Wednesday, 8 June 2016

Selecting the right Heat Meter.

This topic is VERY IMPORTANT
I am aware that the pressure requirements of heat meters is often overlooked - this can be detrimental to a heat pump system.

A heat pump will normally be connected to a radiator or underfloor heat-emitter circuit.  A circulation pump (circulator) is required to ‘force’ the water around this circuit.  Each part of the pipework and each components acts as a restriction to flow. The greater the restriction, the bigger the circulation pump needed.  Indeed, many systems have 2 or more circulators, whilst it should be possible to only have one.

Heat meters  (reading heat in Kw’s) are often required to calculate RHI payments. The main component of a heat meter is a device fitted in a pipe to measure the liquid flowrate through the system. This device will cause a restriction to flow. However, it would be ridiculous for our heat-measuring device to impair energy-efficiency by either causing an inadequate liquid flow-rate, or by increasing circulation pump energy demand.

Let us get a ‘feel’ of the pressures involved.   Metres head (height) of water is a good understandable unit of pressure measurement, but several other units are used;
1 bar pressure (about 1 atmosphere)  = 10.2m head of water = 1,000mB = 100kPa

Most common circulation pumps can produce a maximum of 5 or 6 metres water head  (0.5 to 0.6 bar), however, this is at the pump’s low flow-rate.  For energy-efficient operation, we should operate them at mid-pressure range, ideally no more than a pressure of 2.5m head of water to work against.
Some heat pumps have one circulator to circulate through the heat pump and the emitters.  If the heat pump requires say 1m water head for its required flow-rate, and the emitter circuit requires a further 1m head, then the circulation pump needs to exert a total pressure of 2m head. There is little pressure left  to overcome restrictions for a the heat meter, so the heat meter should not be too restrictive.    Even if your system has 2 circulators, the head meter should not cause a significant restriction so you must SELECT THE CORRECT METER.  

At their rated flow-rate, some  heat meters will require high pressure for circulation.   Some cheaper options can require up to 2.5m head pressure. This is a lot.   However, if a larger heat meter is chosen, its pressure drop can be very low since, for example,  if you halve the flow-rate, you quarter the pressure requirements.   Modern meters can retain high accuracy even at very low flow rates.
E.g. if your system flow is 1 cubic m / hour, your best low-pressure option might be to fit a 2.5 cubic m/hr device.

The example chart  below show how the pressure drop can be very low if the meter is used at a flow-rate well below its maximum.   Note also that some meters are inherently more ‘restrictive’ than others.   The colours are my own opinion, and relate to adding a heat meter to a heat pump system.

The Vortex Flow Sensors have exactly the same issue, and there are fewer pressure ranges to choose from.  However, in the region of 25% of their rated capacity, the pressure drop is usually nice and low.
The message here is – Don’t fit a meter without assessing the pressure drop at your expected flow rate.  Don’t necessarily trust the meter supplier since they don’t all understand the energy loss implications related to heat pumps, and may want to sell you the cheapest option.    It could be perfectly acceptable using a high/medium pressure drop heat meter  on a biomass district heating system feeding radiators, but for heat pump applications, we could be running into trouble unless the pressure drop is low.

Oddly, I see little mention of pressure drop on many heat meter data sheets.

Wednesday, 9 September 2015

Heat Pump Performance Monitoring Examples

On 10th September I will be giving a brief presentation at the Ground Source Heat Pump Expo at the Ricoh Arena on the topic ‘Energy or Performance Monitoring’, so its timely to do a little blog here to elaborate on some of the examples I will be showing from some OpenEnergyMonitor dashboards I have been using

Note; The below are just examples for this blog, and don't necessarily show the whole story.

I have been working with OpenEnergyMonitor for some time, and now have various installs using OEM kit from
In brief, most of the systems I have installed use around 8 temperature sensors, CT power measurement with voltage sensing (real power including power factor), and/or pulse counting from standard electrical kWh meter.  I have used Grundfoss VFS flow sensors, but we are currently working on the direct interrogation of a Kamstrup heat meter, giving heat output

Data is sent via Ethernet to and displayed on dashboards (as samples below). These real-time graphs give a fantastic tool for the installer and the home owner. They show exactly what is happening now, and what has happened over the previous hour/week/month or year. The information can be used to improve the design of a system and also be used to fine tune the user settings.

Let’s start with a SIMPLE dashboard example

This type of dashboard can be accessed on any internet-connected computer using    e.g.

