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

Part 6 — Improper Installation and Poor Workmanship

In an air-to-water heat pump system serving both underfloor heating and fan coils, poor heating performance is often blamed on the heat pump first.

“The heat pump is too small.”
“The leaving water temperature is too low.”
“The circulation pump is not powerful enough.”

However, after checking the equipment and hydraulic design, engineers sometimes discover that the real problem is much simpler:

The system was designed correctly, but it was not installed correctly.

This is Part 6 of our “One Heat Source, Two Terminals” troubleshooting series. In this article, we focus on an issue that can be difficult to identify after the floor has been completed: improper installation and poor workmanship in the underfloor heating system.

1. Why Installation Quality Matters So Much

An underfloor heating system is essentially a low-temperature hydronic heat distribution network.

The heat pump generates thermal energy, but the system must then:

Heat Pump → Main Pipework → Manifold → Floor Heating Loops → Floor Structure → Indoor Space

For this chain to work correctly, sufficient hot water must circulate through every heating loop.

Even when the engineering design is correct, installation mistakes can increase hydraulic resistance, reduce water flow or even partially block individual circuits.

The result can be confusing:

• The heat pump operates normally.

• Leaving water temperature reaches the setpoint.

• The circulation pump is running.

• Some loops or rooms become warm.

• Other rooms remain cold.

• The overall indoor temperature rises very slowly.

In these situations, replacing the heat pump with a larger unit may not solve the problem at all.

2. Common Installation Problems in Underfloor Heating Systems

2.1 Pipe Spacing Does Not Follow the Design

Pipe spacing is normally determined according to the room heating load, floor construction, design water temperature, floor covering and required heat output.

But on some construction sites, installers may change the spacing according to convenience.

For example, a design may require relatively close pipe spacing in areas with higher heat loss, while installers increase the spacing simply to reduce installation time or pipe consumption.

The consequences are straightforward:

Wider spacing → lower installed pipe density → lower effective heat transfer → lower floor surface temperature/output.

This becomes particularly noticeable near:

• external walls;

• large windows;

• doors;

• poorly insulated areas;

• rooms with higher design heat loads.

Underfloor heating should therefore be installed according to the engineering drawing rather than according to an arbitrary “standard spacing” used for every room.

2.2 Heating Loops Are Not Hydraulically Balanced

Loop length also matters.

If one manifold serves several circuits with significantly different pipe lengths and the system is not properly balanced, water naturally tends to follow the path with lower hydraulic resistance.

This may result in:

Short circuit → higher flow

Long circuit → higher resistance → lower flow

The problem then appears as uneven floor temperatures.

One room may heat normally while another room connected to the same manifold remains noticeably cooler.

This is why underfloor heating design should consider not only the total pipe quantity but also:

• individual loop length;

• pipe diameter;

• loop pressure drop;

• manifold configuration;

• design flow per loop;

• balancing valve or flowmeter settings.

After installation, each loop should be commissioned and hydraulically balanced.

3. Pipe Damage During Installation Can Create Hidden Flow Restrictions

One of the more serious installation problems occurs when the floor heating pipe is damaged during construction.

A typical example: pipe deformation at a bend

If a pipe is bent too sharply, it may become flattened or kinked.

The pipe may not be completely closed, so water can still pass through it. This makes the problem difficult to detect.

However, the effective internal cross-sectional area has been reduced.

That creates additional hydraulic resistance.

In simplified terms:

Restricted pipe → higher pressure drop → lower water flow → lower heat delivery

The installer may see water at the manifold and assume the circuit is working correctly. But “water is flowing” does not necessarily mean that the design flow rate is being achieved.

This distinction is extremely important in hydronic heating troubleshooting.

4. Do Not Solve Every Flow Problem by Installing a Bigger Pump

When insufficient flow is discovered, one common reaction is:

“Install a larger circulation pump.”

Sometimes additional pump head is necessary, but it should not be the first response.

If the actual problem is a damaged pipe, blocked circuit or incorrect installation, increasing pump capacity only attempts to overcome an abnormal hydraulic resistance.

The correct sequence should be:

Check the system first → identify the restriction → correct the cause → verify pressure drop → then confirm pump selection.

A circulation pump should be selected for the designed flow rate and total dynamic head, not simply made larger until water begins circulating.

Oversizing the pump can introduce other problems, including excessive velocity, noise and unnecessary electricity consumption.

5. Construction Debris Can Block the Hydronic Circuit

The screenshots highlight another practical issue that deserves much more attention: contamination during construction.

Building sites are rarely clean environments.

During installation, contaminants can accidentally enter open pipework, including:

• sand;

• dust;

• small stones;

• plastic fragments;

• packaging debris;

• construction residue.

For example, when pipe ends are left unsealed during construction, sand or fragments of plastic packaging can enter the pipe.

Once the system is completed, these contaminants may travel with the water until they reach a narrower passage.

Potential blockage points include:

valves, strainers, manifolds, flowmeters, heat exchangers, elbows and smaller pipe sections.

