@editor  Mars, In relation to this customers issue, I would be interested if the next time you do a podcast covering issues simular you could ask the experts what they think is best S plan or Y plan, in my view I only ever do Y plan, S plan is designed for zoning and in my mind the last thing you want to do is zone anything: Low, slow and even temperature delivery across the whole building every time every design, no zoning, no buffers, no second pumps (unless its a pre-plumbed by Mitsubishi and they inverter control all the pumps across their low loss header so no stratification)  

Posted by: @ashp-bobba

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@editor  Mars, In relation to this customers issue, I would be interested if the next time you do a podcast covering issues simular you could ask the experts what they think is best S plan or Y plan, in my view I only ever do Y plan, S plan is designed for zoning and in my mind the last thing you want to do is zone anything: Low, slow and even temperature delivery across the whole building every time every design, no zoning, no buffers, no second pumps (unless its a pre-plumbed by Mitsubishi and they inverter control all the pumps across their low loss header so no stratification)  

If you dont mind can I ask the reasoning - only about the S vs Y bit, the rest of what you say I fully understand. 

I should preface this by saying Im neither a plumber nor a heat pump installer, just a homeowner who was lucky enough to get a good install, in no short measure due to what I have learned on this forum (which helped me to distinguish good installers and specify that I wanted exactly what you have described above).

My understanding (please correct me if Im wrong) is that S plan and Y plan are essentially similar but S plan uses 2 off 2 way valves whereas Y plan uses one diverter valve. 

If this is correct then surely the S plan valves can be wired to be in opposition, thus effectively making it Y plan.  Not better, but not materially worse  either.  Is there anything more subtle than this.  S plan obviously has one more component and slightly more complex wiring, and therefore the potential to get it wrong.  However 2 way valves are, at least when bought retail, cheaper than diverter valves.

Im just trying to make sure that I understand fully, not questioning what you have said.

Also can you expand on your Mitsubishi comment.  I have known for some while that their pre-plumbed cylinder has a LLH, and often wondered why (and why anyone would use their pre plumbed cylinder in any reasonable domestic situation).  It sounds like you may be saying that, if you let the heat pump control both water pumps, it sorts out the flow rates so its automatically 'balanced' and thus delivers max efficiency.  But if it does this what's the advantage of a LLH, why not just direct plumb?

@jamespa Hi James, its only to find out what the other experts think, for me, my reason for only Y plan is because the wiring is very straight forward and the only reason to use the S plan is to create a Y plan with more complex wiring?

Imagine a designer or lets say skilled commissioning engineer that understands the reason for wiring through the aux switch of the other 2 port on an S Plan and can wire / commission the system to work which on an ASHP will be more like a Y plan. They may not be the engineer that takes care of the system or services it, to keep it simple, spring return Y plan DHW priority valve - power on for heating only (easy for any service engineer) "please note this is just my preference" but my preference on design seems to wield very efficient systems with very little resistance in the circuits and simple set ups.

I also think its me following the ASHP mantra of keep it low slow and steady "I just added and simple on design" simple works efficiently. I do admit though I use ESBE valves on my higher capacity installs to get better results on the resistance.    

@cbrenewable Hi CB, sorry me a James took over your posts, its all relevant though. I also wanted to point out and not overlook the pipework network next to the cylinder. I didn't ask about the pipework and 2 port sizes. All of this should be in 28mm or 1" connections no reducers. you have a tightly packed set of circuits there and you want that resistance as low as possible. 

The system will not want to shove 24/Lpm through a 3/4 or 22m 2port valve. but it important its not just the valve that is 28mm or 1" I would design everything in 28mm in the plant room right until the heating circuit near the ceiling as it disappears and further into the house if my my design index circuit needed it.

I hope all this helps a little.

@ashp-bobba and @jamespa - thank you both for all your help with this, it's much appreciated. I fully understand it's not easy to see what's going on from the other side of a screen (especially when half the pipework is nicely hidden!) but everything you've said makes sense to me so we'll see what happens next.

To answer @ashp-bobba 's final question - the plant room (garage) is plumbed entirely in 28mm copper. It only ends up in 22mm plastic pushfit once it leaves the garage and heads into the existing house.

The engineer is coming back on Wednesday. Fingers crossed he's able to make some headway. I have a slight reservation that while the engineer coming out on Wednesday is the one who installed the system he's not the one who designed it so may not be able to answer questions around its design (eg: why are there 2 pumps etc).

I'll let everyone know how it goes in the aftermath - likely once I have a day or two of data to show whether or not anything's changed. It feels odd wishing for bad weather but I hope the cold snap stays around to let me test the system out for a few days afterwards!

Have a good BH weekend all