@johnmo  Agreed on size of buffer tank. They also say that the flow of secondary pump must be higher than primary, therefore they are proposing a kind of thermal store which, intuitively, would need to have some volume. Most manufacturers propose small 4 port buffers or LLHs in their system diagrams. This is the first I have seen specifically mandating a 2 port buffer. There could be some mileage in this in some circumstances. There could be some dilution of output flow temperature theoretically but buffer size (stratification) and minimising reverse bypass flow will mitigate this.

Posted by: @heatgeek

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2 port buffer

2 port buffers by design work very differently from a 3 or 4 port. Heat pump is controlled to only work when the buffer asks for heat, not the house. The heat pumps job therefore becomes keeping the buffer satisfied.

So if you have a house call for heat, it draws energy from the buffer. As the buffer temperature falls, the ASHP is asked to start. If the heating system demand matches ASHP output, the buffer is bypassed either fully or partially, if there is a mismatch in flow/energy the excess flow/energy travels through the buffer. When buffer is hot enough heat pump is cycled off.

@johnmo  Agree entirely.

The other thing I forget to mention, just design out the buffer, save on the cost of a 200L+ buffer, which isn't cheap, plus the additional pump, various isolation valves etc, soon adds up. It's alright the manufacturer adding these lines of text, but the end user picks up the bill. Could quite easily add another £1k to the install bill, so even after the grant the end user just has to pay another £1k, for what? More cupboard space lost.

Just had a look - A Viessmann 6kW heat pump has a retail price of £11k, although Viessmann direct will sell you one for £6k and you still a cylinder, so that takes you to £7800 (retail £14k). So all of the grant has gone already, if installer sells to you for suggested retail price you already have a £7k bill, without install and heating system mods.

Just cannot see the justification in cost even at the reduced price. Swapping out my £1300 heat pump would not save me much, no matter how much better CoP I got.

Think the manufacturer have lost the plot on pricing, or the consumer and tax payer is being taken for a ride, take your choice. Poor consumer who doesn't know or have options, they just have to get a new mortgage. Or it's as some of the press say heat pumps - a rich persons game

Posted by: @Anonymous

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Posted by: @heatgeek

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They state that a buffer tank can "significantly" improve the operation of their heat pump.

Yes it would prolong run times as they say to use a minimum of 200L.  Great if you have space, but then you have lots of radiators so should have lots of water capacity anyway. They also say if running on radiators only.

... but a volumiser would prolong run times by the same amount without any of the downside risk in performance, difficulties with fault diagnosis and extra cost.

I'm still failing to understand why we are even considering buffers unless the use case is somehow exceptional.  Unless there is a concrete reason for system separation which cannot be designed out, why not (as @johnmo advocates), KISS?

Perhaps someone can give me a concrete reason.  It seems installers who use them cant, or at least won't come on here to defend their position.  For the avoidance of doubt my comment applies to domestic installations in 'normal sized' houses, not mansions or commercial neither of which I have thought much about.

@heatgeek Nibe also promote the two pipe buffer, I use this set up when cascading.

you would want flow rates matching best you could though. Although secondary side would probably be pressure dependant pump tbe primary would be fixed speed as it slowed on the secondary it would allow flow through the buffer. The only issue with these diagrams is they don’t take into account any separation on the pumps as they are connected via the flow. I always upsize from the tee, and take into the buffer and too the secondary pump with increased pipe size. 

@damon thank you for posting.

Would you care to tell us in what circumstances you fit a buffer tank, why, and how you set it up to avoid a cop penalty due to system distortion/blending of flow with return? 

So far professional installers who fit buffer tanks have been reluctant to explain their position on this forum, whereas those who don't have been happy to explain quite clearly why not (in a domestic situation other than a mansion).

This is a genuine question, it would be really interesting to know.

Also:

Posted by: @damon

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you would want flow rates matching best you could though. 

How do you do this with heat pumps that modulate their water pump according to energy demand (eg Samsung, Daikin I believe), or don't you fit such heat pumps?

@jamespa 

so I fit Nibe, Viessmann, Vaillant, Grant and Heliotherm. I also fitted a new Worcester the other week but that was a favour.

id use a buffer when the job would dictate.for example on a 13/16 Viessmann vitocal you don’t get much head left on the pump, so a buffer would allow separation and additional pump.

id usually go with a 3 pipe buffer on the flow, upsize the tee and entry point init the buffer.

You then set your secondary pump up to match your index circuit, generally inside the Wilo pico and set it on pressure dependant, these are generally bigger houses so fit trvs and flow will match to suit the system. Distortion on the primary side is limited due to the set up, as some points the st will differ but the Heatpump will control this.

You could use a delta t pump but again it will different to the main pump as some point in the year, as it isn’t control by the units pwm.

you also need inside your buffer correctly, generally you find issues with smaller buffers and header where low volume affects the temps more severely and also the set up, the little buffers under cylinders are useless.

i size my buffer to the minimum out required and number of starts are that point anything Above that tbe system will be pulling it 

you could also use a buffer where you have different circuits and the customer wants control over rooms. 

scop is all well and good but we should also be looking at energy usage, you could be getting a better scop but using more energy.

@damon I live in a 5 bedroom, 2 bathroom, 2 shower room house of 350m2 (about 175m2 downstairs and slightly less upstairs).  It has 35mm pipes from the boiler to both downstairs and upstairs.  Heat loss is estimated at around 15kW.

It would probably need a 16kW Vitocal.  Would the water pump be powerful enough?  How much head does it have?

For comparison, another installer has suggested a Panasonic T-Cap, which he says has a 15m head that can drive almost anything.

Posted by: @grahamf

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Heat loss is estimated at around 15kW.

That's scary. I would be looking at insulation and airtight measures first, to halves that heat loss. Before I started to worry about buffers or not etc.

With that heat loss I would also be looking at design flow temperature of about 35, otherwise you will very deep pockets in the depths of winter. 

Is you heat loss calculation correct?

The 16kw will only give you 12kw around 0° so be careful when looking to have that fitted. 

But you will only have around 15kpa left for the system in the 16kw. 

Posted by: @damon

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scop is all well and good but we should also be looking at energy usage, you could be getting a better scop but using more energy.

Well said, Sir! I have been trying to get this point across to reluctant ears. SCOP measures the efficiency of energy use during ACTIVE heat pump periods. There is an energy saving measure called INTERMITTENCY where energy is not used during inactive periods and this is not measured by SCOP. Intermittency can be introduced through efficient zone control, hysteresis, thermal mass and other measures whilst maintaining accurate room temperatures. Running open loop continuously with inevitable occasional over-heating may achieve headline overall SCOP but does not necessarily achieve overall ENERGY saving or comfort. As an example for a low energy house with UFH and grossly over-sized heat pump, a 300l independently heated (by HP) low temperature thermal store with 8C minimum DT low flow rate to thermostatically controlled rooms will achieve much better energy saving than any form of open-loop working, especially if pumping energy into the floor to raise its temperature for future release. Large solar gain can be used productively by switching off energy to the affected room on a sunny day instead of closing the curtains.