The theory is that buffer tanks help "stabilize" the heat pump's demands and reduce cycling. Most experienced installers in the UK no longer use buffer tanks because they reduce efficiency. We have a buffer tank, and we run on weather compensation. I would definitely move away from the set point/fixed temperature because that will drive your costs up unnecessarily, especially if your set point is over 40°C.

Thanks Mars. I am convinced that my installer's experience only stretched as far as fixed temperature and it did cost us a lot, but not as much as gas would have cost at the peak.

I do think that on weather compensation, the buffer tank must slow down the reaction of the system. However, I suppose that is a small factor compared with optimising the weather compensation curve and I don't pretend that I have got that right yet!

Even if I had, I still have to work out how to balance the secondary circulation pump with the Ecodan pump

Posted by: @davidalgarve

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Thanks Mars. I am convinced that my installer's experience only stretched as far as fixed temperature and it did cost us a lot, but not as much as gas would have cost at the peak.

I do think that on weather compensation, the buffer tank must slow down the reaction of the system. However, I suppose that is a small factor compared with optimising the weather compensation curve and I don't pretend that I have got that right yet!

Even if I had, I still have to work out how to balance the secondary circulation pump with the Ecodan pump

The present opinion seems to be that buffer tanks should be avoided if at all possible, because of the possibility of mixing causing the heat pump to have to supply at a higher Leaving Water Temperature (LWT), thereby reducing its efficiency.

I think that the Ecodan has 5 selectable speeds for the water pump, so to balance the flow through the buffer tank it would be necessary to also change the speed of the secondary circuit water pump if the ecodan water pump speed is changed.

Ideally the flow of water from the heat pump into the buffer tank should be slightly higher than the water flowrate out of the buffer tank to the heat emitters. There should be no throttling of the flow by means of TRV's or solenoid valves, except possibly in bedrooms.

Measure the temperature at the pipework going into and out of the buffer tank, this should give an indication of how well the flowrates are balanced. If the LWT going into the buffer tank is higher than the outlet to the heat emitters, then mixing is taking place and the primary pump speed should be increased or the secondary pump speed should be reduced. Allow time for temperatures to stabilise when taking readings.

Thanks for your response. I have done nothing on this recently, firstly due to Christmas visitors and secondly because I have not been able to find a way of changing the speed of the Grundfos UPM2 25-70 secondary pump. This is supposedly a PWM variable speed pump, but there is nothing connected to what I believe are the PWM terminals.

The ASHP pump is set on 5 but I cannot yet determine or vary the secondary pump speed.

However, that aside and looking at your penultimate sentence, should it perhaps read:

"If the LWT from the buffer tank is higher than the outlet to the heat emitters, then... etc" 

Posted by: @davidalgarve

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Thanks for your response. I have done nothing on this recently, firstly due to Christmas visitors and secondly because I have not been able to find a way of changing the speed of the Grundfos UPM2 25-70 secondary pump. This is supposedly a PWM variable speed pump, but there is nothing connected to what I believe are the PWM terminals.

The ASHP pump is set on 5 but I cannot yet determine or vary the secondary pump speed.

However, that aside and looking at your penultimate sentence, should it perhaps read:

"If the LWT from the buffer tank is higher than the outlet to the heat emitters, then... etc" 

I believe that a PWM capable water pump will default to maximum speed if the PWM signal is not present. You therefore probably have three choices, obtain a PWM signal cable and arrange correct control of the secondary pump, replace the pump with a none PWM version that allows manual control of the pump speed, or bypass the buffer tank and remove the secondary pump, thereby only using the primary water pump. Obviously you would need to confirm that the primary water pump is of sufficient capacity to supply the required flowrate.

Thanks Derek.

We are having a rather mild spell (14C ambient) so I am not sure how meaningful this will be, but temperatures measured earlier today were:

Buffer tank Heat pump Flow 35C & return 33C

Central heating flow 31c return 28C

Most rooms are around 20C

Posted by: @davidalgarve

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Thanks Derek.

We are having a rather mild spell (14C ambient) so I am not sure how meaningful this will be, but temperatures measured earlier today were:

Buffer tank Heat pump Flow 35C & return 33C

Central heating flow 31c return 28C

Most rooms are around 20C

This is a good example of the 'buffer tank effect', in that your heat pump is warming the water to 35C, when without the buffer tank it would only need to produce warm water at 31C. It is therefore probably that the heat pump is operating at lower efficiency.

You could try increasing the speed of the primary water pump to try to balance the flowrates. With a buffer tank the primary circuit flowrate needs to be slightly higher than that of the secondary circuit.

Posted by: @davidalgarve

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Buffer tank Heat pump Flow 35C & return 33C

Central heating flow 31c return 28C

Unless your system volume is very low it's likely, given those those figures, that your buffer tank is costing you 10%+ in efficiency or thereabouts, and absence of WC a further 10%  It would probably be better to omit the buffer tank (and the secondary pump) altogether or replumb it without the secondary pump as a 2 port volumiser in the return.  However without further details it's impossible to be certain.

