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

[IanR]

1 minute ago, ReedRichards said:

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".

 

Unfortunately I haven't been able to explain my understanding of the concept in a way that makes sense to you. There is no mixing in the buffer tank that needs to be avoided, and there is no need to minimise mixing of the heat pump flow with the stored water in the tank, as it will be at the same temp. The flow rates do not need to match, and are unlikely to ever match entirely. Even with those unequal flow rates, the water in the buffer will not mix (it does a little, but not enough to effect performance). When I say the water does not mix, I mean vertically mix. ie. the water at the top of the tank, at flow temp, does not mix with the water at the bottom of the tank, which is at return temp.

 

41 minutes ago, ReedRichards said:

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?   

 

The heat pump has to be sized to cover the entire heating demand. The choice of a buffer is to protect against short cycling, so by definition the heat pump is over-sized for the heat demand.

 

The heat pump will be off because its max return temp has been met, and it can't modulate down any further, so switches off. It will cycle back on before all useable energy is depleted from the buffer - in fact it should cycle back on before the top of the tank falls below its lower hysteresis  boundary and as it comes back on will satisfy 100% of the space heating demand, with the additional energy/flow available, recharging the buffer in parallel. Once the buffer is fully charged again, the heat pump will switch off. 

[ReedRichards]

9 hours ago, IanR said:

 

Unfortunately I haven't been able to explain my understanding of the concept in a way that makes sense to you. 

 

Maybe we are talking at cross-purposes?  I was referring specifically to Caleffi's 3-port buffer tank configuration in my last two posts.

 

Also, my buffer tank is integrated with, and sits directly below, the DHW tank.  It's short and squat, which I imagine are the worst dimensions to avoid stratification.  Here is a picture.  That's the heating flow on the left and return on the right.  The input/output ports to/from the heat pump are round the corner at the same heights.  I don't think this corresponds at all to the buffer tank configuration you have in mind.

 

        

[IanR]

23 minutes ago, ReedRichards said:

Maybe we are talking at cross-purposes?  I was referring specifically to Caleffi's 3-port buffer tank configuration in my last two posts.

 

As was I.

 

23 minutes ago, ReedRichards said:

Also, my buffer tank is integrated with, and sits directly below, the DHW tank.  It's short and squat, which I imagine are the worst dimensions to avoid stratification. 

 

Yes, your 4P buffer looks challenged to work efficiently. But it's not a unique setup so the manufacturer has likely mitigated the compromises of delivering a tight package, as best as possible, assuming the heat pump, controller and tank combination are "manufacturer recommended".

 

Are there indications that your system is not working as expected? ie. heat pump not meeting heating demand, or electrical consumption being higher than calculated?

[ReedRichards]

The cylinder came from Telford.  As best as I can tell my installer bought the cylinder and (LG) heat pump as a package through Telford (as you can now, although a different heat pump brand https://www.telford-group.com/product/clivet-air-source-heat-pump-packages/ ) and Telford included a booklet of instructions on how to set it up.  These included some very unambitious settings for the weather compensation parameters which I changed subsequently.

 

I am perfectly satisfied with my heat pump and it cannot be far off the promised SCOP of 3.2 for heating, although I have no way of knowing exactly.  But electricity has trebled in price since I bought my heat pump so any measures I could take to improve the performance are worth considering (as indeed for any heating system at present).  I replaced an oil boiler without expecting much change in heating costs and as oil has only doubled in price it's a bit galling that I am now paying more for heating than I would have if I had just sat on my hands.    

[IanR]

1 hour ago, ReedRichards said:

I am perfectly satisfied with my heat pump and it cannot be far off the promised SCOP of 3.2 for heating, although I have no way of knowing exactly. 

 

That's the important bit. 

 

I'd trust Telford, as I would the larger heat pump brands that all invest in R&D to have developed the buffer to be optimised for its use. That Telford package appears to fit the retrofit market, where fitting within the constraints of the system being replaced are as important as eking out the last few tenths of SCoP. It perhaps also de-risks the installer from misjudging heating demand by including a buffer that may not be required in all circumstances which could also be at the expense of a few tenths of SCoP.

 

The easiest improvement to efficiency you could make, is bypassing the buffer. You'd need to ensure that in the shoulder months a heating circuit was always open with sufficient volume and demand for a minimum ASHP 10 minute run time (longer would be better).

