What are the design principles informing the, ' which pump and UFH design' question

[JamesPa]

3 hours ago, IanR said:

 

Thanks for clarifying. I've got the same issue taking measurements, I'd need to peel back the insulation to get to somewhere to measure, and it will need taping up to put it back so won't look the same again.

 

I've avoided going down that rabbit hole of data logging so far, but am now looking at what it will cost to buy a multi-channel temp logger and some probes.

 

 

I don't believe you can infer that, from the temps provided.

I agree.   I thought (incorrectly it turns out) that @ReedRichardswas reporting B and C (or A & D).  A & K alone don't really tell us much, as we don't know whether the temp drop is across the buffer, or across the emitters, or split between the two.  We need A/B (either) and C/D (either) to understand what the buffer is doing.

 

Don't most heat pumps have an input for a buffer temperature measurement?  That figure would help a bit.

Edited January 26, 2023 by JamesPa

[ReedRichards]

1 hour ago, JamesPa said:

Don't most heat pumps have an input for a buffer temperature measurement?  

 

Mine doesn't.  The manual states that a minimum water volume of 50 l is required and suggests alternatives of a volumiser, a low-loss header or a buffer tank (a 4-port buffer is illustrated).   if you have a 4-port buffer then the input water to the heat pump has come straight out of the buffer.  And either the water gets completely mixed in the buffer or a probe is only measuring the temperature of that part of the buffer in which it is positioned.

 

Sorry about the confusion over the temperature data I reported.

 

One thing I am thinking is that weather compensation alters the target temperature of the water leaving the heat pump.  But perhaps superior weather compensation would reduce the target temperature differential across the heat pump or alternatively reduce the pump speed as the outside temperature increases.       

[JamesPa]

9 hours ago, ReedRichards said:

Mine doesn't.  The manual states that a minimum water volume of 50 l is required and suggests alternatives of a volumiser, a low-loss header or a buffer tank (a 4-port buffer is illustrated).   if you have a 4-port buffer then the input water to the heat pump has come straight out of the buffer.  And either the water gets completely mixed in the buffer or a probe is only measuring the temperature of that part of the buffer in which it is positioned.

 

Sorry about the confusion over the temperature data I reported.

No need to apologise, thanks to @IanR for pointing out the ambiguity and making it easy to resolve with a diagram!

 

If water is completely mixed in the buffer and pipes well insulated then A-K = B-C = C-H and my conclusion still stands.  However it would be nice to know whether it is completely mixed, which could be ascertained by measuring B-C as well as A-K or C-H.

 

9 hours ago, ReedRichards said:

One thing I am thinking is that weather compensation alters the target temperature of the water leaving the heat pump.  But perhaps superior weather compensation would reduce the target temperature differential across the heat pump or alternatively reduce the pump speed as the outside temperature increases.

 

If the pump speed is constant then DeltaT across the emitters and HP will anyway reduce as the outside temperature increases, because the load on the radiators is less.  My V2 spreadsheet models this (V1 did not).  Note that this statement assumes that the HP targets a specific flow temperature not a specific deltaT (the latter would be a bit stupid)..

 

The optimum strategy (ignoring the energy used by the pump) is to minimise deltaT at the emitters at all temperatures, because that in turn increases the average temp at the emitter for any given HP flow temp and this minimises the heat pump flow temp required to maintain a given output. 

 

Practically, however, the minimum deltaT for any given ambient and emitters is set by the maximum tolerable pump speed before the system gets too noisy. So basically the 'optimum' algorithm is to set the pump speed to the highest tolerable rate and leave it at that rate. 

 

The only way to better this would be to turn the pump speed up above the maximum tolerable rate only when its very cold, if one were prepared to tolerate it in these rare circumstances.  probably not worth the effort though.

 

 

Edited January 27, 2023 by JamesPa

[JamesPa]

9 hours ago, ReedRichards said:

Mine doesn't.  The manual states that a minimum water volume of 50 l is required and suggests alternatives of a volumiser, a low-loss header or a buffer tank (a 4-port buffer is illustrated).   if you have a 4-port buffer then the input water to the heat pump has come straight out of the buffer.  And either the water gets completely mixed in the buffer or a probe is only measuring the temperature of that part of the buffer in which it is positioned.

