In a similar manner to the CH, DHW production also suffers from the effects of those 'nasty' Laws of Thermodynamics, who insist upon moving heat energy from a higher temperature to a lower one. 🙄 This is the cause of heat loss from your home. 😡
When your system switches over to DHW heating, the heat pumps increases the required LWT, to a value high enough to cause heat energy to be transferred into the water inside the hot water cylinder, so as the graph shows, the LWT is at 55C, whilst the water inside the cylinder is at 44C. Heat energy is transferred from the 'leaving water' into the water inside the cylinder, at a rate dependent upon the temperature difference and the water flow rate. As heat energy is transferred, the water flow through the heating coil is cooled, such that the RWT is now 50C.
To ensure that heat energy is always being transferred into the hot water cylinder at a reasonable rate, I suspect that the control algorithms inside the heat pump controller, are programmed to maintain a temperature difference of 5C between LWT and RWT, during the initial phase of heating.
As the temperature of the water inside the cylinder increases, this will cause the RWT to increase, which in turn will cause the LWT to be increased. There is a limit to how high the LWT can be increased, either by parameter setting inside the controller, or just the physical limit of what the heat pump can produce. As the heat pump approaches this limit, it is quite possible that the rate of LWT increase will reduce, so as the RWT continues to increase the DT will start to reduce.
Probably a good analogy would be a vehicle towing two trailers (not to be attempted at home, even with parental supervision), the first trailer being connected to the vehicle by a rope, but the second trailer being connected to the first trailer by a solid bar. As the vehicle (LWT) starts to increase speed, the rope (DT = 5C) starts to extend, until it starts to move the first trailer (RWT), which in turn moves the second trailer (Cylinder temperature). All goes well with the vehicle and the trailers all maintaining the required distance apart, until the vehicle starts to approach the destination. As the vehicle slows, the momentum of the first trailer keeps it going at the original speed, so it moves closer to the vehicle, and the rope becomes slack (no longer maintaining DT = 5C), the distance between the two trailers being constant due to the solid bar. Eventually the brakes are applied and all three come to a stop. 😎
you asked about what our return temperatures are doing across the LLH tank.
So I set up a thermistor to read the return temp and compare with the FTC reading of the Primary Return.
The picture showing the S3 thermostat shows the return temperature of the central heating before it enters the LLH TANK and the THW2 reading shows the return temperature as it leaves the LLH TANK. THERE APPEARS TO BE some consistency as the CH RETURN is also 1c lower than the PRIMARY RETURN.
so the mixing appears to be the same... I think that’s what I was hoping for.?.?.
Just to recap and summarise on earlier posts:
We had seen a small loss of temperature across the Flow Pipes as the water passed through the Low Loss Header tank, into the central heating circuit. This loss was approx 1 to1.5c. This was thought to be a small amount of mixing in the Low loss header tank as the water passed through.
So to check this we tested the return temperatures and confirmed that the Primary Return water was exiting the Low Loss Header Tank slightly hotter than the incoming Central Heating Return water. This increase in Primary Return temperature was also about 1c. This more or less matched the Flow pipe temp. loss, suggesting that a small amount of mixing as might be expected. This mixing could be considerably more if flow rates created by each pump were different.
The test limitation was that the thermistor readings only measure in 1c increments which give a wide ish margin of accuracy.
in Brendon’s article, he tested these two scenarios and found the COP to be 4.71 without flow separation and 3.4 with flow separation. I really don’t understand why the latter figure wasn’t closer to 4.18, even allowing for the need for a secondary pump.
HI Mike
Can you point me in the direction of the article you refer to above.
Also - when removing a LLH, is there a way of calculating the required total water volume required for a given HP installation? Is it linked to HP size etc?
is there a way of calculating the required total water volume required for a given HP
Minimum volume should be in the specs and seems related to size of heat pump. Volume is important for defrost cycles, which use the heat in the water to defrost and to provide sufficient thermal inertia to avoid frequent cycling of the heat pump.
in Brendon’s article, he tested these two scenarios and found the COP to be 4.71 without flow separation and 3.4 with flow separation. I really don’t understand why the latter figure wasn’t closer to 4.18, even allowing for the need for a secondary pump.
HI Mike
Can you point me in the direction of the article you refer to above.
Also - when removing a LLH, is there a way of calculating the required total water volume required for a given HP installation? Is it linked to HP size etc?
From the FTC6 manual.
EDIT - 'Average' means conditions in Strasbourg. 'Colder' doesn't seem to be defined. The reference is here for anyone really interested!!
interesting article. Also flow volume for our Ecodan 8.5 are easily met.
Rad volumes = 111 litres
28mm pipes = 21 litres
22mm pipes = 13 litres
15mm pipes = 3.6 litres
LL header tank est 3 litres
So on the face of it we should be able to retain good flow volume and bypass the low loss header and second pump - assuming the primary pump can cope with the whole circuit.
The article by Brendon highlighted a COP of approx 3.4 before removing the LLH. This is approximately what we are getting with a 21c room temp, 6c ambient daytemp. (COP 3.7 at 8c with a mild night of 5c) However we are only using Radiators albeit with decent sized emitters.
Presuming UFH is a more efficient way to use an ASHP on account of the lower flow temperatures and large areas involved? So perhaps a radiator system with a higher flow temperature will never attain the efficiencies of a good UFH installation?
So if we are going to break into the low COP4s I guess we might still look at getting the LLH removed or bypassed. But perhaps recognise that the gains might not be as good as an UFH installation? Decisions, Decisions.
interesting article. Also flow volume for our Ecodan 8.5 are easily met.