 

You could also reconfigure the buffer plumbing, capping off two of the ports, and configure it as a 2P buffer in the return line.

 

Either of the changes may effect the 7 year Telford warranty.

[JamesPa]

 

14 hours ago, IanR said:

There is no mixing in the buffer tank that needs to be avoided, and there is no need to minimise mixing of the heat pump flow with the stored water in the tank, as it will be at the same temp. The flow rates do not need to match, and are unlikely to ever match entirely. Even with those unequal flow rates, the water in the buffer will not mix (it does a little, but not enough to effect performance). When I say the water does not mix, I mean vertically mix. ie. the water at the top of the tank, at flow temp, does not mix with the water at the bottom of the tank, which is at return temp.

 

If the flow rate of the pump on the heat pump side of a 4 port buffer tank is H, and the flow rate on the central heating side of the tank is C, the H-C must flow from top to bottom (or bottom to top) to balance flows.

 

If H-C is small in comparison with C, the buffer is well designed and the flow rates are sufficiently low that there is no turbulence, then it is indeed plausible that this difference in flow rates of the two pumps is the only source of mixing.  A bottom to top flow will cool the flow to the CH slightly, so the flow temp from the HP will need to be increased a little.  This does impact efficiency, but if the delta T across the emitters is small and the top to bottom flow is also small, then its again plausible that this is a small effect.  A top to bottom flow will warm the return slightly, and increase its volume.  There must be a penalty for this, if only the excess pump energy, otherwise we would have a perpetual motion machine, but again its plausible that this is a small effect.

 

So with reasonably well balanced pumps (and in particular, perhaps, a pump on the HP side that is a little faster than that on the CH side), a well designed buffer tank and a flow rate sufficiently low that there is not material turbulence in the buffer tank, its plausible that the degradation is small and outweighed by the gains.  Some measurements of the temp drop across a 4 port buffer tank (flow to flow), which sadly nobody seems to be in a position to post, would help understand this.

 

Of course if the flow rates are close it still begs the question, why is a 2 port buffer tank (and one fewer pump) not sufficient, but of course there are so many variants of CH design and requirement that there are almost bound to be cases where it isn't!  My enthusiasm for Occam's razor still suggests to me, however, that a 4 port tank and 2 pumps should not be deployed unless there is a clearly identifiable reason to do so.

 

Whilst on buffer tanks I do think the Caleffi 3 port tank is worth a bit more discussion,  For some reason, which I don't currently understand, most of their schematics, including this one, show 2 pumps.  But a variant of this with a single pump and a bypass valve arranged so that the flow goes either direct to the CH system (when there is demand) or through the buffer tank (when there is not) might have some interesting properties.   If anyone does want to explore this perhaps it should be a separate thread!

Edited January 29, 2023 by JamesPa

[JamesPa]

1 hour ago, JamesPa said:

Whilst on buffer tanks I do think the Caleffi 3 port tank is worth a bit more discussion,  For some reason, which I don't currently understand, most of their schematics, including this one, show 2 pumps.  But a variant of this with a single pump and a bypass valve arranged so that the flow goes either direct to the CH system (when there is demand) or through the buffer tank (when there is not) might have some interesting properties.   If anyone does want to explore this perhaps it should be a separate thread!

OK Im being slow today.  The reason the caleffi 3 port buffer diagram has 2 pumps is that, without 2, water from the HP would just circulate through the buffer.  Also, but turning on the CH pump and turning off the HP pump, the Ch can be fed from the buffer.  Likewise by turning on the HP pump but not the CH pump, the HP can dump heat into the buffer.  That's makes for a lot of flexibility that a bypass valve alone can't achieve.

 

Incidentally caleffi have something to say about stratification "Buffer tanks connected to heat pumps tend to have
minimal temperature stratification. This happens because most heat pumps have recommended flow rates of 3 gpm
per ton (12,000 Btu/hr) of capacity. A typical 4-ton air to water heat pump operating at these conditions would “turn over” an 80-gallon buffer in less than 7 minutes. Those flow rates, especially if introduced vertically into the tank, create lots of internal mixing.'.  As per above this may or may not be true (btw 1 ton = 3.5kW in sane units)

Edited January 29, 2023 by JamesPa

[IanR]

1 hour ago, JamesPa said:

If H-C is small in comparison with C, the buffer is well designed and the flow rates are sufficiently low that there is no turbulence, then it is indeed plausible that this difference in flow rates of the two pumps is the only source of mixing. 

