The Hidden Secret to a Successful Heat Pump: Pipe Size Matters

Mars · 2025-10-05T19:47:08Z

When you’re planning a heat pump installation, most of the conversation tends to revolve around one thing: the heat loss calculation. It tells you how much heat your home loses and, in turn, how powerful your heat pump needs to be. But here’s where many installations go wrong... knowing the right size of heat pump is only half the story. If your pipework can’t physically carry the amount of heat that the pump is trying to deliver, the whole system will underperform, no matter how good the heat pump itself is. Heat pumps in the UK typically work by moving warmth around your home through water circulating in pipes that feed radiators or underfloor heating. The amount of heat that can be transferred through those pipes depends on the temperature difference between the flow and return (known as ΔT) and the rate at which the water is moving. Boilers typically operate with a 20C temperature difference, which allows a lot of heat to be carried with relatively low water flow. Heat pumps, however, are designed to work more efficiently at lower flow temperatures, usually with a ΔT of around 5C. To move the same amount of heat with a ΔT of 5C rather than 20C, you need roughly four times more water flow. And when the water has to move that much faster, you need larger pipes to carry it quietly and efficiently. To put this into real terms, we can look at the kind of heat that different pipe diameters can handle at that 5C temperature difference, based on data from the CIBSE Domestic Heating Design Guide. A 15 mm copper pipe can carry roughly 3 kW of heat. A 22 mm pipe can carry about 7 kW. A 28 mm pipe moves around 12 kW and a 35 mm pipe can handle roughly 18 kW at sensible flow velocities of about one metre per second. These are not exact figures (every system has different pressure drops depending on layout and pipe length) but they make one thing clear: pipe size matters, and it matters a lot. If you’re installing a 16 kW heat pump, for example, you’ll likely need at least 35 mm primary pipework to deliver that heat effectively. Try to squeeze all that through 22 mm pipes and the system will struggle. You’ll end up with noisy, turbulent water rushing through the pipes, a stressed circulator pump and a system that can’t deliver its designed output or efficiency. This is why pipe sizing isn’t some minor technical detail – it’s fundamental to system design. Every section of pipe must be able to carry the combined heat load of everything downstream of it. The further you move away from the heat pump, the smaller the load becomes, so the pipework can gradually reduce in size. But if your installer simply reuses existing boiler pipework (often 22 mm copper) for a large heat pump, you can almost guarantee performance issues. Smaller pipes and longer runs mean higher resistance, known as pressure loss, and the pump has to work harder to maintain flow. Over time, that extra effort means more wear, more noise and higher running costs. It’s a false economy to save a few hundred pounds on pipework when it compromises the entire efficiency of a system that costs thousands. It’s also important to remember that at the lower operating temperatures of heat pumps, water becomes more viscous, which increases resistance by up to 50%. So while a low ΔT can make the system more efficient in theory, it also demands higher flow rates and larger pipe diameters to keep performance stable. That’s the delicate balance in good design: move the water fast enough to carry the heat, but not so fast that you create turbulence, noise and wasted energy at the pump. If upgrading your pipework isn’t practical, there are limited workarounds, but they need to be properly engineered, not guessed. Some designers might increase the ΔT slightly to reduce flow rates, though this can dent efficiency. Others may try to spread the load across larger or additional radiators, but that still depends on the pipework’s ability to supply enough energy to each circuit. The truth is, there’s no way around physics: if the pipe is too small, it will throttle your system. You can’t cheat the flow rate, and you can’t expect great performance if the bottleneck is built into the system. For homeowners, the lesson here is simple: don’t let your installer brush past pipework. Ask them to explain what pipe sizes are needed to deliver your system’s output and how those compare with your existing setup. If you’re told that your old 22 mm boiler pipes will “do fine” for a new heat pump, ask for the design data that supports that. Numbers don’t lie, and in most cases, the maths will show that they won’t. Good installers know this and factor it into the design stage, not as an afterthought once problems arise. At the end of the day, a heat pump is only as good as the pipes that feed it. You can have the best machine on the market, a flawless heat loss calculation and even the best intentions, but if your pipework is undersized, you’ll never see the comfort, efficiency or performance y...

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colinc

(@colinc)

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28/10/2025 9:08 pm

@old_scientist and @allyfish , thank you both for this. We currently have a 23yr old Oil boiler, installed in a then new-build. I have no doubt that some of the sludge festering in the pipe and emitter system is of the same vintage.

I am fairly confident we have 22mm F&R from the boiler going up and around the first floor, with 10mm u-bore teeing off to all the rads on both floors. the ratio of 10mm to 22mm copper is unknown and will only be bottomed-out by inspection with a Mk I Eyeball, viz. by lifting the first-floor boards; which no doubt are chipboard, just for added interest to the task.

We are moving into retirement mode, hence will be spending more daytime at home; and given the Oil boiler has passed its prime it will likely need a "tech refresh" in the nearer future too. This why we are considering ASHP for our energy needs; likely looking at PV solar/battery too. Insulation is 'ok', building heat loss is 9.6kW. Windows were replaced 2017 and the loft will be treated to another 150mm duvet.

One plumbing option is to re-pipe the first-floor copper with fit-for-purpose sizes and spec-up rads for a high-flow, lower-temp air-source system. The intent is to route into the ground floor concrete to lay pipe throughout for UFH. Admittedly it'll be a huge disruption, albeit only a few weeks; but we are fortunate to have a contingency for temporary accommodation, namely a caravan.

Mad ? ... borderline ... but more a case of investing in our future energy needs, with lower running costs while delivering increased comfort as our needs change over time. We're not really looking for ROI, per se, although it won't be long into the future where oil-fired heating will reduce a property's marketability and sale price: just my pov.

👍

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