One Heat Source, Two Terminals: Why Is the Underfloor Heating Not Getting Warm? Part 2 — Air Trapped in the UFH Pipes

How Air Locks Reduce Water Flow and Cause Poor Heating in Air-to-Water Heat Pump Systems

An air-to-water heat pump is running normally.

The leaving water temperature is correct.

The supply pipe to the underfloor heating manifold feels hot.

But the return pipe remains cold, the flow meters show little or unstable flow, and the floor simply does not warm up.

What should you check first?

One of the most common causes is surprisingly simple:

Air trapped inside the underfloor heating pipes.

Air inside a hydronic system may look like a minor problem, but it can significantly reduce circulation. In severe cases, an air lock can partially or completely stop water flow through an underfloor heating loop.

This is especially important in a one heat source, two terminal system, where one air-to-water heat pump serves both fan coils and underfloor heating.

If the fan coils work normally but the UFH remains cold, trapped air should be one of the first items on the troubleshooting checklist.

1. A Simple Symptom: Hot Supply Pipe, Cold Return Pipe

One of the easiest ways to identify a possible air lock is to compare the supply and return temperatures.

Imagine this situation:

UFH Supply Pipe → Hot

UFH Return Pipe → Cold or significantly cooler than expected

The heat pump is obviously producing hot water, but that heat is not being transported through the floor loops effectively.

This immediately suggests that the problem may be related to insufficient circulation.

Possible causes include:

• trapped air;

• closed valves;

• blocked strainers;

• insufficient pump head;

• incorrect pump settings;

• blocked UFH loops;

• manifold problems.

Among these, trapped air is particularly common after installation, maintenance, draining/refilling or a long shutdown.

2. Why Does Air Enter a Hydronic Heating System?

Many people assume that a closed water system should contain only water.

In practice, some amount of air or dissolved gas is almost unavoidable.

Air can enter or remain in the system during:

• initial filling;

• maintenance;

• system draining and refilling;

• pipe installation;

• pressure testing;

• leakage or pressure loss;

• improper commissioning.

Water itself can also contain dissolved gases.

As water temperature changes, some dissolved gases can come out of solution and form small bubbles.

These bubbles can travel through the system and accumulate at:

• high points;

• manifolds;

• pipe bends;

• low-velocity areas;

• heat exchangers;

• poorly designed pipe sections.

Over time, small bubbles can combine into larger pockets of air.

This is how an air lock can develop.

3. What Is an Air Lock?

An air lock is a pocket of air trapped inside a water circuit that significantly restricts or prevents water circulation.

Water is effectively incompressible in normal hydronic operation, while air is compressible.

When a circulation pump attempts to push water through a pipe containing a substantial air pocket, part of the pump's energy can be wasted compressing or moving the trapped air rather than establishing stable water circulation.

The result may be:

Reduced flow → reduced heat transfer → cold return water → cold floor

In a severe case:

Air pocket → almost no circulation → UFH loop stops heating

This is why trapped air should not be treated as merely a noise issue.

It is a hydraulic performance issue.

4. Why Is Underfloor Heating Particularly Sensitive to Trapped Air?

Underfloor heating systems can contain many long pipe loops.

A typical manifold may serve multiple circuits distributed across different rooms.

The system therefore contains:

• long pipe lengths;

• multiple bends;

• manifolds;

• valves;

• actuators;

• flow meters;

• different elevations.

This creates many opportunities for air to become trapped.

If one circuit contains air while the others do not, the result may be:

Living room warm

Bedroom warm

Bathroom warm

One bedroom completely cold

The heat pump may be operating perfectly.

The problem is simply that one UFH loop has insufficient circulation.

5. Typical Symptoms of Air in Underfloor Heating Pipes

Engineers and installers should look for several common symptoms.

Hot supply but cold return

The supply manifold receives hot water, but the return side remains unusually cold.

Very low or zero flow meter reading

One or more manifold flow meters show little or no circulation.

Some rooms heat while others remain cold

This strongly suggests a branch-specific problem rather than insufficient total heat pump capacity.

Gurgling or bubbling noises

You may hear sounds such as:

gurgling, bubbling or intermittent water noise

inside pipes or manifolds.

Unstable flow

Flow meter readings fluctuate rather than remaining stable.

Large supply-return temperature difference

A very large ΔT can indicate insufficient water flow.

These symptoms do not prove that air is the only possible cause, but they provide strong diagnostic clues.

6. Why Low Flow Makes the Floor Cold

The relationship between heat transfer, flow and water temperature difference can be expressed as:

$$Q=\dot{m}{c}_p\mathrm{\Delta }T$$

For practical water-side calculations:

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

Where:

• Q = heat transferred;

• Flow = water flow rate;

• ΔT = supply-return water temperature difference.

If air reduces the flow rate substantially, the amount of heat that can be transported to the floor also falls.

