The short version
- Microbore means heating pipe narrower than the usual 15mm, typically 8mm or 10mm, often run to each radiator from a manifold.
- A heat pump usually runs with a smaller temperature drop between flow and return than a boiler, so it needs a higher flow rate for the same heat. Push that through a narrow pipe and the water moves faster, with more resistance.
- The Energy Saving Trust says it is not always the case that heat pumps cannot work with microbore. Its installer guidance says water speed should stay between 0.5 and 1.5 metres per second.
- Short narrow sections near a radiator often pass the check. Long runs and the main pipes from the heat pump are the parts most likely to need replacing.
If someone has told you microbore means “no heat pump”, here is what is actually going on and what your options are.
What is microbore pipe?
Central heating in British homes is typically plumbed in 15mm copper to the radiators, with 22mm (and sometimes 28mm) for the main flow and return pipes. Microbore is the narrower size. The government-funded Electrification of Heat trial defined it as pipework with a diameter less than 15mm. The tube tables the Energy Saving Trust reproduces list microbore at 6, 8 and 10mm outside diameter; in homes you will usually see 8mm or 10mm.
Look for thin, often bendy pipes coming up through the floor to each radiator. One heat pump manufacturer says microbore became popular in the 1970s because it was affordable, flexible and easy to install. In the Electrification of Heat trial it was more common in homes built after 2001, so check whatever the age of your house.
Why pipe size matters more with a heat pump
General engineering explanation. The heat a pipe carries to a radiator depends on two things: how much water flows through it, and how much the water cools down on its way round (the difference between flow and return temperature, often called the “delta T”). The Energy Saving Trust puts it as an equation: power equals mass flow times the specific heat capacity of water times the temperature difference. Grant’s installation manual for its Aerona³ heat pumps gives the same relationship: flow rate equals heat output divided by the temperature differential, divided by 4.2 for water.
The consequence is simple arithmetic. If the temperature drop is smaller, the flow must be bigger to carry the same heat. Halve the drop and you need twice the flow. A quarter of the drop needs four times the flow.
- Heat pumps run with a smaller drop. A manufacturer that makes both boilers and heat pumps says the delta T is typically wider in gas boilers, so their flow rate is lower than for heat pumps. Grant’s manual asks for a temperature difference of approximately 6 to 8°C across each radiator when balancing, and its worked example uses 5K (the same as 5°C) for the whole system.
- More flow through a narrow pipe means faster water. The Electrification of Heat trial explains that without changing pipework, a higher flow rate means higher velocity and therefore more pressure loss.
- The pump has a limit. Every circulating pump can only push against so much resistance (its “head”), which the manufacturer publishes. The same manufacturer says undersized pipework makes the pump work harder and can bring noise, vibration or short cycling.
- Too little flow hurts too. Grant warns that an excessively low flow rate may cause reduced capacity and poor circulation.
So the real question is whether each pipe can carry the flow its radiator needs at a sensible speed, with a pressure drop the pump can manage.
A worked example: one radiator on 10mm pipe
The Energy Saving Trust’s installer toolkit works through an example: a utility room needing a new radiator that gives out 1,308 watts at the design outdoor temperature, with the final metre of pipe reduced to 10mm. Swapping that metre for 15mm would have meant removing fitted units. The design runs a 5°C temperature drop.
| Pipe carrying 0.0625 litres per second | Water held per metre | Water speed | Within 0.5–1.5 m/s? |
|---|---|---|---|
| 15mm (EST) | 0.146 litres | 0.42 m/s | Slower than the range |
| 10mm (EST) | 0.06 litres | 1.04 m/s | Yes |
| 8mm (our calculation) | 0.0347 litres | about 1.8 m/s | No, too fast |
Dividing the 1,308 watts by the 5°C drop times the heat capacity of water gives about 0.0625 litres per second. Divide that by the volume of water in a metre of pipe and you get the speed. The Energy Saving Trust concludes that the 10mm section is acceptable and, because it is short, the extra pressure is limited. The 8mm row is our own illustration using the same method and the tube capacity table the Energy Saving Trust reproduces; it shows how quickly the margin disappears in a narrower pipe. The last column is our comparison against the Energy Saving Trust’s range, not its wording.
