@markspencersmith 

Posted by: @markspencersmith

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ASHP runs on standard WC. Heat flows fully to all rooms, satisfy demand *for whatever reason* and close TRVs. Heat flows to other rooms. A threshold % of TRVs close - WC is decreased,

Before I offer my thoughts I am  not sure I understand what you are saying in the above quote.  Can you clarify the last part 'WC is decreased' (why and by what mechanism)?  Can I also ask if the 'guy you spoke to' has tried this and what the result was, or is this currently a thought experiment?  

Incidentally

Posted by: @markspencersmith

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So, conventional HP wisdom says to balance the rads by dT or dflow as a proxy per the rad size and required output.

is not quite true.  Conventional HP wisdom by those who understand the real world is to balance the radiators for equal room temperature (or the desired variation in room temperature) not equal deltaT.  This is because its unlikely that the design is precisely correct and therefore radiator sizing wont be either.  Also, radiators come in discrete sizes and often are picked with an eye on aesthetics as well as function.

Balancing for equal deltaT is quicker and easier and may be a reasonable approximation given that room share heat, but equally may not be if the rads in all rooms are not 'correctly' sized. 

Posted by: @jamespa

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Balancing for equal deltaT is quicker and easier and may be a reasonable approximation given that room share heat, but equally may not be if the rads in all rooms are not 'correctly' sized. 

Which I can empirically vouch for.  Further, you may even find, as I do, that thermostatically controlling the system can lead to oddities such as having a room at a set point of 19c or 20c or 21c or whateverc and finding the rest of the house is maintaining a fairly constant temp of 22c-23c.  I.e. controlled by the heat loss of the room with the thermostat with very little actual control!

Posted by: @andrewj

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Posted by: @jamespa

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Balancing for equal deltaT is quicker and easier and may be a reasonable approximation given that room share heat, but equally may not be if the rads in all rooms are not 'correctly' sized. 

Which I can empirically vouch for.  Further, you may even find, as I do, that thermostatically controlling the system can lead to oddities such as having a room at a set point of 19c or 20c or 21c or whateverc and finding the rest of the house is maintaining a fairly constant temp of 22c-23c.  I.e. controlled by the heat loss of the room with the thermostat with very little actual control!

 

One of the major challenges with heating generally is that actually we have very limited control, and adding any number of additional controllers doesn't necessarily give us more useful control, no matter how sophisticated they are and how many we add.  This is fundamentally because a house takes a very long time (hours or even days) to respond to changes, different controls are often not connected, and we don't usually use active cooling (so control downwards is dependent on loss not on the heating system).  The manufacturers of controls like us to think differently, but they are largely wrong unless the house in question has the thermal characteristics of a tent, which few do these days.

Fortunately most of the disturbances* such as internal occupancy, cooking, solar gain and wind are fairly small influences on most houses most of the time and anyway get smoothed by response time, or by the fact that low temperature heating is, to an extent, self regulating.  This is why, for many, a simple control strategy based on OAT alone (or OAT with a small tweak of some type whether it be mild auto-adjustment or just limiting) will work better than anything supposedly more sophisticated.  This eliminates what is often the biggest issue, namely time to respond, by getting 'ahead of the game'. 

Its a shame we didn't adopt this control methodology for boilers in the UK, and instead have been brainwashed by the manufacturers into spending lots of money on not particularly effective controls.  The net effect of these, even with a (condensing) boiler, is often to reduce comfort and increase running costs.

*from the simple model of loss proportional to IAT-OAT

@markspencersmith 

"Thinking on this, I have to agree with a guy I saw who concluded this is tosh. Open all the lockshields fully. Smart TRV every radiator. Master thermostat in coldest room. Run the ASHP on WC and 24/7. Have the WC adjusted dynamically according to % of TRVs open. What happens?"

What happened for me is that I had cold rooms at the end  of the heating circuit when I had all the lockshields fully open running 24/7 on pure WC.