This is why a system can have a perfectly acceptable leaving water temperature but still provide poor room heating.

Water temperature alone does not determine heating performance. You also need sufficient water flow.

This is one of the most important concepts in hydronic heat pump troubleshooting.

7. Do Not Immediately Increase the Heat Pump Water Temperature

When the floor does not get warm, a common response is:

“Increase the leaving water temperature.”

For example:

35°C → 40°C → 45°C → 50°C

But if the real problem is an air lock, increasing water temperature does not restore circulation.

You may simply end up with:

Hotter supply water + poor flow + cold floor + lower heat pump efficiency

The correct sequence is:

Check flow first → solve the hydraulic problem → then optimize water temperature.

For a heat pump, this is particularly important because unnecessary high leaving water temperatures generally increase compressor lift and reduce efficiency.

8. Do Not Automatically Install a Bigger Circulation Pump

Another common response is:

“The flow is low, so we need a bigger pump.”

Again, not necessarily.

If air is trapped in the pipe, a much larger pump may still fail to establish proper circulation.

Oversizing the circulation pump can also create other problems:

• excessive water velocity;

• increased electricity consumption;

• pipe noise;

• valve noise;

• excessive differential pressure;

• poor system balancing.

A circulation pump should be selected according to:

Required flow + system pressure drop + hydraulic resistance

not as a substitute for correct air removal.

A bigger pump cannot correct a fundamentally poor commissioning procedure.

9. Purging Each UFH Loop Individually Is Often the Best Solution

When trapped air is suspected, simply opening one automatic air vent may not be enough.

For multi-loop underfloor heating systems, one of the most effective commissioning methods is to purge the loops individually.

A typical procedure is:

Step 1: Switch off the heat source if required by the manufacturer's commissioning procedure.

Step 2: Close all UFH loops except one.

Step 3: Flush water through the open loop.

Step 4: Continue until no visible air bubbles are discharged and the flow becomes stable.

Step 5: Close that circuit and open the next one.

Step 6: Repeat for every UFH loop.

Step 7: Restore all circuits to their design positions.

Step 8: Balance the manifold according to the required flow rates.

This is generally more effective than trying to remove air from every loop simultaneously.

10. Automatic Air Vents Should Be Installed at Important High Points

Air naturally tends to migrate toward high points in a hydronic system.

Therefore, system design should provide suitable air-removal points.

Particular attention should be given to:

• high points in main pipework;

• heat pump plant rooms;

• buffer tanks;

• hydraulic separators;

• manifolds;

• vertical risers.

An automatic air vent can continuously remove small quantities of accumulated air during normal operation.

However:

An automatic air vent is not a substitute for proper initial flushing and commissioning.

A heavily air-locked floor circuit may still need to be purged manually.

11. Do Not Forget the Underfloor Heating Manifold

The manifold is one of the most important locations for both commissioning and troubleshooting.

A well-designed manifold typically provides:

• supply distribution;

• return collection;

• individual loop isolation;

• flow adjustment;

• flow meters;

• actuators;

• filling/draining points;

• air venting.

During troubleshooting, inspect the manifold carefully.

Ask:

Are all loops open?

Do the flow meters show circulation?

Are any actuators closed?

Is air visible in the flow meters?

Is there a large temperature difference between individual loops?

Are the manifold air vents working?

These simple observations can often identify the problem without touching the heat pump.

12. Can a Buffer Tank Help with Air Removal?

A properly designed buffer tank can contribute to air separation because the water velocity inside the tank is much lower than in the main pipework.

As water slows down, entrained air can more easily separate and rise toward an appropriate air-removal point.

But this should not be misunderstood.

A buffer tank is not installed only to remove air.

Depending on the hydraulic design, it may also provide:

• additional system water volume;

• hydraulic separation;

• more stable heat pump flow;

• reduced short cycling;

• thermal energy for defrost;

• separation between primary and secondary circuits.

In some heat pump applications, this can make system operation significantly more stable.

13. Why Primary-Secondary Hydraulics Can Be Helpful

For some projects, particularly systems with multiple terminal types, a primary-secondary configuration can be useful.

A simplified arrangement may be:

Heat Pump → Primary Circuit → Buffer Tank / Hydraulic Separator

then:

Secondary Pump → Underfloor Heating

and/or

Secondary Pump → Fan Coils

This allows the heat pump and terminal circuits to operate with more independent hydraulic conditions.

For example, the heat pump may require a certain minimum flow while the UFH circuit requires a different flow depending on how many zones are open.

Hydraulic separation can make these different requirements easier to manage.

It can also provide convenient locations for:

• air separation;

• dirt separation;

• temperature sensing;

• system filling;

• hydraulic balancing.

14. One Heat Source, Two Terminals Makes Diagnosis Easier

Consider this situation:

Fan coils

Heating normally.

Underfloor heating

Not getting warm.

Heat pump

Operating normally.