The Energy Saving Trust’s key point is about length: longer runs of small pipe have a much bigger effect on pressure drop and flow, so the longer the microbore run, the more likely it needs replacing.
The main pipes carry the flow for the whole house. Grant’s example is a 12kW output at a 5K drop, which needs about 34.3 litres per minute. As our own illustration only (same method and tube table), that flow would move at roughly 1.8 m/s in 22mm pipe, above the top of the Energy Saving Trust’s range, and about 1.1 m/s in 28mm. One manufacturer says boiler primary pipework typically starts at 22mm, whereas on a home air source heat pump it would often be 28mm or 35mm. Your installer should size it from your heat pump’s actual flow rate.
How the installer checks your pipes
An MCS heat pump design starts with a room-by-room heat loss calculation. MIS 3005-D, the MCS design standard, lists design information that must be provided in writing before installation starts (the original version says to you, the redeveloped MCS:2025 version says to the installer; either way, ask for a copy), including:
- each room’s specific heat loss;
- the type and size of every radiator or other emitter;
- the design emitter temperature, based on the worst performing room;
- the design flow temperature leaving the heat pump.
The MCS design and installation standards (MIS 3005-D and MIS 3005-I) do not set a pipe size or water speed for radiator pipework. Instead, MIS 3005-D says equipment should be installable in line with its manufacturer’s instructions, and the manufacturer’s manual sets the flow rates the heat pump needs. The Energy Saving Trust describes how a designer joins those up: work out the flow each radiator needs, use reference tables or calculations to keep the water speed in range, and calculate the pressure needed to push that flow through any narrow sections.
So the surveyor needs to know where the pipes run and what size they are, which can mean lifting a floorboard or tracing the main pipes back to the boiler. At commissioning, the MCS installation standard’s example checklist asks whether all emitters heat up evenly with a similar delta T, which is how a lukewarm radiator at the end of a long microbore run gets caught. Our guide to what to expect at a heat pump survey covers the rest of the visit.
Options if some pipes are too small
None of these is automatic; each comes out of the calculation for your home.
| Option | When it tends to suit | What to bear in mind |
|---|---|---|
| Keep the microbore | Short tails to individual radiators that pass the flow check | Ask to see the figures for the rooms that only just pass |
| Replace the main runs and primary pipes | Long shared runs, or a 22mm primary too small for the heat pump’s flow | Can avoid replumbing every room, as the short tails can stay |
| Replace individual long runs | A distant room whose radiator cannot get enough flow | Can mean lifting floors or boxing in pipes in that room |
| Larger or extra radiators | Rooms where the existing radiator cannot give out enough heat at a low flow temperature | Fixes the output, not the flow: the room’s heat still has to arrive through its pipe |
| Reduce the heat loss | Rooms with obvious gaps in insulation or draughts | Less heat needed means less flow needed in the same pipe |
| Low loss header or buffer | Where the heat pump’s required flow cannot be met through the system pipework | Adds a vessel and usually a second pump; only where the design justifies it |
Radiators. At heat pump temperatures many radiators need to be larger. MCS’s heat emitter guide says that at a 45°C flow temperature a radiator needs a standard rating of 3.1 times the room’s heat loss. General engineering explanation: a bigger radiator does not change how much heat the room needs, so at the same temperature drop it does not reduce the flow its pipe must carry. See do I need new radiators for a heat pump? and heat pump flow temperature explained.
Low loss headers. One manufacturer explains that its combined volumiser and low loss header gives hydraulic separation between the heat pump’s circuit and the heating system, so the flow through the heat pump can be maintained whatever flow the system side needs. General engineering explanation: it does not make a narrow pipe carry more heat; the system side still needs a pump that can move the right flow through it. Our article on whether heat pumps need a buffer tank explains when a buffer, volumiser or header is justified, and when it is not.
