@bontwoody re immersion in summer, his other point was at temperatures like today the heatpump will not start but are there not settings to allow for that? 

When the DHW is heated the valve switches over from the heating circuit to the tank coil so a volumiser should only be needed if that pipework has insufficient volume? Is that right? The manual for the Daikin says 50L if no backup heater or 10l if there is a backup heater, I don't think I would have such a heater but the manual is a little obtuse.

I take your point about being in the heater circuit and anywayI doubt Octopus would do that for me.

@toodles Thank you, I have a similar system controlled by a timeswitch, mine is only 100W but running 24hrs is a fair bit of electricity hence now timed for 3 hours a day. During the energy crisis I installed many smart switches that also measure KwH so with careful timing I shaved a lot off our consumption. For me an eye opener was that my "Theatre sound, Freeview and T v used for about 4 hours a day with the rest on standby consumed more than my fridge freezer. It is on a timer now.

@jancold 😊 Toodles.

@jancold Ive never heard of a ASHP not starting because the temperature was too high. Perhaps someone else on here might know if that was possible?

So Im guessing both circuits would need to meet the minimum volume requirement, but as the hot water circuit has the cylinder, that would be more than ample. Once you start to add up the volumes of radiators and the pipework it soon adds up in the heating circuit. I estimated my small bungalow has 60 litres.

Addendum: Just realised my mistake! unless the cylinder had a splarge pipe the water volume in it is not available, only the water in the coil. But im thinking that the water in the coil would get constantly reheated as it was pumped around to defrost the unit.

@bontwoody When our Daikin heat pump was installed, various settings needed tweaking before the system settled down. One setting had been mis-set or never set in the first place and that was the OA temperature at which the pump would cut out or not start (I am unsure of which it was really). I found that when it was milder (installed in mid Feb ‘23) the pump would not ‘fire up’. I called in the installer and he found a setting was at (I think) 12 -14 degrees; when this was raised to 18, the pump would then run in milder weather. I imagine that something similar but at a higher temperature is required for the DHW circuit as it surely needs to be able to run if the OAT is perhaps 30 degrees C. Regards, Toodles.

@toodles When I went to ultimateRenewables he tried to demonstrate the quietness but it would not start saying the temp was too high initially however he got it going eventually so tweeked a setting perhaps? Stu said he was a software man by training and was in awe of Kraken, so in effect he was loading the data for the seamonster to eat and spit out it's decision. He did say it had bugs (sealice?) Anyway I generally lean to his competence at surveying. He smiled when I pointed out the potential conflict of interest when they also supply the fuel to make a slightly less efficient system and long term profit. 🤔

@jancold Perhaps I am a little naive but feel that Gregg Jackson has an honest head with good ethics on his shoulders as he appears to demonstrate in the way he steers the business and the schemes they promote. I have the same heat pump as OE specified when they surveyed my property but failed to accommodate me due to the work being beyond their scope at the time. Regards, Toodles.

Posted by: @jancold

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It is £2300 for OE (assuming they do not alter it in the survey) and £4200 for BG which was down from £5900 initially

And that's after £7500 being taken off. You have already demonstrated to yourself you run no hotter than 40 degs and be comfortable. So you (well tax payer and you) are effectively paying £10 to 12k for a £2500 heat pump and heat pump cylinder at around £1000 and you have to dig up your path to route the pipes. And you will be forced to have rubbish you don't want. All these big companies do MCS installs for the huge profit margins they make.

Your minimum volume for defrost is on the heating system only - not for cylinder heating. Generally a heat pump will not defrost while doing DHW, mine is blocked from doing by the manufacturer. Main reason, if it defrosted every 20 mins it would spend all day trying to do DHW. You do not need a volumiser for DHW, it is very unlikely you need one for CH either if you are running a single zone.

@johnmo Thanks when they get back to me I will ask them for the justification for the volumiser. It is so good that we cannot trust these er mcs experts. 😩 I could remove the volumiser later and flog it on ebay.

So now UltimateRenewables have found an installer for me so it will be interesting to see what they come up with but it will be a couple of weeks.

@johnmo Thanks for clearing up the matter of defrosts during DHW cycles. I was just guessing really. Wouldnt it be more likely though that the HP would need deforsting during the DHW cycle given the higher flow temperature? What happens it it cant heat the hot water anymore untill it defrosts? Especially it it was less than zero outside.

Posted by: @bontwoody

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What happens it it cant heat the hot water anymore untill it defrosts? Especially it it was less than zero

Never seen an issue, seen plenty of frost, but not seen an issue heating the cylinder, I don't use the immersion either except for PV diversion.

My manual says

"Defrosting is always carried out in winter mode on the utility side, and never on the domestic hot water tank."