Heat Pumps

Technical Overview

This guide describes the principles of operation of heat pump systems and gives some advice as to how owners can use them to achieve the best levels of comfort and economy.

Types of Heat Pump

There are three main heat pump types which are used for heating water in a dwelling. More traditional air conditioning systems can provide warm air as well as cooling but these are outside the scope of this guide. The three main hot water types are:


1)   Ground Source – Extracts heat energy from the ground either directly from water sources or from an anti-freeze mixture in buried pipes,


2)   Air Source – Extracts heat energy from outside air, and


3)   Exhaust Air – Ventilates the dwelling and extracts heat energy from the air as it leaves the building.


Heat pumps may comprise a single unit where energy is extracted from the heat source and output as warm water from a single box, or as a split system where there may be an outside unit and an inside unit and refrigerant gas is exchanged between the two. .

Ancillary equipment


Both heating water and domestic hot water can be provided by heat pump systems. They are well suited to low temperature heating applications such as under-floor heating. If used with radiators then the radiators will need to much larger than normal to provide sufficient heat output.


Domestic hot water is generally limited to around 50°C from heat pump systems. This is quite hot enough for most domestic applications although not as hot as may be experienced from traditional boiler systems.


If heating only is required then this can be connected directly to the heat pump. However, a heating water store (buffer tank) is generally provided which allows more flexibility in the control of individual rooms (or zones) by the use of thermostats.


If domestic hot water is required then a hot water cylinder will be needed to store this water. These are traditionally quite large and heavy when compared to traditional copper cylinders although new generations of stainless steel cylinders are now becoming available and these are much more manageable. Compact heat pumps are available which incorporate a hot water cylinder within the unit. The capacity of these cylinders is generally less than 200 litres as against 300 – 400 litres for stand alone cylinders.


An additional heat source may be required within the system to supplement the heat pump in extreme weather conditions.


An additional heat source will also be required to periodically increase the temperature of domestic water in order to kill off bacteria in the cylinder and prevent growth of legionella. This legionella purge (or pasteurisation process) can take place anywhere between once per day and once every 14 days.


Weather Compensation


Heat pumps generally have two modes of operation. These are ‘Fixed Condensing’ where the output water temperature is set at a fixed value and ‘Floating Condensing’ or Weather Compensated where the heating water temperature from the heat pump is changed in accordance with the outdoor temperature.


When it is warm outside, not very much heat is being lost from the building and so the heating water can be quite cool and still maintain a comfortable temperature inside. As it gets colder outside, more heat is lost from the building and the water in the heating system needs to be hotter in order to maintain comfort levels inside. The line as shown on the graph above defines the relationship between these two temperatures is called the heating curve. This curve can be selected on the heat pump controller, normally as a number and the shape of the curve can be altered in order to optimise comfort and efficiency of the system throughout the year.


For domestic hot water generation, the heat pump is set to fixed condensing operation as the maximum temperature that the heat pump can reach is generally required.

Coefficient of Performance (COP)


The efficiency of a heat pump will vary depending on two main factors. These are the temperature of the source (outside air or Ground loop) and the temperature of the water being generated.


As the source temperature rises, the efficiency of the heat pump increases as more energy can be extracted. As the temperature of the heating water rises, more energy is required and the efficiency of the heat pump falls.


The actual efficiency is calculated by arithmetically dividing the power output by the power input. For example, a heat pump delivering 10kW may have a power input of 2.5kW. Hence the COP is 10/2.5 = 4. This may be presented as an efficiency of 400%.


The quoted value of a heat pump COP is generally calculated with a source temperature of 0°C and an output water temperature of 35°C. An alternative presentation is a seasonally adjusted COP which takes into account average weather conditions over a year in a broad geographical location. It also takes into account the amount of energy required from additional heat sources, pumps, valves and other energy consuming devices. Whilst this can only ever be a best guess as there is no such thing as an average year, it does give a better indication of reality than published benchmark figures.


Ground source heat pumps have a much more consistent COP then their Air Source counterparts. This is because the ground source temperature will only vary by a few degrees through the year whereas the air source temperature varies enormously.


