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?
Let's talk about Buffer vessels for ASHP
Best answer by hydrosam
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.
+3- Carbon Catcher***
Whereabouts do you live ? My advice re increasing the buffer would be to see if there’s a heat geek in your area (check the Find a HeatGeek website). They will definitely have a clue ! Have a look at their website for more info on buffer tanks.
90mm loft insulation is not enough. The usual recommendation is 270-300mm.
Also I’ve read that towel rails aren’t ideal in a heat pump circuit. Having 4 may be part of the problem.
Sorry it’s 900mm insulation not 90. The problem is when the ASHP goes into defrost cycle it basically stops working so my it’s not reaching demand when the thermostats are calling then it’s not getting back up to the set room temperature and it freezes again! So it’s constantly running? I think the idea of a large buffer vessel is it acts as a back up for the ASHP to keep temperatures up while it defrosts? I’m just wondering if this should have been picked up in the design?
- Plan Zero Hero
Hi
Have you searched for “Heat Pump freezing up”? There are other possibilities besides the size of the buffer vessel such as:
- Low refrigerant levels/Refrigerant Leaks
- Malfunctioning defrost cycle
- Bad sensors or thermostats
- Malfunctioning electrical relays for the defrost system
- Stuck cycle reversing valve
- Damaged or poorly operating fan motors
Many of these are easier to diagnose and fix than replacing a buffer vessel.
+3- Carbon Catcher***
Phew.. 900mm loft insulation is massive!!
Back to the questions:
Whereabouts in the country are you?
Is the 20 litre buffer within one of those pre-plumbed cylinders ?
What controls do you have eg Madoka Room Sensor, or a non-Daikin thermostat ?
Are all the TVRs on the rads and underfloor open to get the maximum amount of heated water flowing through the system ?
Are you using weather dependent flow temps ?
Staffordshire, the 20l buffer cylinder is just a round white metal 1, the cheapest by the looks of it, I’m on Heatmiser with 6 zones for the underfloor and a Hive for upstairs. All the actuators on underfloor are fully open and the rads upstairs are balanced out. Yes I m using weather dependent. Regarding BeePee I had the ASHP installed in 2022, I’ve had a Daikin engineer out 2 times and he’s checked it over and said everything is working fine. I spoke to Daikin in the week and I’m reluctant to get an engineer back out because when I asked if they inspect it and find nothing wrong with it would I be charged and he couldn’t answer that question he said. I know someone who got charged £300 by Daikin for a call out. Don’t get me wrong 4 and above I haven’t got a problem. But these units you cannot stop them freezing up and when they do they turn off the temperature carries on dropping on your stats and when it starts back up your starting from scratch again. The only way to make it cheaper is to drop the weather dependent so your flow temp coming in 35 because your burning money trying to get it up to 40 and above. Your told to leave your ASHP all the so it just ticks over then it starts freezing up and it’s not running and working twice as hard to get back up to temperature.
+3- Carbon Catcher***
If your third party controls are closing off areas of the heating circuit the volume of water will be reduced. The hp needs as much volume as it can get. I’m sure you can do without a buffer if you can keep everything open. The defrost cycle shouldn’t cut off the heating as you describe.
+3- Carbon Catcher***
Further thoughts
+3- Carbon Catcher***
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?
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.
Thanks for the reply here
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.
Hi
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
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