The Flamco Baseflex is a Contra-Flex diaphragm heating expansion vessel for sealed central heating and closed-circuit cooling installations — available in 8L, 12L, 18L, 25L, and 35L capacities (MPNs 25300–25305). As water temperature rises in a sealed system, it expands and must have somewhere to go — the Baseflex temporarily absorbs this expanded volume, maintaining system pressure within safe operating limits and preventing repeated activation of the pressure relief valve. Factory pre-charged to 1.5 bar. Maximum working pressure 6 bar; maximum system flow temperature 120°C. EPDM diaphragm, all-welded construction with zinc-plated flange, and a durable red RAL 3002 epoxy powder-coat finish. Certified to EN13831 and the Pressure Equipment Directive 2014/68/EU. Compatible with glycol-based antifreeze up to 50% and ethanol-based antifreeze up to 30%. The Flamco Baseflex is one of the most widely specified expansion vessels in UK central heating installations, trusted by heating engineers for its Contra-Flex clamped diaphragm construction that delivers lower gas permeability and longer service life than standard bladder-type vessels.

⚠ Qualified Installation Required — Pre-Charge Verification Essential — Must be installed by a competent heating engineer in accordance with BS EN 12828 and Building Regulations Part L. The factory pre-charge of 1.5 bar must be verified against the system cold fill pressure before commissioning — the vessel pre-charge should equal the system cold fill pressure. An incorrect pre-charge results in the vessel being waterlogged (pre-charge too low) or providing no useful expansion capacity (pre-charge too high). Check and adjust via the Schrader valve with the vessel isolated and system pressure released.

ℹ Choosing the Right Capacity
Expansion vessel sizing is calculated from the total system water volume, the cold fill pressure, the maximum working pressure, and the maximum system temperature. As a practical guide: an 8L vessel typically suits small systems up to approximately 100L total water content at standard pressures; 12L suits systems of 100–150L; 18L suits 150–250L; 25L suits 250–350L; and 35L suits 350–500L — but always confirm with a proper calculation to EN12828 or use a vessel sizing calculator. An undersized vessel will cause the PRV to activate repeatedly; an oversized vessel is not harmful but wastes money.

8–35 litres

5 sizes

6 bar

Max pressure

120°C

Max system temp

EN13831

Certified

50% glycol

Antifreeze compatible

Features & Benefits

✔ Contra-Flex Clamped Diaphragm — Lower Gas Permeability

The Baseflex uses Flamco's Contra-Flex clamped diaphragm design, where the EPDM diaphragm is mechanically clamped between two vessel halves with a clamp ring, rather than simply floating as a bladder inside the vessel. This clamping arrangement halves the air-to-water surface area ratio compared to conventional bladder vessels — significantly reducing the rate at which the nitrogen or air pre-charge permeates through the diaphragm over time. The result is a vessel that maintains its pre-charge pressure far longer between service checks, a more reliable and extended service life, and reduced risk of the vessel becoming waterlogged prematurely.

✔ All-Welded Construction — Factory Tested

The Baseflex shell is all-welded steel construction — there are no bolted flanges or mechanical seams that could work loose over years of pressure cycling. The zinc-plated flange at the connection provides a corrosion-resistant joint face. Every vessel is factory tested before shipment to confirm structural integrity and correct pre-charge pressure — reducing the risk of receiving a vessel with an incorrect pre-charge or a manufacturing defect. The stove-enamelled outer casing in red RAL 3002 provides corrosion protection for plant room and airing cupboard installations.

✔ EPDM Diaphragm — 120°C System / 70°C Diaphragm

EPDM (Ethylene Propylene Diene Monomer) rubber is the optimal diaphragm material for heating expansion vessels — it has excellent resistance to hot water, steam, and the inhibitor chemicals used in modern sealed systems, and remains flexible across the operating temperature range without the cracking that affects inferior rubber compounds. The diaphragm is rated to a continuous maximum temperature of 70°C (and down to –10°C), while the system itself can operate at up to 120°C — the diaphragm temperature is lower than the system temperature because it is protected from direct contact with the hottest system water.

✔ Glycol & Ethanol Antifreeze Compatible

The Baseflex is approved for use with systems containing glycol-based antifreeze up to 50% concentration (such as Fernox Antifreeze Protector or similar propylene/ethylene glycol inhibitor fluids) and ethanol-based antifreeze up to 30%. This makes the Baseflex suitable for solar thermal primary circuits, heat pump systems with frost protection requirements, and any installation where antifreeze is specified. Many competing expansion vessels are not rated for glycol use, limiting their application in solar and heat pump systems. Confirm the specific antifreeze brand and concentration with Flamco if outside these standard ranges.

✔ 35L Model — Standard Legs Fitted

The 35L Baseflex (MPN 25305) is factory-fitted with standard support legs as part of the unit — this is standard across the Flamco Baseflex 35–80L range. The legs allow the vessel to stand unsupported on a plant room or airing cupboard floor during installation and positioning, making it easier to locate and connect without an installer needing to support the vessel weight while making plumbing connections. The 8L–25L models (25300–25303) are wall-mounted or pipework-supported via their ¾" connection without requiring separate legs.

