Posted by: @prunus

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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 is how my system is arranged, with a three port valve that switches the HP flow either directly to the DHW or to the buffer when in heating /cooling mode.

Posted by: @prunus

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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

Definitely do this.  My dhw operates fine without a buffer or volumiser and your logic explains why.

Posted by: @prunus

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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?

Agree

If you are replumbing why not turn the buffer into a 2 port volumiser in the flow, lose the secondary pump and likely reduce your operating cost by 10-15%.  You have probably violated the warranty anyway so you might as well do the job properly.

Posted by: @jamespa

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If you are replumbing why not turn the buffer into a 2 port volumiser in the flow, lose the secondary pump and likely reduce your operating cost by 10-15%.  You have probably violated the warranty anyway so you might as well do the job properly.

I wondered about that. Wouldn't that potentially cause issues with flow rates? I have no idea what the flow through the radiators is, and I thought the idea of hydraulic separation is that I don't need to care?

Also, I'm concerned about the situation with the fan coils if they are individually valved at the manifold (for reasons of wanting to minimise the places I'm pumping chilled water to minimise condensation), which means that if only one is turned on (quite likely because they make noise) then the flow rate will be limited to the pipe run through that coil only (16mm pipe, most likely).

I need to work out how to 'call for cold' but I was originally thinking that I can let the ASHP and buffer do their sniff cycles (ie they run 24/7 and fire up to cool the buffer when sensing it's warmed up too much), and then I am free to valve individual fan coils via the UFH actuator so they pull cold based on room demand (classic UFH room thermostat style, just with a bit of fan speed control too).

And why would you put the volumiser in the flow and not the return?  Is that intentionally to make the system slower to react?

Posted by: @prunus

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I wondered about that. Wouldn't that potentially cause issues with flow rates? I have no idea what the flow through the radiators is, and I thought the idea of hydraulic separation is that I don't need to care?

Unfortunately even with hydraulic separation you do need to worry about flow rates unless you are happy to increase operating costs by 15%, because unless you match the flow rates you get mixing in the buffer reducing ft to the emitters thus increasing the ft required from the heat pump ( unless you have much larger buffer, in which case you can match average not instantaneous flow rates, which requires a different control loop that few installers know how to implement)

Lots of discussion about this but the general conclusion is that separation is almost never necessary in domestic systems and is primarily for the benefit of the installer not the customer.

Posted by: @prunus

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, I'm concerned about the situation with the fan coils if they are individually valved at the manifold (for reasons of wanting to minimise the places I'm pumping chilled water to minimise condensation),

Most would advise to operate above the dew point unless the system has been specifically designed and insulated to operate below.

Posted by: @prunus

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I need to work out how to 'call for cold'

Don't bother unless it actually turns out to be a problem.  Just switch cooling on or off manually or on a time schedule on the few days you actually need it, operating above dew point. Probably you won't ever get to the point where it is too cold.  Assuming you use windows and curtains sensibly you probably only really need to get from say 28/30 to somewhere in the 20-24 region, IE a point where it's somewhere tolerably warm as opposed to oppressively hot. (I'm assuming you don't hanker after US frigid conditions)

Posted by: @prunus

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And why would you put the volumiser in the flow and not the return?  

There is a very long discussion about the optimum placement elsewhere on the forum.  In summary on the flow so that the slug of cold water you get following defrost is diluted.  However it's not that important, what is important is that it doesn't kick in during the heating and it is not across flow/return.

Posted by: @jamespa

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Unfortunately even with hydraulic separation you do need to worry about flow rates unless you are happy to increase operating costs by 15%, because unless you match the flow rates you get mixing in the buffer reducing ft to the emitters thus increasing the ft required from the heat pump ( unless you have much larger buffer, in which case you can match average not instantaneous flow rates, which requires a different control loop that few installers know how to implement)

Lots of discussion about this but the general conclusion is that separation is almost never necessary in domestic systems and is primarily for the benefit of the installer not the customer.

Agreed, you get loss of optimality - but it continues to work, rather than just breaking.

Posted by: @jamespa

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Posted by: @prunus

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, I'm concerned about the situation with the fan coils if they are individually valved at the manifold (for reasons of wanting to minimise the places I'm pumping chilled water to minimise condensation),

Most would advise to operate above the dew point unless the system has been specifically designed and insulated to operate below.

That's what I'm trying to do: design and insulate it right.

It is probably going to be operating above dew point a lot of the time (ie traditional heatpump: run your deltaT as low as possible, which for cooling means high flow temps) but there are humid days where staying above dew point doesn't get you much cooling.

Posted by: @jamespa

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Posted by: @prunus

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I need to work out how to 'call for cold'

Don't bother unless it actually turns out to be a problem.  Just switch cooling on or off manually or on a time schedule on the few days you actually need it, operating above dew point. Probably you won't ever get to the point where it is too cold.  Assuming you use windows and curtains sensibly you probably only really need to get from say 28/30 to somewhere in the 20-24 region, IE a point where it's somewhere tolerably warm as opposed to oppressively hot. (I'm assuming you don't hanker after US frigid conditions)

We get about 40C internally due to east-facing solar gain (external shutters not feasible due to roof and window design). If I'm doing this I want to do it properly, ie get the same smooth constant temperatures for cooling as you get with a heatpump for heating (targeting about 20C). It also needs to be able to fight internal loads (solar gain, electrical equipment, showers, body heat) which makes it harder work than for heating where those things work with you.