Radiator balancing even with a heat pump is not tosh, certainly not from my experience.

So is the advice still to manually balance rads and IAT limit bedroom/solargain rooms with TRV?

I think the point has a core truth that using IAT systematically to adjust WC curve rather than subjective 'feel a bit cold'. I will be deploying wifi wall stats to most rooms to monitor when my ASHP is commissioned and using that data to inform changes to WC from default.

Posted by: @markspencersmith

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I think the point has a core truth that using IAT systematically to adjust WC curve rather than subjective 'feel a bit cold'.

That 'core truth' is definitely the case.  Adjusting the WC curve is something that is done once, and once only, and needs to be done slowly, with only small adjustments (eg one degree) every 24 hrs, even less as you get close.  Its not a means of day to day control!  Expect the process to take a couple of weeks.

Once adjusted it might be tweaked with several months experience (eg you might tweak the high OAT end), but that should be it. I adjusted mine with reference to actual room temperature because of the way Vaillant arrange their controls. 

Posted by: @markspencersmith

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So is the advice still to manually balance rads and IAT limit bedroom/solargain rooms with TRV?

I would adjust bedroom temperatures down with LSVs unless you need them to be higher during the day than at night, which most people dont.  Balancing doesnt have to be for equal delat T or for equal room temperature, it can be for the desired temperature differences between rooms.  When balancing its important that at least one LSV is fully open, otherwise you are unnecessarily restricting flow.  This will often be the one on the radiator furthest from the heat pump, or maybe several towards the end.  If you need to close down all LSVs to achieve the required temperatures, your WC curve is too high!

TRVs can then be used only for limiting against eg solar gain.

Posted by: @bash

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What happened for me is that I had cold rooms at the end  of the heating circuit when I had all the lockshields fully open running 24/7 on pure WC.

That is also my sense, @bash . As silly things can be done to extensions of heating pipes over the lifetime of a building... 

There may be heating systems where rad balancing is not needed, but I would not rule it out.

You make good points like on variable non ASHP input @markspencersmith , but are you mainly concerned about designing the system rather than making one that was built over time work?

As an aside, the BG survey made me think how much heat can be delivered by a HP on a 15mm pipe. Like are 4 rads of total 9000 btu (if standard gas boiler) too much for a 15mm pipe and could the last one in line be starved? 

@batpred 

BG said they'd like just one radiator off a 15mm run, 2 at a push, any more and they'd prefer to re-pipe.

It was impossible to tell with the ones in the old part of our house, so BG asked if they could re-pipe 4 radiators, including one of the bathrooms and the room below.

Thankfully they were skilled enough to do this without having to take up the bathroom floor.

It was 3 extra man days which they charged me an additional £850 including all the materials, which I was more than happy with.

I now have all the radiators on their own 15mm runs which has made a massive difference to the flow rate, particularly to the farthest radiators.

Posted by: @bash

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@batpred 

BG said they'd like just one radiator off a 15mm run, 2 at a push, any more and they'd prefer to re-pipe.

That's what I sensed BG would be great at! 

We may end up getting a few extra pipes if we get a chance. But I seem to be on my own on excitement with this ASHP "project" so I can only hope it to change once it gets going with warm weather... And we are not fossil fuel fans... 

@batpred 

My wife was not interested at all and certainly not when she had to move out with the children for a week during the install.

Thankfully I'd kept all that to myself until after it was all booked in.

She is however an accountant, so has already seen the financial benefits of the investment,  has appreciated the much warmer house and has now come round to my thinking that it will be worth it in the long run!

Posted by: @bash

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BG said they'd like just one radiator off a 15mm run, 2 at a push, any more and they'd prefer to re-pipe.

er why?

15mm is good for about 3kW at DT5.  Larger rooms in a typical 5-8kW house will need 1kW+ each but not smaller rooms.  This is just being lazy at the expense of more disruption to the householder