 

There is no mixing, assuming inlet and outlet velocities are within the parameters of the 4P buffer.

 

In a running system where the buffer hasn't been allowed to cool off entirely, the tank is stratified with flow temp (minus minor standing losses) water at the top and return temp water at the bottom with a thermocline plane at some height in the cylinder between the two different temps (and different densities) of water,  where there is a rapid temperature change. Depending on how "charged" the tank is determines at what height that thermocline is.

 

1 hour ago, JamesPa said:

A bottom to top flow will cool the flow to the CH slightly, so the flow temp from the HP will need to be increased a little. 

 

I'm not sure what you mean by "bottom to top flow". What happens when the heat pump is off is the is the thermocline plane rises, the tank hold less energy, but the temp at the top stays the same (minus minor standing losses).

 

The outlet to the CH will be within the upper portion where the temp is @ flow temp, so there is no need to increase the temp of the heat pump.

 

1 hour ago, JamesPa said:

otherwise we would have a perpetual motion machine

 

No, you are removing and adding energy to the buffer by increasing and decreasing the portions of the buffer that are at either flow or return temp, so the "average" temp of the buffer is going up and down, but the temps at the top and bottom remain constant (minus minor standing losses)

 

1 hour ago, JamesPa said:

So with reasonably well balanced pumps (and in particular, perhaps, a pump on the HP side that is a little faster than that on the CH side)

 

No, the buffer is to stop heat pump short cycling, so it will run for long periods, but then it will be off completely for long periods. Inlet and outlet fluid velocity needs to be kept within the parameters of the buffer tank by flow rate an pipe sizing.

 

Much of the back and forth of this discussion seems to around the robustness of stratification within a hot water tank. It's being treated like a delicate phenomena that will disperse with any slight disruption, when the reality is far from it. It's used to good effect in DHW cylinders with mains water coming in at the bottom at double or triple the flow rates of a heat pump as well as heating elements within the tank that create convection currents and vertical mixing. Within commercial hot water storage tanks where stratification is not wanted anti stratification pumps have to be included to generate vertical mixing to get rid of it.

 

The following may be of interest. It's not for a buffer, but for a solar thermal storage tank, where a CFD analysis has been run to determine the best baffle shape for the incoming water supply.

 

The tank is equivalent in volume to a moderate-to-large buffer, is oddly rectangular in shape which will make reducing the inlet turbulence more difficult than a circular vessel, and importantly has a 12 l/m flow, which is over double my own 12kW ASHP.

 

The simulated system is defined as: 

 

The analysis results have been verified against a physical prototype.

 

A significant difference is the inlet on the demand side is at the constant water supply temp, that will be lower than the temp in the tank, at the bottom.

 

I'm really hoping the following image doesn't confuse things more, but to me shows unbalanced, and completely unrelated flow rates between heat source and load, having absolutely no effect on the outlet temperature.

 

Also of note, I feel, is the lack of disturbance of the thermocline until it gets into the lower third of the tank where the disturbance of the 12 l/m inlet is causing some turbulence with one design, but is better optimised in the other.

 

The disturbance shown in the lower, cooler part of the tank, wouldn't be there if the incoming water was at the same temp as the body of water under the thermocline. 

 

 

To put the above images in context:

 

Ref. https://iopscience.iop.org/article/10.1088/1757-899X/518/3/032052/pdf

 

Edited January 29, 2023 by IanR

[JamesPa]

37 minutes ago, IanR said:

There is no mixing, assuming inlet and outlet velocities are within the parameters of the 4P buffer

Sorry but so far as I can see there must be if the flow from the pump on the hp side is not equal to the flow from the pump on the ch side.  The only way to balance the flows into and out of the tank if the two pumps are operating at different flow rates, is for there to be a top to bottom (or bottom to top) flow as well as a lateral flow.

