Sizing pipework to a UFH manifold: 22mm vs 28mm primaries in copper and MLCP

The primaries are the flow and return pipes between the boiler, cylinder or heat pump and the underfloor heating manifold. If they are too small, the manifold is starved of flow, the pump works harder and the floor never reaches its design output. If they are oversized, you have spent more on pipe and fittings than you needed.

This guide shows how to work out the flow, how much 22mm and 28mm copper and multilayer (MLCP) pipe can carry, and why the answer is different for a boiler and a heat pump. It is written for installers and confident self-builders. For connecting the pipes at the manifold, see connecting a UFH manifold.

How do I work out the flow rate to the manifold?

Divide the heat load by the temperature drop and the specific heat of water. The formula is:

Flow (litres per second) = heat output (kW) ÷ (ΔT × 4.18)

ΔT is the difference between flow and return temperature. Multiply litres per second by 60 to get litres per minute, the units on most manifold flow meters.

Load	ΔT	Flow needed
6kW	5°C	17.2 litres/min
8kW	7°C	16.4 litres/min
10kW	5°C	28.7 litres/min
6kW	20°C	4.3 litres/min
12kW	20°C	8.6 litres/min

The same 6kW needs four times the flow at a 5°C drop as at 20°C. That is the whole reason heat pump primaries are bigger than boiler primaries. Heat pump design guidance gives the same picture: a 10kW heat pump needs about 30 litres per minute, which calls for 28mm pipe.

UK underfloor heating is commonly designed with a 5 to 10°C drop across the loops. Heat pumps usually target 5 to 7°C. Gas boilers are typically set up for about 20°C.

How much heat can 22mm and 28mm pipe carry?

At a velocity of about 1m/s, 22mm copper carries around 6.7kW at a 5°C drop and 28mm about 11.3kW. Multilayer pipe one size up has almost the same bore: 25mm MLCP matches 22mm copper and 32mm MLCP matches 28mm copper.

Pipe	Bore	Flow at 1m/s	kW at ΔT 5	kW at ΔT 7	kW at ΔT 10	kW at ΔT 20
22mm copper	20.2mm	19.2 l/min	6.7	9.4	13.4	26.8
25mm MLCP	20.0mm	18.8 l/min	6.6	9.2	13.1	26.3
28mm copper	26.2mm	32.3 l/min	11.3	15.8	22.5	45.1
32mm MLCP	26.0mm	31.9 l/min	11.1	15.5	22.2	44.4

We worked these out from the formula above, using 0.9mm wall copper to EN 1057 and 25 × 2.5mm and 32 × 3mm multilayer. They line up with published rules of thumb. The Renewable Heating Hub, quoting the CIBSE Domestic Heating Design Guide, gives about 7kW for 22mm and 12kW for 28mm at a 5°C drop. One heat pump training provider is a little more cautious and says 22mm carries 6kW at a 5°C drop, with 7kW possible if you accept velocity rising to about 1m/s.

Treat 1m/s as a sensible upper figure, not a target. Heat pump guidance gives a typical range of 0.5 to 1.5m/s and says designers often aim for about 0.9m/s, because resistance climbs quickly above that. UFH design guidance suggests keeping velocity under 1m/s.

22mm or 28mm with a boiler?

With a boiler and a blending valve pump pack, 22mm is usually enough. The primary side works at the boiler's larger temperature drop, while the manifold pump circulates the floor at its own flow rate.

The blending valve mixes a small amount of hot boiler water into cooler water returning from the loops. The primary only has to bring in the heat, not the full loop flow. At a 20°C primary drop, 22mm copper at 1m/s could carry about 27kW, more than most domestic UFH manifolds need.

Things that can push you to 28mm with a boiler:

• Shared primaries: the same pipe also feeds radiators or a hot water cylinder.
• A smaller boiler ΔT: check how the boiler is set up. At a 10°C drop, 22mm carries about 13kW at 1m/s, half the figure at 20°C.
• A long run with many bends between boiler and manifold.

Our guide to UFH with a combi or system boiler covers the connections and controls.

22mm or 28mm with a heat pump?

With a heat pump, size the primaries for the full design flow at a 5 to 7°C drop. Above about 6kW, that usually means 28mm copper or 32mm MLCP.

On a heat pump system with a non-mixing pump set, or no pump set at all, the heat pump's water goes straight round the loops. There is no blending valve to reduce the primary flow, so the primary carries the whole flow.

A real example from the Buildhub forum shows the problem. A 6kW heat pump fed a manifold designed for 16 litres per minute across all loops, through about 12m of pipe with several bends. The owner asked whether 22mm would do. The advice was to run 28mm and keep the pressure drop as low as possible. At 16 litres per minute, 22mm copper would run at about 0.8m/s before allowing for fittings, leaving little margin for a long run or a larger heat pump later.

