One Heat Source, Two Terminals: Why Is the Underfloor Heating Not Getting Warm?

Part 3 — Poor Piping Design and Layout

An air-to-water heat pump is operating normally. The leaving water temperature is correct. The circulation pump is running, and there is no obvious air trapped in the underfloor heating loops.

But some rooms still do not get warm enough.

What should we check next?

In many projects, the problem is not the heat pump itself. It is the underfloor heating piping design and hydraulic layout.

Incorrect pipe diameter, excessively long loops, too many circuits connected to one manifold, poor manifold location, inappropriate pipe spacing, or unnecessary mixing arrangements can all increase hydraulic resistance and reduce heat distribution.

The result is a common field complaint:

“The heat pump is running, but the underfloor heating is still not warm enough.”

In Part 3 of our One Heat Source, Two Terminals troubleshooting series, we explain how piping design affects water flow, pressure drop and heat output—and what engineers should check before blaming the heat pump.

1. Why Does UFH Piping Design Matter So Much?

Underfloor heating is a hydronic terminal system.

The heat pump does not heat the room directly. Instead, heat must travel through several stages:

Heat Pump → Hot Water → Main Pipework → Manifold → UFH Loops → Floor → Room

If the hydraulic design is poor, the heat pump may produce enough thermal energy, but the system cannot distribute that energy effectively.

This means:

Good Heat Pump + Poor Hydraulic Design = Poor Heating Performance

This is one of the most important principles in air-to-water heat pump engineering.

2. The First Problem: UFH Loops Are Too Long

One of the most common design mistakes is making individual underfloor heating loops too long.

As pipe length increases, hydraulic resistance also increases.

Higher resistance means the circulation pump needs more differential pressure to maintain the required flow.

If the pump cannot overcome that resistance:

Longer Loop → Higher Pressure Drop → Lower Flow → Lower Heat Transfer

Eventually, the room may struggle to reach its design temperature.

A practical example

Imagine a large living room of 30–40 m².

Instead of dividing it into two appropriately designed loops, the installer tries to cover the entire room with one extremely long loop.

The result could be a circuit approaching 150–190 metres.

This may look simpler during installation, but hydraulically it can be problematic.

The beginning of the loop receives relatively warm water, while the flow resistance becomes excessive and the temperature progressively falls along the circuit.

The far end of the floor may therefore receive much less useful heat.

3. How Long Should an Underfloor Heating Loop Be?

There is no universal maximum loop length applicable to every project because it depends on:

• pipe diameter;

• pipe material;

• required flow;

• design ΔT;

• heat load;

• pump head;

• manifold pressure drop;

• fittings and valves;

• installation method.

However, for common residential UFH systems using approximately 16–20 mm pipe, individual loops are often designed roughly within the 60–100 m range, with around 80–100 m frequently used as a practical design target.

Rather than treating a single number as an absolute rule, engineers should calculate the pressure drop and required flow for each circuit.

The engineering principle is more important than the number:

Do not make one loop excessively long simply to reduce the number of circuits.

Large rooms should normally be divided into multiple hydraulically manageable loops.

4. Pipe Diameter Directly Affects Hydraulic Resistance

Pipe diameter is another critical factor.

For the same water flow, a smaller internal diameter creates higher water velocity and significantly higher pressure loss.

This affects:

• circulation pump selection;

• loop flow;

• manifold balancing;

• noise;

• heat distribution.

Common UFH pipe sizes vary by market and application, with 16 mm and 20 mm being widely encountered.

Larger commercial or special systems may use other dimensions.

But pipe size should never be selected only according to habit.

It must be coordinated with:

Loop Length + Required Flow + Pressure Drop + Heat Load

5. Why You Cannot Solve Every Problem with a Bigger Pump

When a UFH system has insufficient flow, a common reaction is:

“Let's install a larger circulation pump.”

Sometimes this helps—but it does not correct poor hydraulic design.

If the loops are excessively long or pipe diameters are inappropriate, a larger pump may create:

• excessive differential pressure;

• higher pumping energy;

• noise;

• difficult balancing;

• excessive velocity in shorter circuits;

• control valve problems.

