For under-floor gas venting on a new phase of a development, switching the suction of an existing vent unit between zones with actuated valves is usually the better first option. It avoids a new building and a second set of boosters. A separate vent unit makes sense only when the valves cannot be fitted without disruption. That was the advice given to the consulting engineers on a business park in Ireland built on reclaimed land.

The challenge: more buildings, one vent unit

The first phase of the park already had a venting unit in a gas control building. It drew gas from beneath the buildings and released it through a vent stack. The second phase added several more buildings, each needing the same protection from gas collecting under the floor slab.

The designers had to decide how to extend the system. They could share the existing unit, or add a second one. The supplier set out both options so the cost and disruption of each could be compared.

Option 1: actuated valves on the existing unit

In this option the existing vent unit stays where it is. Its suction is directed to one zone of the site at a time, on a pre-programmed basis. The scope was:

• A three-way manifold with one inlet and two outlets, in 100 mm nominal bore galvanised carbon steel, with brackets for floor or wall mounting.
• Two actuated valves, electro-hydraulic and suitable for landfill gas. They work in opposition: when one is open, the other is closed.
• A control panel with a seven-day timer. It has four switching positions a day, two “on” and two “off”.
• Cabling and pipework changes to connect the manifold to the existing unit.

Two options were offered on top. The first connects the valve switching to the site’s new computer-controlled monitoring system. The operator can then see which zone is being vented and keep a log. The second adds an inlet to the manifold, so the first phase can be split into two zones, giving three in all.

Option 2: a new vent unit

The alternative needed a second gas control building and a new skid-mounted unit. The unit had:

• A condensate knockout pot with a polypropylene dewatering filter, so liquid cannot reach the boosters.
• Two boosters, each driven by a 7.5 kW Zone 2 motor. Each can move about 300 m³/h against roughly 150 mbar. The impellers are stainless steel.
• A flame arrester, a pressure switch to detect loss of flow, a gas sample point and a seven-day start/stop timer.
• An auto-dialler that reports a unit number and alarm channel.

Skid mounting lets the supplier test the unit in the factory before it ships. The vent stack and the installation on site were left to others.

Why the valves came first

The reasoning is about disruption. A manifold is small. It fits on the floor or wall of the existing building, and it needs only one or two extra pipes brought in. A second unit needs a new building, new power and a new vent. The supplier’s view was that if the valves could be fitted without disturbing the site, they would be the better choice.

The trade-off is that one unit now serves several zones in turn. Under-floor venting does not need to run everywhere at once, so a timer can share the suction between them. The sizing question is whether the existing boosters have enough capacity for the largest zone. A separate unit removes that question, but at much greater cost and complexity.

The supplier asked the design team questions before finalising the proposal. These are worth copying for your own project:

• Does the existing unit have two inlets, and what size is its inlet manifold?
• Is the pipe for the new phase already in the ground and in the building? Does it fit the second manifold entry?
• Is there room for two valves between the unit and the field pipework?
• How many buildings are in the new phase, and could a further phase follow?
• Can a signal cable reach the existing unit, so the control computer can monitor the boosters and valves?

What to consider for your own project

1. Look at the future phases. A manifold with spare inlets copes with growth more cheaply than a new building.
2. Check the existing boosters. Confirm that they can serve the largest zone on their own.
3. Decide how you will prove it works. Link the switching to your monitoring system so every venting period is logged.
4. Plan for access and servicing. Position the manifold where it can be reached without disturbing other work.

The UK Environment Agency’s guidance on managing landfill gas (LFTGN03) covers gas control more broadly and is a useful reference for any site with a landfill gas risk.

FAQ

What is under-floor gas venting?

It is a system of pipes and a fan or booster that draws gas from beneath a building’s floor and releases it safely outside, so it cannot build up indoors.

Can one vent unit serve several zones?

Yes. Actuated valves and a timer can direct its suction to each zone in turn, provided the unit can handle the largest zone.

Why use two boosters in a vent unit?

Two boosters give duty and standby capacity, so venting can continue if one needs servicing.

Do valves need to be rated for landfill gas?

Yes. The valves here were electro-hydraulic units suitable for use with landfill gas.

Planning gas control for a new development?

Talk to our engineers about extending an existing venting system or designing a new one.