An air-to-water heat pump system may work perfectly when it is first commissioned. The water temperature is correct, the circulation flow is sufficient, and every room reaches the required temperature.
But after several years of operation, something gradually changes:
The heat pump is still running, but the floor does not feel as warm as before.
The homeowner may increase the leaving water temperature from 40°C to 45°C, or even higher. Indoor comfort may improve temporarily, but electricity consumption increases and the original problem remains.
In many cases, the problem is not the heat pump itself.
It may be caused by something much less visible:
The hydronic system has been operating for years without proper cleaning and maintenance.
This is Part 7 of our “One Heat Source, Two Terminals” engineering series, focusing on how sludge, scale, suspended solids and dirty strainers can gradually reduce the performance of underfloor heating connected to an air-to-water heat pump.
A common question is:
“The underfloor heating system is closed. Why does it need cleaning?”
A closed system does not mean a permanently clean system.
Over years of operation, the circulating water interacts with pipes, valves, manifolds, pumps, heat exchangers and other metallic or polymer components.
Depending on water quality, system materials, oxygen ingress, installation cleanliness and operating conditions, deposits can gradually develop.
Typical contaminants include:
The exact mechanism varies from system to system.
The important engineering point is that water quality changes over time, and contaminants can gradually accumulate inside the hydronic circuit.
Underfloor heating contains long pipe circuits operating at relatively low water temperatures.
If contaminants are present, they can gradually settle or adhere to internal surfaces.
This process may be slow enough that the homeowner does not notice a sudden failure.
Instead, system performance deteriorates progressively.
The sequence can look like this:
Contamination → Internal Deposits → Increased Hydraulic Resistance → Reduced Water Flow → Lower Heat Transfer → Colder Floor → Longer Heat Pump Runtime
Eventually, the customer reports:
“The heat pump used to heat the house very well. Now it doesn't.”
That statement is important because it suggests that we should investigate not only equipment capacity but also how the hydraulic system has changed since commissioning.
Mineral scale deserves special attention.
Hard water contains dissolved minerals, primarily calcium and magnesium compounds. Under suitable conditions, these minerals can precipitate and form deposits on heat-transfer surfaces.
Why does this matter?
Because scale has much lower thermal conductivity than metal.
The heat-transfer path changes from:
Hot Water → Heat Exchanger Surface → System Water
to something closer to:
Hot Water → Scale Layer → Heat Exchanger Surface → System Water
The scale acts as additional thermal resistance.
Even a relatively thin layer of deposit on a critical heat-transfer surface can negatively affect performance. The exact efficiency loss depends strongly on the material, deposit composition, thickness, flow conditions and heat exchanger design, so a universal percentage should not be assumed for every system.
The engineering principle, however, is clear:
Deposits on heat-transfer surfaces reduce heat-transfer effectiveness.
This is a common field response.
Suppose the system originally provided adequate heating at a leaving water temperature of 40–45°C.
Several years later, the customer complains that the rooms are cold.
Someone changes the setpoint to 50°C.
The house becomes warmer again.
Problem solved?
Not necessarily.
Increasing water temperature may simply compensate for deteriorating system performance.
The actual problem could still be:
Reduced flow + fouling + poor heat transfer.
For a heat pump system, unnecessarily increasing leaving water temperature is especially undesirable because heat pump efficiency generally decreases as the required water temperature increases, all else being equal.
So instead of immediately raising the setpoint, ask:
Why does the system now require a higher water temperature than it did when it was new?
That question often leads to a much better diagnosis.
When troubleshooting an older hydronic system, the strainer or filter should be high on the inspection list.
Why?
Because the filter is designed to capture contamination before it reaches sensitive system components.
After years of operation, a neglected strainer may contain significant amounts of:
As the filter becomes clogged, pressure drop increases.
The result is:
Dirty Strainer → Higher Resistance → Lower Flow → Reduced Heat Transport
This can produce symptoms remarkably similar to an undersized circulation pump.
Before replacing the pump, check whether the existing pump is simply trying to push water through a heavily restricted filter.
Hydronic heat transfer can be represented approximately by:
Q = ṁ × Cp × ΔT
where:
This equation explains why system cleanliness and water flow are connected to heating performance.
If deposits or clogged components increase resistance, the actual water flow can decrease.
If the terminals do not receive sufficient flow, they cannot transfer the required amount of heat into the building.
So the heat pump may still be capable of generating the required thermal capacity while the distribution system becomes the bottleneck.
This is why we repeatedly emphasize:
Heat generation and heat distribution must be evaluated separately.
A contaminated system does not always lose performance uniformly.
