Frequently ask questions for Householders


Frequently Asked Questions about Heat Pumps

How do heat pumps differ from boilers?

There are no gas or oil connections as they use renewable energy sources. They provide lower flow temperature water (up to 55oC generally) compared to a boiler (generally 70-82oC) and their capacity for supplying useful energy is dependent on input temperature (from ground/air/water). Heat pumps are much more efficient (boiler can achieve no more than 100% efficiency) and have no compulsory annual servicing equipment.

Are heat pumps suitable for UK houses?

Heat pumps can be sited indoors or outdoors. Householders should consider advice from an accredited installer regarding ground conditions for ground source heat pumps as performance is linked to heat extraction potential (wet soil better than dry). Also, it is worth noting that trenching for ground loops requires access to land and a bore hole will require access for drilling equipment.

Air source heat pumps achieve good CoPs due to relatively high ambient winter air temperatures.

Are heat pumps safe?

There is no combustion process involved with the heat pump process (termed as a 'cold' heat medium). There are no risks of gas leaks or oil spills.

The refrigerant commonly used is R407C/R404A and these HFCs are currently acceptable for use in today?s environment. The refrigeration systems tend to be pre-charged.

Are heat pumps energy and carbon efficient?

Heat pumps use a carbon and cost free input energy (from the air or ground) which is converted into useful heat. This means that where a heat pump has a co-efficient of performance of say 4, 75% of the energy is from the environment with 25% from the electrical energy.

The conversion process uses electricity to drive the compressor within the heat pump cycle but so long as the efficiency of the heat pump is greater than 100%, it is energy and carbon efficient. For example, a typical 4kW ground source heat pump is 400% efficient in that it gives 4kW of useful heat for every 1kW of electricity used. This is expressed normally as a spot measurement called Coefficient of Performance (in this case a CoP of 4). Efficiencies of 250-500% are generally available in the UK. Conditions of operation determine the spot measured Co P i.e. Air/ground temperature and Water flow temperature. Those conditions tend to be from laboratory conditions and not real in use testing. If the householder uses a renewable energy tariff then the input energy is also carbon free, creating a truly renewable and zero carbon system.

Are heat pumps more costly to buy and operate?

We know that through the conversion process, environmental energy input is turned into higher kW of heat. Compare that to a gas fired electricity production plant in which an input fuel (at a highly volatile price) is used in a generation process. In general, the cost of electricity power for the heat pump process is less volatile than gas. (recent npower increases have been 5% higher for gas than electric and is consistent with historical price rises)

The most important consideration is the operating costs which are very low. Unlike the requirement for gas boilers, there are no maintenance and servicing costs associated with heat pumps.

Heat pumps are more expensive than gas and oil boiler equivalents but attract only 5% VAT and grants are available to reduce the cost. In England and Wales, the Low Carbon Buildings Programme provides grants of up to £1200 to individual householders for the installation of ground source heat pumps and £900 for air source heat pumps. The air source heat pump grant is expected to be available from around March 2008 once the first air source products and contractors are approved to the Microgeneration Certification Scheme requirements.

In Scotland, grants are available for both ground source and air source heat pumps to a value of up to 30% of the installation cost or a maximum of £4000.

Where does the energy come from and will it always be available?

Heat Pumps can use various sources of energy but the common sources are ground (using solar heat from the earth) and air (ambient air). The energy sources are inexhaustible and the solar heat within the ground is replenished continually.

What type of radiator/heat distributor do I need?

Radiator systems designed for boilers tend not to be optimal for heat pumps. Radiators need higher temperatures (typically 70-80 oc powered by boiler) but heat pumps require flow temperatures around 55 oc. Therefore, underfloor heating is the preferred distribution method. Large surface area radiators are an option and as a rule of thumb, a 20oc reduction in flow temperature requires a 100% oversize of emitter.

How is domestic hot water delivered and what type of hot water cylinder do I need?

