Circulation Pump Runs But Doesn't Heat

One of the most common problems in heating systems is the following:

The circulation pump runs, but the radiators don't get warm.

If the pump is running but the system isn't providing enough heat, this can have many different causes.

This problem is usually seen in the following systems:

• combi boiler systems
• central heating systems
• floor heating systems
• boiler room installations

To correctly identify the problem, all components of the system must be checked.

What Is a Circulation Pump?

A circulation pump is the pump that makes hot water circulate within the piping in heating systems.

Thanks to these pumps:

• water heated in the boiler or combi unit
• is sent to the radiators or floor heating pipes.

The most commonly used pump brands are:

• Grundfos circulation pumps
• Wilo circulation pumps
• Lowara pumps
• DAB pumps

These pumps generally ensure water circulation by running continuously.

1️⃣ Air in the Piping

One of the most common problems is air trapped inside the piping.

When air forms in the system:

• water circulation decreases
• water flow is insufficient even when the pump runs
• the radiators don't get warm.

Symptoms

• gurgling/air noise from the radiators
• the top part of some radiators staying cold
• low flow despite the pump running

Solution

The radiators must be bled.

2️⃣ A Clogged Strainer Filter

Strainer filters found in mechanical piping can become clogged with sediment over time.

When the filter is clogged:

• water flow rate drops
• the pump is strained
• not enough hot water reaches the radiators.

This is especially common in the following systems:

• old installations
• systems with sludge buildup

Solution

The strainer filter must be cleaned.

3️⃣ Insufficient Pump Flow Rate

In some cases the pump runs, but its flow rate is not sufficient.

This situation can occur for the following reasons:

• the pump capacity may be too small
• the pump speed may be too low
• the inside of the pump may be worn

Older pump types in particular can experience this problem.

Solution

The pump speed must be checked, or the pump must be replaced.

4️⃣ Blockage in the Piping

In heating systems used for a long time, the following problems can occur inside the piping:

• limescale buildup
• sludge formation
• rust formation

This significantly reduces water circulation.

This problem is seen more often especially in floor heating systems, since the pipe diameter is smaller.

Solution

A chemical cleaning of the piping must be performed.

5️⃣ Faulty Three-Way Valve

When the three-way valve found in combi boiler systems fails, hot water may not be directed to the correct line.

In this case:

• the pump runs
• water circulation doesn't occur
• the radiators don't get warm.

This fault can especially be seen in the following brands:

• Bosch combi boilers
• Buderus combi boilers
• Viessmann combi boilers

Solution

The three-way valve must be checked.

6️⃣ Thermostatic Valve Problem

When thermostatic valves are set incorrectly, water flow can decrease.

In this case:

• some radiators may close completely
• water flow may drop.

Solution

The valves must be checked.

7️⃣ Seized Pump Rotor

Pumps that have not run for a long time can seize over time.

In this case:

• the pump motor runs
• but no water circulation occurs.

Solution

The pump rotor must be checked.

How Is a Circulation Pump Fault Detected?

To determine whether the pump is faulty, the following signs are checked:

• whether the pump is making noise
• whether the pump is vibrating
• whether there is a temperature difference in the pipes

These checks help identify the fault.

Does the Circulation Pump Need to Be Replaced?

In some cases, replacing the pump may be necessary.

New generation pumps:

• run more quietly
• consume less electricity
• can adjust the flow rate automatically.

The following models are especially preferred:

• Grundfos Alpha2
• Grundfos Magna
• Wilo Yonos
• Wilo Stratos

Circulation Pump Electricity Consumption

Older pump types generally consume high amounts of electricity.

New generation inverter pumps, on the other hand, are far more efficient.

For this reason, replacing the pump saves energy in some cases.

Conclusion

If the circulation pump runs but the system doesn't heat, there can be many causes.

The most common causes are:

• air in the piping
• a clogged strainer filter
• insufficient pump flow rate
• blockage in the piping
• valve faults

Correctly identifying these problems is very important for the heating system to operate efficiently.