Part 8: Low Supply Water Temperature — Finding the Right Balance Between Heating Comfort and Heat Pump Efficiency

Technical Guide | Air-to-Water Heat Pumps | Underfloor Heating | Hydronic System Design

Introduction: Is Higher Water Temperature Always the Solution?

One of the most common complaints in air-to-water heat pump installations is:

"My heat pump is running normally, but the underfloor heating is not warm enough."

In our previous seven articles, we discussed several potential causes of insufficient underfloor heating performance, including trapped air, poor piping design, insufficient heating area, inadequate water flow, improper installation and long-term system contamination.

However, even when the heat pump and hydraulic system are operating correctly, another important factor can affect indoor comfort:

The supply water temperature may be too low for the actual heating requirements of the building.

This creates an interesting engineering challenge.

On one hand, air-source heat pumps generally operate more efficiently at lower leaving water temperatures.

On the other hand, underfloor heating must deliver enough thermal energy to compensate for the building's heat losses.

The goal is therefore not to operate at the lowest possible water temperature, but to find the lowest water temperature that can reliably maintain the required indoor comfort under the prevailing outdoor conditions.

In this article, we will explain how supply water temperature affects underfloor heating performance, why different floor constructions require different temperatures, and how engineers can optimize the system without unnecessarily increasing electricity consumption.

1. How Does Supply Water Temperature Affect Underfloor Heating?

Underfloor heating is a low-temperature hydronic heating system.

Unlike conventional radiators, which typically rely on relatively high water temperatures and a smaller heat-emitting surface, underfloor heating distributes heat across a much larger floor area.

The basic heat-transfer process is:

Heat Pump → Heated Water → Underfloor Heating Pipes → Floor Structure → Floor Surface → Indoor Space

Heat moves from the circulating water through the pipe wall and surrounding floor materials before reaching the occupied space.

The heat delivered to the room depends on several factors:

• Supply and return water temperatures

• Water flow rate through each heating circuit

• Pipe spacing and installation depth

• Thermal conductivity of floor materials

• Floor covering resistance

• Available heat-emitting floor area

• Indoor air temperature

• Building heat loss

A simplified relationship for floor heat output is:

Q = A × q

Where:

• Q = total heat output (W)

• A = effective heated floor area (m²)

• q = heat output per square meter (W/m²)

The heat output per square meter depends strongly on the difference between the floor surface temperature and the indoor environment.

If the supply water temperature is too low, the floor may not reach the surface temperature necessary to offset the room's heat loss.

The result is insufficient indoor heating, even when the heat pump continues operating.

Example: A Room With Insufficient Heating Capacity

Consider a room with:

• Floor area: 20 m²

• Effective heated area: 16 m²

• Design heating load: 800 W

The required average heat output is:

800 W ÷ 16 m² = 50 W/m²

If the installed floor heating system delivers only 35 W/m² under its current operating conditions:

16 m² × 35 W/m² = 560 W

The heating deficit is:

800 W − 560 W = 240 W

In this situation, the indoor temperature may remain below the desired setpoint.

Increasing the supply water temperature may increase the floor heat output, but the correct solution should be based on the floor construction, heating load and manufacturer's performance data.

2. What Is the Recommended Supply Water Temperature for Underfloor Heating?

There is no single supply water temperature suitable for every underfloor heating installation.

The correct temperature depends on the building and the terminal system.

The following values are indicative engineering ranges rather than universal design requirements.

In some well-insulated buildings, a supply temperature around 30°C may be sufficient.

In other buildings, especially those with higher heat losses or thicker floor constructions, the system may require higher temperatures.

The correct water temperature should always be determined by a heat-loss calculation and the floor heating system's thermal performance.

Importantly, increasing water temperature must not cause the floor surface to exceed applicable comfort or safety limits.

3. Wet Underfloor Heating vs. Dry Underfloor Heating: Why Do They Behave Differently?

One of the most important considerations when selecting the supply water temperature is the floor construction.

Underfloor heating systems can generally be divided into two broad categories:

3.1 Wet Underfloor Heating Systems

A wet underfloor heating system typically uses heating pipes embedded in a concrete or cementitious screed.

The structure may include:

1. Structural floor

2. Thermal insulation

3. Heating pipes

4. Screed or concrete layer

5. Floor covering

The heated water transfers energy to the screed, which distributes heat across the floor.

