GWS HeatWave Hydronic Expansion Tanks
GWS HeatWave HWB-4LX
- Established 1975
- Trusted by thousands of customers
The GWS HeatWave™ HWB-4LX is a compact 4 litre inline expansion vessel for small closed-loop heating systems. As the system water heats and expands, the vessel’s air-charged chamber takes up the extra volume, holding system pressure steady and keeping it off the relief valve.
High-grade butyl diaphragm, chromed steel connection and brass O-ring sealed air valve, with a dual-layer polyurethane finish. At 256 × 162 mm it screws straight onto the pipework on its ¾″ BSPT connection. Factory pre-charged to 0.7 bar, rated to 6 bar and 99 °C.
Butyl diaphragm
Inline mounting
¾″ BSPT
Glycol compatible
Model HWB-4LX
More about this product
Product overview
The GWS HeatWave™ HWB-4LX is a 4 litre (1.1 US gal) inline expansion vessel for small sealed heating circuits. Water expands as it heats. Without somewhere to go, that extra volume pushes system pressure up until the relief valve lifts. The HeatWave’s air-charged chamber absorbs it instead, keeping pressure steady through every heat-up and cool-down.
HeatWave™ tanks are built to the same standards as GWS PressureWave™ and Challenger™ vessels, with a high-grade butyl diaphragm, chromed steel connection with welded hex nut, and a brass air valve with O-ring sealed cap. The inline design mounts directly on the pipework, or on the GWS universal wall bracket where the pipe needs support.
Key features
Typical applications
Technical specifications
| Brand / Series | GWS — HeatWave™ |
| Model | HWB-4LX |
| Type | Inline, pipework mounted |
| Nominal volume | 4 L / 1.1 US gal |
| Connection | ¾″ BSPT, chromed steel |
| Factory pre-charge | 0.7 bar / 10 psi |
| Max operating pressure | 6 bar / 87 psi |
| Operating temperature | −10 °C (avoid freezing) to +99 °C |
| Diaphragm | High-grade butyl |
| Air valve | Brass, O-ring sealed cap |
| Finish | Dual-layer polyurethane over epoxy primer, grey |
| Height (A) | 256 mm |
| Diameter (B) | 162 mm |
| Gross weight | 1.7 kg |
| Suitable for | Closed-loop, non-potable hydronic heating and cooling only |
Frequently asked questions
What does a heating expansion vessel do?
Water expands as a heating system warms up. The vessel’s air-charged chamber takes up that extra volume, so system pressure stays steady and the relief valve doesn’t lift every time the heating comes on.
Can I use it on a drinking-water or booster system?
No. HeatWave™ vessels are for closed-loop, non-potable heating and cooling only. For potable hot water expansion, use a GWS ThermoWave™ vessel; for drinking water or pump boosting, a PressureWave™ or Challenger™.
What should the pre-charge be set to?
It arrives at 0.7 bar. Check it with the vessel empty and set it to your system’s cold fill pressure before filling.
Can I use antifreeze?
Yes, propylene glycol at concentrations up to 50%. GWS advise avoiding ethylene glycol.
Is 4 litres big enough?
It suits small sealed circuits such as underfloor zones and HIUs. Whole-house systems usually need 8 to 24 litres; the HeatWave range runs up to 500 litres. Ask our team if you’d like it sized for your system.
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Data Sheet PDFDownload
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 HeatWave HWB-4LX. 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.