Matching the old system. One heat pump manufacturer says its research found modern heat pumps can work effectively with microbore pipework with the correct installation approach, and that the key is matching the heat pump’s operating conditions to the existing system, particularly flow temperature and delta T. Any delta T must still be one the heat pump’s manufacturer allows.
How disruptive is replacing microbore?
In the Electrification of Heat trial, installations took 2 to 4 days on average, including any new radiators and thermal storage but not insulation or other energy efficiency work. Where microbore pipework was replaced, the installation could take around 8 to 10 days.
Many of the trial’s designers and installers were unhappy fitting a heat pump to a system with microbore, because the higher flow would raise velocity and pressure loss. Replacing the pipes added cost and disruption, and some homes did not go ahead because that cost was too high within the project’s limits or the householder found it too disruptive. The summary still says none of these challenges proved insurmountable. Replacing only the main runs, or one problem run, is usually a much smaller job than replumbing the whole house. Our guide to what to expect on installation day covers the wider job.
Pipework changes are one of the things that affect the price, alongside the size of system your home needs, the radiators, where the outdoor unit goes and how far the pipe run is, hot water and any electrical work. See heat pump costs in Manchester. You may be eligible for the Boiler Upgrade Scheme, which offers £7,500 towards an air source heat pump in England and Wales and may cover part of the cost.
What to ask an installer, and where ByEco stands
If you have microbore, ask:
- Which pipes are microbore, how long are the runs, and where do they go?
- What flow rate does each radiator need at the design temperature drop, and what water speed does that give in its pipe?
- Can the heat pump’s pump overcome the pressure drop, or is a second pump, header or buffer needed, and why?
- Which pipes would you replace, which would stay, and what would be lifted or boxed in?
- Which radiators would change, and what is the design flow temperature?
- Will you check at commissioning that every radiator heats up evenly with a similar temperature drop?
We are MCS certified for air source heat pump installation (certificate IAA10330), and we apply for the £7,500 Boiler Upgrade Scheme grant on your behalf. Microbore is one of the things we look at during a free survey: we check the pipe sizes and runs against each room’s heat loss, explain what can stay and what needs changing, and give you an itemised quote. If you are happy with it, we go ahead. Find out more about our air source heat pump service or book a free survey.
Sources
Every figure on this page was checked against primary sources on 3 November 2026. Grant rules, planning rules and prices change — if you are reading this much later, verify before acting.
- Energy Saving Trust — Heat pump installer toolkit 3.5: How to make microbore pipes work
- Energy Saving Trust — Heat pump installer toolkit 3.3: System volume and frost protection
- Energy Systems Catapult for DESNZ — Electrification of Heat Demonstration Project: summary report
- Grant UK — Aerona³ R32 heat pump installation and servicing instructions (DOC 0136 rev 2.4)
- Grant UK — The case for the low loss header
- Mitsubishi Electric — Why smaller pipes are no longer a bore to deal with
- Baxi — Why pipe sizing matters for air source heat pumps
- MCS — MIS 3005-D:2025 Heat Pump Design Standard, Issue 2.0
- MCS — MIS 3005-D The Heat Pump Design Standard, Issue 3.0
- MCS — MIS 3005-I:2025 Heat Pump Installation Standard, Issue 1.0
- MCS — MCS 021 Heat Emitter Guide, Issue 2.1
- GOV.UK — Apply for the Boiler Upgrade Scheme
- GOV.UK — Apply for the Boiler Upgrade Scheme: what you can get
Book your free home survey
Every home is different. The cost depends on the size of system, any radiators that need upgrading, the pipework and where the outdoor unit goes. Get in touch and we will arrange a free survey of your home and give you a clear quote. If you are happy with it, we can go ahead.