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

Part 5 — Insufficient System Water Flow

In an air-to-water heat pump system serving both underfloor heating and fan coils, one of the most common complaints is:

“The heat pump is running and the leaving water is hot, but the underfloor heating still cannot warm the rooms properly.”

When troubleshooting this problem, many people immediately suspect insufficient heat pump capacity.

However, the heat pump may be producing enough heat.

The real problem can be much simpler:

The hydronic system is not circulating enough water.

Insufficient water flow is one of the most common hydraulic problems in air-to-water heat pump and underfloor heating systems. It may be caused by undersized pipes, excessive pressure drop, too many fittings, poor installation, incorrect pump selection, improper pump location, or an unsuitable system architecture.

In Part 5 of our “One Heat Source, Two Terminals” troubleshooting series, we explain how insufficient system flow affects UFH performance and how engineers should diagnose the problem.

1. Why Water Flow Matters in a Heat Pump Heating System

A hydronic heating system transfers heat through circulating water.

The basic relationship is:

$$Q=\dot{m}\times c_p\times \mathrm{\Delta }T$$

Where:

• Q = heat transfer rate

• ṁ = water mass flow rate

• Cp = specific heat capacity of water

• ΔT = supply/return water temperature difference

For practical HVAC calculations using water:

$$Q(kW)\approx 1.163\times Flow(m^3\mathrm{/}h)\times \mathrm{\Delta }T(K)$$

Therefore:

$$Flow(m^3\mathrm{/}h)=\frac{Q(kW)}{1.163\times \mathrm{\Delta }T(K)}$$

This equation explains an important principle:

Heating capacity alone does not guarantee heat delivery. The system must also provide sufficient water flow to transport that heat to the terminals.

2. A Simple Example: 20 kW Heat Pump at ΔT = 5°C

Suppose an air-to-water heat pump is delivering:

20 kW heating capacity

with a design supply/return temperature difference of:

5°C

The required flow rate is approximately:

$$Flow=\frac{20}{1.163\times 5}$$ $$Flow\approx 3.44m^3\mathrm{/}h$$

So the hydronic system must be capable of circulating approximately:

3.4 m³/h

under the actual system resistance.

If the pipework and pump can deliver only 2.0 m³/h, the problem cannot simply be solved by saying:

“The heat pump is 20 kW, so it should be enough.”

The heat may be generated at the source, but it cannot be transported effectively to the floor.

3. Flow Rate and Pump Head Are Different

This is an important distinction.

When selecting a circulation pump, engineers need to consider both:

Flow Rate

How much water must circulate through the system.

Usually expressed as:

• m³/h

• L/min

• L/s

Pump Head

How much hydraulic resistance the pump must overcome at that flow rate.

Usually expressed as:

• metres of water column

• kPa

A pump may theoretically provide enough flow under low resistance, but fail to achieve the required flow once connected to the actual system.

Therefore, selecting a pump only according to nominal flow is not enough.

We need to find the required operating point on the:

Pump Curve × System Resistance Curve

4. Pipe Diameter Is One of the First Things to Check

A very common problem is that the main distribution pipe is too small.

The installer may connect several manifolds to one main pipe without calculating:

• total design flow;

• water velocity;

• pressure loss;

• simultaneous operating demand;

• equivalent pipe length.

As flow increases through a small pipe, water velocity increases.

At the same time, pressure drop increases rapidly.

This can result in:

High Resistance → Reduced Flow → Insufficient Heat Transfer → Cold UFH Zones

This is particularly important in large houses and multi-storey buildings.

5. Do Not Select Main Pipe Size Only by Experience

An installer may say:

“We normally use this pipe size for underfloor heating.”

That is not a sufficient engineering basis.

Main pipe sizing should be based on the actual hydraulic requirement.

A professional calculation should consider:

Required Heating Capacity

↓

Design ΔT

↓

Required Water Flow

↓

Pipe Diameter

↓

Water Velocity

↓

Pipe Friction Loss

↓

Fittings and Valves

↓

Total Pressure Drop

↓

Pump Selection

Skipping these calculations is one of the reasons some systems work well in small projects but fail when the same installation method is copied to larger buildings.