 

Of course as the water flowing from top to bottom moves through the tank it will heat up or cool down by robbing heat from (or donating heat to) the body of water in the tank.  But one way or another it must move through the tank and must exchange heat with the water it mixes with.

 

I'm not suggesting that this means that stratification is necessarily upset, as you say that will depend on flow rates (and times - eg if one pump is off for long enough the stratification will change), but still exchange of energy takes place.

Edited January 29, 2023 by JamesPa

[IanR]

4 minutes ago, JamesPa said:

The only way to balance the flows into and out of the tank if the two pumps are operating at different flow rates, is for there to be a top to bottom (or bottom to top) flow as well as a lateral flow.

 

But no mixing. The warmer water remains above the thermocline, at flow temp, and the cooler below the thermocline at return temp, the proportions of each body of water increase and decrease, moving the thermocline up and down the tank.

 

11 minutes ago, JamesPa said:

Of course as the water flowing from top to bottom moves through the tank it will heat up or cool down by robbing heat from (or donating heat to) the body of water in the tank.  But one way or another it must move through the tank and must exchange heat with the water it mixes with.

 

No, the bodies of water at the two different temps do not mix. Their different densities keep them apart with the lower density water having the top spot. The volume of each of the bodies of water change in inverse relation to each other as either the heat pump adds more flow temp water to it or the HC removes flow temp water from it.

 

17 minutes ago, JamesPa said:

That doesn't necessarily destroy stratification, but still exchange of energy takes place.

 

Your version does destroy stratification where there would then typically be a linear temp gradient from top to bottom of the tank.

 

My version requires stratification to remain in place, otherwise there is no point to a 3P or 4P buffer.

[JamesPa]

Im not now sure if we are agreeing or disagreeing! 

 

Assuming that both HP flow and CH flow are at the top and HP return and CH return at the bottom, lets start with an extreme example.  Suppose we switch off the HP pump but leave the CH pump on.  Nothing will flow through the HP circuit.  Over time cool water will come in at the bottom (from the CH pump) and slowly both the actual water and the thermal gradient will move up to the top.  Eventually the water at the top will be more or less the same temperature as the water coming in at the bottom.  It will take a while to get to this point because the circulating water in the CH system will continue to give up energy to the house, and thus return to the tank cooler, until it has cooled down completely.  That's similar behaviour as a DHW tank when you switch off the element.

 

If we switch the HP back on but it is running at a lower flow rate than the CH pump, then there will be a slower movement of the, initially cool, water molecules from the bottom to the top, at a rate equal to the difference in pump flow rates.  If this movement were not taking place, then more water would be entering at the bottom (at the flow rate of the CH pump) than is leaving at the bottom (at the lower flow rate of the HP pump), and more water would be exiting at the top than is entering at the top,.  This can only be resolved by the movement of water from bottom to top.  The water may well move 'slab-like', but it must move.

 

As the cool water molecules are pushed up they will, if the heat pump is on, exchange energy with the warm water molecules, so the thermal gradient will be maintained.  If everything is left for a while, it will eventually reach a steady state.  The movement doesn't, if its sufficiently slow and is not too turbulent, destroy stratification, but it does involve energy exchange.  In principle the energy exchange could be purely by conduction, but more likely it also involves water mixing in the top stratum as the rising cooler water encounters the turbulence likely caused by the hot water flowing in and out of the ports. 

 

If the movement through the tank is sufficiently slow and not turbulent (other than, possibly, at the top) I would expect that the penalty would be small, quite possibly negligible.  This could be verified by measuring the temperature drop from flow inlet to flow outlet across the buffer tank. 

 

Basically Im saying that I agree that a 4 port buffer tank with little turbulance operated with pumps that have reasonably similar flow rates will not incur a penalty.  However If the flow rates aren't reasonably similar (particularly if the CH flow is faster than the HP flow) or there is significant turbulence, then this may not be the case.  

 

 

[IanR]

5 hours ago, JamesPa said:

If everything is left for a while, it will eventually reach a steady state. 