Some floor heating loops may have:
These circuits are often more sensitive to additional resistance.
As contamination increases, the weakest circuit may be affected first.
The customer may therefore report:
“The living room is warm, but the bedroom is cold.”
or:
“The first floor is okay, but the upper floor takes much longer to heat.”
This does not automatically mean the heat pump is too small.
Engineers should compare individual manifold flow rates and investigate whether some circuits have become hydraulically restricted.
Professional system cleaning should have a clear objective:
restore circulation and remove contamination without damaging system components.
Depending on system condition and manufacturer requirements, maintenance may include:
The exact procedure should follow the system materials, equipment manufacturer's requirements and applicable local water-treatment practices.
Cleaning is important, but prevention is better.
Water quality should be considered during initial system design and commissioning.
Depending on local water conditions and equipment requirements, engineers may need to consider:
This is especially important because water conditions vary greatly between regions.
There is no single water-treatment strategy that should be blindly applied to every heat pump project.
Always check the heat pump manufacturer's requirements and relevant local standards.
In our series, “One Heat Source, Two Terminals” refers to one air-to-water heat pump serving two different terminal systems, typically:
Underfloor Heating + Fan Coil Units
Such a system can contain multiple branches, pumps, valves, manifolds and control devices.
If contamination increases resistance in one branch, hydraulic distribution between the two terminal systems can change.
For example:
Heat Pump
↓
Buffer / Hydraulic Distribution
↙ ↘
Underfloor Heating Fan Coils
If the underfloor heating circuit gradually becomes more restrictive, the flow distribution may shift.
Therefore, troubleshooting should examine the entire hydraulic network rather than treating each component independently.
If an underfloor heating system worked well for several years but now struggles to reach temperature, use a systematic approach.
Confirm:
Compare actual flow against the design requirement.
An abnormal supply/return temperature difference can provide useful information about circulation and heat transfer.
Do not underestimate this simple step.
Compare individual underfloor heating loops.
Air accumulation can produce similar low-flow symptoms.
Look for discoloration, suspended solids, sludge or other evidence of contamination.
Flush and clean the system using an appropriate procedure.
After cleaning, restore the correct hydraulic balance.
Record:
Flow + ΔT + Water Temperature + Room Temperature + Heat Pump Operation
This makes the maintenance result measurable rather than subjective.
Many customers think about cleaning only after the floor stops heating properly.
A better approach is preventive maintenance.
However, there is no universal rule that every underfloor heating system must be flushed on exactly the same fixed interval.
The appropriate maintenance schedule depends on:
Regular inspection is therefore more useful than blindly applying one interval to every installation.
For distributors and HVAC service companies, this also creates an opportunity to move from simply selling equipment toward providing long-term system service.
The customer does not only need a heat pump.
They need the heating system to continue working efficiently for many years.
Our “One Heat Source, Two Terminals” series has now examined seven common reasons why underfloor heating may not reach the expected temperature:
Part 1 — Initial Commissioning or Long Period of Non-Use
Part 2 — Air Trapped in the Underfloor Heating Pipes
Part 3 — Improper Piping Design and Layout
Part 4 — Insufficient Effective Heat Dissipation Area
Part 5 — Insufficient System Water Flow
Part 6 — Improper Installation and Poor Workmanship
Part 7 — Long-Term Operation Without System Cleaning
Notice how these problems are connected.
For Part 7, the failure chain may look like:
Poor Water Quality / Long-Term Contamination
↓
Scale + Sludge + Deposits
↓
Strainer / Pipe / Heat Exchanger Fouling
↓
Higher Hydraulic Resistance
↓
Lower Water Flow and/or Poorer Heat Transfer
↓
Reduced Terminal Heating Output
↓
Underfloor Heating Cannot Reach the Required Temperature
That is why professional heat pump troubleshooting must be system-oriented rather than component-oriented.
When an underfloor heating system has been operating for several years and gradually becomes less effective, replacing the heat pump should not be the first response.
Start with the basics:
Is the water clean?
Is the strainer clean?
Is the design flow still being achieved?
Are the individual floor loops circulating correctly?
Has fouling increased the hydraulic resistance?
Does the system now require a higher water temperature than it did when originally commissioned?
A high-efficiency air-to-water heat pump needs a clean and properly maintained hydronic system to maintain its performance.
The complete engineering chain remains:
Heat Pump → Water Quality → Hydraulic Flow → Distribution → Terminals → Heat Transfer → Indoor Comfort
Maintaining only the heat pump while ignoring the water system is maintaining only half of the heating system.
An air-to-water heat pump system may work perfectly when it is first commissioned. The water temperature is correct, the circulation flow is sufficient, and every room reaches the required temperature.