6. Pipe Length Is Only Part of the Resistance

A common mistake is to calculate only straight pipe length.

Actual system resistance also comes from:

• elbows;

• tees;

• valves;

• strainers;

• check valves;

• manifolds;

• heat exchangers;

• buffer tanks;

• control valves;

• reducers;

• other fittings.

These components create local pressure losses.

Therefore:

$$TotalPressureDrop=PipeFriction+LocalLosses$$

Engineers often convert fittings into an equivalent pipe length for simplified calculations.

For example, a sharp 90° elbow creates considerably more resistance than a gentle change in direction.

This is why good hydronic design should avoid unnecessary fittings and abrupt changes in direction.

7. Why Too Many 90° Elbows Can Become a Problem

In real installations, piping rarely follows a perfectly straight path.

But every unnecessary elbow adds resistance.

When possible, the piping layout should use:

• smoother routing;

• long-radius bends;

• appropriate 45° fittings;

• fewer unnecessary direction changes.

This becomes increasingly important when:

• the main pipe is relatively small;

• the pipeline is long;

• the required flow is high;

• multiple floors are supplied from one plant room.

One elbow alone will not normally destroy a properly designed system.

The problem is accumulated pressure loss throughout the entire hydraulic circuit.

8. PPR Pipe Installation Can Create Hidden Restrictions

Even when the design pipe diameter looks correct on paper, installation quality can change the real hydraulic performance.

This is particularly relevant to PPR piping.

During heat-fusion installation, excessive insertion or poor workmanship can partially reduce the internal flow passage at a joint.

Externally, the pipe may look completely normal.

Internally, however, the effective diameter may become much smaller.

This creates a hidden restriction.

The symptoms may include:

• low system flow;

• large supply/return ΔT;

• weak flow at distant manifolds;

• poor UFH performance;

• pump operating at high speed without improvement.

This is one reason why commissioning measurements are essential.

A correct drawing does not guarantee a correct installation.

9. Large Supply/Return ΔT Can Be a Warning Sign

Suppose the heat pump is producing hot water normally.

You measure:

Supply water: 42°C

Return water: 32°C

Then:

$$\mathrm{\Delta }T=10\mathrm{°}C$$

If the system was designed for approximately 5°C, this larger-than-expected ΔT may indicate insufficient water flow.

The relationship is straightforward:

For a given heat transfer rate,

Lower Flow → Larger ΔT

and:

Higher Flow → Smaller ΔT

This does not mean every large ΔT is automatically a pump problem.

It means that ΔT should be investigated together with:

• actual flow;

• terminal demand;

• heat pump output;

• valve positions;

• pump operating point.

10. Long Distribution Distances Increase the Problem

Another common situation occurs in large villas or commercial buildings.

The heat pump and buffer tank may be installed far away from the heating terminals.

Longer pipes mean:

• greater friction loss;

• more fittings;

• more heat loss;

• higher pump-head requirement.

If the pipes are also undersized, the effect becomes much worse.

Pipe insulation must also be considered.

A long uninsulated or poorly insulated heating pipe can lose a significant amount of useful heat before the water reaches the underfloor heating manifold.

Therefore, for long-distance hydronic systems, engineers should evaluate both:

Hydraulic Loss + Thermal Loss

11. Multi-Storey Buildings Need Special Attention

Consider a three-storey villa.

A common arrangement is:

Heat Pump → Buffer Tank → Main Distribution Pipe → Floor Manifolds

If one central circulation pump is expected to serve all floors simultaneously, hydraulic balancing can become difficult.

The nearest circuit may receive too much flow while distant circuits receive too little.

Symptoms include:

• first floor warm;

• upper floors cold;

• some manifolds receiving strong flow;

• others receiving weak flow;

• rooms heating unevenly.

