on. ie the immersion is just constantly across the mains and yet it can only achieve 57C at its maximum setting. There is no thermostat in circuit beyond the thermostat integrated into the immersion.

The immersion is at the bottom of the cylinder but that should mean that the sensor in the middle of the cylinder and the water drawn off should be hotter than 57. In the draw-off after running the hot tap for a minute or two the highest I could get on my coffee and jam thermometers was about 55-56, which is probably within measurement error. (Hot side of kitchen mixer tap, not anything fancy like a thermostatic mixer)

So either the immersion is faulty or I misunderstood its settings. I can't think of anything else it could be, unless I misunderstood something about how this immersion operates? 

(also to note that supposedly the safety cutout reset is a button on the immersion but seemingly that is pushed in on the pictures above, suggesting the cutout hasn't activated)

Posted by: @prunus

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I think we're talking at cross purposes here. The above test is with the ASHP completely turned off. The setup is mains -> power meter -> contactor -> immersion, and the contactor is turned on. ie the immersion is just constantly across the mains and yet it can only achieve 57C at its maximum setting. There is no thermostat in circuit beyond the thermostat integrated into the immersion.

The immersion is at the bottom of the cylinder but that should mean that the sensor in the middle of the cylinder and the water drawn off should be hotter than 57. In the draw-off after running the hot tap for a minute or two the highest I could get on my coffee and jam thermometers was about 55-56, which is probably within measurement error. (Hot side of kitchen mixer tap, not anything fancy like a thermostatic mixer)

So either the immersion is faulty or I misunderstood its settings. I can't think of anything else it could be, unless I misunderstood something about how this immersion operates? 

(also to note that supposedly the safety cutout reset is a button on the immersion but seemingly that is pushed in on the pictures above, suggesting the cutout hasn't activated)

Ah.  Ok with immersion at bottom then the whole cylinder should eventually heat.  The hottest point will still be by the element, but you would expect everything above that to rise eventually to whatever the thermostat setting is on the immersion or perhaps a tiny bit below if the currents local to the immersion stat cause local hot spots.

Im wondering if there is actually a fault, or whether 57 is the temperature at which the immersion thermostat (possibly nominally set to 60) cuts out.  Obviously at this point it would cease drawing power which you would detect.  I cant see how it can continue to draw power without the tank continuing to heat, unless there is a heat sink!  Otherwise it hat would be violating the laws of thermodynamics.  

I suppose it could be that 57C is the maximum achievable temperature by the immersion heater, but I thought the point of the adjustment is you could set them higher than that?  eg the manual for the 420011210800 "Thermowatt RT 1” ½ IP65 with RTS thermostat - Bipolar double safety disconnection" says:

The immersion heater is supplied with a RTS/RTS PLUS thermostat. Functional temperature
is factory set to 75°C, with adjustable temperature.
To maintain safety, any replacement thermostat must be of the same type.
RECOMMENDATION: low water temperature is recommended to reduce limestone deposit
(Not lower than 60°C).

and that has a picture that looks like the same as the back of mine. But nowhere, either on the product page or in the zipped PDF, does it state minimum/maximum thermostat temperatures.

As you say, if it's powered either the heat has to go somewhere or it's shut itself off - the power meter indicates it's shut itself off, which points to the internal thermostat. Moving the setting on the internal stat does seem to improve the shutoff temperature slightly. But could it be so out of calibration?

Posted by: @prunus

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I suppose it could be that 57C is the maximum achievable temperature by the immersion heater, but I thought the point of the adjustment is you could set them higher than that?  eg the manual for the 420011210800 "Thermowatt RT 1” ½ IP65 with RTS thermostat - Bipolar double safety disconnection" says:

The immersion heater is supplied with a RTS/RTS PLUS thermostat. Functional temperature
is factory set to 75°C, with adjustable temperature.
To maintain safety, any replacement thermostat must be of the same type.
RECOMMENDATION: low water temperature is recommended to reduce limestone deposit
(Not lower than 60°C).

and that has a picture that looks like the same as the back of mine. But nowhere, either on the product page or in the zipped PDF, does it state minimum/maximum thermostat temperatures.

As you say, if it's powered either the heat has to go somewhere or it's shut itself off - the power meter indicates it's shut itself off, which points to the internal thermostat. Moving the setting on the internal stat does seem to improve the shutoff temperature slightly. But could it be so out of calibration?

If the cut out is at 65 I would guess the max design temp is 60, so actual of 57 is plausible albeit at the margins.  Bear in mind these are generally bimetallic strip thermostats so yes it could be that far out of calibration (if they even bother calibrating them, which frankly I doubt).

Well, if that's the right PDF the factory set is supposed to be 75 so in theory it should go up at least that far when cranked up to the max. A bit poor that it could be 18C off, especially where legionella might be a risk. I suppose I'll have to replace it with something better - any recommendations?

Edit: ah, does F.65/S.75 on the label mean 'max functional temp 65C, safety cutout temp 75C'? So it's not possible to set it above 65 +/- lots?

@prunus Hmm

57 vs 65 does seem a way off, but perhaps not impossible.  Given the purpose, one might argue that its not fit for purpose!

I cant advise on immersion heaters TBH, my guess a fair proportion are made in the same factory in China then just branded differently.  They arent expensive, but of course you will need to drain the tank.