Good summary.

Some comments: they will probably dispute that the buffer tank is too small and, even if they replaced it with a larger one, unless its controlled properly it will still mix.  What you can ask them to avoid this argument is

• how is the inevitable mixing (also called 'distortion') in the buffer accounted for in the design
• have they checked that it works as they have designed it ie the temperature drop across the buffer is what they designed for

then await a blank stare or some BS! 

Yes I would ask about the control.  They probably wont understand 'controlling the buffer', perhaps the  simplest way to put it is 'does the call for heat from the UFH also connect to the ASHP as a call for heat, or does the ashp charge the buffer independently? 

An eyeball on the wiring or some experiments with the thermostats could ferret this out and Im not sure I would trust what an installer says anyway without checking it myself!

Posted by: @pash44pump

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(1) I believe the presence of a large container of glycol suggests that there are no antifreeze valves. I assume this is an installer decision that might be sub-optimal but something I have to live with.

Yes and yes unless they are prepared to swap it out or a/f valves. 

That said this may in fact be a design fault, the UFH was designed presumably to work with water which is both less viscous and has a higher heat capacity.  If the UFH is marginal (which it seems it likely is) then this could be contributing to the deficit.  You could ask them how they have accounted, in the design, for the fact that water has been replaced by glycol in the UFH given the increased viscosity and reduced specific heat capacity of the latter.  Again await a blank stare or some BS!

Posted by: @pash44pump

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(2) Are there any leaks? Yes there is a small amount of leakage from the pipework in the following photo. What actually are those valves? How big an issue is this? I would raise this with the installer as another thing that needs resolving.

I think its just a tee'd flow meter.  Very useful!  Small leak not a big issue but best to fix it.

Posted by: @pash44pump

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You won't be surprised to know that the worst performance in the house are bedroom 1 and its en-suite

Do you happen to know where the pipe run is, any possibility its poorly insulated.

Posted by: @pash44pump

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Some posters have mentioned room thermostats as being negative but in a large house, how else do you control the temperature of zones that you don't want to be the same.

they are negative and should generally be set a couple of degrees above the target otherwise you may well not reach the target because in at least some circumstances it will switch off before the fabric has heated properly.  Furthermore and the on/off switching contributes to system inefficiency and risks crippling defrosts if there is insufficient system volume in loop when defrost takes place (that said the buffer tank should deal with this)

In answer to 'how do you control', what you do is balance the loops, much like balancing radiators, so that when run 24*7 they emit the right amount of heat.  There will be valves on the manifolds that allow you to do this.  In fact poor balancing may well be part of the problem.  How many water pumps are there in the system?

I cant currently think of any other design faults but others may be able to comment and I would reserve your position until you have answers to the above, as those answers may reveal other faults.

Hope that helps

@jamespa thanks so much. Very useful

@pash44pump - @jamespa has already come up with some useful comments. At this stage all I would add is:

(a) yes, the analogue flow meter is very useful. It is reading about 17L/min in the photo (reading it the way i read them). The two tee valves: one can used for filling/topping up the system (at least that's what my installer did/does, can't remember which one of the two it is). Fixing the leaks may not be that simple a job as the system is full of fluid but worth attempting.

(b) the floor plans: were they in the scope document? It opened rather oddly here (Libre Office), with some blank pages. If so, there's the beginnings of a heat loss survey there (floor areas and maybe some window information) but it is certainly not the Full Monty.

Let's see what the installers come back with over the full heat loss calculation question.  

@cathoderay floor plans were in the scope document that I posted 👍

@pash44pump Since you will inevitably have to deal with controlling the pump, here is my advice.

Heat pumps produce hot water at a set temperature, which circulates around the house. The temperature of the water in the various parts of the house, varies, but comes from the one and only heat pump. So, if you want to properly heat your "coldest" room, you should control your heat pump, according to the temperature of that room, i.e the water temperature leaving the pump should be high enough for your "coldest" room.

So, how can you achieve this ?

The Midea pump is controlled by its wired controller, which houses the weather compensation curve (which is straight line, but ignore this) and its internal thermostat. These two should be operated in unison. The weather curve should tell the pump the outlet water temperature corresponding to a set of outside air temperatures. So at below zero outside temperatures, I should indicate maximum power (i.e. 60-65 C in your case). The internal thermostat should be at the required room temperature in the "coldest" room, say 21 C. This implies that you should position the wired controller in the "coldest" room.

In order for the above setup to work, you have to enable two settings:

1."Room thermostat" setting should be set to NO. This disables any existing thermostats from the previous installation, other than the internal one.

2. In "Temp. type setting", Room temp. setting should be Yes. This will enable the "coldest" room temperature sensor to stop the pump, in case the water is too hot for the instant climate conditions.

I hope I have not confused you more than needed.