@athlonoc is your heat pump delta T set to match the delta T of your overall emitter circuit or is it still to 5C?

The reason I ask is if your emitters are dropping the flow temperature down by an average of 7C your heat pump may only be raising it again by 5C.

I have 13 emitters 7 of which are old T60s. They work best with a temperature drop (delta T) of approximately 10C and that is what I have balanced them to. Therefore the overall temperature difference at the heat pump is approximately 8C to 10C and so my HP is set to 8C. The HP now replenishes the heat power radiated by the emitters otherwise my flow temp ran cooler than required. The overall system now runs more efficiently not just the HP which is just a component in the system.

Another possible reason is water like electricity always follows the path of least resistance. Although your WC pipe run may be shorter the resistance to your lounge maybe lower so could be starving your WC pipe run?

It could also be a combination of the two

Just my thoughts

Posted by: @athlonoc

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Anyone have any thoughts of why a much shorter run would take twice as long and both have a similar Delta T of 8. The period of time in which these were taken today for this thread is just over 3 hours.

Spacing/total length of pipework in relation to loss of room is the obvious one.  If they small room has the same border around the edges as the large room it will have proportionately less output.

Does it matter, heat pump systems are usually best run open loop 24x7 for max efficiency, minimum cost, and maximum comfort.  Is there a reason not to do this (eg a ToU tarrif)

DT of 8 is quite high, suggesting either insufficient flow rate or that the slab has not yet properly reached equilibrium.  Does the heat pump reach the set FT?  When you tried to achieve DT5 what limit were you coming up against, was the heat pump turning down the flow rate to increase the DT, or was it not reaching flow temperature.  Or did you not leave it long enough for the slab to warm up?  

@jamespa 

Spacing/total length of pipework in relation to loss of room is the obvious one.  If they small room has the same border around the edges as the large room it will have proportionately less output.

The spacing layout seems identical, but I'm not sure what you mean by same border.

Does it matter, heat pump systems are usually best run open loop 24x7 for max efficiency, minimum cost, and maximum comfort.  Is there a reason not to do this (eg a ToU tarrif)

I only wish we could. The system runs to noisy through the pipework. I've tackled this with lowering the flow rate as it's set at 21l/m but when I drop it down to say 12 or 14 it does get quieter but I had so many L9 flow rate errors, I gave up. So I run it during the day when we are awake which is not ideal. Room temp drops are no more than 2 degrees so I have learnt to live with it.

DT of 8 is quite high, suggesting either insufficient flow rate or that the slab has not yet properly reached equilibrium.  Does the heat pump reach the set FT?  When you tried to achieve DT5 what limit were you coming up against, was the heat pump turning down the flow rate to increase the DT, or was it not reaching flow temperature.  Or did you not leave it long enough for the slab to warm up?

All I can say is that I left the whole system on for up to 6 hours or more. As mentioned earlier my flow rate is 21l/m. The upstairs radiators heated upstairs fine. All rooms downstairs with UFH were ok but the WC just took ages. It has the shortest run and is closest to the manifold bar the hallway.

@technogeek 

Thank you for your reply. Not sure what is meant by emitters? 

@athlonoc it is a generic name given to all things that give off heat basically. This can include radiators under floor heating etc 😀

Posted by: @athlonoc

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Spacing/total length of pipework in relation to loss of room is the obvious one.  If they small room has the same border around the edges as the large room it will have proportionately less output.

The spacing layout seems identical, but I'm not sure what you mean by same border.

By border I mean area around the perimeter where there are probably no pipes.

Posted by: @athlonoc

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Does it matter, heat pump systems are usually best run open loop 24x7 for max efficiency, minimum cost, and maximum comfort.  Is there a reason not to do this (eg a ToU tarrif)

I only wish we could. The system runs to noisy through the pipework. I've tackled this with lowering the flow rate as it's set at 21l/m but when I drop it down to say 12 or 14 it does get quieter but I had so many L9 flow rate errors, I gave up. So I run it during the day when we are awake which is not ideal. Room temp drops are no more than 2 degrees so I have learnt to live with it.

OK, thats a shame.  Not much I can say to that!

Posted by: @athlonoc

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All I can say is that I left the whole system on for up to 6 hours or more. As mentioned earlier my flow rate is 21l/m. The upstairs radiators heated upstairs fine. All rooms downstairs with UFH were ok but the WC just took ages. It has the shortest run and is closest to the manifold bar the hallway.

Is it possible the flow rate in this circuit is constrained at the manifold.  There is a flow rate adjustment valve.  The fact you are getting the same DT on a short loop as on a long one suggests it may be, although it also could be that the room is higher loss.  

In the end its either that or something to do with the thermal characteristics such as total pipe length/area, or something about the room itself that makes it higher loss per unit area than the rest of the house.  Is the space under the loo unheated to avoid the possibility that the loo is screwed through the pipes, this can be a fair proportion of a room with a WC only.