The dashboard above shows a bar graph of daily energy input to the heat pump. This can be checked periodically for unusual values. It is showing high use on 15th March. By mousing-over the graph we can see that on this day, 24.2 kWh were used. The reason for this high use could be investigated.  The time period can easily be changed to anything you wish by zooming in or out.  Below this is the outside temperature. This might be interesting on its own right, but may be interesting if compared to energy used per day.   To the right are a few useful dials and figures -  cylinder temperature, room and outside temperature – things any home owner might like to know.    We can see that as from 6th April, the system is switched off.  
This type of simple dashboard is ideal for the home owner, but we can make as many dashboards we like, of varying complexity and detail.  These are very useful for installers and designers, and a far more in depth analysis can be made.

Let’s start with a good example of a GSHP connected to underfloor heating.

This is a 12kW (max output) inverter-drive GSHP operating for a 40 minute period here. The green area represents electrical input and the purple represents heat (direct reading from Kamstrup meter).  The ratio if these two areas gives the COP.  We can see the flow and return temperatures slowly ramping up to a final flow temperature of only 32°C.  Since this is September, the ground collector is exceptionally warm. This, along with the low flow temperature explains why the COP is currently almost 6.  Current conditions are ideal, but from tests earlier in the year, we expect to see average COPs for heating in excess of 4.
This graph is showing us a healthy flow-return dt of 6 degrees.  It is also showing how nicely the speed of the compressor drops in response to the rising flow temperature.

Below is another snip. This time from a fixed-speed GSHP. This one shows the source temperature too.

This shows a period of about 1/3rd of a day, and approx. 30min. run durations which is quite acceptable. The flow and return is nice and low, with average flow temperature around 30C.  The underfloor is a good design here, but he source is dipping below zero. This is not ideal, but a zoom-out of yearly temperatures and knowledge of total heat used would give better understanding.  In this case, since the underfloor is so good, it may be acceptable to have a slightly inferior ground source.

Next, a good example of a GSHP heating a domestic hot water cylinder.  The cylinder is copper with the heat exchanger coil in the bottom section of the cylinder. The heat pump is only 3.5kW and the coil is a nice large 3sq m.  

This example is showing the heat pump electrical input power as the shaded yellow area  (no heat meter fitted). The heating period starts at about 1.3kw input and finishes at around 1.7kw.  It also shows four temperatures;  cylinder top and bottom, and the flow and return temperatures.

This graph is showing the early evening heating period having been off by a time clock.
As we can see, the top of the cylinder is still at a useable 50°C before heating, but the bottom has dropped to 40°C.  The 24 minute heating period shown here starts by heating the bottom water from 40°C.  Indeed, this system has been set up carefully to ensure the system heats from a lower starting temperature.  The heat pump ‘sees’ flow and return temperatures of only 45/40°C at the start.  The 40°C cylinder bottom (not very hot) ‘pulls down’ the heat pump working temperatures, resulting in a high energy-efficiency.  By looking up the heat pumps performance data, we can estimate the average COP with reasonable accuracy; here it is about 3.5 at the start of the heating-up cycle.
As the cylinder warms, we can observe the point just before 18:00 where the bottom is becoming warmer than the top, and natural convection causes the top of the cylinder to rise with the bottom. After about 25 mins the whole cylinder has reached about 53°C.  At the end, the heat pump ‘sees’ temperatures of 55/52°C.  This is getting quite hot, and getting close to the limit of the heat pump’s comfort zone. The COP here may be about 2.8.  (taken from heat pump data).  We can then look at the period of time the heat pump has spent at different COPs, and estimate the COP for the whole DHW heating session. Its somewhere around 3.05.

If the system were enabled 24/7, and the sensor position not optimised,  the lower cylinder would not  drop so far, so the cylinder would  heat  more frequently from a higher starting point. The average working temperature would be much higher, so COP would be lower.  At worse, the COP could be not much better than 2.8.  Added to this, losses from the pipe run, and starting-up losses could result in worse performance.

We can therefore use the monitor to enable us to set the system to operate at a low average temperature, but for the cylinder top to remain at a useful temperature (e.g. say 47°C).
We can also see how nice and close the final cylinder temperature (53°C) is to the maximum flow temperature of the heat pump (55°C). This minimises the need for immersion heater (with COP of only 1).  In this case, the compact copper heat exchanger is exceptionally large compared to the heat pump size, and the coil is also only occupying the lower section of the cylinder.  This gives exceptionally good results, and allows us to heat some of the water  in a ‘batch’ from a colder starting point.

