If this happens on random days then its not insulation.  Is it possible that the diverter valve isnt switching over completely, thus you are actively cooling it whilst you heat the house.  Thats the first thing I would check.  

@jamespa Home Assistant can monitor that setting, I will add it to the graph. As it does it during the mid afternoon heating cycle you would think it could be valve related.

Posted by: @bazcurtis

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@jamespa Home Assistant can monitor that setting, I will add it to the graph. As it does it during the mid afternoon heating cycle you would think it could be valve related.

HA cant monitor whether the valve fully closes or remains a bit open due eg to rubbish in the pipe or because its somehow a bit stuck, you are going to have to find it!

@jamespa I thought that might be the case. I assume you are talking about the rotating valve that is next to the tank. Octopus have been out to look at it once, but as you can imaging there was nothing to look at. This is a good example of the issue. This was today at 3pm for the afternoon top up. It lost 6c before heating back up to 45c. It was at 41c at the start, but then went down to 35c. Where would the cold water come from?

@jamespa I thought that might be the case. I assume you are talking about the rotating valve that is next to the tank. Octopus have been out to look at it once, but as you can imaging there was nothing to look at. This is a good example of the issue. This was today at 3pm for the afternoon top up. It lost 6c before heating back up to 45c. It was at 41c at the start, but then went down to 35c. Where would the cold water come from?

@bazcurtis The ‘Cold’ water of which you enquire would be the ‘Cooler’ water running through your house heating circuit. The DHW would normally be being heated to ~10 or more degrees C higher than the house heating circuit. Therefore, if the 3 way valve is not shutting fully, the DHW tank coil will be dissipating heat into the house heating circuit and ‘diluting’ the heat in that tank. Regards, Toodles. 

@bazcurtis 

Why not do a DHW run and then turn heating off.  Make sure that the circulation has stopped.  That will tell you whether its the valve or not.

do you have a secondary return pump on the cylinder? 

Posted by: @toodles

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@bazcurtis The ‘Cold’ water of which you enquire would be the ‘Cooler’ water running through your house heating circuit. The DHW would normally be being heated to ~10 or more degrees C higher than the house heating circuit. Therefore, if the 3 way valve is not shutting fully, the DHW tank coil will be dissipating heat into the house heating circuit and ‘diluting’ the heat in that tank. Regards, Toodles. 

I had this exact problem this winter (Daikin Altherma HT). With the help of my AI plumber, the problem I had - DHW cooling down far too quickly - was narrowed down to a possible issue with the valve beneath the Honeywell actuator on top of my thermal store. I bought a new brass valve and fitted it myself, replacing the original plastic valve, which was admittedly gunked up a bit, possibly enough to hinder movement. It seems to have worked and the DHW performance is now not giving me any cause for concern.

One curious thing - I have a recirc pump on the DHW pipe. Previously, when it turned on the effect was literally instant. However, with the new valve installed there is a small delay of maybe 30 seconds before the hot water arrives (without the recirc pump, it's minutes). This seemed counterintuitive to me but my helpful AI plumber (excellent rates and I don't have to make it tea) told me that if the valve was faulty the DHW was bleeding into the pipe network, hence the instant performance. Here's what it said (which I know could be total garbage, but it does describe the outcome I've seen)...

The "Leaky Pipe" Paradox (The most likely culprit)

This is the most common reason. Remember how we diagnosed that the old valve was leaking heat from the thermal store into the pipes 24/7?

• Before: Because that valve was failing to seal, hot water was constantly "bleeding" out of the store and into the pipework. This effectively kept the pipes near the valve pre-warmed all the time.

• Now: The new valve is doing its job perfectly—it has hermetically sealed the thermal store. The water sitting in the pipes between the valve and your faucets is now allowed to cool down to room temperature between uses.

• The Result: You now have to flush out a full "dead leg" of cold water before the hot water from the store reaches the tap. Your "efficiency" has gone up, but your "instant gratification" has gone down because the system is no longer accidentally pre-heating your pipes.

@ashp-bobba I don't think so. How can I check?

Posted by: @marzipan71

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Posted by: @toodles

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@bazcurtis The ‘Cold’ water of which you enquire would be the ‘Cooler’ water running through your house heating circuit. The DHW would normally be being heated to ~10 or more degrees C higher than the house heating circuit. Therefore, if the 3 way valve is not shutting fully, the DHW tank coil will be dissipating heat into the house heating circuit and ‘diluting’ the heat in that tank. Regards, Toodles. 

I had this exact problem this winter (Daikin Altherma HT). With the help of my AI plumber, the problem I had - DHW cooling down far too quickly - was narrowed down to a possible issue with the valve beneath the Honeywell actuator on top of my thermal store. I bought a new brass valve and fitted it myself, replacing the original plastic valve, which was admittedly gunked up a bit, possibly enough to hinder movement. It seems to have worked and the DHW performance is now not giving me any cause for concern.

One curious thing - I have a recirc pump on the DHW pipe. Previously, when it turned on the effect was literally instant. However, with the new valve installed there is a small delay of maybe 30 seconds before the hot water arrives (without the recirc pump, it's minutes). This seemed counterintuitive to me but my helpful AI plumber (excellent rates and I don't have to make it tea) told me that if the valve was faulty the DHW was bleeding into the pipe network, hence the instant performance. Here's what it said (which I know could be total garbage, but it does describe the outcome I've seen)...

The "Leaky Pipe" Paradox (The most likely culprit)

This is the most common reason. Remember how we diagnosed that the old valve was leaking heat from the thermal store into the pipes 24/7?

• Before: Because that valve was failing to seal, hot water was constantly "bleeding" out of the store and into the pipework. This effectively kept the pipes near the valve pre-warmed all the time.

• Now: The new valve is doing its job perfectly—it has hermetically sealed the thermal store. The water sitting in the pipes between the valve and your faucets is now allowed to cool down to room temperature between uses.

• The Result: You now have to flush out a full "dead leg" of cold water before the hot water from the store reaches the tap. Your "efficiency" has gone up, but your "instant gratification" has gone down because the system is no longer accidentally pre-heating your pipes.

 

Thanks for the information. Did yours do it all the time. Mine seems to be very intermittent.