Great to see the discussion! We had an ASHP with a buffer tank installed by BG and after quite some time of complaining about our low efficiency they actually came back (Brandon himself 😀) and re-piped the 40l 4-port buffer as a simple volumiser and removing the secondary circulation pump completely. So it's an open loop now.
Without the buffer, the efficiency was significantly improved. The SCOP was 2.4 and I think is currently on track for something around 3. Still not exactly great and I am still puzzling about this, but I think definitely the right thing to do to "remove" the buffer (i.e. to re-pipe as a simple volumiser).
Many thanks to BG and Brendon for actually coming back and changing the pipework! 👍
Hoping now to solve the remaining conundrum (very low dT=low heat transfer and suboptimal COP), hopefully with some help in the forum, but I will post that in another thread.
In my experience the buffer tank definitely was holding back the efficiency and was absolutely not necessary. See comparison of COP (heating only) before and after the change from buffer to volumiser.
Great to see the discussion! We had an ASHP with a buffer tank installed by BG and after quite some time of complaining about our low efficiency they actually came back (Brandon himself 😀) and re-piped the 40l 4-port buffer as a simple volumiser and removing the secondary circulation pump completely. So it's an open loop now. Without the buffer, the efficiency was significantly improved. The SCOP was 2.4 and I think is currently on track for something around 3. Still not exactly great and I am still puzzling about this, but I think definitely the right thing to do to "remove" the buffer (i.e. to re-pipe as a simple volumiser). Many thanks to BG and Brendon for actually coming back and changing the pipework! 👍 Hoping now to solve the remaining conundrum (very low dT=low heat transfer and suboptimal COP), hopefully with some help in the forum, but I will post that in another thread. In my experience the buffer tank definitely was holding back the efficiency and was absolutely not necessary. See comparison of COP (heating only) before and after the change from buffer to volumiser.
Can I just say well done!, (a) for persuading BG to come out and do what (it seems) so many need done to their systems and (b) for collecting the data to prove that it worked! This isnt the first time I have seen removal of a buffer reported, but it is the first time Ive seen it reported and done by one of the 'big ones' and accompanied by evidence of the effect.
Multiple zones (with all that implies), buffer tanks/low loss headers, volumisers (which should be distinguished from buffer tanks as they are totally different beasts) all add to system cost and complexity, both for the installer and the homeowner. All have potentially negative side effects in addition to the 'purpose' that they are intended to fulfil
Octopus install volumisers (not buffers) on the return precisely for this defrost reason - they spell it out - but I argued it for my increased radiator sizing too. Later, when I wanted a small radiator in a cupboard removed the plumber (who was fine with removing it) looked at me and said: 'volume and airing cupboard' - I kept it. It's really useful!
Anyway, after these two subzero weeks at the end of last year and now this week just gone - I realised why my defrosts were at least two hours apart and the longest one only lasted for 16 minutes.
Ever since I switched my heat pump to 'pure weather compensation' - Leaving Water Temperature control in Daikin parlance - it has run like a dream. It's been running for weeks now, without ever stopping or cycling, maintaining an inside temperature 21º- 22º (a 2º LWT set back at night) throughout the changing OATs.
In my Octopus/Daikin FBook group there have been numerous questions about frequent defrosts and spikes in electricity consumption (mine doesn't do these spikes either except to a very small degree). I think the YouTube video (which gets a bit geeky 😁) explains why mine is doing so well and my defrosts are infrequent - water volume!
@lucia I have not attempted to tot up my volume of water but 10 radiators plus a dual fuel towel rail seem to keep the defrosts to perhaps 90 minutes apart and only 6-8 minutes in duration. I notice the first radiator in the circuit will become cool for 2-3 minutes and that’s about it really. According to the Homely Dashboard report. the whole cycle is approx 8 minutes from start to finish. (Daikin EDLA 8 kW.) Regards, Toodles.
This post was modified 1 day ago 2 times by Toodles
Toodles, he heats his home with cold draughts and cooks his food with magnets.
The problem is, unless I have missed something, is he just asserts system volume 'really helps' with defrosts, which I take to mean it reduces duration and/or frequency. But he doesn't explain why. And if I don't see a why explanation, I want one! Nor does he provide any evidence, and when I see an assertion without evidence, I want to see evidence!
@jamespa will do this much better than I will, but surely the speed of frosting up, which in turn will determine defrost frequency, is a functions of how hard the heat pump is working, and ambient conditions, neither of which are directly affected by system volume. When it comes to a defrost, there does need to be enough heat stored in the house to complete the defrost, so there is a minimum system volume in an absolute sense, but the speed of the defrost is determined by how fast the heat is pumped back from the house to the heat pump, and the primary determinant ate the heat pumps operating characteristics, not the system volume.
