Thanks for your help @johnmo. I will research what a Y and S plan boiler is. Noted about the DHW and Central heating not being on together. On our system DHW takes priority over the central heating so both shouldn't be on together.

Have done what you suggested @allyfish and set all zones and time periods to be blank as in your picture. Then set zone 1 ON at 1300 and OFF 1400. Did this about 1000 hrs. Switched the Mode to Auto clicked done and........motorised valve opened, demand for hot water came on and display on the timer said "ON UNTIL 1400". ASHP started up and started to heat the water. The installer replaced the DHW timer 2 weeks ago. It sure is a puzzle. I will inform the installers tomorrow.

Interesting you mentioned 32 possible programmed time settings. With the timer turned off it is possible to get a menu with 1 to 32 settings. Any ideas anyone what this option is for?

Good news - It's working. Following on from comments here it got me thinking. I have set the DHW timer to come on at 1200 hrs and go off at 1300 hrs every day Monday through to Sunday. This seems to satisfy my Heatmiser TM4 controller so far. The dreaded "OFF UNTIL 2400" has not reappeared. I will keep an eye on it though.

Maybe my controller needs an off time every day. Seems a bit odd but if that's what it wants I can live with that. Hopefully I won't be hearing the sound of the ASHP heating the DHW in the early hours again.

I found a link on the internet answering the question of the "EDIT" function on the off screen. According to Heatmiser it does nothing. Link here: electriciansforums.net/threads/heatmiser-tm4-question.198080/

One problem gets solved and another appears. I have noticed that when the DHW gets to temperature the ASHP turns off but the pump keeps running until the OFF time is reached. Today that was for 15 minutes. Not sure if that is normal. Will probably have to raise another post/topic about that.

Thanks for your help everyone. 

@mellow this is good to hear. The tried and tested solution of 'pratt about with it until it finally fixes' seems to have come good for you. 😉

Your secondary pump (the one not inside the ASHP) should switch on and off with either CH or DHW demand from either side until the thermostat is satisfied. It shouldn't run on. If it is, it sounds like it's energised by the TM4 timer, and not the timer + thermostat output in series. If it is running on, all it is doing is removing heat from the DHW cylinder after it has been charged, maybe only a very small amount if all pipes are well insulated, but that's not ideal.

Most Grant systems have a 'low loss header' a rectangular wall mounted box with 4 pipe connections.

GRANTUK.COM "https://www.grantuk.com/media/5367/grant-combined-volumiser-low-loss-header-installer-uk-doc-0184-rev-1-1-april-2023.pdf"

For such systems, the secondary pump should only be energised when either heating or hot water is demanded. The primary pump inside the ASHP does run on, as indicated by the pump graphic on the ASHP controller. This is to remove residual heat and pressure from the refrigeration circuit. It is also energised during cold weather for frost protection.

For systems with an external secondary pump but without a low loss header, (and so without hydraulic separation of primary and secondary circuits), and secondary pump needs to be controlled directly from the ASHP. An auxiliary output contact is provided for this at the Grant PCB Both pumps energise in tandem, since one should not be energised without the other.

Grant are starting to roll out open loop system design schemes without LLHs and using direct circulation, with built-in circulator, or built-in circulator for smaller circuits, plus secondary booster circulator for larger circuits, but they are rather late to the party.

I was more surprised than anyone that I found a solution. Can't take all the credit though as comments here prompted me to try something different - needing an @derek-m OFF time and @allyfish a heating period every day of the week.

I have looked for a "low loss header" but can't see one. There is a 25L buffer tank. Cannot see a secondary pump either.

Very warm today so the water got to temperature 45degC quickly, about 35 minutes. Tap, fan and compressor symbol turned off but pump symbol stayed on for about 15 minutes. (Black triangle in a circle) symbol. I could hear the pump running and the motorised valve which has an indicator light was open.

I will run again and do some more accurate observations.

Might be useful if I upload pictures of our setup in detail.

Liking the technical explanations of various systems - all above my head but after reading through them something usually sticks.

Thanks one and all.

@mellow that will be fine to have the internal pump running on, it will be to remove residual heat and pressure from the condenser inside the ASHP, and thereby the refrigeration circuit. The Grant ASHP controls this pump. It's good to just one internal circulator pump, with no LLH or secondary pump, as that's an efficient set up. Often secondary pumps are fitted where there's little need and the built in circulator can do the job - tends to be a lazy option by installers not familiar or competent at doing basic hydraulic pressure loss calcs to check the capacity of the built-in pump. The buffer vessel is a means of meeting minimum volume in the circuit, usually to aid defrost in winter when the heat source is the water circuit.

@mellow

I doubt that it is a buffer tank if you do not have a secondary water pump, since the system would struggle to work correctly. It may be a volumiser. How many pipe connections are fitted to this tank?

@allfyish Reassuring to know a secondary pump may not be needed - can't see one on my system. Have just read the installation manual and it does state that an additional water pump is an optional extra. Think it's related to "water flow rate" and "difference between flow and return water temperatures".

@derek-m There are 2 pipes attached. It does say "Buffer Tank" on the label and 25L - small white tank on wall to the left.

Thanks.

@mellow

While it may have a label stating buffer tank, it appears to be piped as a volumiser.

Thanks @derek-m - will research volumiser tomorrow. 

Posted by: @mellow

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Thanks @derek-m - will research volumiser tomorrow. 

[/quote

It is merely a tank through which the return water to the heat pump normally flows, in at the top and out at the bottom. When the heat pump is operating the volume of water within the tank is heated, and therefore acts as a store of thermal energy. When the heat pump performs a defrost cycle some of this thermal energy is used by the heat pump to melt the ice.

 

Posted by: @allyfish

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Hi @Mellow, I hope I can help, as I have the same Grant 10kW ASHP, Grant branded Heatmiser 4 channel DHW thermostat and a 210L Grant pre-plumbed cylinder.

As you have a similar system @allyfish just wondering what DHW cylinder thermostat you have. From what you have said yours must be a digital one. Ours isn't - Manual cylinder thermostat, Code 542902 - with a dial 25degC to 65degC. I would be interested to know the details of yours if possible please.

Don't know why but I'm thinking that digital is better perhaps.

Thanks.

Hi @mellow, pics of our Grant thermostat below. Rotary control but digital setting. Standard on their pre-plumb cylinders I believe. First one we had was faulty, Grant had to change it out.