 

For which the time frame would be 10 to 100 times longer than the cycling of the heat pump, while it is providing heat to the central heating, so is not relevant to the performance of the heat pump. This also takes us deeper into the physics of temperature stratification in hot water tanks, where I do not feel this conversation needs to go - there are loads of references for it on t'internet. But, in short, the insulating properties at the boundaries of the stratified layers means that the convection currents from standing losses far exceeds any conduction between the layers so the cooler lower area actually cools quicker than the warmer upper layer and the tank becomes more stratified as it cools. The convection currents are caused by the standing losses at the tank wall which cools a very thin vertical layer of water adjacent to the wall. The water of the vertical layer becomes denser than its surroundings and slips towards the bottom of the tank, reducing the temp at the bottom of the tank.

 

But none of that matters while the pump is cycling as the energy losses are minor and covered by the known standing losses for the tank. The standing losses for my tank, based on it being an average 50% charged during the call for heat period is 17.5W. or at a worst case CoP of 3............     6W of electrical power. 

 

Maybe I've not allowed for the times the buffer is fully charged just as the call for heat ends, so gets wasted, but were talking really small numbers that are well within the potential error size of such predictions.

 

5 hours ago, JamesPa said:

 If this movement were not taking place, then more water would be entering at the bottom (at the flow rate of the CH pump) than is leaving at the bottom (at the lower flow rate of the HP pump), and more water would be exiting at the top than is entering at the top,.  This can only be resolved by the movement of water from bottom to top.  The water may well move 'slab-like', but it must move.

 

If you mean by the water in the the lower portion of the tank, which is at return temp, increasing in volume, while the water in the upper area of the tank, which is at flow temp, reducing in volume so that the boundary between these to stratified layers (the thermocline) rises up the tank a little, the yes it "moves". 

 

 

5 hours ago, JamesPa said:

Basically Im saying that I agree that a 4 port buffer tank with little turbulance operated with pumps that have reasonably similar flow rates will not incur a penalty.  However If the flow rates aren't reasonably similar (particularly if the CH flow is faster than the HP flow) or there is significant turbulence, then this may not be the case.  

 

Unfortunately that's not what I'm saying. There is absolutely no need for the pumps (ASHP & heating circuit) to have any similarity of flow rate or duration, which is lucky because they don't.  The heating circuit pump will run continuously while there is a call for heat, while the heat pump will cycle on and off. In a correctly sized buffer, there won't be sufficient turbulence to cause mixing since the heat pump flow rate and pipe sizing will restrict the flow velocity to within the capabilities of the tank.

 

The images and link I posted above show how robust the stratification in the tank is with the thermocline plane in the bottom 25% of the tank and close to a 12 l/m inlet that has only a plain baffle, flat sides, no porting, no pockets no swirl bowl in the lower face. 

 

Looking at the colour chart I would suggest the top 70% of the tank has remained at the flow temp, or within 0.5°C

 

Edited January 29, 2023 by IanR

[JamesPa]

8 hours ago, IanR said:

If you mean by the water in the the lower portion of the tank, which is at return temp, increasing in volume, while the water in the upper area of the tank, which is at flow temp, reducing in volume so that the boundary between these to stratified layers (the thermocline) rises up the tank a little, the yes it "moves". 

Yes that's exactly what I mean.  For this to happen water must actually move upwards in the tank, but I agree that the integrity of the boundary could be maintained, at least until it pushes so far up the tank that its nearing the top

 

8 hours ago, IanR said:

Unfortunately that's not what I'm saying. There is absolutely no need for the pumps (ASHP & heating circuit) to have any similarity of flow rate or duration, which is lucky because they don't.  The heating circuit pump will run continuously while there is a call for heat, while the heat pump will cycle on and off. In a correctly sized buffer, there won't be sufficient turbulence to cause mixing since the heat pump flow rate and pipe sizing will restrict the flow velocity to within the capabilities of the tank.

 

The images and link I posted above show how robust the stratification in the tank is with the thermocline plane in the bottom 25% of the tank and close to a 12 l/m inlet that has only a plain baffle, flat sides, no porting, no pockets no swirl bowl in the lower face. 

Whist the analogy with DHW is useful to understand turbulence within tanks, it can only be pushed so far.  We run DHW for minutes and in any given run extract only a portion of the tank volume.  We run CH continuously and the volume passing through them is a fairly short period of time well exceeds the tank volume.  

 

I think its time to 'do the math', it may well be that we agree on the outcome, if not actually the mechanism.

