GWS ThermoWave Potable Hot Water Expansion Tanks
GWS ThermoWave Horizontal TWB-20LH
- Established 1975
- Trusted by thousands of customers
The GWS ThermoWave™ TWB-20LH is a 20 litre horizontal expansion vessel for potable hot water. It lies on its side on its own feet and stands only 277 mm high, so it fits in cupboards, plant shelves and other spaces with limited headroom. As stored water heats and expands, the vessel absorbs the extra volume, so the relief valve doesn’t discharge hot water to drain.
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, an O-ring sealed air valve and replaceable tank feet, and needs no maintenance. The connection is ¾″ BSPT. It comes pre-charged to 1.9 bar and is rated to 10 bar and 90 °C.
Horizontal, low height
Butyl diaphragm
Replaceable feet
¾″ BSPT
Model TWB-20LH
More about this product
Product overview
The GWS ThermoWave™ TWB-20LH is a 20 litre (5.3 US gal) horizontal 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 it to drain. That wastes water and the energy used to heat it, and every cycle wears the valve.
The TWB-20LH absorbs that expansion volume and gives it back as water is drawn off. It lies on its side on replaceable feet, standing just 277 mm high with no weight hanging on the pipework. The 20 litre size sits between the 18 and 24 litre inline models and is only available in the horizontal format. Inside, the water is held in a virgin polypropylene liner sealed by a high-grade butyl diaphragm, so it never touches the steel shell.
Key features
Typical applications
Technical specifications
| Brand / Series | GWS — ThermoWave™ |
| Model | TWB-20LH |
| Type | Horizontal, on replaceable feet |
| Nominal volume | 20 L / 5.3 US gal |
| Connection | ¾″ BSPT, 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 |
| Length (A) | 444 mm |
| Height (B) | 277 mm |
| Connection height (C) | 145 mm |
| Gross weight | 5.1 kg |
Frequently asked questions
Horizontal or inline — which should I choose?
Horizontal models stand on their own feet at a low height and keep weight off the pipework. Inline models hang on the pipework and take up less floor space. There’s no inline 20 litre; the nearest inline sizes are the TWB-18LX and TWB-24LX.
Where does it fit?
On the cold feed to the water heater, between the heater and any non-return or pressure-reducing valve. Stand it on a level surface on its feet.
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. Horizontal ThermoWave models come in 12, 20, 24, 35 and 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 Horizontal TWB-20LH. 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.