For the next example we have a complete contrast. This is a very inefficient system!

This one is a 14kW  ASHP.  The heat pump is fine, and function exceptionally well,  but the cylinder heat exchanger is debatably a little small for this big heat pump.  

Looking at the graph, heating starts when the middle of the cylinder is 48C.  the flow temperature s runs up to 60C within 15mins, at which point, the input power drops and the heat pump ‘tracks’ the 60C flow temperature.  After 30mins running, the cylinder is 55C.  The flow temperature here is considerable higher than our previous example. In part due to the heat pump NOT reducing its speed, and in part due to the smaller heat exchanger coil, but it has not done too bad.  
However, the period after 55°C is clearly grossly inefficient.    We can see that the compressor switches off frequently and spends the next 3.5 hrs! attempting to achieve 60°C.  The other thing to mention here is that the distance between the heat pump and cylinder is around 15m. What is actually happening is that most of the heat is simply being lost from the pipe run.  The energy consumed is shown by the yellow area of the power plot.  The final 5 degrees (to 60C) uses several times the power (area) of the first section from 1 to 2.   
The biggest problem here is poor use of the controls.Clearly, it would make a lot of sense to adjust the hot water setting to 55°C so that the heat pump stops.  .   
The final ‘floor heating’ period is just as terrible as the DHW period. Here, only 1 or 2 underfloor zones are open so the flow rate is far too low, as can be seen by the large temperature difference between the flow and return.  This is in excess of 10 degrees. Again, heat dissipated by the floor is far too small for this large heat pump.
This is a clear case of an over-sized heat pump connected to a cylinder and emitter system. A smaller unit would work far better.

Finally, just to top anyone up with a little heat pump theory, I am adding a graph that I used to illustrate heat pump efficiency v the output temperature.   If there is one thing to learn about heat pumps – LEARN THIS.

Here we have the characteristics of 2 sample heat pumps.  The vertical Y axis shows the efficiency, the COP. A 3kW immersion gives 3 kW of heat. It has a COP of 1. Heat pumps give out more heat than they consume because they extract heat from outside.  The X axis shows operating output temperatures ranging from tepid on the left to very hot on the right. 

I am showing 2 typical heat pumps. A typical (R407C refrigerant) unit can reach say 55°C, whilst a ‘high temperature’ (134A refrigerant)  unit may achieve 65°C.  Anyhow whatever type, we can see that to the left, where the water is lukewarm, the heat pump has an easy time, hence the COP is very high (1kw in for 4.5kW of heat). I liken this to driving a car up a slight incline. We should get good fuel economy here, maybe 50mpg .  However, it we heat up to 65°C, the temperature ‘lift’ is great, and this is a little like driving a car up a steep incline – we are in a low gear and the MPG is only 20!.    In the same way that you will NEVER get good fuel economy when driving up a very long steep hill, you will not get a good COP when heating to a high temperature.  That said, it should always be better than using an immersion heater.

So, knowledge of the performance of any heat pump should be understood, and data should be available for all models relating to output temperatures at specific ground source or air-source temperatures. 

If you have a high temperature heat pump, it doesn't mean you always have to operate it at a high temperature.  If you operate it at lower temperatures, the performance should be far better. It is however always a good idea to find out the working limits of you unit.

Some of you may have wondered about the flattening of the curve at very low temperatures.  I have drawn it that way because some heat pumps are not very good at the extremities (limits) of their performance. However, I am finding (and partly guessing) that most inverter heat pumps with electronic expansion valves work very well over a very wide range, so some heat pumps will easily exceed COPs of 5 in ideal conditions (usually late spring or autumn, when not much heating is needed) .Never forget mid-winter conditions - this is the time we need most heat, so the operational area to focus on.

So, it is with an understanding of the characteristics of heat pumps, performance monitoring can be used to great advantage. In general, we want heat pumps to spend as much time at lower output temperatures ( and high source temperatures) as possible.

Monday, 8 June 2015

Temperature sensing with OpenEnergyMonitor

Temperature sensors for monitoring heat pumps

I have been using energy monitors with heat pumps for a few years now and thought I should briefly share my experience.  This covers the practical side of temperature sensors, and is based on my experience.
(If you just want advice about fitting temperatures sensors onto pipes, skip further down the page)

The sensors used in modules are DS18b20. These are Maxim 1-wire digital sensors.

