Do I need to replace my pipework for a heat pump?

One of the most common things homeowners hear about heat pumps is: “You’ll need 28 mm pipework throughout the house.” Closely followed by: “Heat pumps don’t work with microbore.”

Neither is a very useful rule. Some homes genuinely do need sections of pipework upgrading before a heat pump can work properly. Others can keep almost all of the heating pipework they already have.

The important question isn’t simply how big your pipes are. It’s whether those pipes can move enough heat around your particular house. That depends on:

• How much heat the building needs
• How much water needs to circulate
• The temperature difference between flow and return
• The layout of the heating system
• The length of the pipe runs
• Where the pipework splits to different parts of the house

So before someone starts pulling up your floorboards, here’s what actually matters.

The myth: heat pumps need huge pipes because they run at low temperatures

It sounds logical. Gas boilers traditionally send very hot water around a heating system. Heat pumps generally run cooler. So surely cooler water means you need enormous pipes to compensate? Not quite.

The key variable is flow rate. A heating system needs to move a certain amount of energy from the heat source to the rooms in your house. The greater the heating requirement, the more energy has to travel through the pipework — and generally, moving more energy means moving more water. That’s what ultimately decides how much pipe capacity you need.

It’s about heat load, not simply temperature

Imagine a house needs around 5 kW of heating. That relatively small demand can be served with a lower water flow rate, so smaller pipework may be perfectly adequate.

If another building needs 20 kW, considerably more heat has to be moved through the system. That means a much greater flow rate — and potentially substantially larger primary pipework.

So asking “What size pipes does a heat pump need?” is a bit like asking “What size road does a city need?” It depends how much traffic you’re trying to move.

Why do heat pumps usually need higher flow rates than boilers?

There’s another important piece of the puzzle: Delta T. Delta T, usually written ΔT, is simply the difference between the temperature of the water leaving the heat source and the water coming back. With a flow of 40°C and a return of 35°C, the ΔT is 5°C.

Heat Geek’s design guidance describes heat pumps as typically operating with a relatively narrow temperature difference of roughly 5–7°C, with around 5°C as an ideal design target. Traditional gas boiler systems can operate with a much wider temperature difference.

Because each litre of water gives up less heat before returning to the heat pump, more litres have to circulate to move the same amount of energy. In simple terms: a smaller temperature drop means more water flow. That’s why heat pump systems often need higher flow rates than boiler systems — but it still doesn’t mean every pipe in your house needs replacing.

Why your existing pipework might already be big enough

There are several reasons an older heating system may have more capacity than you’d expect.

1. Older systems were often designed for higher flow rates

Older heating systems were commonly designed around narrower flow-to-return temperature differences than many modern boiler systems, which meant higher water flow rates were already needed. So some older properties already have relatively generous pipework.

2. Your house may need much less heat than it used to

Many older properties have been upgraded over the decades. You might now have:

• Loft insulation
• Cavity wall insulation
• Better glazing
• Improved doors
• Draught proofing
• Renovated extensions

Every improvement that reduces heat loss also reduces the energy the heating system has to move around the house. Pipework installed decades ago for a high-heat-loss property may be carrying far less energy today: the pipe diameter hasn’t changed, but its capacity relative to the heating requirement has.

3. Heating systems were often installed using generous rules of thumb

Historically, installers didn’t necessarily carry out the detailed room-by-room heat-loss calculations used in modern heat pump design. Systems were often installed using rules of thumb, which sometimes left pipework and heating equipment larger than the property needed. That extra capacity can work in your favour when moving to a heat pump.

So how do you know if your pipework is big enough?

You need to compare two things: how much heat needs to travel down that section of pipe, and how much heat that pipe can reasonably transport.

This needs to be considered section by section, not simply by looking at one pipe next to the boiler, because the amount of heat being carried falls as the system branches off around the house.

Primary pipework vs radiator pipework

This distinction is really important. Your primary pipework is the main flow and return carrying water away from and back to the heat source. From there, the system branches off towards individual areas and radiators.

Imagine your entire house needs 10 kW of heat. The first section leaving the heat pump may need to carry all 10 kW. But once the pipework splits, one branch might only need 4 kW, another 3 kW and another 3 kW.

Those downstream pipes no longer need to carry the full 10 kW. That’s why you can’t simply decide “this house needs 28 mm pipe”. You need to know where that capacity is required.

You might only need to replace a short section

Imagine the existing heating system has 22 mm primary pipework, but calculations show the new heat pump ideally needs 28 mm pipework immediately after the unit. That doesn’t necessarily mean replacing every 22 mm pipe in the building.

You might be able to install 28 mm pipework from the heat pump to the point where the existing heating circuit splits. After that split, some water travels one way and some another, so each branch carries a smaller load — and the existing 22 mm pipework may be completely adequate.

Only the shared section close to the heat source might need upgrading, rather than the whole property. That can make a huge difference to the disruption and cost of an installation.

A simple example: a 7 kW house with 22 mm pipework

Take a property needing around 7 kW of heat with 22 mm primary pipework. On the table above, 22 mm pipe comfortably carries around 6 kW at a 5°C temperature difference.

At 7 kW, the system might still be workable with a slightly wider temperature difference or a slightly higher water velocity. Pipe sizing isn’t always a hard cliff edge: there’s some engineering tolerance, and the pipe doesn’t suddenly stop working because you’ve gone slightly beyond one figure on a design table.

But go substantially beyond what the pipework can reasonably carry and problems start appearing.

What happens if the pipes are too small?

