static head isn't relevant, the water goes round in a circle, what goes up must come down. What matters is the resistance to dynamic flow.  Given that you have up to 24kw of heat input at 56l/min, into your buffer, you need to move that much heat out of the buffer through the rads: flow needs to try to match the input side.

I can pretty confidently say, that circulating pump is nowhere near enough.To hit 56l/min (2.8m3/h) your dynamic head would have to be less than 2m with that pump, which is highly unlikely for a 14 radiator sized house (unless possibly the radiator side pipework has already been highly optimised / rebuilt.)

suggest you think about:

• what is the pipework layout and sizing on the secondary side? draw it.
• get a heat metering setup installed on the secondary side to actually measure what you are getting through
• as a "can't make it worse, could make it better" - with zero plumbing changes: buy the most powerful circulating pump that you can find (And afford) that fits into the same space (thus making the swap trivial). it looks like a 130mm. then see what you get. 

Thanks @iancalderbank, that's helpful and good shout re the bigger pump. I'll look into that. I might need to start lifting a few floorboards as well!

Whilst I might have 24kW of installed capacity, the reality I've seen is nowhere near that. I've been keeping records of heat generated lately and, even in this recent really cold spell, it's only been managing 10-11kW (and that's with master/slave operation). Frequent defrost cycles mean LWT never reaches it's set point but gets stuck around 43°C ish, and that ends up a few degrees lower by the time it's been through the buffer as well. 

It's been able to keep the house comfortable and stable at around 18°C so in that sense it's working and has given me a good indication of my heat loss with OAT well below zero. I still feel there's a way to go on my efficiency journey though......

Simon

@dunlorn FWIW, I was running at about 9kw also on saturday (-2C here all day), flow rate of 28l/min , that kept us at 21C inside .  pretty sure that is my circulating pump going flat out. I use a UPMM 25-95 130, 8.5m head at 28l/min.

you could also look at a UPML 25-95 130 which doesn't drop below the 9.5M head until about 40l/min. 

surveying the pipes / valves is important too, if there are thin ones, it'll be harder to get the flow rate up just by adding more push.

it does sounds like having read your other posts, your 2x12 setup makes for hard work.

Thanks @iancalderbank. You're not wrong about the hard work. We clearly don't need 24kW but have made some improvements to the house since the system went in so the heating calcs (which I presume exist but have never seen) may have been right once. 

The way the master/slave set up works on the Samsung doesn't seem helpful to me. I wanted the slave to come in to support at OAT below 2°C (which I can set) but the trouble is when the master goes into defrost (or into DHW mode) the slave switches off too! So, at the very time when the extra help is needed it switches off, which means the value of the second unit is diminished. Anyway, that's a whole other story.  In hindsight a 16kW unit and no buffer tank may well have been a better bet but we are where we are! 

@dunlorn would it be feasible to individually control your two units rather than rely on samsung's master slave behaviour?

@iancalderbank Yep, they both have separate controllers (currently linked) but can be reconfigured to run individually, controlled from a single run signal from the thermostat, a Hive in this case. I was going to get my installer to sort this when they come for the annual service in Feb but may bring forward. I don't want tandem running all the while though, really I just need to switch the second one on at end Nov and switch off again in Feb - one unit is more than capable the rest of the time.

@dunlorn I was just pondering a possible idea for you: you could take control of each heat pump individually via modbus, run them as 2 separate rather than a linked pair. By control I mean setting the value of run / don't run, setting target temp (either IAT or LWT depending on mode). Modbus will tell you when its in defrost mode, along with the temps and flow rate. It might then be possible based on that data as input, to write your own control loop that keeps them in antiphase.  (I'm assuming you've done to death with samsung why they stay in phase with the controllers linked and it can't be resolved).

@iancalderbank Joule Samsung have just told me it's not possible to alter the 'slave' to remain on whilst the master goes into defrost. I thought though, if set up in tandem operation, it's perfectly possible for the second pump (heat only) to continue running, while the first pump (heat and DHW) is in a defrost or in DHW mode as they both separately feed a buffer tank.

I don't have the modbus modules fitted so that's £400 for a pair for starters, plus fitting, so I was going to try and get the simple conversion to tandem running done first and see where that gets me. I'm not into coding so may well end up going down the Homely route to get this working better without my endless fettling. 

@dunlorn yeah it should be perfectly possible to treat them just as independent heat sources, decouple the master-slave, if send them both a call for heat, they'll of course both run. you'd be relying on an element of luck that they don't go into defrost mode at the same time. I was trying to find a way for you to make it "better than relying on luck" but as you say if you don't have the modbus modules there is an outlay there. 

one very simple idea that might work (after you've decoupled them  and they are running as seperate heat sources) that requires no coding:

• connect the output 230v of your thermostat to the input "run" terminal of HP1  controller.
• also connect it to an on/off switch (this is the master for HP2 use  / no use).
• then connect from the output of the on/off to a push button timer  - has to be one that breaks rather than makes the connection - timed for X minutes. where X is at least the length of a defrost. output of the push timer to run terminal of HP2.
• If you detect by observation they are in phase : go press the button. delays run signal to HP2 for X minutes. should hopefully get them out of phase.

homely will require modbus by the way and you'd need to ask them if they can control two heat pumps.

@iancalderbank Thanks, that's really helpful!

Posted by: @iancalderbank

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I can pretty confidently say, that circulating pump is nowhere near enough.To hit 56l/min (2.8m3/h) your dynamic head would have to be less than 2m with that pump, which is highly unlikely for a 14 radiator sized house

Hi @iancalderbank. I've done some (crude) monitoring today with the system stable, looking at heat being generated and measuring secondary side flow and return. This yields flow rates between 17-21lpm for my Grundfos, depending on which setting it's on. So, less than even with a single heat pump running (26lpm) but when both are pumping into the buffer this means most inflow is effectively just mixing and being recirculated back! I'm sure this must be acting to throttle performance so the 'benefit' of the second pump is severely limited. Happily, the house has remained comfortable, just, but I'd like a bit more headroom! 

The Grundfos pump curve says delivery head is around 4.5m at 21lpm so getting to 56lpm would mean a massive, and impractical, increase in head, without upsizing a load of pipe. Getting to 30lpm would probably need about 9m of head so may be a sensible aim and should allow a comfortable margin of kW transfer.