What gasket material
for central heating —
how to choose correctly
The choice follows the operating conditions — temperature, pressure and medium. Those three checks usually tell you whether green fibre is enough or whether the joint has moved into aramid or graphite territory. Final selection still depends on the connection type and any approval requirements.
What determines material choice
In this article, we are talking about flat gaskets used in central heating connections — pump unions, heat exchanger fittings, valve bodies and expansion vessel connections. These seal by compression between two metal faces. The material must maintain that seal across the full temperature and pressure range the connection sees in service.
Three parameters determine the correct material:
- Temperature — peak temperature the connection will reach, not average. Boiler connections may briefly exceed normal circuit temperature during start-up or fault conditions.
- Pressure — maximum system pressure. Most domestic sealed systems operate at 1 to 2 bar working pressure, but connections at the boiler may see pressure spikes.
- Medium — heating water, domestic hot water (potable), steam or gas. Each has different material compatibility and approval requirements.
The most common mistake is selecting a gasket by size alone and treating all flat gaskets as interchangeable. A gasket that fits the fitting may not be rated for the temperature or documented for the medium. Size is necessary but not sufficient.
The main material options
The standard material for domestic central heating system connections. Cellulose fibre provides good compressibility — important for flat face connections where surface finish varies — and the NBR binder gives resistance to heating water and moderate temperature cycling.
Has FDA compliance documentation and KTW/UBA drinking-water references, making it suitable to assess for documented water-contact applications within its operating range where those references are required. UK WRAS-required potable-water duty should use REDSEAL PRO 110 or BLUESEAL ULTRA 350 within the relevant WRAS listing. Additional documentation should be checked against the current technical data sheet for the specific grade. Used on standard domestic heating circuit flat face connections including pump unions, radiator fittings and filling loop connections, provided operating conditions remain within its rated range.
The step up from cellulose for applications where operating conditions exceed the cellulose range — commercial heating systems, high-pressure circuits, district heating connections and boiler connections where peak temperatures are higher than domestic norms.
Documented references include BAM, DVGW, WRAS, SVGW, ABS, DNV, EC 1935/2004 and TA-Luft, depending on the exact grade and application. DVGW is the gas-service reference to check where the fitting design requires a flat gasket; TA-Luft is fugitive-emission / leak-rate documentation, not a general gas approval. Higher fibre tensile strength typically helps reduce creep under sustained load, which is useful in connections that are not regularly disturbed or serviced.
Specialist material for high-temperature steam and process heating applications. The graphite component improves high-temperature sealing performance and reduces stick-to-flange behaviour. Documented references include BAM, DIN 28091 and BS 7531 Grade Y.
Not typically required for domestic or light commercial heating. The appropriate choice where steam temperatures exceed 200°C or where process conditions demand the additional performance the graphite matrix provides.
EPDM flat gaskets are suitable for lower-temperature connections that remain within EPDM's temperature limit, such as cold water points and some lower-temperature return-side applications. Not appropriate for connections that regularly reach above 110°C.
In central heating systems, EPDM is more often associated with elastomeric sealing applications than with standard flat fibre gasket duties.
Field check: Before replacing the gasket, check where the joint sits in the system. A pump union on a domestic combi boiler is not the same duty as a district heating connection, even if the thread size is the same. Measure the gasket size, but choose the material from the temperature, pressure and medium.
Decision table — which material for which situation
| Application | Typical temp | Typical material choice | Series |
|---|---|---|---|
| Standard domestic combi boiler — pump union, heat exchanger | Up to 80°C flow | Cellulose + NBR | GREENSEAL PRO 180 |
| Filling loop connection, PRV fitting | Cold to 60°C | Cellulose + NBR | GREENSEAL PRO 180 |
| Expansion vessel connection | Up to 80°C | Cellulose + NBR | GREENSEAL PRO 180 |
| Commercial boiler — high-pressure circuit | Above 120°C possible | Aramid + NBR | BLUESEAL ULTRA 350 |
| District heating connection — higher pressure or long service interval | Up to 130°C+ | Aramid + NBR | BLUESEAL ULTRA 350 |
| Gas connection — only where fitting design calls for a flat gasket and specified DVGW/TA-Luft grade | Ambient to elevated | Aramid + NBR | BLUESEAL ULTRA 350 |
| High-temperature steam above 200°C | 200–280°C | Aramid + graphite + NBR | GRAPHITESEAL ULTRA 350 |
| Cold water connection, low-temp return | Below 60°C | Cellulose + NBR or EPDM | GREENSEAL / RUBBERSEAL |
Temperature values are indicative for typical domestic and light commercial installations. Always verify against actual system operating conditions and boiler manufacturer specifications. For gas applications, DVGW gas-service documentation and any required TA-Luft leak-rate documentation must be confirmed before use.
Why the same fitting can need different materials
Two identical 3/4 inch BSP pump union connections on two different systems can legitimately need different gasket materials. A standard domestic combi boiler running at 65–80°C flow temperature remains within the cellulose fibre range. The same fitting size on a commercial heating system running at 130°C flow is not — and a cellulose gasket fitted there will degrade faster under sustained elevated temperature.
The fitting size determines the gasket dimensions. The operating conditions determine the gasket material. Neither one determines the other.
This is why different connections on the same system can legitimately require different gasket materials — each matched to the conditions at that specific point in the circuit.
For domestic central heating in the UK: many combi and system boiler duties fall within the cellulose fibre range. Flow temperatures are often in the 65–80°C range, depending on the system setup. Many domestic sealed heating circuits operate around 1 to 2 bar working pressure, with PRV set points commonly around 3 bar. Cellulose fibre with NBR binder remains within its rated range under these conditions. Aramid becomes the more appropriate material when the system or specific connection regularly operates at or above the cellulose continuous rating, or where approval requirements demand it.
For typical domestic central heating flat face duties, cellulose fibre is usually the starting material. For higher pressure, higher temperature or gas duties, aramid is usually the next material to assess.
The decision follows the operating parameters at each connection, not a single blanket choice for the whole system. Most domestic boiler connections remain within the cellulose range. Where operating conditions push beyond the cellulose range, aramid is typically the next material to assess. Gas duties should be selected by approval requirement, not by temperature alone.