The correct engineering sequence is:

Calculate heat demand → determine required flow → design pipework → calculate pressure drop → select the pump

Not:

Install pipework first → discover poor flow → install a bigger pump

6. Flow Rate Is Determined by Heat Demand and ΔT

For water-based heating systems, the relationship between heat output and water flow can be expressed approximately as:

$$Q=1.163\times V\times \mathrm{\Delta }T$$

Where:

• Q = heating capacity in kW

• V = water flow in m³/h

• ΔT = supply-return water temperature difference in K

Therefore:

$$V=\frac{Q}{1.163\times \mathrm{\Delta }T}$$

For example, if a UFH zone requires 5 kW and is designed for a 5 K temperature difference:

$$V=\frac{5}{1.163\times 5}$$ $$V\approx 0.86m^3\mathrm{/}h$$

Once the required flow is known, the engineer can evaluate:

• pipe diameter;

• water velocity;

• circuit length;

• manifold size;

• pump head.

This is much more reliable than selecting components based only on floor area.

7. The Manifold Location Is More Important Than Many People Think

The UFH manifold should be positioned so that the individual loops can be kept reasonably balanced in length.

Consider a large house where the manifold is installed at one end of the building.

A nearby room may require:

50 m loop

while a distant room may require:

110 m loop

Now both circuits are connected to the same manifold and circulation pump.

Their hydraulic resistance is very different.

Without proper balancing, water naturally prefers the lower-resistance circuit.

The result can be:

Short loop → too much flow

Long loop → too little flow

Nearby room → too warm

Distant room → too cold

This is a classic hydraulic imbalance.

8. A Central Manifold Position Usually Makes Balancing Easier

Where possible, locate the manifold reasonably close to the centre of the zones it serves.

This can help reduce:

• unnecessary pipe length;

• large differences between loop lengths;

• pressure-drop imbalance;

• excessive dead pipe;

• commissioning difficulty.

For large houses or multi-storey buildings, using several manifolds may be better than forcing every loop back to one location.

For example:

Heat Pump / Buffer Tank

↓

Main Distribution

↙︎　　　　　　↘︎

Manifold A　　Manifold B

↓

Shorter, Better-Balanced UFH Loops

This is often hydraulically superior to one oversized manifold serving the entire building.

9. How Many Loops Should One UFH Manifold Have?

There is no universal number because manifold capacity depends on:

• flow requirement;

• manifold diameter;

• branch size;

• pump capacity;

• project scale;

• control strategy.

However, extremely large manifolds with many circuits should be reviewed carefully.

For many residential projects, a manifold serving approximately 5–8 loops can be convenient for installation, balancing and maintenance, although larger manifolds are commercially available and may be entirely appropriate when correctly engineered.

The important point is not to impose an arbitrary loop limit.

The correct question is:

Can the manifold distribute the required flow to every loop while maintaining acceptable pressure drop and controllability?

That is the engineering criterion.

10. Pipe Spacing Must Follow Room Heat Loss

Another mistake is installing the same UFH pipe spacing throughout the entire building.

Different rooms do not necessarily have the same heat loss.

A room with:

• large glazing;

• exterior walls;

• poor insulation;

• northern exposure;

• high infiltration;

may require more floor heat output than an internal room with small windows.

Therefore, pipe spacing may need to change according to zone heat load.

For example, perimeter zones near large windows may require closer pipe spacing than internal areas.

The correct sequence is:

Calculate Room Heat Loss → Determine Required Floor Output → Select Water Temperature → Determine Pipe Spacing

not simply:

Use the same spacing everywhere.

11. Building Insulation Is Part of the Heating System

This point is frequently overlooked.

When an existing building is converted from a boiler to a heat pump, engineers sometimes focus entirely on the equipment.

But if the building has:

• uninsulated walls;

• poor roof insulation;

• single glazing;

• significant air leakage;

• thermal bridges;

the heating load may be much higher than expected.

In that case, the problem may not be that the floor “cannot heat”.

The floor may actually be delivering heat continuously, while the building is losing heat almost as quickly.

The basic thermal balance is:

$$RoomTemperatureRises$$

only when:

$$HeatOutput>BuildingHeatLoss$$

If the UFH provides 5 kW while the room loses 7 kW under design conditions, the room cannot reach the target temperature regardless of how long the system runs.

