Paul Chaffers, Technical Events Manager at NAPIT, examines the various considerations surrounding heat pump electrical supplies.
Understanding the workings of heat pumps (HP) can be challenging for any electrical contractor unfamiliar with renewable heating systems. Even if you’re only contracted to carry out the electrical installation work associated with the HP installation, knowledge of this type of technology will be helpful.
Since heat pumps are electrical appliances, it is increasingly likely that consumers will seek to install solar PV systems to reduce the operating costs, so you may become involved in contracts that include PV systems and associated inverters and battery storage systems.
HPs are unique because they don’t generate heat like most home heating systems, which either burn fuel or convert electricity directly into heat. They work by moving heat energy – refrigerators and car air-conditioner units are HPs that you will almost certainly already have. For heating systems, the movement of heat is from the outside into your home. Critically, they deliver more heat energy than the electrical energy they consume (by a factor of up to three or four), making them more efficient.
Heat pump types
Broadly speaking, there are three common types of HP:
1. Ground Source HP (GSHP): As its name suggests, it uses the ground as the heat source; basically, the ground acts like a giant battery that stores solar energy. Providing no more heat is extracted than can be replenished, the ground can be considered a sustainable renewable energy source.
2. Air Source HP (ASHP): Again, self-explanatory, using the air as the heat source and providing the outside air temperature remains above the minimum threshold of the ASHP, it can be considered a sustainable source of renewable energy.
3. Water Source HP (WSHP): Some properties with access to a lake, or similar, are able to use this type of HP to provide a sustainable source of renewable energy.
HP design can be very complex, especially for GSHPs, which involve collectors being installed in the ground either horizontally (in trenches) or vertically (in boreholes).
Typically, a sealed pipe is used as the collector containing a water/antifreeze solution which is circulated through the HP heat exchanger. This extracts the heat and passes it to the HP refrigerant, see Fig 1.
The performance of ASHPs is less stable than that of GSHPs due to the outdoor ambient temperature variance throughout the day and year, and this also applies to WSHPs due to the seasonal variations in the water temperature.
The following steps show the basic operation of an ASHP, illustrated in Fig 3:
1. The liquid, which is a refrigerant, warms and turns to gas as the outside air blows over the refrigerant tubes.
2. The gas is then pressurised as it passes through a compressor, adding more heat ( just like a bicycle pump warms in operation).
3. Next, the compressed hot gas passes into the heat exchanger, surrounded by cool air or water, which warms and circulates around the home.
4. Finally, as the heat is extracted from the refrigerant, it condenses back into a cool liquid, ready to start the cycle again.
Electrical supply considerations
The electrical supply needs to be addressed in the same manner as any other installation in terms of the design current and spare capacity within the existing installation. However, it’s likely that an MCS registered installer will carry out the HP installation, and you should be aware that MCS standards need to be adhered to.
For example, HP design standard MIS-3005-D clause 5.7.1 requires that the MCS contractor ensures the electricity supply is adequate for the size of the HP specified.
One common misunderstanding relates to the kW rating of the HP when sizing conductors, because HPs are sold with a kW rating that reflects the heating capacity of the unit. Kilowatt-hour is the unit used by gas and electrical companies to measure the energy we use; this term also applies to the amount of energy we generate, and appliances also use kW for their power rating.
Do not confuse the kW rating of the HP with the nominal current rating when carrying out cable calculations.
The HP manufacturer will specify the nominal current rating, including types and ratings of suitable protective devices for their equipment within their technical specifications. For example, there may be a requirement for a circuit-breaker to have a Type C or Type D time/current curve to allow for start-up currents. Remember, BS 7671 requires electrical equipment to be selected and erected, taking account of the manufacturers’ instructions (Regulation 510.3).
Regulation 132.16 provides the requirements for making additions and alterations. The following is a summary of what will need to be assessed before any work can commence:
- Is the rating of existing equipment adequate for the new addition?
- Is the rating of the distributor’s equipment adequate for the new addition?
