Let's talk about Buffer vessels for ASHP

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? 

Hi @Tommyfr 

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.

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. 

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. 

Further thoughts @Tommyfr Have you asked about this on the renewable heating hub forum? There are lots of similar topics on there discussing buffer tanks. 

@Tommyfr  Did you get anywhere with the renewal heating hub forum ? I had sent you a private message too. 

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?