The solution for individual room control but to maintain minimum flow on an ASHP circuit will be perfect when they produce a valve that only depletes the flow in a controlled manor relayed back to the central control unit to reduce the output of the emitter rather than shuts it off completely. This kind of control has been available for a very long time commercially but is expensive, I am aware some versions of this are starting to hit the domestic market but can be expensive 

I think that maybe what the adia thermal system does, but as you say it's quite expensive and only works with some ashps

Alternatively perhaps someone will come up with something that is relatively crude but 'good enough'.  I do wonder about a completely passive thing, essentially a version of a trv decorators cap, which replaces a trv head and has two adjustments, one set by the installer having complete adjustment, and the second offering the householder limited additional control.  Installer could then balance for deltat which gets close if the sizing is roughly right, householder could then tweak within limits if they need to.  Plastic moulding turned out in millions, cheap.  Might need two versions, one to put on the rads which are fully open and are not to be adjusted, another to put on the other rads.  Still cheap.   Not fully automated but perhaps has potential?  

The solution needs to be cheap for it ever to be considered on domestic designs as the cost of these retro fits are expensive already. It is worth noting that with many of the discussions around the functionality or set up and efficiencies of these systems there is nearly always a better way to design and commission them but it costs more and more. 

Compared to other tech in the commercial industry. TRV's are cheap and were designed to work with boilers for homes as a cheap better form of control, lock shields are cheaper and have been around forever.

All other forms of hydronic local control are more expensive and are often overlooked as the £9,000 to £15,000 bill is already expensive enough and as a nation we are often driven or guided to cheapest quote and not best design. 

It is also worth noting that it is likely that most customers will have the basic installation the 1st time round as it is so expensive, of the £11,000 installation cost £7,500 of this is the retro fit conversion to make the house and the system match up for an ASHP, the remaining £3,500 would be to straight swap the ASHP external unit, just like replacing a boiler but this cost is only possible once the home has been retrofitted.

Once the home is retrofitted, in the future there will be upgrades available when you swap out the systems or mid life as companies invent cost effective solutions to control, we are currently seeing a few software versions already, hardware will probably follow at some point as the market gets bigger.

Capacity control works well on just maintaining DT at the external unit if you have the correct mechanical control at the emitters. Combined with WC capacity control on DT gives a very smooth control curve. Manufacturers have been using this kind of curve since the end of fixed speed compressors in the 90's. These systems are still just AC units with a plate, this tech has been here a very long time but the new part is the water adaption side and control of the water the rest of the system will always work well and always has done on good brands (this is where the fit a buffer to protect the manufacturers kit came in) they knew the systems will work for a very long time if the water flow rate is good and never stops.

Think I attached the wrong link

https://www.draytoncontrols.co.uk/products/radiator-valves/auto-balancing-trvs/auto-balancing-rt414-trv-angled-15mm

Still not quite.  As I read this the body adjusts for fixed flow when on (based on an adjustment made by the installer according to a table that doesn't even include the dt5 condition with which heat pumps are commonly designed) and then the head works like any other head, turning the flow on or off.  We want something that adjusts the flow rate very slowly to achieve the required room temperature, as if adjusting a lsv manually.  It's really a commissioning device not a control device and once set it can stay set.

There is definitely a problem and thus an opportunity.  I've lost count how many times I have explained on this forum how to adjust WC, and then how to adjust lsvs to achieve equal room temps (or the desired different room temps) - in short how to set up an open loop WC based system.  Clearly therefore installers don't always do this which, given how long it takes (elapsed time not actual effort,), is unsurprising.

With a gas boiler installers simply whack up ft to max fit trvs and then let householders sort it out.  It needs to be as simple for an ashp.  There are two (somewhat interlinked) parts of this namely the WC curve set on the heat pump and the flow rates through each radiator.  Balancing for deltaT (which installers may do) is a start, but doesn't finish the job because radiators are inevitably not correctly sized.  Balancing for room temperature (as described also on the heat geek website) is what's needed, but takes time.

Almost but not quite a trv with remote sensor.  A trv with or without a remote sensor operates principally, so far as I know, as an on off device (albeit that there is an element of analogue control). 

No they are not on/off valves, they modulate flow. Exactly what you say you want, they do our the box. Modulate radiator flow based on room temperature. So as to keep room a a set temperature. Add weather compensation, they will continue to modulate flow as needed.