I don't actually know what my cooling loads are, so just planning to start with one fan coil to begin with to see how it goes, but that itself causes flow issues.

Posted by: @jamespa

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Posted by: @prunus

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And why would you put the volumiser in the flow and not the return?  

There is a very long discussion about the optimum placement elsewhere on the forum.  In summary on the flow so that the slug of cold water you get following defrost is diluted.  However it's not that important, what is important is that it doesn't kick in during the heating and it is not across flow/return.

I suppose what I could do is add two three port valves, one for the DHW side (thanks @downfield, much neater) and another one as a buffer bypass. In heating mode the buffer is bypassed on the flow side but remains on the return side, turning into a volumiser. In cooling mode (or maybe never) the pump is in circuit and it operates as a buffer. At least I have both options and it's just one wire to experiment. How does this look?:

 (I'll probably take the opportunity to add heat metering while doing it too)

Posted by: @prunus

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Agreed, you get loss of optimality - but it continues to work, rather than just breaking.

It is very unlikely to break, but of course it just might if there is something unusual about your plumbing.  Its obviously your choice whether to (a) take the risk or (b) do it in a way thats reversible or (c) do nothing in relation to this matterother than taking it out of the DHW loop, which obviously you want to do if its going to be used for cooling.

Posted by: @prunus

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We get about 40C internally due to east-facing solar gain (external shutters not feasible due to roof and window design). If I'm doing this I want to do it properly, ie get the same smooth constant temperatures for cooling as you get with a heatpump for heating (targeting about 20C). It also needs to be able to fight internal loads (solar gain, electrical equipment, showers, body heat) which makes it harder work than for heating where those things work with you.

OK that's quite a serious endeavour, and a seriously high temperature which TBH, suggests suspect house design (or at least house design that didn't take into account solar gain).  However you have what you have. 

I cant comment on this level of cooling other than to say that (as you imply may be the case) I am told by those who specialise in commercial aircon that they size for cooling not for heating, because its more challenging.  I guess this is because the things that happen in an office (and as you point out also a house) will always heat it but never cool it.  You will obviously need to do the same sort of calculations for cooling as you do for heating.  In terms of outside air temperature its the same calculation but the adding in the internal loads.  I personally have no idea how you calculate for solar gain which is perhaps the dominant effect, maybe someone else does.   Are you absolutely sure there is no way to mitigate this at least in part, eg a remotely controlled blind integrated into the window in question, or a projecting shade.  In the end it may pay to spend some money on reducing the gain just as it's a good idea to spend money on insulation.  Of course I accept this may be impossible.

Common thermostats don't deal with cooling and I don't know if Grant ASHPs control for cooling, or just (like at least some) cool.   Sorry I cant help here.

Posted by: @prunus

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I suppose what I could do is add two three port valves, one for the DHW side (thanks @downfield, much neater) and another one as a buffer bypass. In heating mode the buffer is bypassed on the flow side but remains on the return side, turning into a volumiser. In cooling mode (or maybe never) the pump is in circuit and it operates as a buffer. At least I have both options and it's just one wire to experiment. How does this look?:

The idea of making it switchable seems very sound.  If I understand your diagram correctly I think, but am not sure, that the diagram turns it into a kind of three port buffer because there is a third leg not switched by the valves but still connected.  A further shut off valve may be needed.  Obviously the secondary pump would also need to be switchable.

Good spot, I'd missed that 'short circuit' path.  Here's another go:

If no V1/V2/V3 are powered (all NC position) then we're in classic buffer heating configuration. The pump runs when there's a call for heat.

V1 is powered (in the NO position) when DHW is demanded. Because V3 is unpowered (in the NC position) the buffer bypass is closed.

If V2 and V3 are powered then the buffer is bypassed. The pump must be disabled.

Possibly also want to drop the Z2 valve if the actuators are going to do the same job, or replace with a manual isolation valve.

How does this look? What else have I missed?

@prunus,  @jamespa is correct, but removing the buffer could possibly halve your heating bill, possibly more. You have admitted that you have no idea what the flow rate to your radiators is; therefore, your system is designed by reading tea leaves, not following the laws of thermodynamics. You are moving the heat in to your house with water moving down the pipes to the radiators. If you do not know how much water you are moving, you have no idea if your radiators are reverent too much or too little to satisfy the design. Your statement, " the idea of hydraulic separation is that I don't need to care" could not be more wrong, no flow= no heat energy to the radiator=cold house, very simple. 

Balancing and confirming flow rates to radiators was exceptionally difficult, if not impossible in the past, regardless of what plumbers say, today it is simple, there has just been a product launched that you replace the lock-shield with on the return of the radiators, it has a flow meter similar to an under-floor manifold. With this, you can confirm your radiators are receiving the correct flow rate and they will perform to design. 

Cooling can be difficult, however, not impossible in a retrofit scenario. Operating with dew-point control does not work in this country due to exceptionally high humidity levels, you need a dew sensor that will control the compressor of the heat pump, but not the pump. If done correctly, it works very well with under-floor heating and fan coils. 

You can get the valves here: https://renewableheatinghub.co.uk/the-importance-of-radiator-balancing-for-efficient-heating-introducing-the-flow-regulating-valve/