Posted by: @abernyte

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Is there also a danger in the terminology that is being used ie low loss header, buffer and volumiser

Agreed

Posted by: @abernyte

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Not all LLH's act as buffers, correctly specified and installed they operate perfectly correctly as a hydraulic separator. It very much depends on where on the LLH the ports are positioned. 

Surely an LLH suffers from mixing of flow and return to the same extent, probably more because its smaller, than a buffer does, and like a buffer needs to be set up properly to function without degrading efficiency (which seems to be where many installations go wrong).

Can you explain why 'hydraulic separation' is needed in a typical domestic central heating system.  I totally understand why extra volume may be needed, but not hydraulic separation.  if you are doing something complex with a back up boiler or a rather large system then maybe, but in a typical house what value does it add?

Posted by: @derek-m

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Posted by: @abernyte

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Is there also a danger in the terminology that is being used ie low loss header, buffer and volumiser. Not all LLH's act as buffers, correctly specified and installed they operate perfectly correctly as a hydraulic separator. It very much depends on where on the LLH the ports are positioned. 

Are we in danger of vilifying an entire class of products?

As far as I am aware there would be no problem with a LLH or buffer tank, if the primary and secondary flowrates can be balanced to ensure that no mixing of the flow and return takes place. Obviously balancing the flowrates becomes problematic if the heat pump controller is able to vary the primary flowrate, without the necessary adjustment also being performed on the secondary, thereby unbalancing the system.

Having an additional water pump that may not be necessary is also a possible waste of electrical energy.

 

True, but it appears they are frequently not balanced.

My questions to @abernyte (or anyone else) is:

In a domestic installation of a 'normal' scale (say up to 6 bedrooms/300 sq m) with either radiators or UFH but not both

• what value does a LLH add?
• what value does a buffer add that a volumiser doesn't add?

The application of Occams razor is generally a good principle so until we identify the value add then there is no argument for their presence.

The default setup on a Mitisubishi Ecodan solar thermal ported pre plumbed cylinder has a LLH.  My installer, who was one of the highly experienced "better" class of installer when queried regarding this said that the LLH in question has the flow injection plume above the return fitting outlet (not opposite as some seem to have) and as such would operate as a thermal separator to allow the primary and secondary pumps to operate efficiently at different variable speeds.

Since installation the system operates with a flow temp rarely above 35C and has been a resounding success as a heating system. Not a scientific recommendation I admit but I see no reason to doubt his explanation ...so far!  

Posted by: @abernyte

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The default setup on a Mitisubishi Ecodan solar thermal ported pre plumbed cylinder has a LLH.  My installer, who was one of the highly experienced "better" class of installer when queried regarding this said that the LLH in question has the flow injection plume above the return fitting outlet (not opposite as some seem to have) and as such would operate as a thermal separator to allow the primary and secondary pumps to operate efficiently at different variable speeds.

Since installation the system operates with a flow temp rarely above 35C and has been a resounding success as a heating system. Not a scientific recommendation I admit but I see no reason to doubt his explanation ...so far!  

Which simply begs the question - why do I want primary and secondary pumps in the first place (which incidentally must be balanced for the LLH to work without impeding system efficiency).  For the avoidance of doubt I'm talking about a typical fairly simple domestic setup where a heat pump replaces a gas boiler (ie 95% plus of real world scenarios).  Why would I have a second pump, a low loss header/buffer etc.  None of this appears in your typical CH system.  What value do they add.

You may not know I accept, but until someone tells us then the best assumption is no value at all and, since they clearly add trouble and cost, thats a good argument to leave them out.

@abernyte I suspect that arrangement might be similar to the one in our airing cupboard. The flow is the top left pipe and the return is the lower left. The top right is the flow to the secondary pump thence to radiators and the lower right is return from the radiators? Regards, Toodles.

Yes other than Mitsubishi use a 35mm LLH

Posted by: @abernyte

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Yes other than Mitsubishi use a 35mm LLH

But why (bother with a secondary pump and llh) in a typical domestic install?

Does anyone actually know a reason that stands up to scrutiny?  I am not trying to be difficult, I genuinely want to know?  @abernyte, @editor, @derek-m any ideas or is this another appendix from heat pump history that needs to be surgically removed?

Posted by: @jamespa

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Posted by: @abernyte

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Yes other than Mitsubishi use a 35mm LLH

But why (bother with a secondary pump and llh) in a typical domestic install?

Does anyone actually know a reason that stands up to scrutiny?  I am not trying to be difficult, I genuinely want to know?  @abernyte, @editor, @derek-m any ideas or is this another appendix from heat pump history that needs to be surgically removed?

The only reason of which I can think, is that it would be to allow system designers to accommodate 'smart' thermostats and TRV's into their designs, without suffering 'low flow' problems.

Posted by: @derek-m

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Posted by: @jamespa

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Posted by: @abernyte

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Yes other than Mitsubishi use a 35mm LLH

But why (bother with a secondary pump and llh) in a typical domestic install?

Does anyone actually know a reason that stands up to scrutiny?  I am not trying to be difficult, I genuinely want to know?  @abernyte, @editor, @derek-m any ideas or is this another appendix from heat pump history that needs to be surgically removed?

The only reason of which I can think, is that it would be to allow system designers to accommodate 'smart' thermostats and TRV's into their designs, without suffering 'low flow' problems.

 

Wow that's inventive but I'm sure you agree that it can be discarded as a legitimate reason.

Perhaps we ought to ask why should they be included rather than trying to argue that they should be omitted.  No system designer worth their salt includes components without a reason.

Posted by: @jamespa

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Posted by: @derek-m

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Posted by: @jamespa

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Posted by: @abernyte

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Yes other than Mitsubishi use a 35mm LLH

But why (bother with a secondary pump and llh) in a typical domestic install?

Does anyone actually know a reason that stands up to scrutiny?  I am not trying to be difficult, I genuinely want to know?  @abernyte, @editor, @derek-m any ideas or is this another appendix from heat pump history that needs to be surgically removed?

The only reason of which I can think, is that it would be to allow system designers to accommodate 'smart' thermostats and TRV's into their designs, without suffering 'low flow' problems.

 

Wow that's inventive but I'm sure you agree that it can be discarded as a legitimate reason.

Perhaps we ought to ask why should they be included rather than trying to argue that they should be omitted.  No system designer worth their salt includes components without a reason.

 

Which is why I have spent the past three years trying to convince forum members that they should have their poorly designed systems corrected to make them operate more efficiently.

Posted by: @derek-m

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The only reason of which I can think, is that it would be to allow system designers to accommodate 'smart' thermostats and TRV's into their designs, without suffering 'low flow' problems

I think you are both correct, its because systems aren't designed, they are plumbed in by people who don't understand system design but attend a manufacturer's installation course.  So the additions of these components are their to mitigate any system design and it will function in any given scenario, not optimally but it will function.

Posted by: @gary

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Posted by: @derek-m

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The only reason of which I can think, is that it would be to allow system designers to accommodate 'smart' thermostats and TRV's into their designs, without suffering 'low flow' problems

I think you are both correct, its because systems aren't designed, they are plumbed in by people who don't understand system design but attend a manufacturer's installation course.  So the additions of these components are their to mitigate any system design and it will function in any given scenario, not optimally but it will function.

I fear you are right.  However the reason suggested (which is very plausible) is the equivalent of dressing a dirty wound instead of first cleaning it up because the patient complains about the sting of the antiseptic, except that if you did the latter you would likely get struck off.