Thanks @iancalderbank  - and just for my own learnings - primary side, that refer to essentially what's in the house side v the ASHP side?

One of the plumbers here atm who’s trying to follow grants suggestions of trying different speeds on ASHP. Not seeing any change.

mentioned I’ve been told there should be 2 of the Mag Ones, he just went on about they’re being a Y strainer outside (but that’s for the plastic and unrelated to what Grant specifies Mag one wise…)

he’s adamant the flow restrictor is a guage not a restrictor. But doesn’t appear to be working properly.

Plumber has suggested they look at the LLH itself to rule that out before looking elsewhere.  Basically a bit of trial and error.

Running pump at different speeds had zero affect on the temperature differential coming across the LLH

@crimson I don't know what rating the ASHP circulation pump is but there is clear maths in grant's own spec sheets that says the Mag One will be taking a huge fraction of the pump's power . so much that there is almost none left to move water. which is why the flow is slow. If the pump is a large enough spec and can be  turned up to max then you might get a bit higher flow rate. unlikely to be enough to make it work close to properly though.  But the right solution (whilst still keeping a mag filter) is to swap the Mag One for another better filter. Spirovent Spirotrap 1"  or 1 1/4" would have much lower pressure loss. The flow thing is one of these . its job is to slow flow down in a complex heat network where flow needs to be balanced.  It may have a low pressure loss (I can't be bothered to check its spec) but its won't be zero. but you DO NOT NEED IT IN YOUR SYSTEM. simples, as the meerkats say. its on the primary side, there will be a flow gauge in the heat pump itself somewhere, thats what the system actually uses.

a Y strainer is probably unnecessary if you have a good mag filter as well. but would need to see a diagram of where it is. who knows, that could be undersized, and/or perhaps full of cr*p as well.

obviously it'd be better just to rip the whole thing out, including the LLH, and start again but I am trying to give you simple pragmatic things that can be done without total rip out. Changing the mag filter for a better one and removing the tacosetter is an hours work for a competent plumber, and has a decent chance of alleviating a large amount of your performance problem Perhaps a tad more including any drain / refill, but there may be isolation valves that allow that to be minimised I can't tell.

I spent months researching all this before I built my own system (with my own hands). including studying all of @heacol 's published works and system build principles. 

I know , for certain, that what I have said above is a definitive cause of the slow flow. there might be other causes of slow flow that I can't diagnose from x00 miles away and via an internet forum, but that mag filter needs to go. For CERTAIN. 

there isn't anything more I can do until you get this changed, as the dragon's say, I'm out (temporarily at least).

Thanks @iancalderbank  - I'll keep pressing them.  They seem very much blind to it being a problem.

I'll press them to swap to another one, and report back.

I'm not ignoring what you're saying btw, just reporting back what these (not very useful) plumbers are saying.  It's rather frustrating as it seems they have a way of doing it, and can't get out of it, if that makes sense.

I'll press they swap that and remove the balancing valve.

Forgot to add - the pipework is setup so they can easily isolate before the LLH.  I asked how long would it take to replace that and he said not long.  So there's no real time constraint for them to give these things ago.

Thanks again for everyone's help on this - apologies if my posts sometimes just brusque, it's always between working and jumping around dealing with these guys.

Hopefully it will all be of use to someone else who gets in this position.

Posted by: @crimson

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Forgot to add - the pipework is setup so they can easily isolate before the LLH.  I asked how long would it take to replace that and he said not long.  So there's no real time constraint for them to give these things ago.

did they check the flow rate on the primary side as reported on the grant controller, wherever that might be? what was that value? 

one option to prove their effect to this "trial and error" guy is this:

1. measure primary side flow rate with primary pump set to max . presumably a setting for that in the grant controller?
2. replace the tacosetter and mag one with a straight piece of pipe
3. measure primary side flow rate again. it will go up.

then go buy a better mag filter and fit it in place of the straight piece of pipe.

Thanks @iancalderbank  - I'll pass this on.
I've not seen them / or have them mention the primary flow rate (they seem to just do whatever Grant suggests).  I kept saying from what I'm being told on forums, this is a flow issue - and the plumber said he'd expect an error on the ASHP to indicate that.

They're now apparently coming Friday if they get a replacement LLH in time, to replace the LLH.  I'll press they try what you've suggested also.

Thanks again for your time on this

As was writing this he's also just confirmed to me they'll look at Mag one removal as well - so looks like some movement on taking suggestions

@crimson personally I doubt there is anything wrong with the LLH as a device(as a system design, yes, we discussed that to death). Its a metal cylinder or box with 4 holes on the sides for pipe entry. not a lot can be actually wrong with something that simple as a device. But whatever...

Do you have access to the grant controller? with guidance from someone on here who knows grant (please feel free to come in here people who know!) can you check what the primary flow rate actually is?

Yeah I think plumber's blinkers are on.  I'm glad finally some movement into giving what you're saying a try though.  If he's here Friday (suspect Monday tbh), replaces LLH, sees no difference.  Then removes both the Mag one and tacosetter and replaces with a pipe, sees a difference.  It might make it finally twig…

They seem to think perhaps a hole isn't bored correctly in the LLH, no idea how relevant that could be.

I just got excited going into plant room seeing a temp top left of 42C, then quickly realised the hot water cycle is running.  Fact there's still a differential between top right and top left when it's hot water seemed to throw the plumber too.  From a learning perspective, why would the water going out be hotter than going into the hot water system?

…those probes could have pound values wasted at the moment…

Posted by: @crimson

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They seem to think perhaps a hole isn't bored correctly in the LLH, no idea how relevant that could be.

possible. maybe. but doesn't remove the fact that the mag one is a socking great anchor on the system.

Posted by: @crimson

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 From a learning perspective, why would the water going out be hotter than going into the hot water system? 

This is normal if the hot water tank is already quite hot at the point of the coil.

your hot water tank is hotter than 42 at the point of the coil entry. Its at about 45. so its warming up the water that the HP is sending it. As the HP revs up, the top right temp should go up and the temps should go the right way round. you're actually cooling the tank with the HP at the moment. wasting money. the way to avoid it is to let the tank go a lot cooler before you reheat.

@iancalderbank thanks very much for this.

this is the 5-6pm cycle which I’ve been considering just dropping completely leaving us with just two cycles running. For some reason recco from Grant is 4. Will give it a go and see if any lack of hot water on our usage patterns