Bottom line is once you have enough system volume, so you don't run out of heat for the defrost, then that's it, adding extra volume doesn't add anything, or if it does, it only adds a small amount eg a larger system may cool slower, and therefor continue to supply warmer water for longer, and that may shorten the defrost, but only by a small amount?
I'm more asking questions here than supplying answers.
Midea 14kW (for now...) ASHP heating both building and DHW
Further information: here are some defrosts from this morning, about 50 mins apart, duration 4 minutes. But the problem isn't just the defrost, it is the slow recovery from the defrost (with the alternating pattern I've previously noticed). Maybe with a larger system volume (more water to reheat), the recoveries would be even slower?
Midea 14kW (for now...) ASHP heating both building and DHW
Hummm......larger system volume would have more energy stored in the water so the bulk average water temperature reduces less when the defrost cycle happens. I suppose the heat pump controller would then spend less time going flat out to increase the bulk average water temperature, so that should mean less ice accumulation in the fins. Interesting!
Hummm......larger system volume would have more energy stored in the water so the bulk average water temperature reduces less when the defrost cycle happens. I suppose the heat pump controller would then spend less time going flat out to increase the bulk average water temperature, so that should mean less ice accumulation in the fins. Interesting!
Bob
I too have been thinking about this, not least because the only disappointment with my heat pump is that its noisy when recovering from defrost.
I think what you say above is right. The energy needed for defrost remains the same irrespective of the system volume, but larger system volume means that the system water temp will drop less for the same energy extracted. This (a) means that it can start putting heat back into the house earlier and (b) means the heat pump can afford to spend a bot more time 'catching up', which should (i) should mean less ice accumulation in the fins and (ii) means it doesn't have to be as noisy.
Of course the recovery algorithm will not necessarily be sophisticated enough to take advantage of this opportunity, so in practice it may well be that system volume makes little difference once it is sufficient.
@cathoderay
The problem is, unless I have missed something, is he just asserts system volume 'really helps' with defrosts, which I take to mean it reduces duration and/or frequency. But he doesn't explain why. And if I don't see a why explanation, I want one! Nor does he provide any evidence, and when I see an assertion without evidence, I want to see evidence!
He's data obsessed if you follow his channel. However, he points out that minimum volumes are specified in the heat pump set up manuals - he uses the 7kw Vaillant Arotherm as his example - but he's producing his video for new users not to prove a point. He made a film outside of the kit defrosting a few months back because some new owners get a bit stressed by it.
He also uses OpenEnergyMonitor data for his films and I think he refers to this in his defrost discussion.
Octopus state very clearly in their installs that they use the volumiser tank to aid defrosts - I have a feeling it's a warranty condition (but not sure..).
But it fascinates me because mine seem to have minimal impact (except the high pump speed for a few minutes makes my rads hiss - I should adjust the lockshield slightly ) - while others are in almost identical conditions (pump size, OAT etc.,) yet have repeated defrosts in an hour using lots of Kwhs of energy.
Dunno... just something to chew over really. 🤷🏻♀️
@lucia - absence of evidence isn't evidence of absence (of effect) and all that but still he is in effect just making an assertion. Being obsessed with data doesn't mean the data is right, as I know you know only too well.
I don't for a moment doubt there is a minimum system volume required, normally available in the installation manual, but as @jamespa has said, once you have that volume, how much real world benefit do you get from adding more volume? My gut feeling, which of course is not evidence, is that the law of diminishing returns applies.
What would certainly be very interesting is to look at why you have minimal defrosts while others have significant defrosts. To do this you would need to find someone with a similar heat pump (brand/size) and then look at defrost behaviour and system volume for both systems. It would be very worthwhile doing this. We have a lot of people here on the forum, most of whom say what heat pump they have, and many very willingly post their data either spontaneously or on request. Now is a good time to look, while the pain of defrosts is still sharp in people's minds!
Midea 14kW (for now...) ASHP heating both building and DHW
Our system is open no hydraulic separation. The volume is high (18 Large K2 and can’t be bothered to add it up) but the installer still added a 26L in series volumiser. I had stopped quibbling over the details by then but way more than the minimum volume needed.
The result is a COP of 3 on these cold days, and between defrost times of ~40minutes. In warmer weather and low load the on time is ~40minutes and the mark space ratios are ~60-40. Adding more additional volume impacts the low load case most.
in defrost situations it is worst when working hardest to reach the highest LWT (you canna break the laws of physics Jim) so highest system volumes mean the larger radiators (nothing to do with volumisers) run at lower design temperatures.
So a LWT design of 55C will defrost much more than one with a design of 42C for the same outside temperature.