They actually have 3 connections;   0v, 5v and signal.   Since each sensor has its own unique i.d. code, multiple sensors are simply connected into the same 3 terminals on the monitor device.
The great advantage of these digital sensors is that there are no errors due to cabling.  Phone extension cable can be good enough.  Thermistor sensors (variable resistance, as PT100) rely on a specific cable resistance and furthermore some sensors could ‘drift’ out of calibration.   A further advantage of digital sensors is that they do not in themselves generate heat. They can therefore be used in still air with good accuracy.
If several sensors are strapped together (with an elastic band) and tested in a thermos flask over a few hours, most seem to read within 0.2 degrees C of each other.  If you have say 6 sensors, you can chose those that agree most for the most important sensing (e.g. flow and return).   Anyhow, these sensors are perfectly accurate enough for this type of monitoring, and there is comfort in knowing that if you are seeing 35.2C (for example) on a graph, then that sensor tip must be at 35.2 +/-, with a small error margin of say 0.15 degrees.  This is because the digital signal is generated inside the sensor.

What to measure on a heat pump?

Typical temperature measurement could include the following:
Ground source
Water flow and water return from heat pump (hot side)
Ground collector glycol  inlet and outlet from heat pump (cold side) 
Outside (ambient) air temperature.  
Hot water cylinder temperature, and at least one room temperature.

Air source
Water flow and water return from heat pump (hot side)
Outside (ambient) air temperature and air-off (cooled air leaving heat pump) 
Hot water cylinder temperature, and at least one room temperature.

For either of the above there could be a buffer cylinder involved in the design, therefore the flow and return from buffer cylinder to emitter (e.g. underfloor or radiators) may need to be monitored.
There may also be mixing valves (sometimes unnecessary mixing valves) on underfloor manifolds. These may also need monitoring.

For more detailed analysis, the refrigerant internals of the heat pump can be monitored. These may include discharge, suction and liquid temperatures.   This is however a little intrusive, and could affect the warranty. It may be necessary to discuss this with the system installers.

Measuring air and liquids 


Measuring internal air temperature is relatively straight forward, but sensors must be positioned away from any source of heat or radiation source.  Sunlight and close-by appliances and even lighting can greatly affect readings.
The general rule for outside air measurement is to keep the sensor well away from sunshine, or areas that get sunshine.  The sensor can also drop below the air temperature if it can ‘see’ a clear sky.  A small polystyrene roof is a good shield protection from positive or ‘negative’ radiation (i.e. when radiation from sensor is greater than its surroundings).  It is also necessary to keep the sensor dry, unless you purposefully wish to cover it with a wet ‘sock’ to record Wet Bulb Temperature.  There is a lot of information available about this relating to weather stations.


Most heat pump monitoring involves the temperature measurement of water and liquids flowing in the pipes.
For very high accuracy results, as required by heat meter devices, an immersion pocket is required. (Heat Meters measure the difference between two sensors, so both must be very accurate)
The pocket usually involves a ‘tee’ fitting and a hollow pocket that is completely surrounded by the liquid. The sensor is inserted into the pocket.
An easier and cheaper method is to fix the sensor to the outside of a metal pipe. This method is used in all heat pumps for their on-board sensing.
Any sensor that is outside the liquid can be affected by the temperature of the surrounding air, but the error in the reading can be negligible if the sensor is mounted correctly.

There are various methods as follows-
1)  Sensor strapped to metal pipe or metal fitting
2) Sensor inserted in surface pocket (a pipe soldered to the pipe’s surface)

This shows 10mm pipes that have been soft-soldered to copper fittings. Due to the high conductivity of copper, the temperature of the inside of the small pipe is almost exactly the same as the temperature of the liquid inside the pipe. After insulating (lagging) this pipe, the accuracy of the sensor inserted inside the small pipe should be excellent.  Ideally the pocket internal diameter is close to the sensor diameter.  Conductive paste should be used before inserting the sensor
It is a good idea to fit several pockets like this in the right places during installation. Obviously the pipe must be empty.
Many heat pump manufacturers solder pipes on the outside of water or refrigerant pipes so that their sensor probes can easily be inserted to give accurate measurement

Strapping a sensor to a pipe (or a pipe fitting)

 Whilst not as good as surrounding the sensor with a conductive copper pocket, it is possible to simply strap the sensor to a pipe.   Strapping using a copper strap (as used in refrigeration) , or copper wire, will give better results.
The following shows one simple method.

