Just today when i finally got an understanding of all this, daikin sent my commissioning cert after 13months, very coincidental.

Posted by: @newhouse87

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Just today when i finally got an understanding of all this, daikin sent my commissioning cert after 13months, very coincidental.

Have you not found the hidden camera and microphone yet, they are monitoring your every move. 😜 

@derek-m I have a question about the above discussion of the ASHP running 24/7. 

Through this winter ours would have had a call for heat on some days all day, how ever the pumps compressor would have shut off at times when the temperature in the buffer tank has reached the WC temp and only come on again when the temp in the buffer falls by say 5 -10 degrees (not sure what its set to). Would this suggest I need the WC temp lower?

What I'm getting at is are we saying that its really the compressor in the ASHP that ideally should not be cycling, because with mine even though it always has a call for heat and the system is circulating the hot water the compressor may or may not be running?

Posted by: @mookyfoo

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@derek-m I have a question about the above discussion of the ASHP running 24/7. 

Through this winter ours would have had a call for heat on some days all day, how ever the pumps compressor would have shut off at times when the temperature in the buffer tank has reached the WC temp and only come on again when the temp in the buffer falls by say 5 -10 degrees (not sure what its set to). Would this suggest I need the WC temp lower?

What I'm getting at is are we saying that its really the compressor in the ASHP that ideally should not be cycling, because with mine even though it always has a call for heat and the system is circulating the hot water the compressor may or may not be running?

I suspect, though you would have to be the one to prove it, that you are experiencing mixing within the buffer tank. What I think may be happening is that the flow rate from the heat pump, through the buffer tank, and back to the heat pump, is greater than the flow rate out to your heat emitters. Warmed water is therefore coming from the heat pump and a percentage is going through the buffer tank and straight back to the heat pump, and not through the heat emitters where it should be going.

The heat emitters are not getting the full amount of heat energy, and so require a higher LWT to make up for the deficit. The heat pump on the other hand is getting a higher RWT than it should be, so it periodically being stopped until the temperatures fall.

Check the temperatures on the pipework around the buffer tank, and the flow rates if possible.

@derek-m Thanks for the reply. If that was the case as you suggest how can this be improved/stopped?

Posted by: @mookyfoo

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@derek-m Thanks for the reply. If that was the case as you suggest how can this be improved/stopped?

It probably depends upon how your system has been designed and what equipment has been used.

Posted by: @mookyfoo

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If that was the case as you suggest how can this be improved/stopped?

A photo of your buffer tank would be a good start.
Let's have a look at the size and how it's been plumbed.

Per "1) If my understanding is correct using only schedule will mean that your heat pump only performs DHW heating during the scheduled periods, so you know when it will occur. With schedule + reheat, DHW heating will occur, if necessary, during the scheduled period, but could also occur outside the scheduled period if the temperature of the water within the hot water cylinder falls below the reheat setting.

Your present settings of heat DHW to 48C and reheat at 38C, would maintain the water within the cylinder between those two limits, which means that in practice, at 12pm the heat pump will start DHW heating if the water temperature is below 48C, and will continue heating the DHW until the temperature reaches 48C, or the schedule ends at 1pm. If someone runs a bath at 2pm, and the temperature of the water in the cylinder falls below 38C, the heat pump will start heating DHW again until the water in the cylinder is heated back up to 48C. In schedule only mode, the water would not be heated back up to 48C until the next schedule period."

Hi @derek-m I've been monitoring the temp changes in our Daikin 500L thermal store as I now have the DHW set to reheat with a lunchtime scheduled cycle using a set point of 48C and a lower limit of 40C. A single shower in the morning will cause a temp drop from 45C to 38C which seems a little excessive to me - is it? Shower time of 4-5 minutes and yes we have a large Grohe rain shower head - but still a 7C change for a single shower in a 500L tank seems to me a bit high?

I'm thinking of switching back to using solely a scheduled DHW cycle at lunchtime, turning off reheat, and upping the set point to take advantage of our lunchtime PV (we don't have a diverter). Thinking that the drop off during the evening and night from, say, 55 or 60C would still give us warm showers first thing without needing the ASHP to fire up at times when we are not producing solar. 

@marzipan71

The temperature reading that you are seeing, is probably from a sensor in the bottom area of the cylinder, which is where the cold water supply enters. The fact that the reading falls from 45C to 38C does not mean that the full 500 litres is now at 38C, in fact you would probably find that much of the 500 litres is still at 45C.

The sensor is located in the lower area of the cylinder, to ensure that the full 500 litres is heated to the desired temperature, but of course the sensor then quickly responds to the drop in temperature when warm water is drawn from the top section and cold water enters the lower section.

As a test you could run the shower for the normal time that you would do so, but collect the water and measure the volume used, this should indicate how much of the 500 litres has been used, and therefore needs to be reheated.

Iv noticed that even when m dhw tank falls below 38c setpoint, it is not being reheated to48. Sensor was at 33 one morning but shower was still warm enough for me. Its on scheduled and reheat so not sure why its not reheating or maybe just not when im checking it.

Posted by: @newhouse87

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shower was still warm enough for me

Just for the record...

If a DHW system has a mixer-valve to prevent scalding accidents, then these are sold in the UK with a default setting of 42degC.

The default setting for a thermostatically-controlled shower using DHW (ie not an electric unit) is 31degC

Obviously either can be overridden by an installer.
But it's a useful starting point, and explains why a storage tank sensor reading 33degC would still allow an acceptable shower temperature.

Hi @derek-m that all makes sense, thanks. The sensor is 77cm down inside the 155cm high tank, so if its seeing a 7C drop in temp from a single shower, would that mean that around half of the tank is at most 38C at that point? The Daikin manual states 'The cold water which flows out when the hot water is removed is first routed to the storage tank at the very bottom of the heat exchanger, where it cools the lower area of the storage tank down as much as possible. The readiness zone is heated by an external heat generator (condensing boiler, heat pump, solar system, electric immersion heater). Water flows through the heat exchanger for storage charging (SL-WT) from top to bottom. On its way to the top, the domestic water continuously absorbs the heat of the storage tank water. The flow direction, operating on the principle of counter-flow, and the coil-shaped heat exchanger create a pronounced temperature layering in the hot water storage tank. As high temperatures can be maintained for a very long time in the upper section of the storage tank, a high hot water output is achieved even if water is drawn off over a long period of time.'

Good idea about the shower volume test - however, in the short term, I checked our shower head and the manufacturer states there is a limiter set to 9.5l/m. Since we take 4 or 5 min showers, that suggests a shower is about 50L of hot water. I suppose what's happening is that the upper section of the storage tank is at 45C; the shower draws off 50L of water from that layer, which is replaced at the bottom of the tank with cold water that goes through the journey described above. Daikin's point in their last sentence in my reading is that hot water can be drawn off for a relatively long period of time even as cold water is topping up the volume of water in the TS due to temperature layering. The heat pump will cycle on after that first shower as it reads 38C, but there will still be a volume of water in the upper layer of the TS which is hot enough for a shower and has minimal mixing with the colder water entering at the bottom. Is that a reasonable interpretation?

I think per a discussion with @saf1973 it might be best for me to move away from reheat+scheduled to just scheduled at this point to prevent these early morning reheat cycles when we have little or no PV feeding the heat pump. Its good to know how the kit works though and the above had been puzzling me for a while! Thanks for your help, as ever.