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Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls
Last Post by JamesPa 1 month ago
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 2 years ago
Posts: 2753
24/07/2025 8:26 pm  

Posted by: @prunus
↑

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
↑

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
↑

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.

 

  

 

This post was modified 1 month ago 10 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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