In an indirect hot water system, the boiler heats water and circulates it around two separate circuits:
• The radiator circuit (central heating)
• The cylinder coil circuit (used to heat the stored hot water)
Although these circuits are separate, both depend on the boiler firing and circulating water correctly.
In an indirect hot water system, the boiler relies on circulation to pass hot water through the cylinder coil.
If circulation fails due to air, pump failure, or sludge blockage:
• Hot water cannot reach the cylinder coil
• The boiler locks out for safety
• The cylinder cannot be heated by the boiler
At this stage, the immersion heater becomes the only available method of heating the hot water. This is a backup system only and not the normal method of operation.
Trapped air within the central heating pipework, radiators, or boiler will prevent water from circulating correctly.
Airlocks interrupt the normal flow and return of water, leading to no circulation and boiler lockout due to overheating and safety.
A faulty or non-operational central heating circulation pump is one of the most common causes of failed circulation, aside from airlocks.
If the pump is not functioning correctly, hot water cannot be circulated around:
• The radiator circuit, or
• The hot water cylinder coil circuit
This can occur even if the boiler is attempting to fire.
Common physical symptoms of a failing or failed pump include:
• Loud vibrations or excessive noise from the pump
• No indicator lights on the pump (where applicable)
• Visible leaks from the pump body
• Leaks from pump isolation valves
• Boiler overheating or locking out due to lack of water flow
When circulation fails due to pump issues, the boiler will shut down to protect itself, preventing the cylinder from being heated via the boiler.
Circulation can also fail due to restrictions or blockages within the heating system or boiler pipework.
Over time, sludge, debris, or corrosion can partially or completely block:
• Central heating pipework
• Radiators
• Heat exchangers
• Valves
These blockages can restrict:
• The flow of hot water leaving the boiler, or
• The return of cooler water back to the boiler
Any restriction on either the flow or return side will prevent proper circulation and can cause the boiler to overheat and lock out as a safety measure.
Engineers can often determine where the circulation problem lies by checking pipe temperatures:
• If the flow pipe is hot but the return pipe remains cold, this indicates a blockage or restriction on the return side
• If both pipes remain cool, this suggests the pump is not circulating or the boiler is not firing
• Uneven or rapidly overheating pipework near the boiler often indicates restricted circulation
By identifying which pipes are heating up, it becomes possible to diagnose whether the blockage is affecting the flow out or the return back to the boiler.
The boiler can be compared to the heart of the home’s heating and hot water system.
Just as the heart pumps blood around the body, the boiler pumps heated water around the central heating system and through the hot water cylinder coil.
The pipework is equivalent to the body’s cardiovascular system. It carries hot water away from the boiler and returns cooler water back to the boiler to be reheated.
For the system to operate correctly, there must be continuous circulation:
• Hot water is pumped out of the boiler (flow)
• Cooler water returns back to the boiler (return)
• The boiler reheats the returning water
• This cycle runs continuously
This constant “hot out, cold back” circulation is critical. If it stops, the system cannot function safely.
This mirrors how the human body works:
• The heart pumps blood out to the body
• Blood must return to the heart to be re-oxygenated
• If circulation is restricted or blocked, the system fails
The principle is exactly the same in a heating system.
When circulation is restricted or stopped:
• Hot water cannot leave the boiler correctly
• Cooler water cannot return to the boiler
• Heat builds up inside the boiler
• Safety sensors detect overheating or poor flow
• The boiler shuts down (locks out) to prevent damage
This protective shutdown is the heating system equivalent of the body preventing further strain when blood circulation is compromised.
The boiler works like a heart.
The heating pipework works like veins and arteries.
Hot water must leave the boiler and return back to it continuously. If trapped air, sludge, debris, or a failed pump blocks either the flow or the return, circulation stops and the boiler shuts down to protect itself.
Until circulation is restored, the boiler cannot heat the hot water cylinder, and hot water can only be produced via the electric immersion heater
1) Boiler → Heat Exchanger Coil (Indirect Cylinder)
This image shows the boiler heating water in its own loop, then sending that hot water through a coil inside the cylinder which transfers heat indirectly to the stored hot water. The water in the boiler circuit and the water in the cylinder never mix. 2) Central Heating vs Hot Water Flow
This diagram shows how the central heating circuit (to radiators) and the hot water heating circuit (to cylinder) are separate branches of the system, both powered by the boiler’s heat output.
20th August, 2026
18th March, 2026