Heat pumps are least efficient when producing domestic hot water. In winter, Hot water COPs can be a little as 2.

Bi-Valent Point


This rather scary sounding title is probably the most crucial element of a heat pump system in terms of performance and economy.


Four things are important here:


1)   The calculated heat loss of a building at a nominal minimum temperature (say -3°C)


2)   The output power of the heat pump


3)   The average amount of energy required to run the heat pump


4)   The proportion of the year when the temperature falls below the nominal minimum


These factors are all taken into account in order to best predict which size of heat pump is suitable for a property.


In simple terms, the outside temperature drops in winter and so does the output power of the heat pump. The energy required to heat the building increases however and at a given temperature the two values will be coincident. This temperature is called the bi-valent point.


Below this point, the heat pump cannot deliver enough power to heat the building to the designed temperature. Above this point, the heat pump delivers more energy than is required and as a result consumes a little more energy than is required by the heating system.


If additional heating by electric element is activated for too long because the bi-valent point is too high then running costs will soar. If the bi-valent point is too low, then the heat pump will consume more energy through the rest of the year and again, running costs will increase.


The trick is then, to select the heat pump which best matches the calculated heat loss for the property concerned in order to maximise economy whilst maintaining comfort levels.

And this is important because………..


There are a number of circumstances that can have an adverse effect on running costs. These may include but are not limited to the following:


1)   The building heat losses are greater than calculated


2)   The design of the heat pump may not take the bi-valent point into consideration


3)   The heat pump settings may not be optimal


4)   The heat pump may be broken


5)   Additional heat sources are incorrectly connected or programmed


6)   The heat pump has not been correctly sized


7)   Weather patterns change


8)   Other electrical loads


So, looking at these points in more detail:


1)   It is possible to calculate very accurately the heat transfer characteristics of specific materials. It is rather more difficult to predict how these materials behave in installed conditions given the nature of building projects. An awful lot of water is absorbed by a building during its construction. The building will eventually dry out but it takes about a year and increases the calculated heat loss by about 30% in the first year after construction. The amount of shelter or exposure can radically alter the heat losses of a building as wind and rain are very good at stripping heat from a building. Even buildings with a few miles of each other can behave quite differently. Chimneys are useful for removing smoke from open fires but can have a catastrophic effect on building heat losses. Vented cavities can cause massive problems with under-floor heating systems where open frame ‘eco’ joists are used in the construction of a building. In such circumstances air can easily travel across the building under the floor and strip virtually all the heat from the heating system.


2)   Heat pump control systems work quite differently from each other. Even some relatively recent machines may have elements in their control system which gives them a predilection for connection of additional heat where it is not really required or is making up for a deficiency in the building construction. In these circumstances the user needs to ensure that the heat pump is in an operating mode which prevents the use of additional heat. If comfort levels are not maintained in these circumstances above the bi-valent point then further investigations must be made. The alternative is an electricity bill which could run into many £100s.


3)   Heat pumps are set up by the commissioning engineer. These settings should not be changed without full working knowledge of the system and what parameters have been calculated during the design process. Commissioning engineers are human though and even they make mistakes occasionally. If there any doubts about the way a heat pump is set up, we are happy to assist customers in checking system parameters.


4)   Most heat pump systems will have an emergency mode which enables hot water and comfort levels to be maintained without the heat pump even running. This will rely on a back-up heat source such as immersion heaters or a back-up boiler. It is often quite difficult to see when a heat pump has failed. The control unit is often tucked away in a corner or in a cupboard. The best prevention against surprises in the next electricity bill is to disable emergency modes so that the building and/or hot water goes cold. It is then possible to manually over-ride the system and call the installer for assistance.


5)   Hot water temperature should be programmed so that it is within reach of the compressor without the need for additional heat. A setting of 48°C is normally adequate and well within the capability of all heat pump compressors. Cylinder immersion heaters may be activated by the heat pump controller or by an external time-switch. Ensure that these have been correctly identified and set. Cylinder immersion heater are normally only required for the periodic increase for legionella protection.


