One of the most common concerns when considering a heat pump installation is whether an existing heating system with microbore pipework will be suitable.
The simple answer is: microbore pipework is not automatically a problem when installing a heat pump.
However, heat pumps operate differently from traditional boilers, and the difference in operating temperatures and flow rates can make existing small-diameter pipework more of a consideration.
The key question is not simply “Is the system microbore?” but rather:
Can the existing pipework deliver the required flow rate to each heat emitter at the design conditions of the heat pump?
Why does pipe size matter?
Any heating system has to transport a certain amount of energy from the heat source to the radiators, underfloor heating or other heat emitters.
The amount of energy that can be transferred is related to the flow rate of water and the temperature difference between the flow and return — commonly referred to as ΔT (delta T).
The lower the design ΔT, the greater the volume of water that needs to circulate to deliver the same amount of heat.
This is where heat pumps differ from many traditional heating systems.
Typical design ΔTs might look something like this:
These figures are examples of commonly used design conditions rather than universal operating requirements.
Why does a heat pump need more flow?
Let's consider a room that has a calculated heat loss of 1,000 watts at the design outdoor temperature.
The heating system needs to deliver approximately 1,000 watts to the room to maintain the required indoor temperature.
If we use a condensing boiler system designed around a 20°C ΔT, the required water flow is relatively low.
For example, the approximate flow required to deliver 1,000 watts at a 20°C ΔT is:
0.0119 litres per second
At this flow rate, an 8mm copper microbore pipe may be capable of delivering the required energy, depending on the pipe length, pressure drop and design criteria. A suitably sized 12mm plastic pipe would also generally provide sufficient capacity.
Now consider a system designed around a 10°C ΔT.
To deliver exactly the same 1,000 watts, the required flow rate approximately doubles:
0.0239 litres per second
This increases the demands placed on the pipework. In this example, approximately 12mm copper or 14mm plastic may be required, depending on the particular installation.
Now we come to the heat pump.
At a 5°C ΔT, delivering the same 1,000 watts requires approximately:
0.0479 litres per second
That's four times the flow required at a 20°C ΔT.
This is why existing microbore pipework can become a consideration when converting a traditional heating system to a heat pump.
Does that mean microbore cannot be used with a heat pump?
No.
This is an important distinction.
The presence of microbore pipework does not automatically mean that a heat pump installation will require the entire heating system to be repiped.
The existing pipework needs to be assessed against the actual requirements of the proposed system.
The important factors include:
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The heat loss of each room
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The output required from each radiator or other emitter
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The design flow temperature
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The design ΔT
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The length and diameter of the existing pipework
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The material and internal diameter of the pipe
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Pressure loss through the pipework
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The available pump head
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The required flow rate to each circuit
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The total system flow rate
A short run of small-bore pipe supplying a relatively small radiator may perform perfectly well.
A long run of 8mm pipe supplying a high-output radiator, however, may present considerably more of a challenge.
The relationship between flow temperature and radiator size
There is another important part of the equation: the temperature at which the heat pump operates.
Traditional boilers have historically operated at much higher flow temperatures than those normally associated with a heat pump.
A conventional heating system may have been designed around temperatures such as 70–80°C flow temperatures.
Modern condensing boilers benefit from lower return temperatures because this allows them to operate in condensing mode and recover additional energy from the flue gases.
Heat pumps work differently again.
They do not rely on combustion or condensation to produce heat. Instead, they use a vapour-compression refrigeration cycle to move heat from one place to another.
Heat pumps generally operate most efficiently at lower flow temperatures. The lower the required flow temperature, within the limits of the system and equipment, the more efficiently the heat pump can generally operate.
This creates another consideration for existing microbore systems.
The real limiting factors
When assessing whether microbore can be retained, two of the most important factors are:
1. The design flow temperature
2. The size and output of the heat emitters
If the existing radiators are too small to provide the required heat output at the heat pump's design flow temperature, simply retaining the existing pipework will not solve the problem.
The radiators may need to be increased in size or replaced with higher-output emitters.
Once the required emitter output has been established, the pipework can then be assessed to determine whether it can deliver the necessary flow.
What can be done if existing microbore is restrictive?
There are several ways of approaching an existing system where small-bore pipework may limit the available flow.
1. Increase the flow temperature
Increasing the heat pump's design flow temperature can increase radiator output, potentially reducing the required flow through individual circuits.
However, this comes with a trade-off: higher heat pump flow temperatures generally reduce heat pump efficiency.
It should therefore not be viewed as a first-choice solution where lower-temperature operation can be achieved through other measures.
2. Increase the size of the heat emitters
Larger radiators can provide the required room output at a lower water temperature.
This can allow the heat pump to operate at a lower flow temperature while still maintaining comfortable room temperatures.
This is often an important part of a successful heat pump conversion.
3. Improve the building fabric
Reducing the heat loss of the property can significantly reduce the amount of heat that the heating system needs to deliver.
Improving insulation, reducing draughts and addressing other sources of heat loss can therefore reduce the required emitter outputs and water flow rates.
4. Use hydraulic separation where appropriate
In some systems, hydraulic separation can be used to manage different flow requirements between circuits.
This can be particularly useful where the heat pump circuit and the distribution system have different hydraulic requirements.
However, hydraulic separation does not create energy or eliminate pipework pressure losses. The system still needs to be correctly designed so that the required heat can reach the emitters.
So, can microbore work with a heat pump?
Yes — in the right circumstances.
The important point is that microbore should not be judged simply by its diameter.
A heat pump installation needs to be designed around the building's heat loss, the required emitter outputs, the chosen flow temperature and the hydraulic requirements of the system.
A small-bore circuit may be perfectly adequate for one radiator but unsuitable for another, depending on the length of the circuit and the amount of heat it needs to transport.
The correct approach is therefore to calculate the requirements rather than automatically assume that all microbore pipework must be replaced.
In Summary
Heat pumps generally operate with a much smaller ΔT than many traditional boiler systems.
This means that more water needs to flow through the heating system to deliver the same amount of heat.
That increased flow requirement is where small-bore pipework can become restrictive.But that does not mean that every microbore system is unsuitable for a heat pump.
The question should always be: Can the existing pipework deliver the required flow at an acceptable pressure drop, while providing the required heat output from the emitters at the chosen heat pump flow temperature?
If the answer is yes, existing microbore may be retained. If the answer is no, the system may require alterations — such as larger pipework, larger radiators, improved insulation, or changes to the hydraulic design.
Ultimately, good heat pump design is about matching the heat source, pipework, emitters and building heat loss as one complete system.
Microbore is simply one part of that equation.
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Want to know whether your pipework is suitable?
The best way to establish whether your existing pipework can work with a heat pump is through a proper heat-loss calculation and system design.
At Thermodynamics South Yorkshire, we assess the heat loss of each room and the existing pipework before determining what needs to be changed.
The result may be that you need new pipework. It may also be that you don't.
The important thing is to know before the installation begins.
Heat Pump Installations with Thermodynamics
At Thermodynamics, we specialise in air source heat pump installations and support homeowners through every stage of the Boiler Upgrade Scheme in Sheffield, Doncaster, Worksop, Retford, Chesterfield and Gainsborough. From assessing your home’s suitability and system design to managing the BUS grant application and completing a high-quality installation, we handle the entire process.
If you’re considering upgrading your boiler and want to take advantage of the £7,500 BUS grant, get in touch with Thermodynamics today for expert advice and a personalised quote.