Advantages

• Good heat distribution

• High thermal mass

• Stable indoor temperature

• Suitable for continuous low-temperature heating

• Strong compatibility with air-to-water heat pumps

Limitations

• Slow temperature response

• Longer initial warm-up time

• Greater sensitivity to floor thickness and thermal resistance

• Less suitable for frequent large temperature setbacks

A thick screed may require several hours to warm up after a prolonged shutdown.

However, once heated, it can provide stable thermal output.

3.2 Dry Underfloor Heating Systems

Dry underfloor heating systems typically use lightweight panels, insulation boards or heat-spreading plates rather than a thick concrete screed.

Common applications include:

• Renovation projects

• Timber floor construction

• Lightweight buildings

• Projects requiring reduced floor height

• Buildings where structural weight is limited

Advantages

• Lower thermal mass

• Faster response

• Potentially lower installation weight

• Easier integration into certain retrofit projects

Limitations

• Heat distribution depends heavily on plate and panel design

• Performance is sensitive to pipe spacing

• Poor heat-spreading components can create uneven surface temperatures

• Heat output varies considerably between products

A properly designed dry system can operate efficiently at low water temperatures.

However, a poorly designed system may require higher supply temperatures to achieve the same room heating performance.

Engineering takeaway: Wet and dry systems should not be compared using supply temperature alone. Their actual thermal output must be evaluated under equivalent operating conditions.

4. Why Does Floor Covering Material Matter?

Even when two rooms use the same underfloor heating pipes and supply water temperature, their heating performance may differ.

One important reason is the thermal resistance of the floor covering.

Consider these common materials:

For example, a room with ceramic tiles may transfer heat more effectively than a room with thick carpet and insulating underlay.

This does not mean carpet cannot be used with underfloor heating.

It means that its thermal resistance must be included in the design calculation.

What Happens When Floor Resistance Is Too High?

The heat must pass through more resistance before reaching the room.

Consequently:

• Heat output may decrease.

• Floor warm-up may take longer.

• Higher average water temperatures may be required.

• Heat pump efficiency may decline.

Before increasing the supply water temperature, engineers should confirm whether the floor covering matches the original system design.

5. Why Can a Heat Pump Deliver 35°C Water but the Room Still Feel Cold?

A heat pump delivering 35°C water is not necessarily malfunctioning.

The problem may be that the heating system requires more heat output than it can deliver at that temperature.

There are several possible reasons.

Reason A: Building Heat Loss Is Higher Than Expected

The building may have:

• Poor wall insulation

• Single-glazed or poorly insulated windows

• Air leakage

• Insufficient roof insulation

• Thermal bridges

• Higher-than-expected ventilation losses

For example, if a room requires 70 W/m² but the floor heating system can deliver only 45 W/m², the room may not reach its design temperature.

Reason B: Floor Construction Creates Excessive Thermal Resistance

A thick screed, deep pipe installation or insulating floor covering may reduce heat output at the selected water temperature.

Reason C: Insufficient Effective Heating Area

Furniture, fixed cabinets and other obstructions may reduce the useful heat-emitting area.

Reason D: The System Is Operating Below Its Required Design Temperature

A system designed to operate at 40°C during very cold weather may not maintain indoor comfort if the controller limits the supply temperature to 30°C.

Reason E: The Actual Manifold Temperature Is Lower Than the Heat Pump Outlet Temperature

In a one-heat-source, two-terminal system, the heat pump may supply water at 40°C while the UFH manifold receives cooler water because of mixing, bypass flow or hydraulic configuration.

Therefore, engineers should measure the temperature at the actual underfloor heating supply manifold rather than relying only on the heat pump display.

6. One Heat Source, Two Terminals: Why Temperature Control Becomes More Important

In a combined heating system, one air-to-water heat pump may serve two different terminal types:

Underfloor Heating + Fan Coil Units

These terminals may have different operating requirements.

For example:

These are indicative characteristics, not fixed equipment specifications.

Why Does This Matter?

Suppose a fan coil circuit requires 45°C supply water under design conditions, while the underfloor heating circuit needs only 35°C.

If both circuits are connected without appropriate temperature management, one of two problems may occur:

Situation 1: Supply Temperature Is Too Low

The fan coils may fail to deliver sufficient heating capacity.

