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Tommyfr
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February 9, 2024
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Let's talk about Buffer vessels for ASHP

  • February 9, 2024
  • 12 replies
  • 1967 views

Hi, I’ve got a Daikin 11kw ASHP for my underfloor heating and radiators upstairs, floor area 130ms 6 rads and 4 towel rads upstairs combined kw outage 7.5kw. I’ve got a 20 litre buffer vessel for my unit. My problem is whenever the temperature drops to 4 or below like yesterday my ASHP is constantly on because of the defrost cycle and can cost me upwards of £30 a day for my electricity. I’ve rang Daikin and explained that this surely can’t be right, I’ve insulated between walls, joists there’s 90mm insulation in all my loft areas I’m on the weather dependent curve to be cost efficient but the defrost cycle is killing me! My electricity bill for January was £500! The only thing Daikin have told me to do is to increase the buffer size of the buffer vessel? I’ve looked up and rule of thumb for buffer vessels is 10 litres to every kw of size of unit? So in theory I should need at least 100 litr buffer vessel not the minimum 20 litres that my installer fitted to gain warranty? Will changing my buffer vessel bring my electricity usage down and save me money in the long run? On cold days I use u to 100kw per day? What is the advantages of having a 100 litre buffer vessel to a 20 litre? Because I’ve been pricing up and a decent 100 litre buffer vessel tank is coming in at around £450? Obviously I’d like to know as much as I can before I start shelling out? Daikin aren’t very helpful and my installer hasn’t got a clue?

    Best answer by hydrosam

    Updated on 16/05/25 by Ben_OVO

    A few quick comments on this

    The 16kw Daikin will not output 16kw at your design temp (around -2C to -3C), it’ll be more like 12-13kw. The data is hidden in the stats sheets somewhere. 
    The 20ltr buffer is not a buffer, it’s a volumiser to add volume to the circuit during defrost cycles. A buffer won’t solve your issues.

    The minimum modulation output of the heat pump will be around 4-5kw. If zones are switching off you’ll likely drop below this output requirement which will cause the heat pump to cycle and efficiency plummet. Zoning is bad for heat pumps. See here.

    The other thing might be your heat loss calculations could be too high, and the chosen heat pump too big. A detailed look at my system is here. 

    Defrost cycles are normal, but will happen more if you have a higher leaving water temperature. If you share some settings like your weather compensation curve, and whether you run heat on a schedule or leave on 24/7 I can try and help. 

    One other Q. Is total floor area 130m2, or is that the first floor? If total 16kw heat pump is likely well over sized. 

     

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    Abby_OVO
    Community Manager
    Abby_OVO
    Community Manager
    February 22, 2024

    Thanks for the reply here @hydrosam, great input as always!

     

    @Tommyfr How have you been getting on? 

     

    I’ve actually got a 16kw Daikin fitted not a 11kw. My heat loss report which I received after the insulation shows my heat loss at 10.8kw at a maximum flow of 55degrees and has got the Daikin code for a 11kw unit to be fitted hence why I thought it was a 11kw but it’s actually a 16kw unit? This sounds way too big just to run UF and Rads? I don’t understand why if the heat loss report is showing 10.8 and a 11kw to be fitted, why have I ended up with a 16kw?

     

    Did you make any headway on this? Maybe some of hydrosam’s reply would be helpful if you’ve still got questions.

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      hydrosam
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      February 24, 2024

      Hi @Abby_OVO 

      We took the conversation offline. It looks like there are several things not quite right with the system. Probably the first to investigate is the original heat loss calculations that decided on the size of the heat pump. He’s doing the right thing and getting an independent expert (a registered heat geek engineer) to come and check it out. 

      Fingers crossed things get sorted for Tommyfr 

      15kwh grid-tied battery, ASHP Daikin 11kw R410 mono + 300l Cylinder. 3.3kw Solar PV. Kia e-Niro EV
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