A partial blockage can be especially difficult to diagnose because the circuit may still have some flow — just not enough flow.

6. Why Insufficient Flow Means Insufficient Heat

The basic relationship for hydronic heat transfer can be expressed as:

Q = ṁ × Cp × ΔT

Where:

• Q = transferred heating capacity

• ṁ = water mass flow rate

• Cp = specific heat capacity of water

• ΔT = supply/return water temperature difference

This equation explains why water flow is so important.

Suppose the heat pump is capable of producing sufficient heating capacity, but installation defects restrict the water flow through the floor loops.

The heat source may be operating correctly, but the distribution system cannot transport the required thermal energy to the floor.

This is why troubleshooting should never focus only on the heat pump leaving-water temperature.

Engineers should also measure:

actual flow rate + supply temperature + return temperature + pressure differential + individual loop condition.

7. Poor Installation Can Look Like a Heat Pump Capacity Problem

This is one of the biggest traps in troubleshooting.

Imagine a house where the heat pump runs continuously but indoor temperature cannot reach the setpoint.

The immediate conclusion may be:

“The heat pump is undersized.”

But consider another possibility.

If the heat pump can generate 100% of the required thermal energy while the hydronic network can only transfer part of that energy because of insufficient flow, the terminal system cannot use the full available capacity.

Increasing heat pump capacity does not remove the hydraulic bottleneck.

Before increasing equipment size, engineers should determine whether the problem is on the:

heat-generation side
or
heat-distribution side.

8. Site Supervision Is Part of System Engineering

A good engineering drawing alone does not guarantee a good heating system.

Installation quality must also be controlled.

For underfloor heating projects, especially larger residential or commercial projects, we recommend maintaining proper construction records.

Before pipes are covered by screed or flooring, record:

• manifold location;

• pipe routing;

• loop identification;

• pipe spacing;

• individual loop length;

• bend conditions;

• pressure-test results;

• flowmeter settings;

• concealed joints, if any;

• photographs of completed pipework.

Taking photos from several angles at each important construction stage can be extremely valuable.

Once the pipes are covered, visual inspection becomes impossible. A simple photographic construction record can save many hours of troubleshooting later.

9. Protect Open Pipe Ends During Construction

This is a small detail with potentially large consequences.

Any unfinished pipe should be temporarily capped or sealed.

Do not leave open pipe ends exposed while other construction work continues.

This helps prevent:

Sand / debris → pipework → valves or manifolds → restriction → insufficient flow → poor heating performance

Before final commissioning, the system should also be properly flushed according to the system design and equipment manufacturer's requirements.

Filters and strainers should be inspected after initial operation because construction debris may accumulate during the first circulation period.

10. Commissioning Checklist for Suspected Installation Problems

When an underfloor heating system cannot reach the expected temperature, technicians should avoid immediately changing the heat pump parameters.

A more systematic diagnostic sequence is:

1. Confirm the heat pump's actual leaving and return water temperatures.

2. Confirm the total system water flow.

3. Check individual manifold flowmeters.

4. Compare flow between different floor heating loops.

5. Confirm that all valves and actuators are fully operational.

6. Check strainers and filters for contamination.

7. Inspect for air trapped in the system.

8. Verify circulation pump operating conditions.

9. Check whether pipe loops were installed according to the drawings.

10. Investigate possible crushed, kinked or blocked pipework.

11. Rebalance all heating circuits.

12. Compare actual room heat loss with available floor heating output.

The key principle is simple:

Measure first. Diagnose second. Modify the system last.

11. Special Considerations for “One Heat Source, Two Terminals”

When one air-to-water heat pump supplies both underfloor heating and fan coil units, troubleshooting becomes more complex.

The two terminal types have different operating characteristics.

Underfloor heating typically requires relatively low water temperatures and stable continuous circulation, while fan coils generally have different flow, temperature and control requirements.

Therefore, poor installation in one branch can affect the hydraulic behavior of the entire system.

Engineers should evaluate the complete system:

Heat Pump → Buffer / Hydraulic Separation → Pumps → Distribution → Fan Coils + Underfloor Heating → Controls

Correct hydraulic separation, pump selection, balancing and control logic are especially important when different terminal circuits operate independently.

12. The Bigger Engineering Lesson

When underfloor heating does not get warm enough, there is rarely only one possible explanation.

In this series, we have already discussed several common causes:

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

These problems are often interconnected.

For example:

Poor installation → pipe restriction → higher hydraulic resistance → insufficient flow → reduced floor output → low room temperature

That is why professional troubleshooting should examine the system as a whole rather than immediately blaming one component.

Conclusion

A high-quality heat pump cannot compensate for poor hydronic installation.

For an underfloor heating system to perform correctly, design, equipment selection, installation, commissioning and control must work together.

For contractors and distributors, this also highlights an important principle:

The performance of an air-to-water heat pump project depends not only on the heat pump itself, but on the quality of the entire hydronic system.

Good drawings are important.

Correct equipment selection is important.

But what is actually installed on site ultimately determines whether the design becomes a successful heating system.