Situation 2: Supply Temperature Is Too High

The underfloor heating circuit may receive unnecessarily hot water, creating comfort and floor-temperature control problems.

A properly designed system may therefore require:

• Separate temperature control

• Independent circulation circuits

• A suitable mixing arrangement

• Hydraulic separation where necessary

• Appropriate zone controls

• Correctly configured heat pump setpoints

However, mixing valves should not be added automatically to every installation.

If both terminal systems are designed to operate at the same low supply temperature, a simpler arrangement may be possible.

The system should be selected according to the actual terminal requirements.

7. Why Is Lower Supply Water Temperature Better for Heat Pump Efficiency?

An air-source heat pump transfers heat from outdoor air into the heating water.

The compressor must raise the refrigerant temperature sufficiently to transfer heat to the water.

As the required leaving water temperature increases, the refrigerant condensing temperature generally increases as well.

This raises the compressor's operating pressure ratio and reduces efficiency.

The basic efficiency indicator is the coefficient of performance:

COP = Heating Capacity ÷ Electrical Input

For example, consider a hypothetical heat pump operating under the same outdoor conditions:

These values are illustrative only and do not represent a specific heat pump model.

For a constant heating demand of 10 kW:

At COP 4.5:

Electrical Input = 10 ÷ 4.5 = 2.22 kW

At COP 3.3:

Electrical Input = 10 ÷ 3.3 = 3.03 kW

The electrical input is approximately 36% higher in this example.

Actual results depend on outdoor temperature, compressor technology, water flow, operating conditions and equipment performance.

The engineering objective is to maintain sufficient heating output while minimizing unnecessary water temperature.

8. Does Increasing Indoor Temperature by 1°C Increase Energy Consumption?

Indoor temperature settings also influence energy consumption.

The greater the difference between indoor and outdoor temperatures, the greater the building's heat loss, all other conditions being equal.

A simplified steady-state heat-loss relationship is:

Q = H × (Ti − To)

Where:

• Q = building heat loss (W)

• H = overall building heat-loss coefficient (W/K)

• Ti = indoor temperature (°C)

• To = outdoor temperature (°C)

Consider an example:

Outdoor temperature: 0°C

Indoor temperature: 20°C

Temperature difference: 20 K

If the indoor setpoint increases to 21°C:

Temperature difference: 21 K

The temperature-driven heat loss increases by approximately:

(21 − 20) ÷ 20 × 100% = 5%

This is a simplified example, not a universal annual energy-consumption percentage.

Actual electricity consumption also depends on heat pump COP, solar gains, internal gains, ventilation and system controls.

Nevertheless, unnecessarily high indoor temperature settings can increase heating demand.

For many residential applications, approximately 20–22°C is a common comfort target, although actual preferences, occupancy, activity and applicable standards may differ.

9. A Real-World Example: Why Low Water Temperature Can Still Maintain Comfortable Rooms

Consider a well-insulated office building equipped with a wet underfloor heating system.

Suppose the system operates with:

• Supply water temperature: approximately 28–30°C

• Continuous heating operation

• Good building insulation

• Proper hydraulic balancing

• Adequate effective heating area

Under suitable outdoor conditions, the indoor temperature may remain around 21–23°C.

This is possible because the building heat loss is relatively low and the floor provides a large heat-emitting surface.

However, the same supply water temperature may be insufficient in an older building with poor insulation.

This demonstrates an important principle:

Supply water temperature cannot be selected independently of building heat loss and terminal capacity.

The performance of one building should never be treated as a universal design standard for another.

10. Why Continuous Operation Often Works Better Than Frequent On-Off Control

Wet underfloor heating systems generally have substantial thermal inertia.

A thick concrete screed takes time to absorb heat.

It also takes time to release that stored heat.

This creates a relatively slow response to changes in the thermostat setpoint.

For this reason, many heat pump systems benefit from stable operation and appropriately configured weather-compensated control.

Example: Frequent On-Off Operation

A homeowner turns off the heating system every morning and restarts it in the evening.

The floor cools down during the day.

When the system restarts, the heat pump must reheat both the floor structure and the indoor space.

The homeowner may then increase the water temperature to accelerate the process.

This can reduce heat pump efficiency.

A More Suitable Strategy

For many well-insulated buildings with high-mass underfloor heating:

• Use stable room temperature settings.

• Avoid unnecessarily large temperature setbacks.