Copper or brass conduct heat well.  The temperature difference between the inside of the pipe and the outside is tiny, so always fit sensors onto metals.  Plastic pipes are poor thermal conductors, so the effect of surrounding air can affect the sensor temperature. Furthermore, if the pipe is changing in temperature, the sensor may ‘lag’in time behind. i.e. it will respond very slowly to temperature changes.  This may or may not be a problem.

This is not the best position for a sensor since there is 3mm of plastic between the liquid inside and the probe, however, the copper wire strap is making the best of a bad job.  Heat will conduct along the copper. There is a lot of copper in contact with the plastic, and this will conduct and will transfer heat to/from the ‘surrounded’ probe.  If this is insulated well on the outside, it may give good-enough results for ground source comparisons.  Time response may be a little slow, which may not necessarily bad.   Fitting the probe onto a metal fitting is a far better bet.

Thursday, 6 March 2014

Getting the best from underfloor heating

Many years ago, when heat pumps were not so common, I found it a real struggle to get any of the main underfloor heating suppliers to embrace the need for low water temperatures for use with heat pumps.
15 years on, and I’m sure that things have changed dramatically, but I’m still stumbling across things that make me doubt that.
When I visited Germany over 10 years ago, I got the feeling they commonly use much closer spacing between floor pipes than we do in the UK, and I have read of very close spacing (50mm) in Austria.  I realise these are potentially cold countries, but even recently visiting a Spanish heat pump company, they were surprised that we don’t use closer pipe spacing (more pipes).

Over the last 6 months I have heard several references in the trade to; ‘never use closer than 150mm pipe spacing’.   However, CIBSE clearly give ratings for 100mm spacing, as does the MCS Emitter Guide, so why are some in the UK so resistant to putting more pipe into floors.

To basics  -   COP v heat pump water temperature
Heat pumps are more energy-efficient if the water temperature is low.
Coefficient of performance (COP) = heat output / electrical power input

This graph of an air source heat pump shows how beneficial a low flow temperature is.   Units like this with electronic expansion valves are particularly efficient at very low water temperatures.

How to get a low flow temperature with underfloor heating

The values for the example graph above have been extracted from CIBSE, and show a general trend for a fixed heat output.  It demonstrates what we all should know  - the closer the spacing, the lower the required water temperature for the same heat output.
Putting the two together we could plot pipe spacing v COP.  It would clearly indicate that closer spacing improves COP, so why do some seem reticent to embrace close spacing?  Is that added cost for more pipe really that much??
I have gleaned that some people have the notion that close spacing could lead to warm floors and an overly hot house.  The whole point here is that with closer spacing, we can turn the water temperature setting (heating curve) down, and get the same heat output, thus improving the COP.
I think some of the fears about hot  floors stem from boiler systems, many of which had plenty of extra output capacity for quick warm-up, and hot patches could result. However, heat pumps are generally slower response, with lower water temperatures, so are much more forgiving in this respect.  I recently visited a system where I had used 50mm spacing in an always-open loop in a bathroom.  I asked the owner if they had ever thought the bathroom floor was too warm.  The answer was no, never. 
The reason why I had used 50mm spacing was that we didn’t want a buffer cylinder, so I was trying to ensure there was adequate water quantity, and flow-rate in the floor – effectively using the floor as a buffer.  
Another hot (excuse the pun) topic here is floor coverings. People are now seeing that a carpet will drop the star rating for RHI, so there is an added financial incentive for having solid (tiled etc.) floor, as seems far more common in Germany, instead of carpet.
However, by designing a house where some rooms are tiled, and some are carpet, you are somewhat limiting any changes that future occupants might wish.  (e.g.  if 100mm spacing in a carpeted room, and 200mm in a tiled room).   Underfloor pipes are literally set in stone, so can never be changed.  The only solution that I could think of here would be to interlace the loops (given that there is often more than 1 loop per room).  There is always the option of turning off one of the interlaced loops.  I like multiple interlaced loops.
I have come across an installation where a new owner had fitted thick pile carpet in one room without thinking the affect it would have.  The result was a cold room, remedied only by turning up the water temperature, thus  increasing the running cost, and the reliance on room thermostats to limit heat to the tiled rooms.  
The general design approach for underfloor is to consider the heat required (e.g. watts/sq m) , but unless a buffer cylinder is fitted, we should also consider what heat is being delivered to the floor, given that some zones will be closed for some of the time, and the outside temperature is seldom at design temperature (-2C etc.).   For most of the time, we have far more available heat than the floor needs.  Even with modulating heat pumps, there can still be a tendency for the flow temperature to stray above the theoretical flow/return temperatures. This is another reason for favouring more pipe in the floor.  Furthermore,  MCS requires the heat pump to provide at least 100% of the design at -2C etc.  Due to models only being available in certain size jumps, the heat pump installed is often oversized, so this is an added reason for ensuring that there is adequate pipe in the floor.
For any thinking that buffer cylinders are the perfect answer – they may be an answer, but they are seldom perfect, as shown by this piece of monitoring.     (