 

Lets consider an 8kW heating system operating at approximately full load, with a 200l tank and (since this is a UFH thread), a deltaT across the emitter of 4C.  Roughly 0.5l/s of water must be delivered to the load. 

 

If the HP pump and the CH pump are operating at exactly the same speed, the tank water will remain static except at top and bottom where 0.5l/s will pass laterally between input flow and output flow/input return and output return.  A static thermal profile will establish.  Lets call this the 'baseline'

 

If the HP pump is stopped, then it will take 400s, 6.6 mins, for the contents of the tank to cycle round the CH system, the original thermocline will have moved fully through the tank (and possibly replaced by a new one at a lower temp) and the water exiting the tank will be (roughly) at the initial return temperature. 

 

If, alternatively, we have both the HP pump and the CH pump on and the HP pump is operating at half the speed of the CH pump, it will take 800s, 13 mins, for the water initially at the bottom to rise to the top.  But rise to the top it must because otherwise there is more water leaving at the top than entering at the top.  The thermocline will be pushed up the tank relative to the baseline.  Eventually half of the water exiting the tank at the top will have come from the bottom, heating up as it is pushed up, but not quite as high as the original temperature.  If the system remains like this it will eventually reach a new steady state where there is a flow temp difference, however small, across the buffer tank.

 

If,  we have both the HP pump and the CH pump on and the HP pump is operating at twice the speed of the CH pump, it will take 800s, 13 mins, for the water initially at the top to rise to the bottom, cooling as it falls.  Eventually half of the water exiting the tank at the bottom will have come from the top, cooling down as it is pushed down, but not quite as low as the original temperature.  If the system remains like this it will eventually reach a new steady state where there will be a return temp difference, however small, across the buffer tank.

 

I imagine what happens in a properly designed system is actually none of these.  I expect that the HP can deliver more than the maximum load required by the CH.  If the HP pump switches off for a time whilst the CH pump continues running (because the demand from the buffer tank is satisfied but not the demand from the load), the thermocline moves up with the excess water entering at the bottom.  However the HP pump switches on again before the cool water reaches the top.  The HP pump then, for a while, delivers water at a faster rate than the CH pump, and the thermocline moves down again as hot water is pushed down the tank.  This cycle can continue indefinitely, the control system of the HP effectively balancing the input and output to the tank even though the pumps themselves are not balanced.  The water in the centre of the tank gently oscillates up and down the tank, but plausibly never leaves it.  Thus, over time, the appearance of a quasi steady state is maintained (it would be interesting to build a glass tank, introduce some dye into the water, and watch it move!)

 

Is this more or less correct in your view?

[ReedRichards]

9 hours ago, IanR said:

 

Unfortunately that's not what I'm saying. There is absolutely no need for the pumps (ASHP & heating circuit) to have any similarity of flow rate or duration, which is lucky because they don't.  The heating circuit pump will run continuously while there is a call for heat, while the heat pump will cycle on and off. In a correctly sized buffer, there won't be sufficient turbulence to cause mixing since the heat pump flow rate and pipe sizing will restrict the flow velocity to within the capabilities of the tank.

 

Just on a point of information, that's not how my heating system works.  When there is a call for heat the heating pump turns on.  About a minute later the ASHP internal pump turns on (or there is a delay in this being reported on the controller).  After another couple of minutes the compressor is reported as turning on.

 

As soon as the compressor turns off, all pumps cease operation.  This does sometimes happen if there is a room thermostat calling for heat but because (I presume) the return water temperature gets too high.  I have a third party controller which restricts the number of cycles to 3 per hour.  It doesn't know I have a buffer tank so doesn't know that it would be okay to leave the heating pump running.  So it doesn't.

 

If a thermostat is still calling for heat the sequence repeats 20 minutes after the start of the previous cycle. 

[JamesPa]

3 hours ago, ReedRichards said:

Just on a point of information, that's not how my heating system works.  When there is a call for heat the heating pump turns on.  About a minute later the ASHP internal pump turns on (or there is a delay in this being reported on the controller).  After another couple of minutes the compressor is reported as turning on.

 

As soon as the compressor turns off, all pumps cease operation.  This does sometimes happen if there is a room thermostat calling for heat but because (I presume) the return water temperature gets too high.  I have a third party controller which restricts the number of cycles to 3 per hour.  It doesn't know I have a buffer tank so doesn't know that it would be okay to leave the heating pump running.  So it doesn't.