• Apply weather-compensated water temperature control.

• Allow sufficient time for the floor to respond.

• Maintain appropriate system water flow.

Continuous operation is not automatically the most energy-efficient strategy in every building.

The optimal schedule depends on insulation, occupancy, climate, electricity tariffs and system characteristics.

However, excessive cycling and frequent large temperature changes should generally be avoided in high-mass floor heating systems.

11. What Is Weather Compensation and Why Is It Important?

Weather compensation is one of the most useful control functions for modern air-to-water heat pumps.

Instead of operating at a fixed leaving water temperature, the heat pump adjusts its water temperature according to outdoor conditions.

For example:

This is an illustrative heating curve. Actual settings must be calculated and commissioned for the building and terminal system.

When outdoor conditions are mild, the heat pump can operate at a lower water temperature.

When outdoor conditions become colder, the controller increases the supply temperature to compensate for the building's higher heat loss.

Benefits of Weather Compensation

Properly configured weather compensation can:

• Reduce unnecessary high-temperature operation

• Improve seasonal efficiency

• Maintain more stable indoor temperatures

• Reduce excessive thermostat cycling

• Improve comfort in slow-response heating systems

A heating curve that is set too low may cause insufficient room heating.

A curve that is set too high may waste electricity and cause overheating.

Correct commissioning is essential.

12. How Can Engineers Determine Whether the Supply Temperature Is Too Low?

Before increasing the heat pump setpoint, engineers should carry out a structured diagnosis.

Step 1: Measure Actual Indoor Temperature

Check whether the room reaches the required design temperature.

Use a calibrated thermometer placed away from direct sunlight and localized heat sources.

Step 2: Measure Supply and Return Water Temperatures

Record temperatures at:

• Heat pump outlet

• Heat pump inlet

• UFH manifold supply

• UFH manifold return

This helps identify unexpected temperature losses or mixing effects.

Step 3: Check Water Flow

Verify that the system meets the heat pump's minimum flow requirement and the terminal circuits receive their design flow.

Insufficient flow can create poor heating performance even when the supply temperature is correct.

Step 4: Measure Floor Surface Temperature

Use a suitable contact thermometer or calibrated thermal imaging equipment.

Thermal imaging can help identify:

• Uneven heating patterns

• Poorly performing circuits

• Large temperature differences

• Possible installation or flow problems

However, infrared measurements must account for surface emissivity, reflections and measurement conditions.

Step 5: Check the Floor Construction

Confirm:

• Pipe spacing

• Pipe installation depth

• Screed thickness

• Floor covering

• Thermal insulation below the pipes

Step 6: Compare Actual Output With Building Heat Loss

Determine whether the installed floor system can deliver the required heating output at the current mean water temperature.

Step 7: Adjust Water Temperature Gradually

If the hydraulic system and terminal design are correct, the heating curve or supply temperature may be adjusted in small increments.

Allow sufficient stabilization time before evaluating the result.

For high-mass floors, this may require many hours or longer.

13. Why Increasing Supply Water Temperature Is Not Always the Correct Solution

Suppose an underfloor heating system cannot maintain the desired indoor temperature.

The homeowner increases the supply temperature from 35°C to 45°C.

The room becomes warmer.

Does that mean the problem has been solved?

Not necessarily.

The original problem may have been:

• Insufficient water flow

• Trapped air

• Improper pipe spacing

• Excessive floor thermal resistance

• Poor hydraulic balancing

• Incorrect control settings

• Undersized heating terminals

Increasing the water temperature may compensate for some of these problems, but it may also conceal them.

In addition, excessive supply temperatures can create floor surface temperatures that exceed comfort limits or floor-covering manufacturer requirements.

A professional heating system should be optimized through correct design and hydraulic commissioning, not simply by raising the temperature setpoint.

14. What About Maximum Floor Surface Temperature?

Underfloor heating systems must satisfy both heating capacity and thermal comfort requirements.

The floor surface temperature should not be increased without limits.

EN 1264 provides a framework for the design and performance assessment of water-based surface heating systems.

Commonly referenced maximum surface temperatures include:

• Approximately 29°C for occupied floor areas

• Approximately 33°C for bathrooms

• Approximately 35°C for designated peripheral zones

These figures depend on the applicable standard, zone classification and project requirements.