✔ EN13831 & PED 2014/68/EU — White Finish Option

Certification to EN13831 (the European harmonised standard for closed expansion vessels) and the Pressure Equipment Directive 2014/68/EU (UK CA post-Brexit) provides the compliance documentation required for installation in systems subject to Building Regulations notification and for commercial and insurance purposes. The standard red RAL 3002 powder coat finish can be specified in white RAL 9010 as an alternative — suitable for visible or aesthetic installations where the red finish is not desired. Both finishes are the same durable epoxy powder coat.

Model Range — MPN 25300 to 25305

MPN	Model	Capacity	Pre-charge	Max Pressure	Notes
25300	Baseflex 8/1.5	8L	1.5 bar	6 bar	G ¾" M · wall/pipe mount
25301	Baseflex 12/1.5	12L	1.5 bar	6 bar	G ¾" M · wall/pipe mount
25302	Baseflex 18/1.5	18L	1.5 bar	6 bar	G ¾" M · wall/pipe mount
25303	Baseflex 25/1.5	25L	1.5 bar	6 bar	G ¾" M · Ø290 × H463mm · 4.0kg
25305	Baseflex 35/1.5	35L	1.5 bar	6 bar	G ¾" M · legs fitted as standard

Technical Specifications — All Models (Common)

Manufacturer / Range	Flamco — Baseflex
Factory Pre-charge	1.5 bar (adjustable via Schrader valve)
Max. Working Pressure	6 bar
Max. System Flow Temperature	120°C
Diaphragm Material	EPDM
Diaphragm Temperature Range	–10°C to +70°C (continuous)
Diaphragm Design	Contra-Flex — clamped between two vessel halves (clamp ring)
Construction	All welded · zinc-plated flange
Outer Finish	Red RAL 3002 epoxy powder coat (white RAL 9010 available)
System Connection	G ¾" male BSP — all models
Antifreeze Compatibility	Glycol-based up to 50% · Ethanol-based up to 30%
Application	Sealed heating (EN12828) · closed cooling · solar thermal primary
Quality / Testing	Factory tested before shipment
Certifications	EN13831 · PED 2014/68/EU

Frequently Asked Questions

How do I know if my expansion vessel has failed?

The most reliable sign is the pressure relief valve repeatedly releasing small amounts of water — this means the vessel can no longer absorb expansion. To confirm: with the system cold and at its fill pressure, attach a tyre gauge to the vessel Schrader valve. A healthy vessel will show the pre-charge pressure (typically 1.0–1.5 bar). If the reading is zero or water comes out, the diaphragm has failed. Another test: tap the vessel — a healthy vessel sounds partly hollow; a waterlogged vessel sounds solid throughout. Vessels should also be checked annually as part of a boiler service; most last 10–15 years with correct installation and pre-charge, but may fail sooner in hard water or high-cycling systems.

What is the correct pre-charge pressure for my system?

The vessel pre-charge pressure should be set to match the system's cold static fill pressure — the pressure at the vessel location when the system is cold and fully charged but before the boiler has been running. For most domestic systems this is 0.5–1.5 bar depending on the height of the system above the vessel, the incoming mains pressure, and whether a filling loop or pressurisation unit is used. The factory 1.5 bar pre-charge suits many typical installations, but it must be checked and adjusted on site. To adjust: isolate the vessel, drain the system side of the vessel to zero, check the gas charge with a tyre gauge, and inflate or deflate to the correct value before reconnecting.

Can I use this vessel on a solar thermal primary circuit?

Yes — the Flamco Baseflex is suitable for solar thermal primary circuits with the following considerations: the EPDM diaphragm is compatible with glycol-based solar antifreeze up to 50% concentration, which covers the standard propylene glycol solutions used in UK solar thermal systems. The maximum system temperature of 120°C accommodates stagnation temperatures in typical evacuated tube and flat-plate collectors. However, on solar thermal systems the maximum temperature can be higher during stagnation (up to 150–200°C in the collector) — the expansion vessel should be located on the return (cooler) side of the circuit, away from direct stagnation temperatures, with a connecting pipe of sufficient length to limit vessel temperature. Consult the solar system designer for final specification.

What is the difference between the Baseflex and a standard bladder expansion vessel?

A standard bladder expansion vessel uses a free-floating rubber bladder inside the vessel shell. The entire outer surface of the bladder is exposed to the gas charge, giving a large gas-to-water interface area through which gas can permeate over time — causing the vessel to gradually lose its pre-charge. The Flamco Contra-Flex clamped design clamps the diaphragm between the two vessel halves, so only the inner surface of the diaphragm faces the water. The gas charge is on the outer side. This reduces the gas-to-water diaphragm surface area by approximately half, dramatically reducing the permeation rate and extending the time between pre-charge top-ups. In practice, a well-installed Baseflex will maintain its pre-charge for significantly longer than a standard bladder vessel of the same size.

Can I use the Baseflex on an open-vented system?

No — the Flamco Baseflex is designed for sealed (closed) systems only. Open-vented systems manage expansion via a feed and expansion tank in the loft that is open to atmosphere — they do not use expansion vessels. The Baseflex is for sealed heating systems (as described in EN12828) and closed cooling circuits where the system is pressurised and relies on the expansion vessel to accommodate water volume changes. If you are converting an existing open-vented system to sealed, the open-vent and feed pipes must be correctly removed or capped, a filling loop and pressure gauge added, and a correctly-sized expansion vessel like the Baseflex installed — this is a significant system modification that should be designed and installed by a qualified heating engineer.

Heating Expansion Vessels