One Heat Source, Two Terminals: Why Is the Underfloor Heating Not Getting Warm? — Causes, Diagnosis & Solutions

A Practical Troubleshooting Guide for Air-to-Water Heat Pump Systems with Underfloor Heating

An air-to-water heat pump is running. The leaving water temperature looks normal. The circulation pump appears to be operating.

But the underfloor heating is still not getting warm.

This is one of the most common problems encountered during the commissioning or restart of hydronic underfloor heating systems.

It becomes particularly interesting in a “one heat source, two terminals” system—for example, one air source heat pump serving both fan coils and underfloor heating.

If the fan coils are heating normally while the floor remains cold, it is tempting to conclude that the heat pump is undersized or defective.

In many cases, however, the heat pump itself is not the problem.

The problem may be found in the water circuit, circulation pump, valves, manifold, trapped air, water quality, floor construction, building moisture or commissioning procedure.

This article explains the first major category of problems:

Why does underfloor heating fail to warm up properly during initial commissioning or after a long period of shutdown?

1. First Determine Whether the Problem Is the Heat Source or the Terminal

Before changing any parameters, engineers should identify where the problem actually exists.

In a system with one heat pump supplying two terminal types:

Air-to-Water Heat Pump → Fan Coil Circuit + Underfloor Heating Circuit

the first diagnostic question should be:

Is the heat pump unable to produce heat, or is the heat unable to reach the underfloor heating circuit?

This distinction is extremely important.

If the heat pump can maintain the required leaving water temperature and another terminal circuit is operating normally, the heat source is probably functioning.

Attention should then shift downstream to:

Heat pump → pump → valves → main pipe → manifold → UFH loops → floor structure → room

This simple diagnostic sequence can prevent unnecessary refrigerant-side troubleshooting or premature replacement of the heat pump.

Cause 1: Initial Commissioning or Long-Term Shutdown

A hydronic floor heating system does not always respond immediately when it is switched on.

This is particularly common under two conditions:

Initial commissioning

The system has just been installed and is being operated for the first time.

Restart after long-term shutdown

The system has been unused for several months—or, in some projects, considerably longer.

Under both conditions, slow temperature rise can result from several completely different causes.

2. Check All Valves Before Blaming the Heat Pump

This sounds simple, but it is surprisingly common on real projects.

During installation, pressure testing or construction, some valves may remain closed or partially closed.

Possible locations include:

• heat pump supply valve;

• heat pump return valve;

• primary circuit isolation valves;

• secondary circuit valves;

• underfloor heating manifold valves;

• individual loop valves;

• zone valves;

• balancing valves.

The heat pump may therefore operate normally while little or no hot water actually reaches the floor circuit.

Quick diagnostic method

Check the water temperature progressively along the circuit:

Heat pump outlet → main supply → manifold inlet → individual UFH loops → return manifold → heat pump return

If the pipe is hot before a valve but significantly colder immediately after it, the restriction is likely close to that point.

This is much faster than randomly changing heat pump settings.

3. Check Whether Supply and Return Connections Are Correct

Another installation error is reversed supply and return piping.

This can occur at:

• the heat pump;

• circulation pump;

• buffer tank;

• mixing assembly;

• manifold;

• individual terminal circuits.

Some hydronic systems will still circulate with incorrectly connected piping, which makes the problem more difficult to identify.

But incorrect flow direction can interfere with:

• check valves;

• balancing valves;

• thermostatic valves;

• flow meters;

• mixing valves;

• pump operation;

• automatic control logic.

During commissioning, never assume that the pipe labels are correct.

Verify the actual direction of water flow.

4. Check the Circulation Pump — “Running” Does Not Always Mean “Pumping”

A circulation pump can make noise, display a running symbol and consume electricity while still providing insufficient flow.

Several conditions can cause this.

Air locking

Air trapped around the impeller or within the hydronic circuit can significantly reduce circulation.

Pump seizure after long-term shutdown

A pump that has remained unused for a long period can become mechanically stuck.

Incorrect pump direction

The pump may have been installed against the intended system flow direction.

Incorrect speed or control mode

Variable-speed pumps can be set too low or use an inappropriate constant-pressure/proportional-pressure mode.

Insufficient pump head

The pump may simply be unable to overcome the hydraulic resistance of long UFH loops, manifolds, valves and pipework.

Therefore:

Never judge pump operation only by touching the pump or listening to it. Verify actual water flow.

Where available, check the manifold flow meters and compare the measured flow with the design flow.

5. Water Quality Can Cause a Pump to Seize

Water quality is often overlooked during heat pump installation.

A newly installed hydronic system may remain filled with water for weeks or months before final commissioning.

In some projects, the delay can be much longer.

During this period, untreated water can contribute to:

• corrosion;

• scale;

• sludge;

• magnetite;

• suspended particles;

• pump seizure;

• blocked strainers;

• restricted valves;

• reduced heat exchanger performance.

Hard-water areas require particular attention.

If a project uses treated or softened water during normal operation but untreated water was originally used during construction and pressure testing, the commissioning engineer should consider whether the temporary water should be drained and the system properly flushed before final operation.