But after several years of operation, something gradually changes:
The heat pump is still running, but the floor does not feel as warm as before.
The homeowner may increase the leaving water temperature from 40°C to 45°C, or even higher. Indoor comfort may improve temporarily, but electricity consumption increases and the original problem remains.
In many cases, the problem is not the heat pump itself.
It may be caused by something much less visible:
The hydronic system has been operating for years without proper cleaning and maintenance.
This is Part 7 of our “One Heat Source, Two Terminals” engineering series, focusing on how sludge, scale, suspended solids and dirty strainers can gradually reduce the performance of underfloor heating connected to an air-to-water heat pump.
A common question is:
“The underfloor heating system is closed. Why does it need cleaning?”
A closed system does not mean a permanently clean system.
Over years of operation, the circulating water interacts with pipes, valves, manifolds, pumps, heat exchangers and other metallic or polymer components.
Depending on water quality, system materials, oxygen ingress, installation cleanliness and operating conditions, deposits can gradually develop.
Typical contaminants include:
The exact mechanism varies from system to system.
The important engineering point is that water quality changes over time, and contaminants can gradually accumulate inside the hydronic circuit.
Underfloor heating contains long pipe circuits operating at relatively low water temperatures.
If contaminants are present, they can gradually settle or adhere to internal surfaces.
This process may be slow enough that the homeowner does not notice a sudden failure.
Instead, system performance deteriorates progressively.
The sequence can look like this:
Contamination → Internal Deposits → Increased Hydraulic Resistance → Reduced Water Flow → Lower Heat Transfer → Colder Floor → Longer Heat Pump Runtime
Eventually, the customer reports:
“The heat pump used to heat the house very well. Now it doesn't.”
That statement is important because it suggests that we should investigate not only equipment capacity but also how the hydraulic system has changed since commissioning.
Mineral scale deserves special attention.
Hard water contains dissolved minerals, primarily calcium and magnesium compounds. Under suitable conditions, these minerals can precipitate and form deposits on heat-transfer surfaces.
Why does this matter?
Because scale has much lower thermal conductivity than metal.
The heat-transfer path changes from:
Hot Water → Heat Exchanger Surface → System Water
to something closer to:
Hot Water → Scale Layer → Heat Exchanger Surface → System Water
The scale acts as additional thermal resistance.
Even a relatively thin layer of deposit on a critical heat-transfer surface can negatively affect performance. The exact efficiency loss depends strongly on the material, deposit composition, thickness, flow conditions and heat exchanger design, so a universal percentage should not be assumed for every system.
The engineering principle, however, is clear:
Deposits on heat-transfer surfaces reduce heat-transfer effectiveness.
This is a common field response.
Suppose the system originally provided adequate heating at a leaving water temperature of 40–45°C.
Several years later, the customer complains that the rooms are cold.
Someone changes the setpoint to 50°C.
The house becomes warmer again.
Problem solved?
Not necessarily.
Increasing water temperature may simply compensate for deteriorating system performance.
The actual problem could still be:
Reduced flow + fouling + poor heat transfer.
For a heat pump system, unnecessarily increasing leaving water temperature is especially undesirable because heat pump efficiency generally decreases as the required water temperature increases, all else being equal.
So instead of immediately raising the setpoint, ask:
Why does the system now require a higher water temperature than it did when it was new?
That question often leads to a much better diagnosis.
When troubleshooting an older hydronic system, the strainer or filter should be high on the inspection list.
Why?
Because the filter is designed to capture contamination before it reaches sensitive system components.
After years of operation, a neglected strainer may contain significant amounts of:
As the filter becomes clogged, pressure drop increases.
The result is:
Dirty Strainer → Higher Resistance → Lower Flow → Reduced Heat Transport
This can produce symptoms remarkably similar to an undersized circulation pump.
Before replacing the pump, check whether the existing pump is simply trying to push water through a heavily restricted filter.
Hydronic heat transfer can be represented approximately by:
Q = ṁ × Cp × ΔT
where:
This equation explains why system cleanliness and water flow are connected to heating performance.
If deposits or clogged components increase resistance, the actual water flow can decrease.
If the terminals do not receive sufficient flow, they cannot transfer the required amount of heat into the building.
So the heat pump may still be capable of generating the required thermal capacity while the distribution system becomes the bottleneck.
This is why we repeatedly emphasize:
Heat generation and heat distribution must be evaluated separately.
A contaminated system does not always lose performance uniformly.
Some floor heating loops may have:
These circuits are often more sensitive to additional resistance.
As contamination increases, the weakest circuit may be affected first.