Floor covering manufacturers may impose lower limits, particularly for certain wood and resilient flooring products.

Engineers should therefore verify both the heating output and allowable surface temperature.

15. Practical Recommendations for Heat Pump Installers

For air-to-water heat pump systems serving underfloor heating, the following principles are recommended.

1. Calculate the building heat loss first.

Do not select supply water temperature based only on experience or floor area.

2. Confirm the actual floor heating output.

Check pipe spacing, floor construction, covering resistance and effective heated area.

3. Use the lowest practical supply water temperature.

The selected temperature must still satisfy the design heating load.

4. Measure temperatures at the correct locations.

The temperature shown on the heat pump controller may differ from the actual UFH manifold supply temperature.

5. Verify water flow and hydraulic balancing.

Water temperature and flow must be evaluated together.

6. Consider different terminal requirements.

In a one-heat-source, two-terminal system, fan coils and underfloor heating may require different control strategies.

7. Configure weather compensation correctly.

A properly commissioned heating curve can improve both comfort and seasonal performance.

8. Avoid unnecessary high-temperature operation.

Higher water temperatures generally reduce heat pump COP.

9. Respect floor surface temperature limits.

Thermal comfort and flooring protection are essential.

10. Educate the homeowner about thermal inertia.

Underfloor heating is designed to deliver steady, comfortable heating rather than rapid temperature changes.

16. Frequently Asked Questions

Q1: Is 35°C supply water temperature enough for underfloor heating?

It can be sufficient for many well-insulated buildings, but not all installations.

The required temperature depends on building heat loss, floor construction, pipe spacing, flow rate and floor covering.

Q2: Why is my underfloor heating cold even though the heat pump shows 40°C?

Possible causes include low water flow, trapped air, incorrect mixing valve settings, poor hydraulic balancing or a difference between heat pump outlet temperature and actual manifold supply temperature.

Q3: Can an air-source heat pump operate with 30°C water?

Yes. Many modern air-to-water heat pumps can operate at approximately 30°C leaving water temperature, subject to their operating limits.

Whether that temperature is sufficient to heat the building is a separate design question.

Q4: Does increasing the water temperature increase electricity consumption?

Generally, yes, for the same outdoor conditions and heating demand.

Higher leaving water temperatures usually reduce heat pump COP, although the exact effect depends on the equipment and operating conditions.

Q5: Should underfloor heating run continuously?

Many high-mass underfloor heating systems benefit from stable operation with modest temperature adjustments.

However, the most economical control strategy depends on building characteristics, occupancy and energy tariffs.

Q6: What is the difference between wet and dry underfloor heating?

Wet systems usually embed heating pipes in screed or concrete and have greater thermal mass.

Dry systems use lightweight panels or heat-spreading plates and generally respond more quickly.

Both can work efficiently with heat pumps when properly designed.

Q7: Can underfloor heating and fan coils use the same water temperature?

Yes, if both terminals are selected to deliver their required capacity at that temperature.

Otherwise, separate temperature control or hydraulic arrangements may be necessary.

17. Conclusion: The Best Supply Temperature Is the Lowest One That Meets the Heating Demand

Low supply water temperature is one possible reason why underfloor heating cannot maintain the desired indoor temperature.

However, the solution is not always to increase the heat pump setpoint.

A well-designed air-to-water heat pump system must balance three essential factors:

Heating Capacity + Indoor Comfort + Energy Efficiency

Supply water temperature should be selected based on the building's actual heat loss and the thermal performance of the heating terminals.

In a one-heat-source, two-terminal system, engineers must also consider how the different terminal circuits interact.

A properly configured heat pump, combined with adequate flow, correct floor construction and weather-compensated control, can provide comfortable heating while minimizing unnecessary electricity consumption.

The key engineering principle is simple:

Do not aim for the highest possible water temperature. Aim for the lowest water temperature that reliably delivers the required heating performance.

About This Technical Series

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

This technical series explores common causes of poor underfloor heating performance in air-to-water heat pump systems.

• Part 1: Initial Commissioning or Long-Term Non-Use

• Part 2: Air Trapped in 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 Workmanship

• Part 7: Long-Term Operation Without System Cleaning

• Part 8: Low Supply Water Temperature

Our goal is to help heat pump distributors, HVAC contractors, system designers and installers better understand the relationship between heat pump performance and complete hydronic system design.

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