Simply installing a larger pump is not always the best solution.

12. Bigger Pump Does Not Automatically Mean Better Heating

This is another common misunderstanding.

When the floor does not heat properly, some installers immediately replace the circulation pump with a larger one.

Sometimes this helps.

Sometimes it creates new problems.

An oversized pump can cause:

• excessive water velocity;

• hydraulic noise;

• unnecessary electricity consumption;

• valve noise;

• poor control authority;

• hydraulic imbalance.

The correct objective is not:

“Install the biggest possible pump.”

It is:

Select a pump that delivers the required design flow at the calculated system resistance.

13. Circulation Pump vs Booster Pump

These two concepts should not be confused.

Circulation Pump

Its main purpose is to circulate water around a closed hydronic loop.

Booster Pump

Its primary purpose is to increase available pressure when the existing pressure is insufficient for the required distribution condition.

In heat pump systems, pump selection and location must follow the actual hydraulic architecture.

Adding a pump without understanding the system can create pressure interaction rather than solve the problem.

14. Pump Location Matters

Pump installation position can significantly affect system behaviour.

For example, if a buffer tank provides hydraulic separation, the heat pump side and terminal side should be analysed as separate hydraulic circuits.

A typical arrangement might be:

Primary Circuit

Heat Pump → Buffer Tank → Heat Pump

and:

Secondary Circuit

Buffer Tank → Circulation Pump → UFH/Fan Coils → Buffer Tank

This is fundamentally different from simply placing multiple pumps randomly in series.

Each pump should have a clearly defined hydraulic responsibility.

15. Why a Buffer Tank Can Help

A correctly designed buffer tank can provide several functions:

• hydraulic separation;

• additional system water volume;

• more stable heat pump operation;

• reduced short cycling;

• easier integration of multiple terminal circuits;

• improved zoning flexibility.

In a one heat source, two terminals system, the heat pump may supply both:

Underfloor Heating + Fan Coils

These two terminal types can have very different:

• flow requirements;

• water temperature requirements;

• control logic;

• operating schedules.

Hydraulic separation can therefore make the system easier to control.

However:

A buffer tank does not compensate for incorrect pipe sizing or an incorrectly selected circulation pump.

Every circuit still needs adequate design flow.

16. Primary-Secondary Systems Can Be Better for Large Projects

For larger villas or multi-zone projects, a primary-secondary hydronic architecture may be more practical.

For example:

Primary Side

Heat Pump → Buffer Tank

Secondary Side

Buffer Tank → Zone Pump 1 → Ground Floor UFH
Buffer Tank → Zone Pump 2 → First Floor UFH
Buffer Tank → Zone Pump 3 → Second Floor UFH
Buffer Tank → Separate Circuit → Fan Coils

This architecture allows different zones to operate according to demand.

If only one floor requires heating, there is no need to force full design flow through every terminal circuit.

This can improve:

• hydraulic stability;

• zoning;

• comfort;

• control;

• pumping efficiency.

17. Why Return-Side Pumping Is Often Used in Secondary UFH Circuits

In some secondary underfloor heating arrangements, the circulation pump is installed on the return side of the terminal circuit.

The exact pump location should always follow the hydraulic design, expansion-vessel connection point, pressure relationships and manufacturer requirements.

The important principle is not simply “supply side” versus “return side.”

It is:

The pump must be positioned so that the intended circuit receives stable differential pressure and design flow without interfering with other circuits.

This is particularly important when multiple pumps share a buffer tank or common header.

18. Hydraulic Balancing Is Essential

Even with correct pipe sizes and pumps, an unbalanced system can still have flow problems.

Water naturally follows the path of least resistance.

Therefore, shorter circuits may receive too much flow while longer circuits receive too little.

Hydraulic balancing may involve:

• manifold flow meters;

• balancing valves;

• differential pressure control;

• variable-speed pumps;

• zone valves;

• commissioning adjustments.

The objective is not to make every circuit receive the same flow.

The objective is to make every circuit receive its design flow.