 

Just a thought, if your system is as illustrated by @IanR and you have only one pump you might want to run a pipe from B to C (with stop taps at B, C and in the middle of the pipe).  This would enable you to bypass the buffer on the flow and, if there is complete mixing in the buffer as you suppose, reduce your WC curve by ~4C gaining perhaps 8-15% system efficiency.  If it doesn't work open the taps at B and C and close the mid pipe tap.  A summer job i would suggest.

[IanR]

15 minutes ago, JamesPa said:

Just a thought, if your system is as illustrated by @IanR and you have only one pump you might want to run a pipe from B to C 

 

I had omitted pumps and valves in my sketch, to focus on the measurement points. A 4 port buffer requires a second pump on the circuit to the emitters, that some non-buffer installations can avoid.

 

23 minutes ago, JamesPa said:

if there is complete mixing in the buffer as you suppose,

 

I can't imagine why a 4 port buffer would be designed to mix. The internals of mine are designed very much to not mix.

 

12 hours ago, JamesPa said:

Don't most heat pumps have an input for a buffer temperature measurement?  That figure would help a bit.

 

My controller and buffer allows for three buffer temps to be used, top, centre and bottom, but I'm not using that option on my setup. The fact that there are 3 pockets within the buffer for temperature measurement suggests the manufacture believes there will be stratification within their buffer.

[JamesPa]

5 hours ago, IanR said:

 

6 hours ago, JamesPa said:

if there is complete mixing in the buffer as you suppose,

 

I can't imagine why a 4 port buffer would be designed to mix. The internals of mine are designed very much to not mix.

 

18 hours ago, JamesPa said:

Don't most heat pumps have an input for a buffer temperature measurement?  That figure would help a bit.

 

My controller and buffer allows for three buffer temps to be used, top, centre and bottom, but I'm not using that option on my setup. The fact that there are 3 pockets within the buffer for temperature measurement suggests the manufacture believes there will be stratification within their buffer.

I agree with all of this which is why it would be good if we had some actual measurements of temp drop across a buffer tank so we could see how well design translates into practice.  Others have suggested that there is no or almost no stratification and some facts would help. 

 

Of course I'm not suggesting you should make the measurements, only that it would be good if someone did (for the avoidance of doubt I don't have a buffer tank so cant make a measurement).

Edited January 27, 2023 by JamesPa

[JohnMo]

5 hours ago, IanR said:

can't imagine why a 4 port buffer would be designed to mix.

They probably aren't designed to mix when heat pump is delivering XX l/min and system is absorbing the same XX l/min.

 

The mixing will occur when heat pump is still delivering XX l/min, but 1 or more zones have closed and the heating system is only taking X l/min.

 

Or at any time where ASHP pump and secondary pump flow rates differ even slightly, then flow and return will take a direct flow path through the buffer as it becomes the path of least resistance.

[JamesPa]

3 hours ago, JohnMo said:

They probably aren't designed to mix when heat pump is delivering XX l/min and system is absorbing the same XX l/min.

 

The mixing will occur when heat pump is still delivering XX l/min, but 1 or more zones have closed and the heating system is only taking X l/min.

 

Or at any time where ASHP pump and secondary pump flow rates differ even slightly, then flow and return will take a direct flow path through the buffer as it becomes the path of least resistance.

Is the take away from all of this, if you don't require mixing for some identifiable reason, use only one pump and either a 2port buffer on no buffer?

[IanR]

7 hours ago, JohnMo said:

They probably aren't designed to mix when heat pump is delivering XX l/min and system is absorbing the same XX l/min.

 

I'm pretty sure they'd be designed for the parameters in which they operate and I'd imagine the primary circuit flow rate very seldom matches the heating circuit flow rate.

 

4 hours ago, JamesPa said:

Is the take away from all of this, if you don't require mixing for some identifiable reason, use only one pump and either a 2port buffer on no buffer?

 

Perhaps I've missed previous discussions, but where is the evidence that correctly sized 4 port buffers do not operate as designed?

 

Any buffer is an additional expense and will reduce the efficiency of the overall system, so if they can be avoided there's no point in installing them. If you are confident in your energy performance calcs, can leave a zone open that provides sufficient ASHP run time, and don't wish to run an MVHR wet duct heater/chiller or a fan coil heater/chiller, then there's no need for a buffer/volumiser/accumulator.

[ReedRichards]

7 hours ago, IanR said:

Perhaps I've missed previous discussions, but where is the evidence that correctly sized 4 port buffers do not operate as designed?