In this example of a system with a simple buffer tank, the heat pump flow/return needs to be approx. 4 degrees hotter than UH flow/return.    This could reduce energy efficiency by 10%, plus the added energy to run a second circulation pump.
A buffer can usually be avoided IF enough zones are always on AND if there is plenty of pipe in the screed.
Another potential worry is the thermal mass of the screed.   I was recently involved in the design of a passive house where a buffer cylinder was not wanted, so I proposed  to use the screed as the buffer, and fitted 9 x 100m loops with average 100mm spacing in a  floor that was 200mm thick.  One might expect some temperature overshoot, and I may not have been so bold as to propose it if it were not for the fact that due to the interlaced nature of pipework,  we could shut off ½ the zones if we needed to.
The result was surprisingly good with all room zones open and control on one master thermostat. The whole house has very even temperatures.  This demonstrates the self-regulating nature of very low temperature underfloor heating.
Another concern I have heard of is the pumping power required  for such a lot of pipe.  In the above passive house example.  If the heat pump needs 10 lit/min to give a 5 degree flow-return dt , then the flow rate is about 1.11 litres/min for each pipe loop.  If we used a more standard 5 loops, the flow rate per loop would be 2 litres/ min.  For the same heat (kW) and same dt, more pipe actually means less pumping power since the heat output per m of pipe is lower, and the flow-rate for each loop  is lower.
Finally, I have also come across the notion that mixer valves and pumps are desirable, even for a heat pump.  Here is an example to illustrate the potential penalty of having a mixer even if the mixer never actually mixes (i.e. its fully open).

The arrangement here with recorded temperatures shows that the flow to the floor is always equal in temperature to the return to the heat pump. In this example, the flow from the heat pump is 7 degrees higher  than the flow to the floor.  The mean floor temperature  was only 23°C (average of 28 & 18).
If the mixing valve and pump were removed, the heat pump’s flow could go directly to the bottom manifold.  
To get the same floor heat output, the heat pump setting could be adjusted down so as to give a working flow of 26 and return of 20.  This is a reduction in flow temperature of 9 degrees, potentially (according to our first graph at top of page ) saving 13%.  
(see top graph - The COP at 35°C is 3.1, at 26C its 3.1, saving 13% in COP). 

All these little details can eat into potential savings. They should be dealt with at design stage.

Thursday, 20 February 2014

Potential perils of plastic pipe

A college tutor recently suggested to me that the current environment in the heating industry does not encourage thinking.   Maybe I should therefore be less amazed when I come across plumbers who think copper and plastic pipe are interchangeable. 

Environmental considerations aside, a more direct issue is that of flow rate and pressure drops.  Bore size of plastic is considerably smaller than copper, and it seems that this is often overlooked.  I am not against plastic, but if installers select plastic pipe instead of copper without checking the size correctly, this could have a negative effect on heat pump’s COP.  

(If you drop-off ½ way through this - read the scenario at the end )

With respect to the materials of plastic and copper, to my surprise I found a very informative document that discusses primary energy of these two materials in some detail, and concludes that the total energy involved in mining and manufacturing copper is far greater than the total energy (including the crude oil) to manufacture plastic pipe.  Looking at other potentially less biased general data on copper and plastic, it seems to confirm that more oil is used processing copper than would be used to produce plastic.  

Another factor to consider, that mainly affects pipe runs to hot taps etc, is the heat capacity of the pipe material. Plastic has a relatively high specific heat, and the wall is thicker, but it’s light. The net thermal capacity of the two is fairly similar.
(The cold feel of copper is more to do with conductivity from the hand than heat capacity)

However, obvious factors aside, one of the biggest issues that could affect installations involving heat pumps relates to the internal bore diameter.  This could have a noticeable effect on the energy-efficiency of the system.