6)   An over-sized heat pump will have a higher than expected running cost over the year but only by a few percent. An undersized machine will require excessive additional heat to be provided during relatively mild winter conditions. Bear in mind that the size of the heat pump will have been calculated based on building design information. This calculation is rarely in error.


7)   In 2010 the United Kingdom experienced the coldest winter for 30 years. Temperatures of 10 – 20 degrees below zero were common for several months. In these conditions, some air source units will no longer work, relying totally on back-up heat sources, the output of the units that will run at those temperatures is severely reduced. Heat losses of buildings massively increase. Many of my customers reported an increase of up to 20% in energy usage of their chosen heating fuel during that period as that was in the relatively mild South-West. Remember, any reduction in temperature below the bi-valent point will increase use of additional heat.


8)   Check the electrical load of other equipment in the house. Halogen lighting is particularly greedy with electricity, also showers, cookers and home entertainment systems can consume large amounts of electricity over time.

Hot Water Loops


The installation of a secondary hot water loop is a great way of ensuring hot water is available more or less instantly at any outlet. This is well suited to water conservation. However, it should be borne in mind that a continuously circulating loop is effectively a giant radiator and the loop will lose heat when it is hot. For reasons of economy, it is wise to reduce the operating time of hot water loops to peak use periods and ensure that the thermostat fitted to the return pipe is set just below the normal return temperature of the water (normally about 43°C). All hot water loop pipe-work must be insulated in order to preserve comfort levels and reduce heat loss. In some systems, hot water is only produced a short period in each hour. It is not a very good idea to throw away all that heat between hot water heating periods.

Towel Rails


Under-floor heating systems often require supplementary heat in bathrooms due to the small heated floor area. Towel rails are an ideal heat source for this function.


There are five main ways of heating a towel rail. These are:


1)   Direct electricity – clean, convenient and very reliable


2)   As part of the heating system – Permanently hot in winter, permanently cold in summer and warmish the rest of the time, not a great solution.


3)   A combination of the above using a dual fuel towel rail


4)   Connected to the secondary hot water loop – Always hot when the loop is running but can quickly diminish the stored water temperature if there are many towel rails. Bear in mind that these are heated during the heat pump’s least efficient operating periods. Towel rails connected to a hot water loop must be made of non ferrous metals.


5)   Connected to the primary hot water heating coil and switched on when the heat pump is not heating hot water – Similar in operation to 4 above but towel rails can be made of ferrous metals and hence are cheaper (but probably have a much shorter life).


Please note the comments made in the Hot water Loop section which are equally applicable to towel rail circuits.

Making the Most of Your Heat Pump


There are two basic rules to getting the best out of your heat pump system. These are:


1)   Never let the system use an additional heat source unless it is really necessary, and


2)   Always run the system as cool as possible.


Taking the last point first. It is vital that the system is set up to suit the individual dwelling. No two buildings are the same and it is important to get to know how your building with its heating and hot water installation behaves.


The starting point is your under-floor heating system. This will have been designed to provide an acceptable level of comfort with the designed heat input at -3°C. The flow rates provided on the installation drawing will provide heat output normally between 100% and 120% of the required output. There is some scope here to tune individual rooms to perform better or worse in order to balance the heat output with the heat loss for each room. The specification is a starting point. Do not be afraid to adjust the flow rates in each zone in order to achieve this balance. The following process may take several days but will be well worth it in the long run.


But first, try this experiment. Please do not do this whilst driving. Get into your car on a cool day or after dark when the temperature is below about 15°C or less. After the car is warm, turn the heater to maximum temperature but turn that heater fan off – You will remain cold. Now increase the fan speed until you are comfortable. Now turn the heater temperature down until you start to feel cold again. Now, increase the fan speed until you are comfortable again. Repeat this process until the fan speed is at maximum. The outcome of this experiment is that you should notice that there is a balance between air temperature and air flow. The trick is to find the lowest temperature which gives you comfort and finding an air flow rate to match it. Now this is exactly what needs to be done to the under-floor heating system, and yes, it works with radiators too but to a lesser extent. Remember heat pumps are most efficient at low water output temperatures.


