Do you need a buffer tank or volumiser with UFH and a heat pump?

Buffer tanks and volumisers are two of the most argued-about parts of a heat pump system. Some installers fit a buffer on every job as a habit. Others avoid them unless the numbers prove one is needed. For underfloor heating (UFH) the second approach is usually right, because a wet floor already holds a lot of water and a lot of stored heat.

This guide explains what each vessel does, how to check your system volume, when a buffer is still justified, and how zoning choices on the manifold change the answer. It is written for homeowners planning a heat pump and for installers sizing one. For the wider picture, see our guide to underfloor heating with a heat pump.

What is the difference between a buffer tank and a volumiser?

A volumiser simply adds water to the circuit, while a buffer tank also separates the heat pump circuit from the heating circuit. That difference is what decides how each one affects efficiency.

Vessel	How it is piped	What it does	Main downside
Volumiser	In series, usually on the return	Adds litres to meet the minimum volume	Takes space; some heat loss from the vessel
Four-pipe buffer	Heat pump on one side, heating on the other	Adds volume and separates the two circuits	Mixes flow and return, needs a second pump

A four-pipe buffer works like a large low loss header. When the heating side draws a different flow from the heat pump side, warm and cool water mix inside it. The heat pump then has to produce a higher flow temperature to give the floor the temperature it needs, and it runs less efficiently. There is also an extra pump to buy and run, and standing heat loss from the tank.

A volumiser sits in line with the pipework, so the water passes straight through it and there is no mixing point. That is why design guides for heat pumps favour a volumiser when the only problem is too little water.

Why does a heat pump need a minimum system volume?

A heat pump needs enough water in the circuit to run steadily and to supply the heat it uses for defrosting. Every air source heat pump has to defrost its outdoor coil from time to time, and it takes that energy from the water in the system.

The Energy Saving Trust's installer toolkit says enough liquid volume must be permanently available to store that defrost energy. MCS's heat pump design standard, MIS 3005-D, says an air source system should be able to hold the internal design temperatures across multiple defrost cycles, and that equipment must be installable to its maker's instructions. In practice, the minimum volume in the installation manual is the figure to design to.

The minimum is not the same for every model or setting. The toolkit notes that it is set by the lowest flow temperature the system will run at, not the design flow temperature. It reproduces one maker's table in which a 7kW unit needs 20 litres at one minimum flow setting and 55 litres at the other, and a 10kW unit 45 or 150 litres. Always read the figure for your exact model and set-up.

Does underfloor heating hold enough water on its own?

Often it holds a useful amount, but you need to count it rather than assume it. The water that counts is the water the heat pump can always circulate, even when room thermostats have closed their loops.

You can work out the loop volume from the pipe size. Our 16 x 2.0mm PERT-EVOH pipe has a 12mm bore, which holds about 0.113 litres per metre. A 100m loop therefore holds about 11 litres. 12 x 2.0mm pipe has an 8mm bore and holds about 0.05 litres per metre.

Floor	Loops	Water in loops (16mm)
Small ground floor	4 x 90m	About 41 litres
Typical house, one floor	8 x 90m	About 81 litres
Two floors	14 x 85m	About 134 litres

Add the primary pipework, the manifold, any radiators and the heat pump's own volume. The Energy Saving Trust notes that pipework alone usually will not give the required volume, and that towel rails or bathroom radiators without thermostatic valves can be counted because they never close.

The worked method is simple. Take the maker's minimum, subtract what the system always holds, and the gap is the volumiser size. One design guide's example is a 200 litre minimum against 170 litres held in the pipes, emitters and equipment, which leaves a 30 litre volumiser.

How does zoning the manifold change the answer?

The more loops that can shut at the same time, the less water the heat pump can rely on. On a manifold with an actuator on every port, the worst case is a single small room calling for heat while every other loop is closed.

There are three ways to deal with this:

1. Zone less. Heat pumps work best running long and steady. Many heat pump designs use one thermostat per floor, or leave large living areas without actuators, so most of the floor stays open all the time.
2. Keep an open circuit. A towel rail, bathroom radiator or one large loop with no actuator gives a permanent path and permanent volume.
3. Add a volumiser for the gap. Size it on the volume that is always open, not the total in the floor.