What does this tell us?

It strongly suggests that the heat source is capable of producing heat.

The troubleshooting focus should therefore shift to the UFH side:

UFH pump → mixing valve → manifold → actuators → flow meters → air → floor loops

Now consider the opposite:

Fan coils

Not heating.

Underfloor heating

Not heating.

Then the problem may be further upstream:

Heat pump → primary pump → buffer tank → main valves → control system

This is why a one-source, two-terminal system actually provides useful diagnostic information.

15. More Air Vents Are Often Better Than Too Few — But Location Matters

Adding appropriate air-removal points during installation is generally much easier than trying to solve chronic air problems after the building is completed.

However, simply installing many vents randomly is not good engineering.

They should be positioned where air naturally collects.

Typical locations include:

High points + risers + manifolds + hydraulic separators/buffer tanks + plant room air separators

Good system design should make air removal predictable and serviceable.

A beautifully installed heat pump system that cannot be properly filled, flushed and vented is not a well-designed hydronic system.

16. Why Air Can Damage the Circulation Pump

Air inside the system does more than reduce heating performance.

If a circulation pump operates with significant air content, it may experience:

• unstable operation;

• noise;

• reduced hydraulic performance;

• loss of lubrication/cooling in some pump designs;

• cavitation-like conditions;

• premature wear.

This is another reason why persistent bubbling or gurgling should not be ignored.

If the pipework sounds like water and air are continuously moving together, investigate it.

17. A Practical Troubleshooting Sequence

If you suspect air in the UFH pipes, use a systematic process.

Step 1 — Check heat pump leaving water temperature

Confirm that the heat source is actually producing hot water.

Step 2 — Compare supply and return temperatures

A hot supply and abnormally cold return may indicate insufficient flow.

Step 3 — Check manifold flow meters

Identify circuits with zero, low or unstable flow.

Step 4 — Check valves and actuators

Make sure the affected loops are actually open.

Step 5 — Check circulation pump operation

Confirm real flow, not simply whether the pump motor is running.

Step 6 — Inspect filters and strainers

Rule out dirt blockage.

Step 7 — Purge UFH circuits individually

Remove trapped air loop by loop.

Step 8 — Check automatic air vents

Confirm that vents are installed correctly and functioning.

Step 9 — Restart and observe

Check whether flow becomes stable.

Step 10 — Rebalance the manifold

Adjust each loop to its required design flow.

Only after these checks should you start changing pump size or heat pump water-temperature settings.

Quick Diagnostic Table

Frequently Asked Questions

How can I tell if there is air in my underfloor heating pipes?

Common signs include gurgling noises, unstable or zero manifold flow, some loops remaining cold, and a hot supply pipe combined with an unusually cold return pipe.

Can air stop underfloor heating completely?

Yes. A sufficiently large air pocket can create an air lock that severely restricts or stops circulation through an individual loop.

Will a bigger circulation pump remove an air lock?

Not reliably. The correct solution is to properly purge and vent the circuit. Pump selection should be based on required flow and system pressure drop.

Why is the heat pump producing hot water but the floor is still cold?

Hot water production confirms heat generation, but it does not confirm heat distribution. Insufficient flow caused by trapped air, valves, pump problems or blockage can prevent heat from reaching the floor.

Should every underfloor heating manifold have an air vent?

Proper air-removal provisions should be incorporated into the hydronic design, particularly at manifolds and high points. Exact installation should follow the system design and local requirements.

Does a buffer tank remove air?

A buffer tank may assist air separation because of lower water velocity inside the vessel, particularly when combined with appropriate air-removal components. However, its primary function depends on the hydraulic system design, and it does not replace proper purging.

The Engineering Principle to Remember

When the underfloor heating is cold, don't ask only:

“Is the water hot enough?”

Ask:

“Is enough hot water actually flowing through the floor?”

A heat pump can produce perfectly hot water while the building remains cold if that thermal energy cannot be transported through the hydronic system.

For an air-locked UFH circuit, the troubleshooting logic should be:

Heat Source OK → Supply Temperature OK → Check Flow → Identify Air → Purge Loops → Verify Pump → Rebalance → Recheck ΔT

And in a one heat source, two terminal system, always compare the behavior of the fan coil and UFH circuits. That comparison can quickly tell you whether the problem is on the heat-source side or the terminal-distribution side.

One Heat Source, Two Terminals — Troubleshooting Series

Part 1: Initial Commissioning or Long-Term Shutdown
Part 2: Air Trapped in Underfloor Heating Pipes
Next: More practical causes of poor underfloor heating performance

At EcoHeat Pump, we believe good heat pump performance depends not only on the heat pump itself, but also on correct hydraulic design, installation, commissioning and terminal-system matching.

We also welcome HVAC engineers, installers and heat pump professionals to share their field experience and help identify anything that could be added or improved in this series.

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

www.ecoheat-pump.com