- Are the earthing and bonding arrangements adequate (where necessary for the protective measure used)?
In order to address the first two points listed above, an assessment of maximum demand will be required. Regulation 311.1 permits diversity to be taken into account when determining the maximum demand of an installation.
For domestic and similar installations, a simple method of assessing maximum demand using diversity could be to take 100% of the highest-rated circuit-breaker and 40% of all other circuits (see worked example, below).
Determine the maximum demand using the diversity of an installation which has an 8-way consumer unit with the main switch rated at 100 A. It has two spare ways and supplies six existing circuits. The intention is to add a 20 A HP supply to one of the spare ways:
l One 40 A cooker circuit
l Two 32 A ring-final circuits
l One 16 A immersion heater
l Two 6 A lighting circuits
l One 20 A heat pump
40 + (0.4 x (32 + 32 + 16 + 6 + 6 + 20)) = 84.8 A
Providing the maximum demand does not exceed the service cut-out rating or the rating of the CU main switch, then the loading is deemed acceptable.
However, in order to determine the adequacy of the distributor’s equipment, further checks may be required with the Distribution Network Operators (DNO).
DNOs are responsible for delivering the power from transmission networks to the nation’s homes, businesses, and industries. There are seven DNOs across 14 different districts that handle the distribution of power across the country. These are shown in Fig 4.
Energy Networks Association (ENA)
The ENA represents all the DNOs and manages the infrastructure which delivers electricity and gas.
It supports the DNOs by managing, creating and maintaining a vast array of electricity codes, standards and regulations that support the day-to-day operation of the energy networks.
There is often a need to contact them to ascertain the suitability of connecting low-carbon technologies. Even if the service cut-out rating is known, the ratings and utilisation of all local distribution network assets require consideration.
The ENA maps the increased network load associated with the roll-out of such technologies allowing accurate modelling of the network to prevent unintended stress on assets and ineffective and uneconomic asset replacement programmes.
The process for notifying HP installations to the ENA is similar to that required for the EV charging infrastructure; in fact, the same forms are used. The ENA has produced a combined installation process flowchart, which can be found on their website: www.energynetworks.org.
The flowchart delivers two outcomes, which are ‘Connect & Notify’ (within 28 days), or ‘Apply to Connect’.
The application form is the same whether the installation is a ‘Connect & Notify’ or an ‘Apply to Connect’ and the form can be downloaded in the further information box below.
Basically, where the new maximum demand exceeds 60 A (13.8 kVA) per phase or where there are any deficiencies with the DNO’s equipment, an application to connect will be required prior to the HP installation.
Earthing and bonding
The last consideration of Regulation 132.16 regarding the adequacy of earthing and bonding arrangements must not be overlooked. The Regulation refers to earthing and bonding being adequate, if necessary, for the protective measure applied for the safety of the addition or alteration.
It’s likely that the protective measure used for protection against electric shock will be automatic disconnection of supply (ADS) and, therefore, the earthing and bonding must be fully compliant with the latest requirements of BS 7671.
Building Regulations notification
Additional to the ENA notification, in England and Wales notifiable work under the Building Regulations must also be notified within 21 days of completion. Examples of notifiable work associated with HP installations are shown in Table 1 with the level of membership scope required.
Whether you are an MCS registered business, certificated for multiple scopes of membership, or a subcontractor carrying out one aspect of the installation, hopefully this article has reinforced the need to communicate and ensure that the correct procedures are followed.
MCS contractors have to issue a comprehensive handover pack after commissioning, including details of all notifications and certificates.
Lastly, the installation must be registered on the MCS Installation Database (MID) no later than 10 working days after commissioning and an MCS Certificate generated. Further Information As part of the launch of BS 7671:2018+A2:2022, all of NAPIT’s comprehensive collection of technical guidance books have been updated. They’re available, as always, in both electronic and paper format at: www.napitdirect.co.uk
Get more details about NAPIT membership and benefits here