In order to prevent external influences affecting the adjustments, turn all of your room thermostats up to maximum and make sure that all external doors and windows are kept closed. Ensure that the heat pump room sensor is located in an area away from direct sunlight or drafts. If you have an adjustable room thermostat for the heat pump control, ensure that this is set to 20°C or is in its central position and the heat pump is under automatic control.


Now set the heat curve on your heat pump to just below the mid curve. If, after 24 hours the building is generally too hot or too cold, adjust the heat curve up or down to compensate. You will reach a point where most of the building will be about right but some areas will be warmer and some cooler. Increase the flow rates to the areas which are cooler and decrease the flow rates to the warmer areas.


It is unlikely that you will achieve perfect balance but is should be possible to get very close to the desired comfort levels.


If there is one stubborn area that is unacceptably cool, you will need to increase the heat curve until this area is acceptable (having set the water flow rates to maximum in this area). You will then need to reduce the flow rates in the remaining areas to achieve the balance point.


Now you can set the thermostats in each room to the desired temperature and these will serve to limit the heat output from the floor if there is a temporary excursion above the desired temperature due to a log fire, solar gain or other external influence.


Minor variations may occur during the year and if so, this indicates that the level of the curve is correct but the shape is slightly wrong. This can be altered by choosing a higher or lower heat curve and increasing or decreasing the heat curve offset to compensate. Your commissioning engineer can advise in greater detail if you need to do this.


Hot water temperatures should generally be set at about 48°C and no higher than 50°C. Depending on the heat pump type, the Legionella purge can be set either daily, weekly or for an alternative fixed period


Additional heat sources can normally be disabled. This can be done manually or automatically based on the bi-valent point. Make sure that doing this does not affect the Legionella purge however or the heat pump may sit doing nothing waiting for an event that can never happen. Talk to us about this for further details


If your heat pump has a summer switch off temperature then make sure that this suits the needs of your building. Also check the bi-valent point setting. You may well get away with reducing this temperature if your building is well constructed.


Other Things to Bear in Mind


Screed floors need less heat than wooden floors because they are more efficient at transferring the heat. The larger the heated area, the more heat is transferred to the room. Carpets and underlay reduce heat output.


Most electricity will be used at night as the flow temperatures will automatically increase during the hours of darkness. Finding an off peak tariff where night time electricity is cheapest is the most suitable but you will also need to consider your other energy usage before choosing the best tariff.


Make sure that all pipes are lagged, that there are no leaks in the system and no corrosion on any parts.


Keep the air flows clear for Air Source Heat pumps.


Have your installer check over the system once per year. Servicing is not mandatory but it is wise to have the following checks made annually:


Brine (antifreeze) in ground loops should be sampled and tested to ensure the strength of the antifreeze is still correct, that there has been no build up of microbiological contamination and that corrosion inhibitors are still at the correct level. We can send you a sample bottle and will test this free of charge or we can sample this for you for a modest fee.


Dirty filters, coils, and fans reduce airflow through the system. Reduced airflow decreases system performance and can damage the system’s compressor. Check, clean or change filters during annual maintenance or as needed (e.g. 3 monthly for exhaust air units), and maintain the system according to manufacturer’s instructions.


Clean outdoor coils whenever they appear dirty; occasionally, turn off power to the fan and clean it. Remove vegetation and clutter from around the outdoor unit.


High pressure air, should be used once a year to blow air in reverse through air source heat exchanger. The will require removing the fan protector and hence the machine must be isolated and locked off prior to this action being taken.


Annual service shall include:


Inspect ducts, filters, fans, and any indoor coil for dirt and other obstructions

Clean strainers on brine and heating circuits, ensure system is re-pressurised and introduced air is flushed from system.

Diagnose and seal duct leakage

Verify adequate airflow by measurement

Verify correct refrigerant charge by indirect measurement

Inspect electric terminals, and if necessary, clean and tighten connections, and apply nonconductive coating

Verify correct electric control, making sure that all installed functions operate according to the system design

Verify correct thermostat setting and operation for cylinder immersion heaters

Checking of all sensor calibration