The usual rule of thumb for heat pumps is simple: 22mm is fine for about 5kW, 28mm is needed around 10kW, and 20kW needs 42mm. See underfloor heating with a heat pump for flow temperatures and design.

What size multilayer pipe should feed a manifold?

Use 25mm MLCP where you would use 22mm copper and 32mm MLCP where you would use 28mm. If you are close to the limit, go up a size.

MLCP is measured on the outside, and its wall is thicker than copper's, so the bore is smaller than the number suggests. 25 × 2.5mm MLCP has a 20mm bore. 32 × 3mm MLCP has a 26mm bore.

Fittings matter too. Press fittings for multilayer pipe have an insert that sits inside the pipe and narrows the bore at every joint. Heat pump designers often advise going up a pipe size with plastic pipe for this reason. Keep joints and elbows to a minimum on long primaries, and use the pipe's ability to bend instead of fitting an elbow at every change of direction.

Our pre-insulated PEX-AL-PEX pipe comes in 25mm and 32mm, red for flow and blue for return, so the primaries arrive at the manifold already lagged. MLCP to copper adaptors join 25mm MLCP to 22mm or 28mm copper and 32mm MLCP to 28mm copper. Our MLCP pipe guide covers cutting, calibrating and pressing.

Does the length of the primary run matter?

Yes. Pressure drop rises with length and with every bend, valve and fitting. On a long run, choose the larger pipe.

• Boiler and blending pack: the boiler pump only has to push the primary flow as far as the blending valve. The manifold pump handles the loops.
• Heat pump direct: the heat pump's own pump, or the pump set, has to overcome the primaries and the loops. Every bit of extra resistance on the primary takes pressure away from the floor.
• Glycol: if the system runs glycol for frost protection, the usual guidance is to take about 10% off the pipe's carrying capacity.

For quick decisions, this table sums up the rules above. Check any borderline case with the formula.

Situation	Usual primary size
Boiler, blending pump pack, UFH only	22mm copper or 25mm MLCP
Boiler, primaries also feed radiators or cylinder	Size for the total load, often 28mm
Heat pump up to about 5 to 6kW	22mm copper or 25mm MLCP
Heat pump about 6 to 11kW	28mm copper or 32mm MLCP
Heat pump above about 11kW	Larger than 28mm, by calculation

How do the primaries connect to a 1" manifold?

Reduce at the manifold, not along the run. Bring 22mm or 28mm all the way to the cupboard, then fit an adaptor to the 1" ball valves.

The 1" thread on the manifold or pump pack is a connection size, not a guide to pipe size. A 1" male fitting on 28mm copper or 32mm MLCP keeps the full bore up to the valves. Fit a 1" ball valve set, so the manifold can be isolated without draining the primaries. Copper joints can be made with copper M-press fittings. Our guide to press, solder and compression fittings compares the options.

Any connection to a gas boiler must be made by a Gas Safe registered engineer.

Products for this job

• Pre-Insulated PEX-AL-PEX Multilayer Pipe (16 to 32mm, red and blue)
• PEX-AL-PEX Multilayer Pipe (20, 25 and 32mm)
• MLCP Press Fit to Copper Brass Tail
• MLCP Press Fit Male Straight Union
• Copper M-Press Male Straight
• Manifold Ball Valve Set 1"
• UFH Non-Mixing Pump Set with Grundfos UPM3 Flex

Frequently asked questions

What size pipe feeds an underfloor heating manifold?

Usually 22mm or 28mm copper, or 25mm or 32mm multilayer. Work out the flow from the load and temperature drop, then pick the size that carries it at around 1m/s or less.

Is 22mm big enough for a heat pump to UFH?

For a small heat pump, up to about 5 to 6kW at a 5°C drop, usually yes. Above that, 28mm is the safer choice, especially on a long run.

Why do heat pumps need bigger pipes than boilers?

Heat pumps run a smaller flow and return difference, about 5°C against about 20°C for a gas boiler. To move the same heat with a smaller drop, they need much more flow, and so bigger pipe.

Does the manifold's 1" connection mean I need 1" pipe?

No. The thread size is just the fitting. Size the pipe to the flow and use an adaptor at the ball valves.

Can I use 20mm MLCP to feed a manifold?

Only for a small load. 20mm multilayer has a smaller bore than 22mm copper, so it suits a single room pack or a short, low-load run. Check the flow with the formula first.

Should the primaries be insulated?

Yes, wherever they pass through spaces that do not need the heat. Pre-insulated multilayer pipe saves fitting lagging on site.

What velocity should I design to?

Aim for about 0.9m/s and stay under about 1m/s on domestic primaries. Above that, resistance and pressure drop rise quickly.

Sources: Renewable Heating Hub: pipe size matters (CIBSE figures), Installer Online: flow rates in a heat pump system, EN 1057 copper tube dimensions, Buildhub: UFH manifold flow and return pipe size