Domestic hot water is normally stored at 60-65oc and as the heat pumps can generally only lift temperatures to around 45-50 oc, an additional water heating element will be required. This is usually an electric heating element in the water cylinder for boost. It is normal to try to use an off peak tariff for hot water provision by the heat pump and this can reduced the costs for boost with appropriate control. Larger coil cylinders are required for heat transfer but both direct and indirect cylinder types are suitable. Some manufacturers supply high temperature heat pumps to satisfy total hot water demand but there will be a reduction in CoP.

As a rule of thumb, ensure 200W/person is added to the design load if the heat pump is to be used for domestic hot water production.

Surely it's too cold in the UK for air source heat pumps?

Air source heat pumps can provide efficient benefits at temperatures as low as -20oc. Average temperatures in the UK range from 2 oc to 22 oc. The record low was -10 oc.

The efficiency of a heat pump is determined by two critical factors. Efficiency reduces as temperatures falls to very low levels e.g. 0 oc, and hot water system temperature increases.

Can heat pumps be used for cooling purposes?

Heat pumps are available in reverse mode to provide summer cooling. The waste heat (i.e. the heat extracted in the cooling process) can be used to pre-heat domestic hot water.


Frequently Asked Questions on Thermostatic Mixing Valves

What is a Thermostatic Mixing Valve (TMV)?

TMVs accurately control water temperatures for bathing, showering, hand-washing and bidets. They are designed to maintain the desired water temperature, even when the incoming water pressures/flow rates change. By contrast, a mechanical mixing valve cannot adjust to changes in supply temperature, pressure or flow rate.

Digital mixing valves are also available. For different types of TMV see questions 2 and 4.

TMVs can be fitted under baths and basins, be part of shower fittings, or as a feature of exposed hot and cold water mixers.

How does a TMV work?


Hot and cold water entering the valve is mixed to a temperature pre-selected by the user or installer. A thermally sensitive mechanism within the valve automatically proportions the amount of hot and cold water entering to produce the required blend. The mechanism then automatically compensates for any reasonable variations in supply pressures or temperatures within the design range to maintain the pre-selected temperature. In the event of cold water supply failure, the thermostatic mixing mechanism will automatically shut down the flow to prevent discharge of dangerously hot water. The flow will also be shut down in the event of a hot water supply failure to prevent thermal shock. In the event of a sudden supply temperature increase (‘spike’), the TMV will quickly (but not absolutely instantly) adapt and deliver water at a safe temperature. For more on temperature spikes see question 15.

Main components

Thermostatic element
A temperature sensitive element which expands or contracts depending on the temperature of the water surrounding it. When the thermostatic element senses a temperature change, it moves a piston which changes the proportion of hot and cold water being mixed in the valve. This movement enables the valve to remain stable and to shut down in case of cold or hot water failure.

Usually connected to the thermostatic element, the piston moves back and forth over the cold and hot ports of the valve, changing the proportion of hot and cold water entering the valve depending on the temperature of the water.

Return spring
When the thermostatic element expands it moves the piston under its own energy and compresses the return spring; when the thermostatic element is cooled, the thermostatic element contracts and the return spring pushes the piston back.

Temperature adjustment
Many thermostatic mixing valves have a separate temperature adjustment (usually beneath a lock shield cover). Typically, this can be adjusted to change the position of the piston and therefore the proportion of hot and cold water entering the valve.

Digital Mixing Valves

Digital thermostatic mixing valves control water temperature and flow by means of accurate temperature measurement and electrical/electronic control. As with all TMVs they can comply with TMV2 and TMV3 approval schemes and maintain the same level of thermostatic control. Digital products can utilise manual or touch free control by the user, the latter providing a degree of infection control for surface transmitted infections. The digital nature of these products can also provide additional duty flushing or thermal disinfection regimes, programmed by facilities management personnel.

Where are TMVs fitted? What distance can they be from an appliance?

TMVs are fitted within or near a tap or shower unit.