Simple description of a trv

https://www.traderadiators.com/blog/how-do-thermostatic-radiator-valves-work

Er - that description you posted says that the trv shuts off the flow when the room is at the set temperature, which is also what seems to happen in practice at least with my Drayton trvs.

Think I attached the wrong link

https://www.draytoncontrols.co.uk/products/radiator-valves/auto-balancing-trvs/auto-balancing-rt414-trv-angled-15mm

Almost but not quite a trv with remote sensor.  A trv with or without a remote sensor operates principally, so far as I know, as an on off device (albeit that there is an element of analogue control). 

No they are not on/off valves, they modulate flow. Exactly what you say you want, they do our the box. Modulate radiator flow based on room temperature. So as to keep room a a set temperature. Add weather compensation, they will continue to modulate flow as needed.

Simple description of a trv

https://www.traderadiators.com/blog/how-do-thermostatic-radiator-valves-work

Er - that description you posted says that the trv shuts off the flow when the room is at the set temperature, which is also what seems to happen in practice at least with my Drayton trvs.

Hence the reason everyone is saying minimise their usage.

Auto balance anything, is just a band aid on a fully open system. They really have no place. People should either self install or get a professional install, the professional install should balance they system, if they don't call them back, you have paid good money for an install.

Almost but not quite a trv with remote sensor.  A trv with or without a remote sensor operates principally, so far as I know, as an on off device (albeit that there is an element of analogue control). 

No they are not on/off valves, they modulate flow. Exactly what you say you want, they do our the box. Modulate radiator flow based on room temperature. So as to keep room a a set temperature. Add weather compensation, they will continue to modulate flow as needed.

Simple description of a trv

https://www.traderadiators.com/blog/how-do-thermostatic-radiator-valves-work

It's literally the same as adjusting the lsv manually, something you do slowly over hours or days until it's right, then leave forever.

Is that really the case?  Won’t the rate of heat loss in the room change as it cools/warms outside and thus what it is balanced for today at 1c isn’t the same as tomorrow when it’s -3c?  The flow temperature would have increased but is the radiator still properly balanced in the changed scenario?

Yes it really is the case to a good (good enough) approximation.  Once emitters are balanced open loop WC with all emitters permanently on works pretty well, and all this is is open loop WC with an automated balancing instead of manual balancing.

That's good to know - and in which case, adding anything fancy to the system to "automate" it sounds like (a) an unnecessary complication; and (b) likely to not work very well and/or just become aggravating (in my somewhat jaded and world-weary opinion)

In principle I agree.  However how else can you correctly adjust the lsvs?  Installer won't spend the hours that it takes to do it (they may adjust for DT but won't in most cases adjust for room temperature) and most homeowners not capable/not willing.  The alternative (as I fear the default) is fitting trvs which are a bad option.  

It's a real problem that needs a solution that works for the average householder, not just the ones on this forum!

It's literally the same as adjusting the lsv manually, something you do slowly over hours or days until it's right, then leave forever.

Is that really the case?  Won’t the rate of heat loss in the room change as it cools/warms outside and thus what it is balanced for today at 1c isn’t the same as tomorrow when it’s -3c?  The flow temperature would have increased but is the radiator still properly balanced in the changed scenario?

Yes it really is the case to a good (good enough) approximation.  Once emitters are balanced open loop WC with all emitters permanently on works pretty well, and all this is is open loop WC with an automated balancing instead of manual balancing.

That's good to know - and in which case, adding anything fancy to the system to "automate" it sounds like (a) an unnecessary complication; and (b) likely to not work very well and/or just become aggravating (in my somewhat jaded and world-weary opinion)

It's literally the same as adjusting the lsv manually, something you do slowly over hours or days until it's right, then leave forever.

Is that really the case?  Won’t the rate of heat loss in the room change as it cools/warms outside and thus what it is balanced for today at 1c isn’t the same as tomorrow when it’s -3c?  The flow temperature would have increased but is the radiator still properly balanced in the changed scenario?

Yes it really is the case to a good (good enough) approximation.  Once emitters are balanced open loop WC with all emitters permanently on works pretty well, and all this is is open loop WC with an automated balancing instead of manual balancing.