DOMESTIC HEATING O
ver the next five years, the numbers of UK homeowners looking to move away from fossil fuels is likely to increase sharply. So, the question of whether to replace microbore pipes when fitting heat pumps is one that installers will come up against more and more often. While it’s certainly true that heat pumps can work well with small pipes and in buildings of all types and ages, it takes careful design and a clear understanding of system dynamics. Currently, I get called at least once a week
to reconfigure a heat pump set-up because the original installer didn’t accurately calculate the building’s heat loss, radiator outputs, separation requirements, flow rates, pressure losses, and water volume. Without that data you simply cannot judge if microbore will cope or determine the right configuration.
The importance of pressure loss
Microbore pipes cause greater pressure loss due to increased friction from higher flow velocity. However, this phenomenon is less significant with copper pipework - common in pre-1980s homes. The internal diameter of 10mm copper piping is larger than that of 10mm plastic, giving a lower pressure loss of around 45mm/m (roughly 300 kPa). So, in a well-insulated property with radiator outputs around 700W, 10mm copper might work, while plastic would probably need replacing. Copper is quieter too, for similar reasons. Another factor affecting pressure loss is whether or not glycol is present in the circuit. This thicker fluid means the pump must work harder to maintain the required flow rate, cutting efficiency and increasing head loss. If the pump can’t overcome the index circuit (the section with the greatest resistance), you may need to split the system and
www.heatingandventilating.net
Microbore and heat pumps – when it comes to pipe size, how low can you go?
Pipework measuring 8mm and 10mm remain common across the UK’s housing stock. But with heat pumps needing around four times the flow rate of a typical gas boiler, it can make refits challenging for both installers and householders. Viessmann partner Liam Barry, projects director at Custom Renewables, shares his tips for ensuring every installation is a success
add a buffer tank or low-loss header with an additional pump, if space allows. As well as making the installation more expensive, this also means higher running costs due to distortion and mixing. For maximum efficiency, at a 5°C Delta temperature, a 5kW unit needs about 14l/ min; a 7kW requires 19l/min; a 10kW around 29l/min; and a 12kW about 35l/min. Pressure loss also rises with length, so the aim should be to keep microbore runs as short as possible before branching into larger pipework. Older houses often have long, winding circuits that add resistance. Underfloor loops of 80–100m in 10–12mm pipe will almost certainly require separation and extra pumping. Microbore also reduces total water volume,
increasing system cycling and running costs, especially during defrost cycles. Larger radiators, a volumiser, or a buffer tank can counteract this.
Controls count
A home’s heat is a bit like sand in a bucket. Heat loss is the hole in the bottom: it widens as the outside temperature drops and shrinks when it’s milder. A well-controlled heat pump adds sand at the same rate it leaks out. Integrated weather compensation and high-quality system controls are therefore vital, while poor third-party controls can let the bucket empty faster than it’s filled, forcing the system to work harder to catch up. Similarly, “one-size-fits-all” pre-plumbed
units are rarely a good idea. They can include unnecessary components that prevent the heat pump delivering heat directly to the circuit, causing mixing and inefficiency. We often recommend Viessmann’s Vitocal ASHP range because it offers intuitive app-based controls that match heat input precisely to demand.
Skills and support Above: A Vitocal 150-A recently installed by Liam Barry in Aylesbury, Buckinghamshire, UK.
To fit heat pumps successfully, low-temperature system design training is essential. Specialist courses such as those run by Northampton Heating Academy or Heat Geek are ideal. Viessmann also offers design guidance through its MCS umbrella scheme, Viessmann Access. Nearly every microbore challenge can be solved with the right design, but there’s usually a cost. Deciding how to proceed is therefore a trade-off: accept some efficiency loss and invest in mitigation measures or face the disruption and expense of a full re-pipe. Taking the time to get the calculations right and investing in specialist training can ensure reliable, efficient low-carbon heating, whatever the pipe size.
30 November 2025
DOWNLOAD THE HVR APP NOW
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60