The customer may therefore report:
“The living room is warm, but the bedroom is cold.”
or:
“The first floor is okay, but the upper floor takes much longer to heat.”
This does not automatically mean the heat pump is too small.
Engineers should compare individual manifold flow rates and investigate whether some circuits have become hydraulically restricted.
Professional system cleaning should have a clear objective:
restore circulation and remove contamination without damaging system components.
Depending on system condition and manufacturer requirements, maintenance may include:
The exact procedure should follow the system materials, equipment manufacturer's requirements and applicable local water-treatment practices.
Cleaning is important, but prevention is better.
Water quality should be considered during initial system design and commissioning.
Depending on local water conditions and equipment requirements, engineers may need to consider:
This is especially important because water conditions vary greatly between regions.
There is no single water-treatment strategy that should be blindly applied to every heat pump project.
Always check the heat pump manufacturer's requirements and relevant local standards.
In our series, “One Heat Source, Two Terminals” refers to one air-to-water heat pump serving two different terminal systems, typically:
Underfloor Heating + Fan Coil Units
Such a system can contain multiple branches, pumps, valves, manifolds and control devices.
If contamination increases resistance in one branch, hydraulic distribution between the two terminal systems can change.
For example:
Heat Pump
↓
Buffer / Hydraulic Distribution
↙ ↘
Underfloor Heating Fan Coils
If the underfloor heating circuit gradually becomes more restrictive, the flow distribution may shift.
Therefore, troubleshooting should examine the entire hydraulic network rather than treating each component independently.
If an underfloor heating system worked well for several years but now struggles to reach temperature, use a systematic approach.
Confirm:
Compare actual flow against the design requirement.
An abnormal supply/return temperature difference can provide useful information about circulation and heat transfer.
Do not underestimate this simple step.
Compare individual underfloor heating loops.
Air accumulation can produce similar low-flow symptoms.
Look for discoloration, suspended solids, sludge or other evidence of contamination.
Flush and clean the system using an appropriate procedure.
After cleaning, restore the correct hydraulic balance.
Record:
Flow + ΔT + Water Temperature + Room Temperature + Heat Pump Operation
This makes the maintenance result measurable rather than subjective.
Many customers think about cleaning only after the floor stops heating properly.
A better approach is preventive maintenance.
However, there is no universal rule that every underfloor heating system must be flushed on exactly the same fixed interval.
The appropriate maintenance schedule depends on:
Regular inspection is therefore more useful than blindly applying one interval to every installation.
For distributors and HVAC service companies, this also creates an opportunity to move from simply selling equipment toward providing long-term system service.
The customer does not only need a heat pump.
They need the heating system to continue working efficiently for many years.
Our “One Heat Source, Two Terminals” series has now examined seven common reasons why underfloor heating may not reach the expected temperature:
Part 1 — Initial Commissioning or Long Period of Non-Use
Part 2 — Air Trapped in the Underfloor Heating Pipes
Part 3 — Improper Piping Design and Layout
Part 4 — Insufficient Effective Heat Dissipation Area
Part 5 — Insufficient System Water Flow
Part 6 — Improper Installation and Poor Workmanship
Part 7 — Long-Term Operation Without System Cleaning
Notice how these problems are connected.
For Part 7, the failure chain may look like:
Poor Water Quality / Long-Term Contamination
↓
Scale + Sludge + Deposits
↓
Strainer / Pipe / Heat Exchanger Fouling
↓
Higher Hydraulic Resistance
↓
Lower Water Flow and/or Poorer Heat Transfer
↓
Reduced Terminal Heating Output
↓
Underfloor Heating Cannot Reach the Required Temperature
That is why professional heat pump troubleshooting must be system-oriented rather than component-oriented.
When an underfloor heating system has been operating for several years and gradually becomes less effective, replacing the heat pump should not be the first response.
Start with the basics:
Is the water clean?
Is the strainer clean?
Is the design flow still being achieved?
Are the individual floor loops circulating correctly?
Has fouling increased the hydraulic resistance?
Does the system now require a higher water temperature than it did when originally commissioned?
A high-efficiency air-to-water heat pump needs a clean and properly maintained hydronic system to maintain its performance.
The complete engineering chain remains:
Heat Pump → Water Quality → Hydraulic Flow → Distribution → Terminals → Heat Transfer → Indoor Comfort
Maintaining only the heat pump while ignoring the water system is maintaining only half of the heating system.
住所
違う 違う34, 南路,ヨンフェン工業公園,シュンデ地区,フォシャン528000,広東省,中国人民共和国
Tel
86-757-22860323
電子メール
luke@ecoheat-pump.com