Interesting.  Lets see how @IanRresponds to my latest attempt (above) to rationalise what is going on.  If I have it about right then it might be important that the HP both knows about the presence of, and also knows the temperature of, a 4 port buffer (but not so a 2 port buffer), because its that knowledge that effectively compensates for differential pump flows and ensures that, over time, the thermocline doesn't drift to the top or the bottom of the tank.

[IanR]

10 hours ago, JamesPa said:

If the HP pump is stopped, then it will take 400s, 6.6 mins, for the contents of the tank to cycle round the CH system, the original thermocline will have moved fully through the tank (and possibly replaced by a new one at a lower temp) and the water exiting the tank will be (roughly) at the initial return temperature. 

 

Thankfully, it won't happen, since a heating load at that level will keep the ASHP on. The heat pump will only switch off when the heating load is lower than the pump can modulate down to.

 

10 hours ago, JamesPa said:

If, alternatively, we have both the HP pump and the CH pump on and the HP pump is operating at half the speed of the CH pump, it will take 800s, 13 mins, for the water initially at the bottom to rise to the top.  But rise to the top it must because otherwise there is more water leaving at the top than entering at the top.  The thermocline will be pushed up the tank relative to the baseline.  Eventually half of the water exiting the tank at the top will have come from the bottom, heating up as it is pushed up, but not quite as high as the original temperature.  If the system remains like this it will eventually reach a new steady state where there is a flow temp difference, however small, across the buffer tank.

 

Again, it shouldn't happen, as the heat pump will fulfil the heat demand, and not run at a lower rate than is required by the demand. It switches off once it's at the bottom end of its modulation, but the return temp is still rising.

 

It has made me think about what happens when a defrost happens. There is a chance this could happen when the buffer has less that a full charge, and if the defrost takes longer than it takes for the heating demand to deplete the "flow temp" layer of water at the top, then then heating circuit will start to draw off water at what was the original return temp and will start to return water to the buffer at some lower temp. Since defrosting happens after the pump has been running for a while, the buffer is likely to be fully charged. Hopefully it takes longer for the heating demand to deplete the useable energy from the buffer than it does to complete a defrost event.

 

10 hours ago, JamesPa said:

If,  we have both the HP pump and the CH pump on and the HP pump is operating at twice the speed of the CH pump, it will take 800s, 13 mins, for the water initially at the top to rise to the bottom, cooling as it falls.  Eventually half of the water exiting the tank at the bottom will have come from the top, cooling down as it is pushed down, but not quite as low as the original temperature.  If the system remains like this it will eventually reach a new steady state where there will be a return temp difference, however small, across the buffer tank.

 

The heat pump will often run faster than the heating demand, increasing the volume of flow temp water at the top of the tank and reducing the volume of return temp water at the bottom, with the thermocline boundary coming down the tank. The warmer water above the thermocline remains at flow temp, it doesn't cool. Since the flow temp water at the top does not mix with the return temp water at the bottom, the temp returning to the ASHP from the buffer continues to return at "return temp", until the buffer is fully charged and the thermocline gets to the bottom of the tank, at the same level as the inlet from the heating circuits and outlet to the ASHP. Now the return water from the heating circuit will briefly mix with flow temp water and this returns to eth ASHP where it can't modulate down any further but has warmer return water coming to it which would push up the flow temp above target, so it would switch off.

 

10 hours ago, JamesPa said:

I imagine what happens in a properly designed system is actually none of these.  I expect that the HP can deliver more than the maximum load required by the CH.  If the HP pump switches off for a time whilst the CH pump continues running (because the demand from the buffer tank is satisfied but not the demand from the load), the thermocline moves up with the excess water entering at the bottom.  However the HP pump switches on again before the cool water reaches the top.  The HP pump then, for a while, delivers water at a faster rate than the CH pump, and the thermocline moves down again as hot water is pushed down the tank.  This cycle can continue indefinitely, the control system of the HP effectively balancing the input and output to the tank even though the pumps themselves are not balanced.  The water in the centre of the tank gently oscillates up and down the tank, but plausibly never leaves it.  Thus, over time, the appearance of a quasi steady state is maintained (it would be interesting to build a glass tank, introduce some dye into the water, and watch it move!)