Rad volumes = 111 litres
28mm pipes = 21 litres
22mm pipes = 13 litres
15mm pipes = 3.6 litres
LL header tank est 3 litres
So on the face of it we should be able to retain good flow volume and bypass the low loss header and second pump - assuming the primary pump can cope with the whole circuit.
The article by Brendon highlighted a COP of approx 3.4 before removing the LLH. This is approximately what we are getting with a 21c room temp, 6c ambient daytemp. (COP 3.7 at 8c with a mild night of 5c) However we are only using Radiators albeit with decent sized emitters.
Presuming UFH is a more efficient way to use an ASHP on account of the lower flow temperatures and large areas involved? So perhaps a radiator system with a higher flow temperature will never attain the efficiencies of a good UFH installation?
So if we are going to break into the low COP4s I guess we might still look at getting the LLH removed or bypassed. But perhaps recognise that the gains might not be as good as an UFH installation? Decisions, Decisions.
If you look at the Ecodan databook numbers you can get some idea of the effect on COP of lowering your LWT by 1.5 degrees. It's not going to be a lot; I doubt enough on its own to for you to break into the 4s.
The attached sheet came from the Ecodan Data files Posted by @mjr ( thanks for that) And is for the Ecodan PUZ-WM85-VAA
Can anyone explain.
This might be a daft question but I’m curious about how the different outputs are selected? Between Max, Nominal, Mid and Min. Are these four output groups selected automatically by the FTC or are they somehow derived by any of the settings?
The attached sheet came from the Ecodan Data files Posted by @mjr ( thanks for that) And is for the Ecodan PUZ-WM85-VAA
Can anyone explain.
This might be a daft question but I’m curious about how the different outputs are selected? Between Max, Nominal, Mid and Min. Are these four output groups selected automatically by the FTC or are they somehow derived by any of the settings?
It is not possible to select a particular output, they are included within the test data to indicate the likely power consumption and efficiency when operating at different levels of loading. The notes below indicate how they should be interpreted. When operating the heat pump will vary the compressor speed (frequency) to achieve the required output energy.
Definition of terms Max :Performance at Maximum compressor frequency Nominal :Performance at Nominal compressor frequency Mid :Performance at Medium compressor frequency (80% of Nominal) Min :Performance at Minimum compressor frequency :This icon means injection circuit is active. NOTES: • The reference data at water outlet temperatures of 35°C,40°C,45°C,50°C,55°C and 60°C are shown. • Gray highlighted data means integrated data including defrost operation. • Actual performance may vary depending on operating conditions. • These data are measured based on EN14511-2013. Nominal heating capacity A7W35: Heating outside air DB 7°C/WB 6°C, Water outlet temperature 35°C (ΔT=5°C) A7W45: Heating outside air DB 7°C/WB 6°C, Water outlet temperature 45°C (ΔT=5°C) A2W35: Heating outside air DB 2°C/WB 1°C, Water outlet temperature 35°C ( ΔT=5°C)
We had seen a small loss of temperature across the Flow Pipes as the water passed through the Low Loss Header tank, into the central heating circuit. This loss was approx 1 to1.5c. This was thought to be a small amount of mixing in the Low loss header tank as the water passed through.
So to check this we tested the return temperatures and confirmed that the Primary Return water was exiting the Low Loss Header Tank slightly hotter than the incoming Central Heating Return water. This increase in Primary Return temperature was also about 1c. This more or less matched the Flow pipe temp. loss, suggesting that a small amount of mixing as might be expected. This mixing could be considerably more if flow rates created by each pump were different.
The test limitation was that the thermistor readings only measure in 1c increments which give a wide ish margin of accuracy.
Can anyone help with some maths on estimating percentage of energy loss of DT….?
After a summer break we are now instigating some changes to our HP by making the 2 circuit (DHW and secondary heating circuits) layout into a single circuit, driven off a single pump.
But before I discus this with our installer it would help to clarify in my own mind what potential losses are occurring through the LLH.
The above extract describes a 1c to 1.5c loss of Flow temperature as the LWT crosses the LLH and exits into the secondary circuit becoming the Secondary Flow.
Previously I didn’t think this was a major loss. Since the flow temperature was 37 or 40c. However I’m doubting that thought now:-
My thought now is that the loss is a continuous 11/2 degrees loss of the DT5c.
If this were a true loss this equates to a potential 33% reduction of the DT due to water mixing within the LLH.
HOWEVER I realise the energy isn’t lost, it is recycled by passing back into the Primary Return pipe and so the return temperature is raised by an equivalent 11/2c. Also it would appear that the HP is having to raise the DT by 6 or 7 degrees in order to provide only a 5C DT at the emitters…. so there must be some dynamic losses in this situation which I can’t quantify?
So my question is how do I calculate the actual energy inefficiency as a result of mixing through the LLH.
I’ve attached a sketch of the pipes through the LLH with a simplified 1c loss of DT
@sunandair if your sketch is accurate, and the DT on both sides is the same, and the flow rate on both sides is the same, then heat energy transfer is the same both sides, as the maths for this is DT times flow rate.
however, don't forget that one of the major causes of energy loss with an LLH, is the fact that the flowT must drop across it. That means that to achieve a given flowT on the radiator side (which you need to get the emitters providing the required heat output to your house), you need a higher flowT on the source side. Then where the source is a HP, that higher flowT on the source side means the HP is running less efficiently.
to put it another way, for the same DT and flow rate (= heat energy output) at the HP, it costs more in input (electrical) energy , the higher the absolute values of the flowT (and returnT) .
If you have no LLH, the flowT of the HP and the flowT of the radiators are by definition the same, thus your HP is as efficient as it can be given whatever other conditions are in play.
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