Thank You to both Derek M and JamesPA.

To clear up a couple of points made. 

1-The primary pump is already set to max speed (5), so I have to look to reducing the secondary circulation pump speed. I haven't found a way, yet, of doing so with the installed Grundfos UPM2 and may have to consider replacing the pump.

2- I am using weather compensation, but as mentioned, the temperatures given were whilst we are having a rather mild spell.

Turning to James's points, I don't feel that I could commit to the extensive changes suggested and it would no be possible to find a contractor here in the Algarve to do the work, so I have to do the best I can, with what I have.

May I ask James two questions, i.e.

In your view, am I wasting my time in trying to find a way to vary (and potentially reduce) the secondary pump speed?

Assuming I could vary the speed of the secondary pump, what flow and return  temperature differentials at the buffer tank might be reasonable? (I would hope to test with colder ambients than the 14C today) 

Happy New Year to All  

Posted by: @davidalgarve

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May I ask James two questions, i.e.

In your view, am I wasting my time in trying to find a way to vary (and potentially reduce) the secondary pump speed?

Assuming I could vary the speed of the secondary pump, what flow and return  temperature differentials at the buffer tank might be reasonable? (I would hope to test with colder ambients than the 14C today) 

Before answering the question I think its worth rehearsing the theory of how buffer tanks are supposed to work.

Firstly as @derek-m says any temperature drop between the primary and secondary flow will reduce efficiency.  Very, very roughly the reduction is 2-3% per degree C  which is why Im saying a 4C drop will cost 10% or maybe a bit more.  You can check the COP curves for your machine at the actual flow temperatures to get a more precise value.

The flow-flow temperature drop is caused by mixing between flow and return in the buffer tank.  In a sufficiently well designed, sufficiently tall, sufficiently large and sufficiently well controlled buffer tank there is negligible mixing, in the worst case of one which is poorly designed/set up the mixing will cause a flow-flow temperature drop equal to the temperature drop across the emitters (which is close to the situation you have).  It definitely is possible for a buffer tank to operate correctly, people have posted actual measurements, but its probably more likely that it doesn't unless the installer really knows what they are doing!

This desired situation is achieved because hot water enters ands leaves at the top and cold water enters and leaves at the bottom at, on average, exactly the same rate.  A temperature gradient establishes top to bottom and, while it may gently move up and down the tank, it never reaches the bottom or the top because both secondary and primary pumps are either a)operating at exactly the same speed (which of course wont in practice happen) or b) modulated on and off by the heat pump controller (or another mechanism) so that the average flow rate is the same and the difference in instantaneous flow rate never causes the tank either to be dominated by secondary water or dominated by primary water. 

Additional to the above if, as @derekm says, the secondary runs a bit faster than the primary, then its the secondary which turns on/off to achieve this state, thus avoiding the throttling of the heat pump output which would occur if it were the primary that is turned on/off.  However without the control/feedback loop described above, you might actually be better having the secondary operating very slightly slower than the primary, so that flow water dominates the content of the tank not return water (@derek-m do you agree or is this wrong?)

The above also depends on the buffer tank being tall enough that the thermal gradient can establish (horizontal buffer tanks don't work properly for this reason), the inlets/outlets being sufficiently well designed internally that there is little internal turbulence, and of course the heat pump being correctly connected to the pumps and thermocouples in the buffer tank that it can control both correctly (In principle a pair of thermostats connected to the call for heat of the heat pump could also control the pump on/off, but I think that this is a bit difficult with WC also operating - others may correct me though).

The ecodan controller expressly supports buffer tanks (I dont have the connection diagram to hand nor do I know exactly how it controls and to what extent it achieves the ideal situation described above).  I think it also needs to be set up on the FTC unit in installer mode.  From the sounds of it, I doubt your installer has bothered to implement the control properly.  This is, in my view, unforgivable - if you are going to impose a (very likely superfluous) buffer tank at least have the decency to ensure that its properly connected and controlled!

So the answer to your question is possibly not, because without the feedback loop through the Mitsubishi FTC to ensure that the thermal gradient never reaches the top/bottom of the tank, you will still get mixing unless the pumps are perfectly matched, which is unlikely to occur.  However you may find that tweaking it does improve things a bit, or even quite a lot, so I would give it a try if its easy to do.  Any reduction in the flow-flow temperature drop is good, if you could get it to within say 1C Id celebrate and have a good cup of tea!    

Hope that helps, others may be able to add some refinements/simplifications.

(BTW the establishment of a temperature gradient also happens in DHW tanks which is why you can get close running all the hot water out before it starts to run cold.) 

@jamespa

I would agree with your statement James.

If the water from the heat pump is flowing into the buffer tank at a slightly higher flowrate than the  water flowing out of the buffer tank to the heat emitters, then there should be little chance of mixing in the upper section of the buffer tank. The water flowing in and out should therefore be at approximately the same temperature.