A filter spigot is the small dedicated tap that delivers filtered water separately from the main faucet, and it is the visible half of any under sink installation. It looks like the simplest part of the job and it generates a surprising number of problems, most of them decided at installation and awkward to correct afterwards.
Air gap or standard, decided first
Two body types exist and they are not interchangeable. A standard spigot carries only the filtered water line. An air gap spigot carries three connections: filtered water plus the reverse osmosis drain routed up through the spigot body and back down, which prevents drain water siphoning back into the system. Some jurisdictions require the air gap arrangement, so check local code before drilling. Air gap spigots also gurgle audibly in normal operation, which surprises owners who were not told to expect it.
Making the hole
Many sinks have a spare knockout for a sprayer or a soap dispenser, and using it saves the whole problem. Where a hole must be made, the material decides the method: stainless steel takes a step drill or a hole saw with cutting fluid and patience, granite and quartz need a diamond hole saw with water cooling, and ceramic and enamel crack readily and are best left to someone who does it regularly. Check clearance underneath for the mounting hardware before committing to a position.
Positioning it so it works
Reach matters more than looks. The spout must clear the tallest jug or bottle the household actually fills, and must swing over the bowl rather than towards the edge. Keep it far enough from the main faucet that hands and handles do not clash. And leave room underneath for the mounting nut and the tube connections, since a spigot set tight against the sink upstand or the wall becomes very difficult to tighten and later to service. Sit it in place before making anything permanent.
Why they drip and what to do
Most drips are the ceramic cartridge inside the spigot rather than anything wrong with the filter system, and the cartridge is usually replaceable separately. A weep at the base is normally the mounting seal, which needs the spigot removing and reseating rather than simply tightening harder. Persistent dripping just after a cartridge change is trapped air working through and settles within a day. And on air gap models, an intermittent trickle into the sink means the drain path is partly blocked, which is a drain line problem rather than a spigot fault. Our under sink installation guide covers the connections.
Questions people ask about water filter spigot
Do I need an air gap spigot?
In some jurisdictions, yes, to prevent drain water siphoning back into the system. Check local code before drilling, since the bodies differ.
Why does my air gap spigot gurgle?
That is normal. The drain water passes through the spigot body by design, and the noise is the air gap doing its job.
How do I make a hole in a granite worktop?
A diamond hole saw with water cooling. Stainless takes a step drill with cutting fluid, and ceramic cracks readily and is best left to a specialist.
Why is the spigot dripping?
Usually the ceramic cartridge inside it, which is often replaceable separately. A weep at the base means the mounting seal needs reseating.