For TMVs not within a unit, a useful guide to recommended distances from a unit is the Health Technical Memorandum 04-01, published by the Department of Health, paragraph 9.49 which reads as follows:

 “Particular attention should be given to ensuring that pipework containing blended water is kept to the minimum.  Generally, the downstream dead-leg should not exceed 2m, and the complete length of the spur should not exceed 3m.”

Furthermore, the TMVA Code of Practice (available here), Section 4.2 ‘Group Mixing’, states:

  1. The operation of one or more outlets should not affect the operation of any other outlet.
  2. When one valve is used to supply mixed water to a number of outlets the length of the pipe run and the volume of mixed water after the valve should be kept to a minimum.
  3. The maximum pipe run after the mixing valve should be such that the required mixed temperature, at the furthest outlet, should be reached within 30 seconds.

Can TMVs be set to a low temperature?

Yes. There are a number of valves available that can be set between 20-46°C.

How quickly does a TMV adapt when a user changes the tap or shower water temperature?

TMVs will respond quickly to changes in selected temperature. This is verified by the respective TMV schemes that ensure certified products provide water within known anti-scald temperature ranges by conducting third party testing.

It is not recommended that users make very sudden changes to their shower temperature control. However, if a user tries to change their water temperature straight from cold to the maximum temperature, occasionally this can cause a temporary temperature increase of 7°C above the chosen temperature, potentially supplying water at a higher temperature than the recommended maximum. The NHS D08 TMV Model Engineering Specification makes reference to this, and permits a 7°C temperature ‘spike’ to last for 1.2 seconds, with 50°C permitted for a maximum of 0.5 seconds. Therefore as long as a TMV ensures temporary temperature spikes last no longer than these times, it will still be safe.

How should TMVs be maintained? Do they need to be serviced?

The BuildCert TMV2 scheme states:

“It is a requirement that all TMV2 approved valves shall be verified against the original set temperature results once a year.

 When commissioning/testing is due the following performance checks shall be carried out.

  • Measure the mixed water temperature at the outlet.
  • Carry out the cold water supply isolation test by isolating the cold water supply to the TMV, wait for five seconds if water is still flowing check that the temperature is below 46°C.

    If there is no significant change to the set outlet temperature (±2°C or less change from the original settings) and the fail-safe shut off is functioning, then the valve is working correctly and no further service work is required.”

In essence, what is required by TMV2 is an annual functionality check, but many BEAMA members recommend annual preventative servicing.

Several factors may determine the required service frequency of a TMV, most notably water hardness and frequency of use.

TMV3 valves must also receive a regular in-service test to check they are performing adequately. If they are not, the valve must be serviced. In the absence of other instructions or guidance, the regime described in D 08 (an NHS Model Engineering Specification) should be used. D 08 states that the first test must take place 6-8 weeks after commissioning, and the second after 12-15 weeks. Following this, there is no fixed required test frequency, but the valve should be tested at intervals between which there is shown to be little or no change in mixed water temperature. Depending on the change in mixed water temperature between commissioning and the second test, D 08 recommends the third test to take place between 18 and 28 weeks after commissioning.

Are regulations for TMVs the same in all parts of the UK?

While there are some differences between Building Regulations in Scotland and Northern Ireland compared to England and Wales, the maximum hot water regulation of 48°C applies in all areas of the UK for TMV2s, and was established in the Scottish regulations first.

The NHS in England, Northern Ireland, Scotland and Wales all endorse the health guidance note that recommends TMVs as a way to help health employers meet their duty of care.

Can the heating system influence the performance of TMVs?

It is important that the installation method in the manufacturer’s instructions is respected as failure to do so may prevent the valve from operating correctly. Common installation errors are:

i. Failure to use flow regulators when installing the TMV product to a combi boiler system. This may cause performance issues during winter months.

ii. Failure to remove flow regulators when installing to a vented system, ie a traditional immersion heater. This may reduce the water flow.

 All products should be installed according to the manufacturers’ instructions, including supply conditions. If you have any doubt regarding product installation, then please contact the manufacturer.