12. Window Performance Can Make a Major Difference

Windows are particularly important in heating-load calculations.

Two identical rooms can have very different heating requirements if one uses:

• modern double or triple glazing;

while the other uses:

• old single-pane glazing.

Window size also matters.

A room with a large glass façade may need significantly more heat than a room of the same floor area with only one small window.

This is why UFH design should not be based solely on:

“X watts per square metre for the whole house.”

Room-by-room heat-loss calculation is much more reliable.

13. North-Facing and Perimeter Rooms May Need Different Design

Building orientation can also influence heating demand.

Depending on climate and solar exposure, rooms facing different directions can have different heat-loss and solar-gain characteristics.

Perimeter rooms, corner rooms and spaces with large external wall areas are usually more demanding than internal rooms.

Therefore, designers may need to adjust:

• pipe spacing;

• loop flow;

• zoning;

• control;

• floor output.

Good UFH design is room-specific, not simply building-wide.

14. Do Air-to-Water Heat Pumps Need a Mixing Valve for UFH?

This is another important design question.

Traditional boiler systems may produce water at:

70–80°C

while underfloor heating typically requires much lower temperatures.

Therefore, a mixing valve is commonly used to reduce the supply temperature.

But an inverter air-to-water heat pump can normally control its leaving water temperature directly.

For example, it may supply:

30°C, 35°C, 40°C or 45°C

depending on system demand and design.

If the heat pump is serving only low-temperature UFH at one common temperature, an additional mixing station may therefore be unnecessary in many systems.

15. When Is a Mixing Valve Still Necessary?

This does not mean mixing valves are never used with heat pumps.

They can still be required when different terminals need different water temperatures.

For example:

Radiators: 45–50°C

UFH: 30–35°C

In this case, a mixing circuit may be needed to reduce the UFH supply temperature while the heat pump operates at the temperature required by the higher-temperature terminal.

Similarly, a one-source, two-terminal system may require hydraulic temperature separation depending on its design.

Therefore:

Do not install a mixing valve simply because “UFH always needs one.”

First determine whether the system actually requires different water-temperature levels.

16. Unnecessary Components Can Make Troubleshooting Harder

A common engineering principle is:

Use every component that is necessary—but avoid components that have no clear hydraulic or control function.

Every additional component creates:

• additional pressure drop;

• additional control logic;

• another potential failure point;

• additional installation cost;

• additional commissioning work.

For example, an incorrectly selected or incorrectly adjusted mixing valve can cause the UFH supply temperature to remain too low even when the heat pump is producing sufficient heat.

The result may be:

Heat Pump = 40°C

↓

Incorrect Mixing

↓

UFH Supply = 28°C

↓

Insufficient Floor Output

The installer may then incorrectly blame the heat pump.

17. A One-Source, Two-Terminal System Needs Hydraulic Coordination

When one heat pump serves both fan coils and underfloor heating, the two terminals may have different requirements.

Fan coils

Typically require:

• relatively higher heating water temperature;

• faster response;

• different flow characteristics.

Underfloor heating

Typically requires:

• lower water temperature;

• stable continuous flow;

• slower response;

• careful hydraulic balancing.

Therefore, a good design may require:

Heat Pump

↓

Buffer Tank / Hydraulic Separation

↓

Secondary Distribution

↙︎　　　　　　　　　↘︎

Fan Coil Circuit　　UFH Circuit

Each circuit can then be designed according to its own:

• required flow;

• pump head;

• temperature;

• control strategy.

18. Why the Longest or Most Difficult Loop Matters

When selecting a circulation pump, engineers should not simply add the pressure drops of every UFH loop together.

Because the loops are normally connected in parallel, the pump must provide:

• the combined design flow of all operating circuits;

• sufficient head for the most hydraulically demanding circuit plus common pipework and components.

This distinction is important.

For example, if six UFH loops operate simultaneously, their flow rates add together.

But their individual pressure drops do not simply add because the loops are parallel.

The pump selection should therefore consider the system curve correctly.

19. Hydraulic Balancing Is Essential

Even a well-designed UFH system still needs commissioning.

Suppose five circuits have different pressure losses.

