Buffers, hot water and cooling

Prunus · 2025-07-19T15:03:23Z

I have a 13kW Grant ASHP, installed to the letter of Grant's instructions at the time. 50 litre buffer, 200 litre DHW cylinder, radiators (with TRVs which are never used), wired as classic S-plan. After I rewired it to use the native ASHP controls since the installer followed the (very mistaken) Grant instructions, it works well for heating and hot water. The ASHP is maybe a bit oversized (deliberately, for noise reasons) but it seems to work fine - no short cycling. On mild days it'll run for a short time to heat the buffer and then circulate that heat for an hour or few around the house (ie the buffer is doing its job). It's set to run 'sniff' cycles every 20 minutes, ie run the internal pump to take the temperature of the buffer water by sending it past the outdoor unit's temperature probe and decide whether to turn on or not*. Here's a diagram: (* I have actually wired a buffer thermistor so it can measure the buffer temperature directly, but it seems the only mode it'll use that is when it tries to keep the buffer at a constant temperature like a gas boiler - no weather compensation or anything, which isn't what I want. When I disable that mode the buffer temperature always reads 25C - seems using sniff cycles is the only option in more sensible modes.) I'm pondering installing some fan coils for cooling. Current thinking is to install them on a second zone (S-plan+) like an upside down underfloor heating system: UFH manifold, UFH actuators, and no valvegear in the fan coils (easier to manage condensation that way). Sketch: Are there any reasons not to go with this plan? I've tested the ASHP by switching the cables to the DHW and Z1 zones and running it in cooling mode - it makes a nice tank of chilled water (although the coldest water sinks to the bottom of the tank and the draw is at the top, so you can't actually chill the whole tank to 7C even if that's what it's making). One thing I wondered was about the buffer when switching between DHW and cooling. If you've just made a tank of hot water for a shower then want to switch to cooling mode. What happens when you engage cooling is the unit runs its internal pump and the fan with no compressor to cool down the buffer circuit to outside temperature, and then runs the compressor to cool it below ambient. But this is inefficient as every time you switch from hot water to cooling you have to cool that 50+ litres, and heat it again when you want hot water. Typically I have it set to just run the hot water any time the water temperature gets a bit low, which can be at any point in the day. Let's assume that I just want a fit and forget option, ie I don't want to micromanage scheduling the hot water at 3am to avoid conflicts with cooling. It seems less than ideal to have hot water and cooling fighting like this. One option would be to move the DHW before the buffer, on the basis that there will always be a constant flow through the DHW circuit. This would then move the buffer into the heating/cooling side only, and allow a switch between heating and cooling modes without having to shift the buffer water. Something like: What would be the pros and cons of this approach? I need to do the calculation of the ASHP's minimum flow against flow through the hot water cylinder coil, but let's assume that's ok. I'd have thought there would be enough heat in the DHW side to scavenge for defrost cycles when in DHW mode, and in heating mode there's still the buffer. If one of the valves stuck presumably the ASHP would shut off to protect itself for lack of flow. Any other things to watch out for, or reasons why this isn't a good idea? Edit: maybe I'll need to move the expansion tank to before the Z1 and Z2 valves, to manage pressure when in cooling mode too?

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Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

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JamesPa

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24/07/2025 8:26 pm

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.

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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