Posted by: @biagg

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This is the box i was told to use if i needed to "top up" the heat of the HW tank, I've turned that off.

- that is the immersion heater element, so that's the right thing to do. There should now be nothing which is trying to add heat to the HW cylinder.

Posted by: @biagg

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I have cut off the Cold water, but the loft CW tank will still be full so i assume the HW tank will be full still. 

I'd expect you to not have either of those things any longer, unless they are used just for a shower? With a sealed HW cylinder, the cold feed for it comes directly from the mains, as the cylinder is under pressure. Your hot water and cold water should come out at the same rate, roughly, rather than in a vented system where the HW pressure is based on the water in the HW tank.

There ought to be a WRAS blue handled ball valve which you can turn thru 90° on the pipework to the DHW cylinder which will stop the cold feed to it. Then you could have cold water flowing elsewhere, but without the feed to the HW cylinder, your HW taps won't release any water. This ought to stem the leak, but it depends a little on how bad it is.

So long as you've turned off all heat options, then you can await the plumber next week.

@mikefl I noticed it comes from the mains after posting. 

I have the blue handled ball valve i'll turn that off and give the water a go, if it leaks still I'll just have to deal with it for two days. Thanks for the help.

Edit: Blue valve fixed the leak for now, so at least I can make a cuppa.

Posted by: @mikefl

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Parameter 01 31 shows the HW cylinder water temperature - or it should if you've had the sensor correctly fitted, and parameter 51 07 set to 1 (if it's zero then I think the Grant guesses based on flow temps to the DHW cylinder).

Hi @mikefl, Grant don't normally connect anything to terminals 7&8 to enable the parameter, and rely on the external 3rd party tank temperature thermostat and programmable timer to schedule DHW production. Another example of Grant dumbing down the installation for S plan boiler bashers? The value of 01 31 defaults to 10degC irrespective of actual DHW temperature. Tank temperature is indicated by the digital thermostat on the pre-plumbed cylinders, but is very inaccurate reading.

@allyfish Thanks for the info. For whatever reason, I didn't get the standard Grant HW cylinder with my install (it's a ukcylinders slim one) and they did wire up a temp probe on it, connected to terminals 7-8 on the HP. The installation guy just didn't set the parameter to indicate this, but once I set 51 07 to 1, 01 31 gives me the cylinder water temp ... albeit to only 1C precision, and the probe is towards the bottom of the tank and seems erratic at best (e.g. after a shower it'll read 18C even tho the water is still hot to the hand). There needs to be more than 1 temp probe on a DHW cylinder to give a better overall value (or the water needs to be mixed more). It's too inaccurate how they measure currently.