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China Guangdong Wotech Renewable Energy & Technology Co., Ltd.
Guangdong Wotech Renewable Energy & Technology Co., Ltd.
Guangdong Wotech Renewable Energy & Technology Co., Ltd.(hereinafter referred to as Wotech) was founded in 2005, integrating research and development, manufacturing, marketing, and after-sales service of heat pumps. FOSHAN SHUNDE HUAMING RENEWABLE ENERGY&TECHNOLOGY CO.,LTD. (hereinafter referred to as DUHM) is a subsidiary of Wotech，was founded in 2011. We combine renewable energy with new technology to offer efficient solutions so that we can create a more sustainable future together!
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Latest company news about One Heat Source, Two Terminals: Why Is the Underfloor Heating Not Getting Warm? Part 2 — Air Trapped in the UFH Pipes
2026-09-14

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

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=m˙cpΔTQ=dot{m}c_pDelta T For practical water-side calculations: Q(kW)≈1.163×Flow(m3/h)×ΔT(K)Q(kW)approx1.163times Flow(m³/h)timesDelta 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 Symptom Possible Explanation Recommended Check Supply hot, return cold Very low circulation Air, pump, valves, blockage One UFH loop cold Local air lock Purge that loop Several loops cold Manifold/pump issue Pump, valves, air Flow meter reads zero No circulation Actuator, valve, air, blockage Flow fluctuates Air in circuit Purge and vent Gurgling sound Air moving through pipe Air-removal system Bigger pump gives little improvement Air/restriction remains Purge and inspect system Fan coils work but UFH does not UFH-side hydraulic problem Secondary circuit Both terminals are cold Possible primary-side problem Heat source and primary circuit 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  
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Latest company news about One Heat Source, Two Terminals: Why Is the Underfloor Heating Not Getting Warm? — Causes, Diagnosis & Solutions
2026-09-11

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

  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: Confirm heat generation — Is the heat pump actually producing the required leaving water temperature? Confirm valves — Are all required supply, return, manifold and zone valves open? Confirm flow direction — Are supply and return connections correct? Confirm pump operation — Is the pump producing actual flow, not merely running? Check filters and strainers — Is there construction debris or sludge? Remove air — Are individual UFH loops fully purged? Check manifold flow — Is every loop receiving adequate water flow? Measure ΔT — Compare supply and return water temperatures. Check floor construction — Screed thickness and floor covering affect response. Check building condition — Is this a new, damp or long-unheated building? Allow sufficient warm-up time — Do not diagnose a high-mass floor system after only a few hours. 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 Symptom Possible Cause What to Check Heat pump hot, UFH completely cold Closed valve / no flow Valves, pump, manifold Pump running but no manifold flow Air lock / seized pump / blockage Purge, strainer, pump Some loops warm, others cold Hydraulic imbalance / trapped air Flow meters, loop balancing Supply hot, return very cold Insufficient flow Pump, blockage, valves Supply and return nearly equal but room cold Low heat transfer / building load Floor surface, flow, load New house warms very slowly Moisture + thermal mass Allow gradual continuous heating System unused for long period Pump seizure / deposits Pump, water quality, filters Fan coils hot but UFH cold UFH-side problem Secondary circuit and controls 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  
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