Original or alternative boiler part: where it is safe, where it is not, and what it means for your warranty

Short answer: for universal parts outside the combustion process, such as an automatic air vent, a condensate trap, an expansion vessel, a room thermostat or a circulation pump of the same type, an equivalent alternative is technically sound and often identical to what the manufacturer buys in itself. For everything that determines or monitors combustion, such as the gas valve, burner, ignition and ionisation electrode, fan with venturi, circuit board and flue gas parts, use only the part specified by the manufacturer: the settings, software and dimensions are specific to each appliance, and you will only notice a deviation when a CO alarm goes off. The manufacturer's warranty on the appliance requires original parts and fitting by a qualified installer with virtually every brand; the part itself is always covered by the statutory warranty, original or not.

Three kinds of parts

Original (OEM) is the part supplied by your boiler's manufacturer, in its packaging and under its manufacturer part number. The manufacturer rarely makes it itself: a Grundfos or Wilo pump, a Honeywell or Sit gas valve and an EBM fan come from suppliers and are given a Remeha or Intergas part number. Equivalent is the same part from the same supplier, under the supplier's own part number, sometimes without the brand-specific plug or setting. Alternative or aftermarket is a part from a different manufacturer offered as "suitable for". With the first two you know what you are getting; with the third it depends on who made it.

By part type

Part	Alternative acceptable?	Why
Automatic air vent, drain cock, filling valve	Ja	Universal sizes (3/8", 1/2"); no effect on combustion
Condensate trap and condensate drain	Yes, if it fits	Only the connection size and the height matter; branded traps sometimes have a click connection that only the branded part fits
External expansion vessel	Ja	Capacity and pre-charge pressure matter, not the brand. Internal vessel: only the branded part fits in the boiler
Room thermostat	Ja	OpenTherm is an open standard; see which thermostat fits
Circulation pump	Equivalent yes, aftermarket no	The same Grundfos UPM3 or Wilo Para under its own part number is identical; check the installation length, connection and plug (PWM signal)
Temperature sensor (NTC), water pressure sensor, flow sensor	Only equivalent with the same characteristic	An NTC with a different resistance curve makes the boiler fire too hot or too cold; plug and cable length differ
Diverter valve and motor	Equivalent sometimes	Brand-specific housing; a motor from the same supplier is possible, the valve itself usually only original
Gaskets and O-rings	Only from the branded kit	Size, material and temperature resistance are specific to each appliance; the wrong burner gasket leaks flue gas
Ignition and ionisation electrode	No	Length, angle and distance from the burner determine flame monitoring
Fan and venturi	No	The speed characteristic and venturi determine the gas-air ratio
Gas valve and burner	No	Adjusted per appliance and gas type; the core of safety
Circuit board, burner control unit, display	No	Software per model and year of manufacture; parameters and safety logic
Heat exchanger	No	Casting, insulation and burner door are unique to each appliance
Flue gas outlet and air supply	Certified system only	Must be approved for the appliance; individual parts must not be replaced with other materials

Why the line is drawn at combustion

A modern condensing boiler controls the gas-air ratio with the fan, the gas valve and the venturi; the ionisation electrode monitors the flame and the circuit board intervenes if anything deviates. Each of these parts is set up for that appliance and matched to the others. A fan with a slightly different characteristic delivers a different amount of air; an electrode that is two millimetres shorter measures a different flame signal. The boiler will still burn, but with the wrong CO2 value and therefore more carbon monoxide, or it keeps shutting down on a fault that nobody can explain. That is why, in many countries, work on this part is legally reserved for certified gas engineers, see which parts you may replace yourself, and that is why they only fit the specified part here.

What it means for your warranty

There are two warranties. The manufacturer's warranty on the appliance (with Intergas, Remeha and Vaillant 5 years on the appliance, often 10 years or more on the heat exchanger) applies under the manufacturer's conditions: installation and servicing by a qualified installer, and repairs with original parts. If you fit an alternative gas valve and the heat exchanger fails a year later, the manufacturer may reject the claim. The warranty on the part itself always applies, original or alternative: by law you can expect the part to do what it promises, and the supplier is responsible for that. Once the appliance's manufacturer's warranty period has ended, only the second one still applies. Our warranty page and the warranty FAQ sets it all out.

Practical rule: as long as your boiler is within the manufacturer's warranty, choose original parts and have a qualified installer fit them. After that, weigh up each part using the table above.

What we supply

The parts in our catalogue are genuine manufacturer parts and carry the brand's manufacturer part number; every product page shows the origin and, under "Suitable for", the appliances the part fits according to the manufacturer's data. For universal parts such as air vents, expansion vessels and thermostats, you will find the supplier brands (Caleffi, Flamco, Honeywell) alongside the branded parts. If you are unsure whether a part is equivalent, send us the number of the old part; we will check whether it is the same part under a different number. How to find that number is explained in How to find the right part.

Frequently asked questions

Is an original pump really the same as the standalone Grundfos?

Often, yes, provided the type, installation length, connection size and plug match; the manufacturer buys in the same pump. Watch out for boilers where the pump is controlled via a PWM signal: then the plug and the signal must be correct.

Why is the original part more expensive?

You pay for the manufacturer's guarantee that the part fits your appliance and has been tested, for the branded packaging and for distribution. For safety parts, that is the price of certainty; for an air vent costing €6 it makes little difference.

Can an installer refuse to fit a part I bought myself?

Yes. Many installers only fit parts whose origin they know, because they are liable for safe operation. Discuss it beforehand and let them check the part number.

What is a reconditioned part?

A used part that has been inspected and put back on the market, mainly circuit boards. For old boilers for which new circuit boards are no longer available, this may be the only option; buy it from a supplier that gives a warranty.