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Heat Pump Pipe Sizing Question

Air Source Heat Pumps (ASHPs)

Last Post by Mars 28 minutes ago

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trebor12345

(@trebor12345)

Estimable Member Member

Joined: 2 years ago

Posts: 148

Topic starter 07/05/2026 4:55 pm [#2987]

I watched Mars' video yesterday on his heat pump install "What Does a Sub-Par Heat Pump Installation Actually Look Like?"

Got me thinking about pipe sizing from, the outdoor unit to the cylinder inside, for my new 3.5kw vaillant arotherm being installed in a few weeks time.

The proposal was for 32mm PEX/PE-RT/Multilayer Pipe, has oxygen barrier. The run is 10m in total length.

In this case 32mm (26 mm ID) might be too large, and 25mm (20 mm ID) might be a better size.

Can I have your thoughts please?

This topic was modified 53 minutes ago by trebor12345

This topic was modified 52 minutes ago by Mars

Hitachi Yutaki SCombi Heat Pump
(Indoor Unit ) RWD-3.0RW1E-220S-K
(Outdoor Unit) RAS-3WHVRP1

2024 build bungalow, Southern england, 179 m2, 14w/m2
Underfloor heating all fully open
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels

👍

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pipe sizing

Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4560

07/05/2026 5:06 pm

Great that you're thinking about this before the installation... let's run the numbers.

For a 3.5 kW unit, plug it into the formula. Aiming for an efficient delta T of 5C:

Required flow = 3.5 ÷ (5 × 0.0698) = 10 litres per minute

That's a very modest flow requirement. At 10 L/min, even 22mm pipework would probably handle it comfortably... so 25mm with a 20mm internal diameter is absolutely fine for a run of 10 metres. Pressure drop and flow velocity will both be well within sensible limits.

The 32mm proposal isn't wrong... it's just oversized for a unit of this output, and on a short 10 metre run the difference in heat loss is negligible either way.

To my understanding, where oversizing external pipework becomes a real consideration is on much longer runs with larger units, where the additional thermal mass of water sitting in wider pipe can actually slow system response. Not really a concern at 3.5 kW over 10 metres IMO, but would be interested to hear what others think.

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4560