Without balancing, water follows the easiest path.

The shortest loop may receive too much flow while the longest receives too little.

Manifold flow meters and balancing valves allow the engineer to set each circuit according to its design requirement.

For example:

These numbers are only illustrative—the correct values must come from the actual room heat load and loop design.

The important point is:

Equal flow is not always correct flow.

Each loop should receive the flow it actually needs.

20. How to Diagnose Poor UFH Piping Design

If the heat pump is operating correctly but certain rooms remain cold, use a systematic process.

Step 1 — Calculate room-by-room heat loss

Determine whether the UFH system is physically capable of covering the heating load.

Step 2 — Check loop lengths

Identify excessively long circuits.

Step 3 — Check pipe diameter

Confirm that pipe dimensions match required flow and pressure drop.

Step 4 — Check manifold location

Look for large differences in loop length caused by poor positioning.

Step 5 — Check manifold flow rates

Compare actual flow with design values.

Step 6 — Check pipe spacing

Determine whether high-load areas have sufficient floor output.

Step 7 — Check pump head

Confirm that the pump can overcome the critical circuit resistance.

Step 8 — Check hydraulic balancing

Make sure short loops are not stealing flow from long loops.

Step 9 — Check mixing valves

Confirm whether they are actually required and whether the settings are correct.

Step 10 — Check building insulation

Do not ignore windows, walls, roof and infiltration.

21. Quick Troubleshooting Table

22. Frequently Asked Questions

Why is my underfloor heating not getting warm even though the heat pump is working?

If the heat pump produces the correct water temperature, the problem may be related to insufficient UFH flow, excessive loop length, poor balancing, trapped air, incorrect pipe spacing, manifold layout or high building heat loss.

Can underfloor heating pipes be too long?

Yes. Excessive loop length increases pressure drop and can reduce water flow. The acceptable maximum depends on pipe size, required flow, design ΔT and pump head.

Is 100 metres the maximum length for an UFH loop?

Not universally. Around 80–100 m is a common practical design range for many residential systems, but the actual limit should be determined through hydraulic calculations.

Should every UFH loop be the same length?

No. Exact equality is not necessary, but very large differences make balancing more difficult. Each loop should be designed for the room heat load and then hydraulically balanced.

Does an air-to-water heat pump need a mixing valve for underfloor heating?

Not always. If the heat pump directly supplies the UFH at the required low water temperature, an additional mixing valve may not be necessary. Mixed-temperature systems may still require one.

Can a bigger circulation pump solve a poorly designed UFH system?

Not necessarily. A larger pump may increase flow but cannot correct excessive loop length, incorrect pipe sizing, poor manifold layout or inadequate floor output.

Why are some rooms warm while others stay cold?

This commonly indicates hydraulic imbalance, different loop lengths, insufficient flow, incorrect pipe spacing or different room heat losses.

Final Engineering Principle

When underfloor heating does not get warm, do not look only at the heat pump.

A successful hydronic heating system depends on the entire chain:

Building Heat Load

↓

Required Heat Output

↓

Required Water Flow

↓

Pipe Diameter & Loop Length

↓

Manifold Layout

↓

Pump Head

↓

Hydraulic Balancing

↓

Floor Heat Transfer

↓

Indoor Comfort

If any one of these steps is incorrectly designed, even a high-performance heat pump may appear to perform poorly.

This leads to one of the most important lessons in air-to-water heat pump engineering:

The heat pump produces the heat. The hydraulic system determines whether that heat reaches the room.

One Heat Source, Two Terminals — Troubleshooting Series

Part 1 — Initial Commissioning or Long-Term Shutdown

Part 2 — Air Trapped in Underfloor Heating Pipes

Part 3 — Poor Piping Design and Layout

We will continue this series by examining more practical causes of insufficient underfloor heating performance in air-to-water heat pump systems.

At EcoHeat Pump, we believe good system performance depends not only on COP, compressor technology or heat pump capacity, but also on correct system design, hydraulic engineering, installation and commissioning.

We also welcome HVAC engineers, installers and heat pump professionals to review these technical discussions, point out anything that could be improved, and share your own field experience.

For more air-to-water heat pump engineering knowledge and application solutions:

www.ecoheat-pump.com