Expansion Vessel Installation and Connection

Picture a boiler with its own pump, and behind it two or three circuits, each with another pump. Without a header, the pumps push against each other. One circuit starves another of water, radiators heat unevenly and the boiler keeps switching on and off. The header creates a zone where the water moves very slowly. The boiler side (primary circuit) and the heating side (secondary circuits) meet there without affecting each other's pressure.

A header also collects air at the top and sludge at the bottom. That is why it usually has an automatic air vent at the top and a drain valve at the bottom for flushing out dirt.

A low loss header makes sense mainly in these situations:

• The house has several heating circuits with their own pumps
• Radiators and underfloor heating run together at different water temperatures
• Several boilers work together (a cascade), or a boiler is combined with another heat source
• The boiler or heat pump manufacturer requires a steady minimum flow through the unit

A simple system with one boiler and one radiator circuit usually does not need a header. A heating engineer decides whether yours does, based on the system layout and the boiler manufacturer's instructions.

The process depends on whether this is a new system or an addition to an existing one. When a header is added to a working system, the job usually goes like this:

1. The heating engineer inspects the system: number of circuits, boiler output, pipe material and size, and the condition of pumps and valves.
2. They choose the header size and connection diameter to match the output and flow rate.
3. They switch off the boiler and pumps, close the valves and drain the part of the system they need to work on. For a small local job they may freeze the pipe instead of draining it. Freezing blocks the flow in the pipe for a short time.
4. They fix the header vertically on a wall bracket. The pipework alone should not carry it.
5. They connect the primary side (flow and return from the boiler) and the secondary side (flow and returns to the circuits). Isolating valves and unions go on the connections so the header can be serviced later.
6. They fit the air vent and drain valve and, if the controls need one, a pocket for the temperature sensor.
7. They refill the system, bleed it and bring it up to working pressure.
8. They check every joint for leaks with cold water and again with hot water.
9. They set the pump speeds and the controls so the boiler flow and the circuit flows match.
10. They insulate the header and its connections so it does not waste heat into the boiler room.

If the boiler runs on gas, the heating engineer does not touch the gas side during this job. Any change to the boiler settings belongs to a qualified, registered gas engineer. Wiring for new pumps and controls must be done by a qualified electrician. If you smell gas near the boiler, do not switch it on and do not use light switches. Open the windows, leave the room and call your gas emergency line or 112.

Low loss headers come in several designs. The choice mostly depends on the boiler output, the number of circuits and the space in the boiler room.

• A standalone header suits systems that already have a manifold, or where you plan to fit one separately
• A header combined with a flow and return manifold saves space and makes fitting simpler when a new boiler room with several circuits is being built
• A factory-insulated header saves the work of insulating it afterwards
• A header with a magnetic filter catches metal debris, which helps older systems with steel pipework in particular

With a condensing boiler, have the size and layout designed carefully. If more water flows through the header from the boiler than the circuits draw off, hot water goes straight back to the boiler. The return water is then warmer, the boiler condenses less and uses fuel less efficiently.

Several things affect how much work the job involves:

• Access to the boiler and free wall space in the boiler room
• Pipe material: copper or plastic pipes connect differently from old steel pipes, which have to be cut and threaded or welded
• Whether the whole system has to be drained, or local isolation or pipe freezing is enough
• The number of circuits and whether new pumps, valves or a manifold are needed
• The state of the water in the system: dirty heating is best flushed before connecting
• Whether a boiler replacement or other heating work is happening at the same time

In practice, fitting a header is often just one part of a larger boiler room upgrade. If you are also planning a new boiler or extra circuits, it makes sense to do everything in one go.

In your own house, changes in the boiler room do not normally need planning permission or building approval. If the work is part of a bigger renovation, check with your local building authority. In a block of flats with communal heating, nobody should touch the shared pipework without the agreement of the building management or owners' association.

• The header is fitted horizontally or at an angle, so it does not separate air or sludge
• Primary and secondary connections, or flow and return, are swapped
• The header is too small, so water moves fast inside it and the pumps start interfering again
• Isolating valves are missing, so the whole system has to be drained for servicing
• The header hangs on the pipework with no bracket
• The pumps are set without regard to each other, so the boiler keeps cycling on and off
• The temperature sensor sits somewhere other than where the boiler controls require
• Insulation is missing, so the boiler room overheats and heat is lost

Many of these mistakes do not show up straight away. They typically appear in winter, when all the circuits run at once.

Ask the installer to show you the work with the heating running. Check that:

• The header is vertical and firmly fixed to its bracket
• All joints stay dry with both cold and hot water
• The air vent and drain valve on the header work
• Isolating valves and unions are fitted on the connections
• System pressure stays within the boiler manufacturer's range once the water is hot
• All circuits heat up and the boiler does not switch on and off at short intervals
• The pumps are set and the installer has shown you their settings
• The header and the pipes around it are insulated
• You have received the wiring and piping diagram and the header's manual