Good practice

Before final commissioning:

Flush → clean → inspect strainers → refill correctly → remove air → verify pressure → verify flow

For closed hydronic systems, water treatment should follow local standards and the equipment manufacturer's requirements.

6. Check the Strainer Before Increasing Pump Speed

When underfloor heating flow is insufficient, many engineers immediately increase the circulation pump speed.

Before doing this, check the strainer.

Construction debris can easily enter a newly installed system.

Typical contaminants include:

• metal particles;

• welding residue;

• sealing material;

• plastic fragments;

• pipe debris;

• corrosion products.

A partially blocked Y-strainer or magnetic filter can create substantial pressure drop.

The result can be:

Heat pump operating → supply water hot → pump running → insufficient UFH flow → floor remains cold

Increasing pump speed may temporarily hide the symptom without solving the root cause.

7. Air Must Be Removed from the Underfloor Heating Loops

Air trapped inside UFH circuits is another frequent commissioning problem.

Because floor heating circuits can contain many long loops, trapped air may prevent individual circuits from circulating correctly.

Symptoms can include:

• some rooms warm while others remain cold;

• unstable manifold flow meters;

• noise inside the pipework;

• high supply temperature but low floor temperature;

• large temperature differences between loops;

• fluctuating system pressure.

Each circuit should be purged correctly.

In difficult cases, loops may need to be isolated and flushed individually rather than attempting to purge the entire manifold simultaneously.

8. A New Building Can Take Much Longer to Heat Than Expected

This is one of the most misunderstood issues.

A newly completed house is not thermally equivalent to an occupied, dry building.

The floor slab, walls, plaster and other construction materials can contain significant moisture.

When floor heating is first started, part of the heat supplied by the heat pump is not immediately raising room air temperature.

It is heating and drying the building structure.

The floor itself also represents a large thermal mass.

Therefore:

A cold, damp building may require a substantial amount of energy before the indoor temperature begins to rise noticeably.

This is not necessarily a heat pump fault.

9. Floor Construction Changes the Warm-Up Time

Not all floors respond at the same speed.

For example:

Tile or stone

Usually provides relatively good heat transfer, although the heavy floor structure can still have significant thermal mass.

Engineered timber or wood flooring

Typically has greater thermal resistance and may respond more slowly.

Thick screed

Stores a large amount of thermal energy and therefore takes longer to reach steady-state temperature.

Carpet

Can significantly increase thermal resistance depending on the carpet and underlay.

Consequently, two rooms supplied by the same manifold can have noticeably different surface temperatures and warm-up times.

This does not automatically mean that one UFH loop is faulty.

10. Do Not Raise the Water Temperature Aggressively During Initial Commissioning

When a customer says:

“The house is still cold. Increase the heat pump water temperature.”

the instinct may be to immediately increase leaving water temperature.

That is not always the correct approach.

For a newly commissioned floor heating system, gradual warm-up is generally preferable.

A typical commissioning strategy might start around:

35°C supply water

and then increase the target progressively according to the floor construction, heating requirement and applicable commissioning procedure.

The exact temperature and ramp rate should follow the floor-system, screed and heat-pump manufacturer's requirements.

Why?

Because underfloor heating is a high thermal-mass system.

The objective is not to make the pipe hot as quickly as possible.

The objective is to gradually bring the floor structure and building envelope into thermal equilibrium.

11. Do Not Expect Immediate Room Temperature Response

This is a major difference between fan coils and underfloor heating.

A fan coil can produce warm air within minutes.

Underfloor heating cannot.

The process is approximately:

Heat pump heats water

↓

Water heats UFH pipe

↓

Pipe heats screed

↓

Screed heats floor finish

↓

Floor transfers heat to the room

↓

Walls, furniture and building mass absorb heat

↓

Room temperature gradually stabilizes

This is why comparing fan coil response directly with UFH response can lead to incorrect conclusions.

A fan coil is a relatively fast-response terminal.

Underfloor heating is a slow-response radiant terminal with high thermal inertia.

12. Initial Heating May Need Several Days

For a cold or newly constructed building, proper initial warm-up should be planned in advance.

Do not start the system late in the evening and expect normal indoor conditions the following morning.

Depending on:

• outdoor temperature;

• building moisture;

• slab thickness;

• insulation;

• floor finish;

• initial indoor temperature;

• heating load;

• heat pump capacity;

the system may require several days to approach stable conditions.

For newly completed or very damp buildings, the stabilization period can be even longer.

Therefore, commissioning should be scheduled before final customer acceptance, not immediately before it.

13. Outdoor Conditions Matter During Commissioning

The same UFH system will behave very differently under different outdoor conditions.

Commissioning during mild weather can make heating output difficult to assess because the building requires very little heat.

Commissioning during extremely cold weather can create the opposite problem: the heat pump must simultaneously heat a cold building mass and offset a high ongoing building heat loss.

