GWS ThermoWave Potable Hot Water Expansion Tanks
GWS ThermoWave Inline TWB-18LX
- Established 1975
- Trusted by thousands of customers
The GWS ThermoWave™ TWB-18LX is an 18 litre inline expansion vessel for potable hot water. Stored water expands as it heats. The vessel absorbs that expansion, so the relief valve doesn’t discharge hot water to drain and system pressure stays steady.
The water only touches a virgin polypropylene liner and high-grade butyl diaphragm, never the steel shell. It has a patented stainless steel water connection and an O-ring sealed air valve, and needs no maintenance. It fits straight onto the pipework on its ¾″ BSPT male connection. Factory pre-charged to 1.9 bar, rated to 10 bar and 90 °C.
Polypropylene liner
Butyl diaphragm
Inline mounting
¾″ BSPT
Model TWB-18LX
More about this product
Product overview
The GWS ThermoWave™ TWB-18LX is an 18 litre (4.8 US gal) inline expansion vessel for potable hot water. When a water heater warms its stored water, the water expands. Without an expansion vessel, the relief valve vents that extra water to drain. That wastes the water and the energy used to heat it, and every cycle wears the valve.
The TWB-18LX takes up that expansion volume and gives it back as water is drawn off. Inside, the water is held in a virgin polypropylene liner sealed by a high-grade butyl diaphragm, so it never touches the steel shell. At 364 × 279 mm, it screws straight onto the pipework on its ¾″ BSPT male connection, with no bracket needed.
Key features
Typical applications
Technical specifications
| Brand / Series | GWS — ThermoWave™ |
| Model | TWB-18LX |
| Type | Inline, pipework mounted |
| Nominal volume | 18 L / 4.8 US gal |
| Connection | ¾″ BSPT male, stainless steel |
| Factory pre-charge | 1.9 bar / 28 psi |
| Max operating pressure | 10 bar / 150 psi |
| Operating temperature | −10 °C to +90 °C |
| Liner | Virgin polypropylene |
| Diaphragm | High-grade butyl |
| Air valve | O-ring sealed |
| Finish | Two-part polyurethane, primed |
| Height (A) | 364 mm |
| Diameter (B) | 279 mm |
| Gross weight | 4.0 kg |
Frequently asked questions
Why does a water heater need an expansion vessel?
Water expands as it heats. In a sealed hot water system that extra volume has nowhere to go, so pressure rises until the relief valve opens and dumps hot water to drain. The vessel absorbs it instead.
Where does it fit?
On the cold feed to the water heater, between the heater and any non-return or pressure-reducing valve.
What should the pre-charge be set to?
It arrives at 1.9 bar. Check it with the vessel empty and set it to your incoming cold supply pressure before filling.
Which size do I need?
It depends on the volume of stored hot water and your supply pressure. The ThermoWave range runs from 2 to 60 litres. Check the water heater manufacturer’s recommendation or ask our team.
Can I use it on central heating?
It’s designed for potable hot water. For sealed central heating systems, use the GWS HeatWave™ range.
Standard delivery:
Standard delivery is free on most orders. Some items may incur a delivery charge depending on weight, size or value. Any applicable charge will be displayed at checkout.
Deliveries will typically be made between 7am and 6pm, Monday to Friday (excluding public holidays).
Next day delivery:
Next working day delivery is available for a £16.50 + VAT delivery charge.
Orders must be placed before 12 noon to qualify for next working day delivery. Orders placed after 12 noon on Thursday but before noon on Friday will be delivered the following Monday. Orders placed after 12 noon on Friday will be delivered on Tuesday.
Pallet/Large/Difficult items:
Some large, heavy or difficult to transport items may incur an additional delivery charge. Please contact us for a delivery quote before ordering.
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Frequently Asked Questions
Quick answers to common questions about the GWS ThermoWave Inline TWB-18LX. For technical specifications, see the product details above; for sizing or installation advice, contact our team. If you have any other questions, we're happy to help.
What is an accumulator tank?
An accumulator tank is a pressurised vessel that connects directly to the mains supply and stores water under pressure. When a tap or shower opens, water discharges immediately from the vessel at full mains pressure rather than drawing only from the street main. This delivers stronger, more consistent flow at peak times. Accumulators have no pump and no moving parts so they are maintenance free and silent.
When should I use an accumulator instead of a pump?
An accumulator is the right solution when static mains pressure is adequate – above 2 bar – but flow is poor or drops during peak demand. It boosts available flow without adding a pump, energy use or noise. If static pressure is below 2 bar, a booster pump is needed instead, or an accumulator combined with a charge pump such as the WaterBoost ACF1.
What size accumulator do I need?
Accumulators are typically sized on a 60 per cent usable rule – a 300 litre vessel delivers around 180 litres of boosted water per cycle. As a guide, a 2 to 3 bedroom home suits a 300 litre vessel such as the GWS GCB-300LV. Larger 4 to 5 bedroom homes or properties with multiple bathrooms typically need 450 to 600 litres. Vertical models save floor space, horizontal models suit lofts and roof spaces.