Technically, you can force more and more water through a pipe. The problem is what happens when you do: the water has to travel faster, and that higher velocity creates more resistance and friction. Potential consequences include:

• Increased system noise
• Greater resistance to flow
• Difficulty achieving the required heat pump flow rate
• Increased risk of erosion inside pipework

Eventually, the circulating pump may simply struggle to move enough water around the system — and if the required flow rate can’t be achieved, the heat pump may not operate correctly.

Around 0.9 m/s is a commonly targeted design velocity, within wider typical limits either side. So pipe sizing isn’t about whether water physically fits through the pipe; it needs to move through the system sensibly.

What about the pipes going to individual radiators?

These are usually much smaller than the main heating pipes. Many radiators are supplied with 15 mm pipework, which the sizing figures put at up to roughly 3 kW of demand. Most individual rooms need significantly less heat than that, so standard radiator branches can often stay.

The correct process is still the same:

1. 01
   Calculate how much heat the room needs.
2. 02
   Work out how much energy that branch must carry.
3. 03
   Check whether the existing pipework can support it.

But what about microbore?

Microbore is one of the biggest sources of heat pump anxiety. A house with small pipes behind the walls can make homeowners assume they’re facing a complete replumb. But once again, bore size alone doesn’t tell you whether the system will work.

The important question is how much heat each section actually needs to carry. A small pipe feeding a low-heat-loss bedroom has a completely different requirement from a small pipe supplying several large downstairs rooms. So microbore shouldn’t be condemned automatically because a heat pump is being installed — it needs to be assessed as part of the overall hydraulic design.

Could insulation be cheaper than replacing pipework?

Suppose your calculations show that one area of the house needs more heat than the existing pipework can comfortably carry. One solution is obvious: install larger pipes. But there may be another option: reduce how much heat the room needs.

Adding insulation reduces the heat loss of the property. Once that heat demand falls:

• Less energy needs to travel through the pipes
• Existing pipework effectively gains spare capacity
• Radiators can potentially run at lower temperatures
• The required heat pump output can fall

So spending money on insulation can sometimes solve several problems at once, rather than spending it on larger pipework. It won’t always remove the need for pipe upgrades, but it shows why the heating system and the building should be designed together.

Another option: hydraulic separation

Engineers sometimes separate the heat pump circuit hydraulically from the existing heating circuit, using equipment such as:

• A buffer
• A low-loss header
• Closely coupled tees
• A plate heat exchanger

In simple terms, the heat pump circulates water through one circuit, and a second pump moves water through the house, so each side can run with different hydraulic characteristics. Sounds ideal — but engineering rarely gives you something for nothing.

The compromise

If the two sides run at significantly different flow rates, the heat pump may have to produce hotter water than would otherwise be necessary — and hotter water generally means lower heat pump efficiency.

So hydraulic separation is a compromise rather than the automatic answer to undersized pipework. It can solve hydraulic problems, but good design weighs the effect on efficiency too.

A few extra things that affect pipe sizing

Even if the basic numbers look good, there are some important caveats.

Plastic pipe

Plastic heating pipe can have a smaller internal bore than the equivalent copper size, particularly where inserts and fittings restrict the flow. Don’t apply copper sizing figures directly to plastic systems; they need additional allowance.

Very long pipe runs

Distance matters too. A long run creates more resistance than a short one, so an extension at the far end of a heating circuit might need larger pipework than the same heating load close to the main circuit. A pipe diameter that works well over a short distance may not behave the same way over a very long one.

Glycol

Some heat pump systems use glycol as antifreeze protection. Glycol carries heat less well than plain water, so a glycol system may need slightly higher flow rates — and additional allowance when sizing the pipework.

So, do you need 28 mm pipework for a heat pump?

Maybe. But that’s the wrong question. A better one is: does each section of my existing heating system have enough capacity for the heat that needs to pass through it?

For one house, the answer might mean replacing a substantial amount of pipework. For another, it could mean upgrading only the first metre or two leaving the heat pump. For another, the existing system may already be completely suitable.

That’s why blanket statements like “all heat pumps need 28 mm pipes” or “heat pumps don’t work with microbore” aren’t particularly helpful. Heat pump pipework needs to be calculated, not guessed.

What should you ask your heat pump installer?

If an installer says your pipework needs replacing, you shouldn’t necessarily be suspicious — there may be a very good reason. But they should be able to explain it. Ask:

What is the calculated heat loss of my property?

This decides how much heat the system needs to move.

What flow rate does the heat pump need?

This is one of the main factors that decides primary pipe size.

Which sections of my existing pipework are actually undersized?

The answer shouldn’t automatically be “all of it”.

Why does this section need upgrading?

Ask what load it carries and what problem the existing pipe would cause.

Can the existing pipework stay after the system branches?

The main pipe close to the heat pump often carries far more energy than the branches downstream.

Would improving insulation change the requirement?

Reducing the building’s heat loss can reduce the flow the heating system needs.

Is hydraulic separation being proposed?

If so, ask why it’s needed and what it could do to the system’s efficiency.

The bottom line

Installing a heat pump doesn’t automatically mean ripping up your floors and replumbing your house. Yes, heat pumps generally need higher water flow rates than traditional boiler systems, and some homes have pipework that genuinely isn’t big enough. But that doesn’t add up to a universal requirement for giant pipes everywhere.

Look at the heating system as exactly that — a system. Calculate the heat loss. Work out the required flow. Understand where that flow travels and how the existing pipework branches. Then upgrade the parts that actually need upgrading.

The best heat pump installation isn’t the one that replaces the most equipment. It’s the one that understands what you’ve already got — and changes only what genuinely needs changing.