Therefore, engineers should record at least:

• outdoor temperature;

• indoor temperature;

• heat pump leaving water temperature;

• return water temperature;

• ΔT;

• total system flow;

• manifold flow;

• compressor operating frequency/load;

• room temperature trend.

Do not evaluate performance from a single temperature reading.

Trend data is much more useful.

14. A Practical Diagnostic Sequence

When an underfloor heating system is not warming up, avoid randomly adjusting parameters.

Use a systematic troubleshooting sequence:

1. Confirm heat generation — Is the heat pump actually producing the required leaving water temperature?

2. Confirm valves — Are all required supply, return, manifold and zone valves open?

3. Confirm flow direction — Are supply and return connections correct?

4. Confirm pump operation — Is the pump producing actual flow, not merely running?

5. Check filters and strainers — Is there construction debris or sludge?

6. Remove air — Are individual UFH loops fully purged?

7. Check manifold flow — Is every loop receiving adequate water flow?

8. Measure ΔT — Compare supply and return water temperatures.

9. Check floor construction — Screed thickness and floor covering affect response.

10. Check building condition — Is this a new, damp or long-unheated building?

11. Allow sufficient warm-up time — Do not diagnose a high-mass floor system after only a few hours.

12. Only then evaluate heat pump sizing and system design.

This sequence can save significant troubleshooting time.

15. Why “Hot Supply Pipe but Cold Floor” Is an Important Clue

Suppose the heat pump outlet is 40°C.

The main supply pipe is also hot.

But the floor remains cold.

This immediately tells us something useful:

The heat pump can generate heat.

The next investigation should focus on heat transport and heat emission.

Possible causes include:

Low flow → closed valve → blocked filter → air lock → pump problem → manifold imbalance → insufficient loop flow → high floor thermal resistance → high building thermal mass

This is a much more efficient diagnostic approach than immediately checking refrigerant pressure or increasing compressor output.

16. One Heat Source, Two Terminals: Use the Other Terminal as a Diagnostic Reference

A system containing both fan coils and underfloor heating gives engineers a useful diagnostic advantage.

Suppose:

Fan coils = heating normally

Underfloor heating = not warm

This strongly suggests that the heat pump is capable of producing useful heat.

The fault is more likely to be associated with:

• UFH secondary pump;

• mixing valve;

• manifold;

• actuator;

• flow setting;

• air;

• floor loop;

• control logic;

• hydraulic balancing;

• floor thermal inertia.

Conversely, if both fan coils and underfloor heating fail to heat properly, the investigation should move upstream toward:

• heat pump capacity;

• leaving water temperature;

• primary flow;

• buffer tank;

• system control;

• heat pump operating mode;

• outdoor design condition.

This is why troubleshooting should always move logically from heat source → distribution → terminal → building.

Quick Troubleshooting Table

Frequently Asked Questions

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

A running heat pump does not guarantee that sufficient heat is reaching the floor. Check leaving water temperature, circulation flow, valves, strainers, air in the loops, manifold settings and circulation pump operation.

Why are my fan coils heating but the underfloor heating is cold?

If the fan coils work normally, the heat source is probably operating. The problem is more likely located in the UFH circuit, such as the pump, mixing valve, manifold, actuator, air lock, insufficient flow or control settings.

How long does underfloor heating take to warm a cold house?

Underfloor heating has much higher thermal inertia than fan coils. A cold slab or newly constructed building can require many hours or several days to approach stable operating conditions, depending on construction, insulation, moisture and outdoor temperature.

Should I increase heat pump water temperature if the floor is not warm?

Not immediately. First confirm adequate water flow and correct hydraulic operation. Excessively increasing water temperature can reduce heat pump efficiency and may not solve a circulation problem.

Why can a circulation pump run but still provide insufficient flow?

Possible causes include air locking, pump seizure, incorrect speed setting, insufficient pump head, blocked strainers, closed valves and excessive hydraulic resistance.

Should an underfloor heating system run continuously during initial warm-up?

Continuous operation is often preferable during the initial stabilization of a cold, high-mass building, but the temperature ramp and commissioning procedure should always follow the UFH, floor/screed and heat pump manufacturer's requirements.

The Engineering Principle to Remember

When underfloor heating does not warm up, do not immediately conclude that the heat pump is too small or defective.

Troubleshooting should follow the energy path:

Heat Generation → Water Circulation → Hydraulic Distribution → Floor Heat Transfer → Building Thermal Response

If heat is generated but cannot circulate, investigate hydraulics.

If hot water reaches the manifold but not individual loops, investigate distribution.

If the loops are circulating normally but the room temperature rises slowly, investigate floor construction, building thermal mass, moisture and actual heating load.

That diagnostic logic is especially valuable in one heat source, two terminal systems, where fan coils and underfloor heating can behave very differently even when connected to the same air-to-water heat pump.

About EcoHeat Pump

We provide professional air-to-water heat pump solutions for residential and commercial heating, cooling and hydronic applications, including underfloor heating, fan coils, buffer tank systems and customized OEM projects.

For more heat pump engineering guides and application solutions, visit:

www.ecoheat-pump.com