All metric pipes are measured by their outside diameter. As can be seen, with common pipe sizes (outside diameters), equivalent plastic pipes have considerably smaller internal area to copper. This has a dramatic effect on flow characteristics.

 The graphs below illustrate the relative internal dimensions of common pipes.

 (see AECB Water Standard)

A brief note about smoothness - It seems a common belief that plastic is ‘smoother’ than copper, but the inner wall ‘smoothess’ of the two is the same.  However, plastic can be one-piece with slow sweep bends. This is certainly ‘smoother’ than copper with tight elbows.   Re inner surface, we can assume the two materials are the same.

Whilst it is fairly easy to look-up the pressure drop resulting from a specific flow rate with a specific pipe, we can see from the 2nd graph at-a-glance the relative flow capacity since the cross sectional area loosely indicates flow capacity.

If a certain flow rate is required, then we can look-up the required pressure that is required across the pipe length (beginning of pipe to end of pipe length).  The internal bore must be chosen such that the circulation pump is not overly large and energy-wasteful.

This graph shows the pressure required to maintain a certain flow rate for a fixed length of pipe.  As can be seen, the pressure drop along the example pipe varies very dramatically, so the wall thickness makes very big difference. 
In this example, we can see that a 15mm copper pipe could be used with a common central heating pump (shown at 3.6m head, 36kPa). However, if plastic were chosen, then one would need almost 9m head to achieve the required flow - far beyond the capability of normal circulators. On the other hand, if 22mm plastic were chosen, the pressure requirements would be only 1m head (10kPa) which is likely to achieve very low circulation pump energy.

In real life, we tend to have a pump connected to a pipe system, and the flow rate that results is dictate by the balance between the pressure produced by the pump and the ‘restriction’ of the entire pipe work circuit.

For our final graph, we consider a pipe circulating with a fixed-pressure electronic pump (Alpha etc)

The above graphs show relative changes in flow rate that would result from a fixed pressure. If the pipe chosen were too small, then a larger circulation pump may be needed in an attempt to compensate for the extra restriction caused by the small internal bore.

The point here is that by choosing plastic instead of copper of the same nominal size, the system could potentially suffer unless the sizing is checked. There is of course no problem using plastic if it’s the right diameter.  Indeed, 28mm plastic may be an ideal choice for the connections from a heat pump simply to minimise noise transmission.  The best final solution is often a mixture of both plastic and copper for a multitude of reasons.

All this emphasises how dramatically the wall-thickness affects flow rates and pump pressure requirements, but how does this translate into reduced COPs?

Scenario (based on something I observed on a barn conversion)

Let us consider a radiator at a far distance from the heat pump.  The flow-rate relates to the pressure drop, which relates directly to the pipe-run length, and of course, the required flow-rate relates to the room size (bigger the heat demand, the more flow required).  In this instance the room is large.
The default pipe size choice would be normal 15mm (outside diameter), but if the sums are done, it may become apparent that the choice should be between either 15mm copper or 22mm plastic. 

How could a pipe with too small bore affect the COP? 

Radiators should be balanced, in general by throttling valves (lockshield)  on smaller radiators, and those with shorter pipe runs. However, this is actually quite a difficult thing to achieve with a heat pump because the temperature difference (water inlet to water outlet) may only be 5 degrees (°C).  (It’s much easier to measure and adjust when the temperature differences are larger).

It is not ideal, or easy, to have to throttle most other radiators on a system, and there is the added risk of the circulation pump not being big enough to cope.
The likely result of any ‘restrictive’ pipe-runs would be a reduced flow rate to the radiator.  This would result in a considerable area (the bottom) of the radiator being cool, and a reduced heat output to that room.
The obvious action to redress the short-fall of heat would be to increase the flow temperature by increasing the heating curve setting of the heat pump. i.e. increasing the flow temperate from say 40 to 45°C.    Now the heat pump has to heat ALL water to a 5 degrees higher level.  This is likely to reduce the COP by 10 to 12%.

This all indicates that one must never assume copper and plastic are interchangeable without considering the pressure drops and diameters.  That aside, we have also highlighted the importance radiator balancing.  Ideally the pipe runs to radiators would be laid out so that the flow rates are naturally about right without the need for much valve adjustment.  A little extra work on the design makes life much easier thereafter.

Anyone intersting in further reading on water flows - this site is very intersting