Choosing a UFH pump: Grundfos UPM3 Auto vs Flex vs Wilo vs Bastion

The pump is the only moving part in most underfloor heating systems, and it decides whether every loop gets its share of flow. The four pumps we fit to our packs all do the job. The differences are how they are controlled, how much head they have and what they cost.

This guide compares them, explains the pump settings in plain English, shows how to check the head you need and covers swapping a pump. For which pack to put the pump in, see mixing valves explained or non-mixing pump sets for heat pumps.

How do the Grundfos, Wilo and Bastion UFH pumps compare?

The main difference is control: the UPM3 Flex takes a PWM speed signal, the UPM3 Auto adjusts itself, and the Wilo and Bastion pumps run on settings you choose, with a PWM option on the Bastion.

PP, CP and CC are proportional pressure, constant pressure and constant curve, explained below. All four are 230V and 130mm port to port with 1½in unions, so any of them fits our valve-only and no-pump packs.

What does each pump cost?

Bastion is the lowest cost, Grundfos the highest, and a pump bought inside a pack costs less than buying the pack and pump separately.

Prices inc VAT, October 2026. The same pumps come in non-mixing sets from £175 (Bastion 6m) to £215 (Wilo 6m), and in single room mixing packs from £201.

What do the pump settings mean?

Pump settings decide how the pump reacts when zones open and close. Grundfos's data booklet recommends constant pressure for underfloor heating, because most of the resistance is in the loops themselves.

Why constant pressure suits UFH: when one zone's actuators close, the other loops still need the same pressure across them. A constant pressure pump keeps it there, so the remaining rooms don't get extra flow and noise. Grundfos also notes that with an automatic bypass fitted, the constant pressure setting should be higher than the bypass valve's opening pressure.

Grundfos UPM3 Auto or UPM3 Flex: which should I choose?

Choose the Flex when the heat source will control it, and the Auto when the pump will run on its own.

• UPM3 Flex. Has power and PWM signal connectors. Grundfos lists it for the heating side of heat pumps and for gas and oil boilers. On the heating PWM profile it runs at full speed below a 10% signal and varies speed between 10% and 84%. Only use PWM where the heat pump or boiler manual describes the connection.
• UPM3 Auto. No PWM input. Grundfos's booklet lists proportional pressure, constant pressure and constant curve, each with three levels, plus AUTOADAPT modes. Grundfos lists underfloor heating among its applications. Choose it for a boiler-fed manifold where you want the pump to tune itself.

With a heat pump that has no PWM output, the Auto set to constant pressure is often the simpler choice.

What pump head do I need for underfloor heating?

Most single domestic manifolds need less than 6m, so a 6m pump covers them, and a 7m or 7.5m pump gives margin on bigger systems. Check it with these steps:

1. Find the longest loop, including its tails to the manifold. This is the index loop that sets the head.
2. Work out its flow: L/min = loop output in kW × 14.35 ÷ ΔT. A 1.2kW loop at ΔT 7°C needs about 2.5 L/min.
3. Look up the pipe maker's pressure loss for that flow and multiply by the loop length.
4. Check the loop is under 250 mbar. The Danfoss handbook says not to exceed this, because the pump also has to cover the manifold, valves and pipes.
5. Add the losses of the manifold, pump pack, valves and primary pipes, then convert: 1m head is about 98 mbar.
6. Choose a pump whose curve gives that head at the total flow of all loops, with some margin.

A 250 mbar loop is about 2.5m of head, which leaves room on a 6m pump for the rest of the system. Choose 7.5m when the manifold is far from the heat source, loops are close to 100m for 16mm pipe, or one pump serves a large floor. Our guide to manifold ports and loop lengths helps keep loops short and even, which keeps the head down.

How much electricity does a UFH pump use?

Very little. The Grundfos UPM3 70 models have a maximum power input of 52W, and they draw less than that most of the time because they slow down as zones close.

Worst case: 52W running flat out for 24 hours a day for 200 heating days uses 0.052 × 24 × 200 = 250kWh. In practice a modulating pump uses well under this. EU ecodesign rules for circulators (Regulation 641/2009, amended by 622/2012) set a maximum energy efficiency index (EEI) of 0.23, with the final stage in force from August 2015. Grundfos gives 0.20 as the benchmark for the most efficient pumps, and our Grundfos UPM3 Flex is listed at 0.20 or less.

How do I replace an underfloor heating pump?

Most UFH pumps can be swapped like for like, because our packs all use the standard 130mm pump position with 1½in unions.

1. Isolate the electrical supply and have an electrician disconnect the pump.
2. Close the isolating valves either side of the pump, or the primary and manifold ball valves.
3. Undo the two unions and lift the pump out, catching the water left in it.
4. Fit the new pump with new union washers, with the flow arrow pointing the same way as the old one.
5. Open the valves slowly, check for leaks and vent the air from the pump.
6. Reconnect the power cable, and the PWM cable if used, then set the pump mode.

Each pump uses its own plug-in cable. If you're swapping brands, order the matching cable: Grundfos UPM3 (£10), Wilo, or Bastion power (£10) and PWM signal (£9) cables are listed separately.

Which pump should I choose for my job?

• Boiler-fed manifold, best value: Bastion 6m, in a blending pack for £201.
• Boiler-fed manifold, pump sets itself: Grundfos UPM3 Auto blending pack, £266.
• Heat pump with a PWM output: Grundfos UPM3 Flex, in the pre-assembled non-mixing set for £207.
• Large floor or long loops: Bastion 7.5m, from £181 in a non-mixing set.
• Replacing a failed pump: the same brand as the old one, so the cable matches.

Products for this job

• Grundfos UPM3 Flex 25-70 Circulation Pump
• Bastion A-Rated UFH Circulating Pump (6m or 7.5m)
• Wilo A-Rated UFH Circulating Pump 6m
• Blending Valve Pump Pack with Grundfos UPM3 Auto
• Non-Mixing Pump Set with Grundfos UPM3 Flex
• Grundfos Replacement Pump Power Cable (UPM3)
• Bastion Replacement Pump Cables
• All pumps, actuators and valves

Frequently asked questions

What is the best pump for underfloor heating?

There isn't one best pump. The Grundfos UPM3 Flex suits PWM control from a heat pump, the UPM3 Auto suits a hands-off boiler system, and the Bastion 6m gives the lowest cost for a standard manifold.

What setting should my underfloor heating pump be on?

Constant pressure suits most UFH manifolds, according to Grundfos. Start on a middle level and step up only if the longest loop can't reach its design flow on the flow meter.

Is a 6m pump enough for underfloor heating?

For most single manifolds with loops of 100m or less, yes. Choose a 7m or 7.5m pump for large floors, long loops or a manifold far from the heat source.

Can I use any 130mm pump on my UFH pump pack?

On our packs, yes. They take any 130mm pump with 1½in unions. Check the cable fits the new pump.

Why is my underfloor heating pump noisy?

Air in the pump or loops is the usual cause. Vent the pump and manifold, then check the pump isn't set higher than the system needs. High speed with most zones closed can also cause noise.

Does the UFH pump need to run all the time?

No. The wiring centre switches the pump on when any zone calls for heat and off when all zones are satisfied.

Can a heat pump control a Bastion pump?

Yes, if the heat pump has a PWM output. Add the Bastion PWM signal cable, which is sold separately.

Sources: Grundfos UPM3 data booklet, Grundfos UPM3 Auto operating guidance (Oilon), Danfoss underfloor heating handbook