 

Well I believe everything here is based on the Brendon Uys study and the inference some of us have drawn from it:

 

On 24/01/2023 at 08:30, JohnMo said:

A recent test (simulation) was showing a reduction of 1 on CoP, with buffer compared to without buffer.  Both running WC, but with buffer HP required increased HP flow temps to overcome the mixing within the buffer.

 

The actual change was from a COP of 2.7 in the Test 2 configuration to 3.4 in the Test 3 configuration where the buffer was removed.  Another change was that in Test 3 the heat pump was allowed to do Load Compensation (or something akin thereto) rather than just Weather Compensation.  

 

Quote

The third...the heat pump controlled by the heat pump’s controller, maintaining a constant temperature in the thermal store (house).

 

The "missing link" in the study would have been a test where only one of the two changes made between Test 2 and Test 3 was made but that didn't happen.  

[JohnMo]

The test was completed in steady state conditions there were no transitions to different temperatures; as far as the simulator house was concerned.  So even if a load compensation was involved, it would do nothing. The graph of output temperature was very stable.

 

There has been other sources of information on buffers, well prior to the Brendon test. Just go on to heat geek , as an example, they's been saying avoid a buffer and or LLH for ages.

 

Even just having a second pump could be cost £50+ a year in running costs.

[JamesPa]

9 hours ago, IanR said:

Perhaps I've missed previous discussions, but where is the evidence that correctly sized 4 port buffers do not operate as designed?

Nowhere so far as I know.  But 4 port buffers are designed for mixing (otherwise why have 4 ports?).  They may well have baffles to encourage some stratification but, if there is mixing, water must be forced past the baffles.

 

Mixing inevitably reduces heat pump efficiency but, if you need it you need it, and thus are obliged to suffer the degradation.  If you don't need it then its easily avoided by using a 2 port buffer if system volume is the issue, or no buffer if it isn't.  Basically don't use 4 ports unless you need 4  ports.

 

Am I missing something or is it as simple as that?

 

(for simplicity I've left out discussion of the 'idronics' 3port combined buffer/bypass mentioned in another thread.  That deserves a bit more attention as it may have some interesting potential in systems where volume is marginal but mixing is not required).

[ReedRichards]

1 hour ago, JohnMo said:

The test was completed in steady state conditions there were no transitions to different temperatures; as far as the simulator house was concerned.  So even if a load compensation was involved, it would do nothing. The graph of output temperature was very stable.

 

There has been other sources of information on buffers, well prior to the Brendon test. Just go on to heat geek , as an example, they's been saying avoid a buffer and or LLH for ages.

 

Even just having a second pump could be cost £50+ a year in running costs.

 

I think Load Compensation might free the heat pump from the constraint to maintain a 5 C flow differential between flow and return and that might well be beneficial in real-world circumstances, although I'm not so sure about the test chamber; such a pity that Brendon squandered the opportunity to find out.

 

My second pump is within the heated fabric of the building and only runs when the thermostat calls for heating so the issue with the running cost would be that it is £50+ a year at a COP of 1 rather than £16.66 at a COP of 3 (or whatever).  I think it's probably running at 50 W on the medium setting.  It won't run for as much as 20 hours a day but let's say that it does because that's 1 kWh = 34p per day during the heating season.  So £50 per year is certainly of the right order of magnitude.

 

I don't need mixing but I do need (I believe) a volume of warm water to defrost the heat pump, even when the valves to my radiators are closed.  

[JamesPa]

 

22 minutes ago, ReedRichards said:

I don't need mixing but I do need (I believe) a volume of warm water to defrost the heat pump, even when the valves to my radiators are closed.  

 

It might be that one pump, a 2 port buffer in the return and a bypass valve is the optimum in this (quite common) circumstance, or the 3-port combined buffer/bypass (which I don't yet fully understand) described here:

 

https://idronics.caleffi.com/magazine/27-air-water-heat-pump-systems

 

The former, at least, would guarantee no mixing and eliminate the need for the second pump.

 

Of course there may be other reasons for the second pump in your system and we don't yet know whether any mixing actually occurs, so this is pure speculation.  But it is what I would now default to with the benefit of the wisdom on this forum, whereas 2 years ago, although I was uncomfortable with the 'always fit a 4 port buffer tank and 2 pumps' mantra, I couldn't objectively defend my discomfort). 

