Posted by: @prunus

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DT according to the ASHP seems to be a constant 3C, either in heating or when chilling the DHW tank as a test (both with the secondary pump set to 1.1m3/h (=18l/min), I should try with it at max speed).

 

Thats presumably heat pump DT, what about radiator DT and, importantly, temp diff between flow from heat pump and flow to rads (ie temp diff across the buffer).

Posted by: @prunus

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Installer's heat loss calculation came in at 8kW, and system has glycol in it.

Interesting, so why did he fit a 12kW heat pump? 

Given that installers tend to overestimate the actual house loss may be perhaps 6kW.  Does the system cycle a lot even when its pretty cold (say 4C or less?).  8kW can be achieved with single 22mm primaries by pushing the envelope a bit!  

Posted by: @prunus

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I'm not seeing how I can get anywhere near the 15 l/min minimum flow without having a buffer? 

15l/min is ~4m/s through a pipe with ID of 9mm.  Thats, high but its either perfectly straight or at worst very smooth bends through the radiator, and its short.  The water pump may be capable of doing this but 3 off 10mm dia pipes (ie 3 fancoils in circuit simultaneously) and you are well within range no question.  If you get fancoils that don't shut off the water just modulate the fan (which may be the norm - I dont know), they will all be in circuit simultaneously anyway.

There is absolutely no point in having the ability to move heat (which is the only thing that min flow rate is about) unless there is somewhere for it to go/come from.  Unless you have sufficient fancoils in circuit you wont dump enough heat into the water and so you will get cycling, possibly short cycling, whatever the flow rate.  Not sure switching from volumiser for heating to buffer for cooling is going to help in reality because it doesn't change the load

Buffer or volumiser will both equally help with the short cycling, increasing the period but not changing the on/off ratio.  However your 50l buffer may not be large enough to make a material difference; with a 12kW system you typically would have 200l+ system volume.

Given the (over) size of the heat pump, and your idea that you can run only a small number of fancoils, you do need to do some calculations, however back of the envelope, about minimum turn down in cooling, likely load in cooling, and the required system volume so that you get a reasonable cycle period for a reasonable hysteresis in the flow temperature.  I am guessing it will work out quite a bit larger than 50l + the volume of a few fancoils, given the capacity of your heat pump.

Alternatively work above dew point and keep the rads in circuit so they increase the system volume.

This needs some basic thermodynamic and flow design else you risk having to experiment quite a lot.  It may 'just work', but equally it may not.

Seems a lot of work with hit or miss results and potential condensation issues, no condensation means very little cooling as the first thing any cooling does is remove moisture! You'll then need a drain or drain pump from the drip tray. It would be vital to insulate well all pipe work and the pump, and if the same pump is used for any heating shortly after the cooling mode it will condensate. I'd consider having a real AC system fitted as relatively cheap and super efficient.

Posted by: @dgclimatecontrol

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Seems a lot of work with hit or miss results and potential condensation issues, no condensation means very little cooling as the first thing any cooling does is remove moisture! You'll then need a drain or drain pump from the drip tray. It would be vital to insulate well all pipe work and the pump, and if the same pump is used for any heating shortly after the cooling mode it will condensate. I'd consider having a real AC system fitted as relatively cheap and super efficient.

I agree it seems like a lot of work particularly as its going to be needed for only a few days per year, at least for now ie until Climate change really takes off (probably pretty soon!)

Can I ask some questions about A2A (ie (presumably) what you call 'real AC'). 

When I've had quotes for this in a business context it seems to come out roughly as expensive as A2W for multi-room setups.  The ODUs are simpler and cheaper, but the IDUs are more expensive than radiators, and more expensive even than fan radiators, so it pretty much evens out.  Furthermore there seems to be a bit of a preference in the AC industry for 1 ODU to 1 IDU, as opposed to multi-split, and the latter seems to be more expensive (I never understood why).  In a domestic context 1 ODU to 1 IDU is pretty much a non starter, unless the house is very open plan or you have lots of space for an array of ODUs, so multi split seems to me to be almost inevitable.  

Also the refrigerant pipes still need routing to each IDU just as water pipes do in A2W.  Granted they are smaller so easier to route, but they still need insulating SOFAIK

Finally where cooling is needed I have always been told that a condensate drain is needed for each IDU even with A2A, unless of course you operate above dew point which you are arguing against.

Have I got any of the above wrong?

I guess what these questions amount to is there really that much difference between an A2W and an A2A multi room setup for cooling and if so what are the essential differences and why? 

PS I do grant that A2A will probably be more efficient; whether that matters for light cooling in a domestic setting is perhaps a moot point.

PPS FWIW Ive seen a couple of reports of good results for cooling above dewpoint with UFH.  The reports weren't 'perfection' (ie not not American style freezing temperatures), but appear to offer a significant comfort increase.  If you already have UFH or a fancoil or two, it seems to me like a good option just to put your heat pump into above-dewpoint cool mode.  Obviously its a different circumstance if, like OP, you need to do a lot of additional work to get off the starting block.  Like many things the optimum strategy depends on where you start!

@jamespa Your reasonably close! Especially with warmer summers and unpredictable weather. A2A indoor are more expensive than A2W wall mounts because they control the outdoor unit and have more features and modes thanA2W, also much quieter and more compact per kW output. They also need to withstand high pressures (40 bar with R32) However trade price on a 3.5kW indoor is £230 ish, so price isn't an issue.

Pipe wise its much easier and quicker to run 1/4" and 3/8" pipe than say 15mm.

Multisplits are far cheaper than any A2W system, we install both, so lets say a typical 3 bed semi has 5 indoor units to one outdoor at around £5000 installed an A2W unit would be about £10,000 (less any grant) The A2A while it doesn't heat water it will provide faster more controllable heating to any property. As many have found out in the last few years they will use the AC mode which is a blessing even up here north of the border.

Single splits use the same indoor units, and the single outdoor is only slightly cheaper because millions are produced as opposed to say thousands with multi split units. We fit multi splits to many homes and sometimes a single or two to heat or cool most of a property. They are often not needed in every room.

@dgclimatecontrol thanks, that's helpful.