Three phase or single phase heat pump: which to choose and what supply you need
“Three phase heat pump” is searched 50 times a month in the UK; in France “pompe à chaleur monophasé” and “triphasé” 260 times each. The short answer: single phase is fine only for small units, and even then it is not the best choice. For a home switching to a heat pump we recommend three phase.
01
Single phase and three phase: the difference
Single phase brings one phase at 230 V into the house; three phase brings three phases, 400 V between them. The same power on three phase is shared over three wires. One of our 12 kW heat pumps, at -7 °C with radiator water at 55 °C, delivers 10.09 kW at COP 2.06: it draws 4.9 kW. On single phase that is 21 A on one phase; on three phase, 7 A per phase. How much a heat pump uses over a year is in the article on running costs.
02
What supply a UK home has
Most UK homes have a single-phase supply with a 60 or 100 A main fuse (Energy Saving Trust). A heat pump drawing up to 32 A can be connected and then notified to the network operator within 28 days, provided the home’s maximum demand stays under 13.8 kVA per phase and the model is listed for connect and notify (Energy Networks Association); otherwise you apply before connecting. Upgrading to three phase is a paid service: Electricity North West gives an average cost of £5,370 for 80 A per phase.
03
At the end of the line the voltage drops
A house close to the substation has no problems. The further away it is, the more the cable matters: a large load on one phase pulls the voltage on that phase down and unbalances the three-phase network. A case from one of us: the last house on the line, 1,500 metres from the substation, cables fifty years old, and on the line heat pumps, air conditioners, electric cookers and solar panels. By day the solar panels exporting push the voltage up, in the evening the loads pull it down: on one phase it ran from 192 to 253 V. The EN 50160 standard asks for 207-253 V for 95% of the time and never below 195.5 V. It was solved with three phase, splitting the loads across the three phases. Solar panels and heat pumps together are in the article on solar.
04
Single phase only suits small units
Small heat pumps also come in single phase. But it is not the best choice: all the current runs on one wire, the voltage on that phase drops and the supply is left tight for the rest of the house. Homes that switch to a heat pump usually add an induction hob, solar panels, sometimes an electric car: on three phase these loads are shared. Our units from 18 kW up are three phase only.
05
Is three phase worth it?
If the upgrade costs a few hundred pounds, up to about £1,000, three phase is clearly worth it. At £5,000 it is a lot: then a single-phase unit within the 32 A connect and notify limit can be the sensible choice, as long as the house is not at the end of a long line. For an opinion on your home, ask us for information.
06
Frequently asked questions
Single phase or three phase heat pump?
Three phase if you can: the current is shared over three wires, the voltage drops less and the supply stays free for cooking and lighting. Single phase suits small units.
Can I connect a heat pump to a single-phase supply?
Yes, if it draws up to 32 A and the home stays under 13.8 kVA per phase: under the ENA connect and notify rules you connect it and notify the network operator within 28 days.
How much does a three phase upgrade cost?
It depends on the network operator and the street: Electricity North West gives an average of £5,370 for 80 A per phase. Up to about £1,000 it is clearly worth it.
Why does my voltage drop in the evening?
If the house is far from the substation, the loads on the same phase along the line pull the voltage down; by day solar panels exporting push it up. With three phase the loads are split across the phases.
How much current does a 12 kW heat pump draw?
Our 12 kW unit at -7 °C with water at 55 °C draws 4.9 kW: about 21 A on single phase, about 7 A per phase on three phase.
All the technical tips are on the series page.