[IanR]

29 minutes ago, ReedRichards said:

Well I believe everything here is based on the Brendon Uys study and the inference some of us have drawn from it:

 

For me his study is far from conclusive and leaves too many questions unanswered. We don't know anything about the 4 port buffer that was used, whether it was correctly sized, or even if it was manufacturer recommended for use with the heat pump. We do know it was installed in an environment with a ΔT of 38°C, so would incur 2.5 to 3 times the standing losses of an install within the thermal envelope.

 

The test rig used was a rig built for another purpose, and missed the most important data points, the temps across the buffer. I can speculate that was because the original rig didn't have a buffer, and therefore no sensors set up for it and there was limited investment to re-hash the rig for the new test. Perhaps the buffer used was one that was "laying around" and unsuitable for the installation it was being tested within in, unfortunately we don't know, so that report is open to being questioned.

 

In order to state the major brands operating in this arena are developing, manufacturing and selling an accessory that doesn't perform as specified, better science is needed.

 

There is also the earlier study that stated "very little difference was seen in the predicted COP between the different configurations".  There's not enough detail from either study to know which is correct.

 

1 hour ago, JohnMo said:

Even just having a second pump could be cost £50+ a year in running costs.

 

Yes, a 4 port buffer will require an additional pump. Some installations will require that additional pump anyway. £50 a year, I would suggest, is at the top end of an estimate on the power used. I appreciate I'm probably towards the lower end of the range, but at today's prices and based on 90 - 100 heating days a year, running an average 10 hours a day the annual running costs for the pump is under £12.

 

For all buffers/volumisers/accumulators there are also additional standing losses, which while they will be inside the thermal envelope, are not controllable so can't be ignored. It's an additional drain on the system of a similar size to the pump.

 

Moreover, there is a capital cost to including a buffer/volumiser/accumulator, so there has to be a justified reason for doing so. [Personally, DHW re-heat times make it an easy justification, ie. specifying a heat pump size larger than the space heating requirement for reasonable DHW performance]

 

45 minutes ago, JamesPa said:

But 4 port buffers are designed for mixing (otherwise why have 4 ports?). 

 

I don't believe that's the case, they are designed for stratification, ie. no vertical mixing. Through stratification the heating circuit, including circuits to MVHR wet duct heater/chillers or fan coil units receive their flow at the ASHP flow temp (minus minor standing losses), while increasing the system volume to avoid short cycling. As the energy in the 4P Buffer is depleted (while the ASHP is off), there's no reduction in flow temp to the heating circuits until nearly all the usable energy from the buffer has been depleted. ie. the temp gradient to the heating circuit is changed from linear to something approaching exponential. The ASHP should of course have cycled back on before the heating circuits see any of that temp drop off.

 

A 4P buffer therefore maximises ASHP run time while maintaining a constant flow temp to the heating circuits, if correctly sized. This comes at the cost of possibly an additional pump, and, compared to a system with no buffer/volumiser/accumulator, some additional standing losses.

 

16 minutes ago, JamesPa said:

It might be that one pump, a 2 port buffer in the return and a bypass valve is the optimum in this (quite common) circumstance,

 

I feel they have different use cases. If increasing the volume of your heating circuit with a 2 port buffer gives sensible heat pump run times, and that's all you need, then a 4P buffer doesn't need to be considered. A 2P buffer in the return line reduces the standing losses as far as possible, so is ideal for this scenario. However, it wouldn't be of any use to a low energy home that could benefit from 1kW of distributed heat input, in an evening during the shoulder months of what would otherwise be a non-heating day.

 

[JamesPa]

2 minutes ago, IanR said:

 

1 hour ago, JamesPa said:

But 4 port buffers are designed for mixing (otherwise why have 4 ports?). 

 

I don't believe that's the case, they are designed for stratification, ie. no vertical mixing.

Sorry but  I'm missing why you would have 4 ports if you don't need mixing?  When I say that they are 'designed for mixing' what I'm really saying is 'thats part of their function'. Of course it's also called 'hydronic balancing' but doesn't that inherently imply mixing?

 

If the flow from the HP is switched off and the flow to the ch is on, then the water circulates vertically through the buffer defeating the stratification.

[JamesPa]

21 minutes ago, IanR said:

I feel they have different use cases. If increasing the volume of your heating circuit with a 2 port buffer gives sensible heat pump run times, and that's all you need, then a 4P buffer doesn't need to be considered. A 2P buffer in the return line reduces the standing losses as far as possible, so is ideal for this scenario. However, it wouldn't be of any use to a low energy home that could benefit from 1kW of distributed heat input, in an evening during the shoulder months of what would otherwise be a non-heating day.