Fewer zones does not have to mean less comfort. A screeded floor stores heat for hours, so it evens out room temperatures far more than radiators do. Our guide to zoning and wiring UFH covers how to group rooms. A manifold with automatic balancing valves, such as our auto-balancing stainless steel manifold, also helps here, because it holds each loop at its set flow as other loops open and close.

When is a buffer tank still the right choice?

A buffer is justified when the heat pump and the heating really do need to be separated, not just topped up with water. The cases that come up most are:

• Heavily zoned systems where most of the circuit can close and you cannot or will not change the zoning.
• Fixed-output heat pumps, twin-compressor units and cascades of several heat pumps.
• Mixed emitters at different temperatures, for example radiators upstairs and UFH downstairs, where each side has its own pump.
• A heat pump alongside another heat source such as a wood stove with a back boiler or solar thermal.
• The heat pump maker requires one for the model or for the hydraulic layout you have chosen.

Where you do fit one, keep the pump flows on each side matched as closely as you can, so that as little water as possible mixes inside. Insulate the vessel well. If the heating side has its own pump, the UFH manifold can use a non-mixing pump set rather than a blending valve, because the water is already at floor temperature.

What should a heat pump and UFH layout look like without a buffer?

The simplest layout runs the heat pump's own pump straight through the manifold, with a volumiser on the return if the sums need one. There is no mixing point, so the water the heat pump makes is the water the floor gets.

1. Heat pump flow to the hot water diverter valve, then on to the UFH manifold.
2. Manifold with flow meters on every loop, and either no pump set or a non-mixing set if the heat pump's pump cannot carry the floor on its own.
3. Return from the manifold, through the volumiser if fitted, back to the heat pump.
4. Open path through loops without actuators or a towel rail, so there is always flow.

Make sure the primary pipes are big enough. Heat pumps run a small temperature drop, so they need much more flow than a boiler. Our guide to UFH manifold primary pipe size shows the sums. Use oxygen-barrier pipe in the floor, such as our 5-layer PERT-EVOH pipe, so oxygen does not reach the heat pump's metal parts.

If you want a Boiler Upgrade Scheme grant, the heat pump must be designed and fitted by an MCS-certified installer, who will set the final hydraulic layout. The 0% VAT rate on heat pumps applies only when a qualifying installer supplies and installs them, until 31 March 2027; kit bought supply-only, including all MaxHeatUFH products, carries 20% VAT. MaxHeatUFH does not install, but we can pass your project to a vetted approved installer through our installation enquiry.

Frequently asked questions

Do I need a buffer tank with underfloor heating and a heat pump?

Usually not. Check the heat pump's minimum system volume against the water the system always holds. If it falls short, a volumiser on the return is normally enough. A buffer is for cases that need the circuits separated.

Where should a volumiser go?

Usually on the return to the heat pump, piped in series so all the water passes through it. Follow the heat pump maker's instructions, which may show a set position.

How big should a volumiser be?

Take the maker's minimum system volume and subtract the volume that is always open in the pipes, loops, manifold and emitters. The difference is the size you need. Round up to the next vessel size.

Does a buffer tank reduce heat pump efficiency?

A four-pipe buffer can. It mixes flow and return water when the flows on each side differ, so the heat pump has to run hotter to give the floor its target temperature. It also adds a second pump and standing heat loss.

Can I count the water in my underfloor loops?

Only the loops that stay open. If every loop has an actuator and they can all close, count the smallest set that will ever be open, not the whole floor.

Does a buffer tank stop short cycling?

Extra volume helps a heat pump run longer, but a correctly sized heat pump, sensible zoning and weather compensation do more. Short cycling is often a sign the heat pump is oversized or the controls are fighting it.

Do I need a buffer if I have radiators and underfloor heating?

Not always. If both can run at the same flow temperature, they can share one circuit. If they need different temperatures or each has its own pump, separation such as a buffer or low loss header is more likely.

Sources: Energy Saving Trust heat pump installer toolkit: system volume and frost protection, MCS MIS 3005-D heat pump design standard, GOV.UK Boiler Upgrade Scheme, HMRC VAT Notice 708/6