 

Is this more or less correct in your view?

 

Pretty much. 

 

When using flow temp control, there does have to be a breakdown of the stratification at the point the buffer is full in order to send warmer water to the ASHP to switch it off, but stratification will reform as soon as the ASHP switches off. Thermocouples at 25/75 of the tank height could avoid this, but would require more setting up on the control. I can't see an issue with it, so there's probably no reason to add the extra kit.

 

9 hours ago, ReedRichards said:

As soon as the compressor turns off, all pumps cease operation.  This does sometimes happen if there is a room thermostat calling for heat but because (I presume) the return water temperature gets too high.  I have a third party controller which restricts the number of cycles to 3 per hour.  It doesn't know I have a buffer tank so doesn't know that it would be okay to leave the heating pump running.  So it doesn't.

 

If a thermostat is still calling for heat the sequence repeats 20 minutes after the start of the previous cycle. 

 

Unfortunately I don't understand the logic to your 4P Buffer. What you describe happening can be explained by the buffer being fully mixed, so it could just be a volumiser in your flow or return line (return would be better). The their part controller does add to the evidence that your package is a one-size fits all package that de-risks the installation for the installer. Not really a problem as long as its able to keep the property warm.

Edited January 30, 2023 by IanR

[JamesPa]

1 hour ago, IanR said:

12 hours ago, JamesPa said:

I imagine what happens in a properly designed system is actually none of these.  I expect that the HP can deliver more than the maximum load required by the CH.  If the HP pump switches off for a time whilst the CH pump continues running (because the demand from the buffer tank is satisfied but not the demand from the load), the thermocline moves up with the excess water entering at the bottom.  However the HP pump switches on again before the cool water reaches the top.  The HP pump then, for a while, delivers water at a faster rate than the CH pump, and the thermocline moves down again as hot water is pushed down the tank.  This cycle can continue indefinitely, the control system of the HP effectively balancing the input and output to the tank even though the pumps themselves are not balanced.  The water in the centre of the tank gently oscillates up and down the tank, but plausibly never leaves it.  Thus, over time, the appearance of a quasi steady state is maintained (it would be interesting to build a glass tank, introduce some dye into the water, and watch it move!)

 

Is this more or less correct in your view?

 

Pretty much. 

Phew, we got there in the end but...

 

1 hour ago, IanR said:

11 hours ago, ReedRichards said:

As soon as the compressor turns off, all pumps cease operation.  This does sometimes happen if there is a room thermostat calling for heat but because (I presume) the return water temperature gets too high.  I have a third party controller which restricts the number of cycles to 3 per hour.  It doesn't know I have a buffer tank so doesn't know that it would be okay to leave the heating pump running.  So it doesn't.

 

If a thermostat is still calling for heat the sequence repeats 20 minutes after the start of the previous cycle. 

 

Unfortunately I don't understand the logic to your 4P Buffer. What you describe happening can be explained by the buffer being fully mixed, so it could just be a volumiser in your flow or return line (return would be better). The their part controller does add to the evidence that your package is a one-size fits all package that de-risks the installation for the installer. Not really a problem as long as its able to keep the property warm.

it does seem like correct buffer 'control' (by the heat pump) might matter.  Otherwise the buffer tank de-stratifies because either the CH flow or the HP flow dominates.  Many of the HP control units for which I have read the instructions can have buffer temp probes.  Are these important I wonder.

[ReedRichards]

My buffer tank has no temperature probes nor any other form of electrical connection.  I'm not aware of any setting for the heat pump that can tell it that it has a buffer tank, let alone report the temperature if a probe was present.  Occasionally I see a power blip in the middle of the night which could be the heat pump circulating water to check that the return water is not too cold; that's the only means it has of getting some idea of the buffer temperature.

 

I suppose the buffer tank guarantees that I don't void the warranty by operating the heat pump with less than the specified minimum volume of water (which is 50 l).  And it provides a reservoir of hot water with which the heat pump can defrost itself as necessary.

 

My main problem with the buffer tank is that it adds an audible internal pump whereas the external oil boiler I replaced kept the central heating completely silent.            

Edited January 30, 2023 by ReedRichards