Is the latter really the case.  Surely system volume (which determines hp run time) is system volume whether the buffer is 2, 3 or 4 ports or indeed if the volume is in the emitters.  If the HP switches off but the circulator pump keeps running, heat is still delivered to the emitters however many ports the buffer has.  Again I admit I may be missing something, what is it that I'm missing?

[IanR]

17 minutes ago, JamesPa said:

Sorry but  I'm missing why you would have 4 ports if you don't need mixing?  When I say that they are 'designed for mixing' what I'm really saying is 'thats part of their function'. Of course it's also called 'hydronic balancing' but doesn't that inherently imply mixing?

 

If the flow from the HP is switched off and the flow to the ch is on, then the water circulates vertically through the buffer defeating the stratification.

 

The design of internal ports and baffles or pockets is to avoid vertical mixing. The cooler water coming in at the bottom pushes the warmer water out at the top, but remains as a layer of cooler water under the warmer water, ie. stratified. "Mixing" is not part of their function, they are designed to minimise mixing.

 

I'm not clear on 'hydronic balancing' within this context. I've only heard it used with regards separate heat emitters and ensuring they all receive the same flow temp.

 

With regards to "why 4 ports", it's because 4 ports, in combination with stratification:

 

27 minutes ago, IanR said:

the heating circuit, including circuits to MVHR wet duct heater/chillers or fan coil units receive their flow at the ASHP flow temp (minus minor standing losses), while increasing the system volume to avoid short cycling.

 

ie. not at a mixed temp, but at the ASHP flow temp (minus minor standing losses)

 

4 minutes ago, JamesPa said:

Is the latter really the case.  Surely system volume (which determines hp run time) is system volume whether the buffer is 2, 3 or 4 ports or indeed if the volume is in the emitters.  If the HP switches off but the circulator pump keeps running, heat is still delivered to the emitters however many ports the buffer has.  Again I admit I may be missing something, what is it that I'm missing?

 

Yes, with all things equal, system volume determines ASHP run time. If that additional volume is fully mixed, and the heat pump is off, then the heating circuit flow temp will reduces linearly until the heat pump comes back on, and will then increase linearly, inline with the hysteresis of the heat pump control. With a 4P buffer, if you were to accept that it remains stratified, the flow temp to the heating circuits does not reduce (apart from minor standing losses) between heat pump cycles***. A stratified 4P buffer allows for a wider hysteresis, which can then allow longer cycle times of the heat pump, if required.

 

*** I would expect a small drop in flow temp to the emitters when the heat pump cycles back on, until it gets up to the desired flow temp since the inlet to the buffer from ASHP is at the same or similar level to the outlet to the heating circuit.

 

25 minutes ago, JamesPa said:

Again I admit I may be missing something, what is it that I'm missing?

 

I don't know you are missing anything. You are believing 4P buffers mix, and I'm believing they remain, to a large extent, stratified. As you have pointed out, there's not much point in a 4P Buffer that mixes, if they did there are other alternatives that could avoid an additional pump.

[JohnMo]

@IanR do you have a buffer? Do you have any real numbers to share?

 

Everything mentioned above is good, when you have all zones open and are running in a steady state. Cannot see how it's true when the input and output of the buffer isn't balanced with respect flow or pressure. All the flow diverters in the world will not stop mixing, when the system is not balanced.

[IanR]

20 minutes ago, JohnMo said:

@IanR do you have a buffer? Do you have any real numbers to share?

 

I do have a 4P buffer, feeding two UFH manifolds, each with multiple zones, as well as feeding an MVHR wet duct heater/chiller. The buffer is used for both heating and cooling. I also shut the buffer off from the UFH circuits to allow me to redistribute solar gain through the property via the UFH without having the HP on.

 

I have no data logging, but have had no reason to question the systems performance. I did attempt to confirm the install was working within expected parameters during the first winter, but that wasn't easy. I had to run tests over night, since my property benefits well from solar gain and the background electrical consumption is more easy to control and calculate so that I could be confident in my power consumption estimates for the heating system from only the property's electricity meter. When I crunched the numbers, making allowance for standing losses, an extra pump, the longer run of pipes from ASHP to buffer that I have, I was surprised to see that, based on the heat pumps own heat meter, the HP operating at a higher CoP than the manufacturer suggested. I had thought that was likely to be me over-estimating losses and perhaps the buffer hadn't entirely cooled to ambient before I started the test. The Brendon Uys report may have provided another explanation which is his suggestion the CoP from the manufacturer is based on the H4 boundaries, where as I had assumed it was on the H3 boundaries, hence me making allowances for the standing losses and pump.