19. A Practical Troubleshooting Sequence

When an underfloor heating system is not reaching temperature, do not immediately increase the heat pump setpoint.

A systematic diagnosis is much more effective.

Step 1 — Confirm Heat Pump Output

Check:

• leaving water temperature;

• return water temperature;

• operating frequency/capacity;

• alarms;

• actual heating demand.

Step 2 — Measure ΔT

Compare actual supply/return ΔT with the design condition.

An unusually high ΔT can indicate low flow.

Step 3 — Check Actual Flow

Use:

• heat pump flow data;

• external flow meter;

• manifold flow meters;

• commissioning instruments.

Do not rely only on touching the pipes.

Step 4 — Check Pipe Diameter

Verify whether the main distribution pipe can carry the required total flow.

Step 5 — Calculate Pressure Drop

Include:

• straight pipes;

• elbows;

• valves;

• strainers;

• manifolds;

• heat exchangers;

• fittings.

Step 6 — Inspect Installation Quality

Look for:

• partially closed valves;

• blocked strainers;

• PPR fusion restrictions;

• kinked pipes;

• incorrect connections;

• air locks.

Step 7 — Check Pump Selection

Compare the required operating point against the actual pump curve.

Step 8 — Check Pump Position

Confirm that the pump is serving the intended hydraulic circuit.

Step 9 — Balance the System

Adjust each circuit to its required design flow.

Step 10 — Recheck Room Performance

Only after the hydraulic system is operating correctly should terminal heating performance be evaluated.

20. Quick Diagnostic Table

21. An Important Engineering Principle: Flow Before Temperature

When a room is cold, increasing the heat pump leaving water temperature is often the first reaction.

But if the real problem is insufficient flow, this approach does not address the root cause.

A better troubleshooting sequence is:

Heat Demand

↓

Required Heat Output

↓

Required Water Flow

↓

Pipe Diameter

↓

System Pressure Drop

↓

Pump Operating Point

↓

Hydraulic Balancing

↓

Terminal Heat Output

Only after these parameters are verified should we consider increasing the water temperature.

22. Why Correct Flow Also Improves Heat Pump Efficiency

Hydraulic design affects more than comfort.

It also affects heat pump performance.

Insufficient flow can contribute to:

• unstable leaving water temperature;

• excessive ΔT;

• reduced heat transfer;

• frequent cycling;

• high-pressure protection under some conditions;

• poor defrost recovery;

• reduced seasonal efficiency.

An air-to-water heat pump should therefore never be considered as an isolated machine.

It is part of a complete hydronic system.

A high-efficiency heat pump connected to a poorly designed water system cannot deliver a high-efficiency heating system.

Final Engineering Takeaway

For a one heat source, two terminals system combining underfloor heating and fan coils, insufficient water flow is one of the most important problems to diagnose.

The root cause may be:

Undersized Pipe

→ Excessive Pressure Drop

→ Insufficient Pump Head

→ Poor Hydraulic Balancing

→ Insufficient Terminal Flow

→ Insufficient Heat Transfer

→ Room Cannot Reach Set Temperature

The solution is not simply a larger heat pump or a larger circulation pump.

The correct approach is to treat the entire system as a hydraulic network and calculate:

Capacity + ΔT + Flow + Pipe Size + Pressure Drop + Pump Head + Balancing

When all of these parameters work together, the heat produced by the heat pump can actually reach the rooms where it is needed.

One Heat Source, Two Terminals — Troubleshooting Series

This article is Part 5 of our engineering series:

Part 1 — Initial Commissioning or Long-Term Shutdown
Part 2 — Air Trapped in Underfloor Heating Pipes
Part 3 — Poor Piping Design and Layout
Part 4 — Insufficient Effective Heat Dissipation Area
Part 5 — Insufficient System Water Flow

More practical articles will continue to examine air-to-water heat pumps, underfloor heating, fan coils and hydronic system design from a real engineering perspective.

www.ecoheat-pump.com