 

55 minutes ago, JohnMo said:

Cannot see how it's true when the input and output of the buffer isn't balanced with respect flow or pressure. 

 

For me a 4P buffer's ability to remain stratified is not the difference in flow rates between the primary and heating circuits, but the actual flow rate. The higher the flow rate the larger the buffer needs to be to keep the disturbance within the horizontal plane.

[ReedRichards]

Does anyone know how to achieve logic operations with mains voltage? 

 

When my heating is running I only (perhaps) need a volumiser, so I could close a valve to remove access to the buffer ports on the flow or return.  I would only open the valve for a 4-port configuration when both of the two heating zones are closed.  So that's A NOR function, isn't it?  I guess that could be difficult. 

 

Alternatively I could open a valve to bypass the buffer with an OR function, in fact my internal pump works that way so maybe I would just need a connection to the bypass-valve in parallel with the pump?  But that would rely on water preferring to bypass the buffer rather than go through it even though it could do either.  I don't have the plumbing experience to know where water will want to go of its own accord. 

[JamesPa]

16 minutes ago, ReedRichards said:

Does anyone know how to achieve logic operations with mains voltage? 

 

Relays are certainly one option.  I think you need 2 to make a NOR.

 

Bypass valves rely on differential pressure (I think), so (crudely speaking) a bypass valve opens when the alternative route is blocked.  Put a tee in the flow, one leg goes to the heating circuit, one to the buffer.  The one to the buffer has a bypass valve in it.  The bypass will open and water flow via the buffer tank when the heating circuit shuts down (I think).  It might be worth checking out the three port configuration advocated by caleffi https://idronics.caleffi.com/magazine/27-air-water-heat-pump-systems  I still don't quite get these but the logic seems prima facie believable.

Edited January 28, 2023 by JamesPa

[ReedRichards]

I have a good deal of respect for Caleffi as I have previously read some of their other reports (and they make some useful plumbing parts that you can't get in the UK).  I looked at Caleffi's 2 & 3 port configurations and what I don't know for sure is how the pump on the heating system manages to take the hot water as it arrives from the heat pump but not the cooler water in the buffer tank.  It's got to depend on how well-matched the flow rates are between the heat pump pump and pump on the heating side.  I'm not sure I how I would go about matching flow rates if I wanted to try.

[IanR]

2 hours ago, ReedRichards said:

I looked at Caleffi's 2 & 3 port configurations and what I don't know for sure is how the pump on the heating system manages to take the hot water as it arrives from the heat pump but not the cooler water in the buffer tank.  It's got to depend on how well-matched the flow rates are between the heat pump pump and pump on the heating side.  I'm not sure I how I would go about matching flow rates if I wanted to try.

 

I'm probably sounding like a broken record here, but the flow rates don't need to match - the top of the tank is at flow temp (minus minor standing losses). If the flow rate to the heating circuits is higher than that from heat pump, some hot water will be removed from the tank to support the heating circuit load. If the heating circuit flow rate is lower than the heat pump flow rate, then as well as supplying the heating circuit directly, the heat pump will push some additional heat into the tank, not necessarily increasing the temp at the top of the tank, but increasing the volume of water at the top of the tank which is at flow temp.

[ReedRichards]

Yes, I understand that @IanR.  But if you want to minimise the potential mixing that goes on in the buffer tank or minimise mixing water from the heat pump with water from the buffer tank then you do need to match the flow rates.

 

For example, if the flow rate in the heating system is, say, twice that from the heat pump then the system will draw half its water from the buffer tank and the buffer tank will be replenished entirely from water that has passed through the heating system.  So initially, say, water at 40 C from the heat pump would mix with water at, say, 30 C in the buffer to make water at 35 C.  If that loses, say, 5 C going though the heating system then it will be at 30 C coming back so the buffer remains at a constant 30 C.  This is very probably an extreme and unrealistic example but it illustrate what, potentially, could "go wrong".

 

Is the ideal to make the flow rate from the heat pump just a bit greater than that around the heating system so the buffer tank warms up gradually?  It seems that way but maybe there is a flaw in my reasoning?