Hot fill or cold fill: what your kitchen needs before you order a boiling tap

The short answer

A boiling water tap is a tank in your cupboard with a tap on top, and the tank has to be fed from somewhere. Feed it from the cold supply and it is a cold fill installation. Feed it from the hot supply and it is a hot fill. Which one you can have is decided by the pipework already under your sink, not by the tap you fancy, and it is worth knowing before you order rather than on the morning of the fit.

Most UK kitchens have both a hot and a cold pipe under the sink, so most buyers can pick either. The households that need to read this carefully are the ones with only a cold feed at the sink, which is common in flats with electric water heating, in kitchens that were moved during a renovation, and in any home where the hot pipe was capped off when a combi went in.

Cold fill: one pipe in, hot and boiling out

On a cold fill installation the tank takes mains cold water, heats it and holds it. The tap gives you boiling water from the tank and cold water straight off the mains. If you also want ordinary hot water at that sink, the tank has to supply it, which is what a mixing valve is for. The valve blends boiling tank water with cold to produce hot at a normal temperature.

The capacity question is the one people get wrong. A tank does not give you its own volume in usable hot water, it gives you more, because every litre of boiling water blends with cold to make roughly a litre and a half of hot. Franke publishes the figure for its Maris system: a 4 litre boiler on cold fill yields about 6.5 litres of hot water. That is a sink full of washing-up water, not a bath, and it refills as it goes.

Cold fill suits a kitchen with no hot pipe, an island sink a long way from the cylinder, and any installation where you would rather not run new pipework across a floor. The trade-off is that heavy hot water use at that sink now runs through a 4 litre tank instead of your boiler.

Hot fill: the tank takes the hot feed

On a hot fill installation the tank is fed from the existing hot supply. Your boiler or cylinder does the bulk heating and the tank finishes the job, so hot water at the sink is effectively unlimited and arrives faster, because the tank is already holding hot water at the outlet rather than waiting for it to travel.

Hot fill is the better answer in most conventional kitchens with both pipes present. It puts less work on the tank, it keeps the tank's energy use to holding temperature rather than heating from cold, and it removes the ceiling on hot water at that tap.

When you need a mixing valve

Franke sets out the decision as a short series of questions in its 2026 catalogue, and the logic applies well beyond Franke.

The six metre and thirty second test is the useful part. It is easy to check: run the hot tap at the kitchen sink with a stopwatch. If you are standing there for half a minute waiting for it to warm up, a boiling tap with a mixing valve fixes a daily irritation that has nothing to do with tea. If hot water arrives in a few seconds, the valve is money spent on a problem you do not have.

Mixing valves are sold as accessories rather than included, so check whether your quote has one in it. On the Franke range the valve is a separate line item and it fits both the mechanical and the electronic Maris Water Hub.

What this rules in and out before you shop

Some taps make the decision for you. The Franke Mythos Water Hub 6-in-1 needs both a hot and a cold supply, because it mixes mains hot and cold alongside its four filtered functions. If your sink has only a cold feed, that tap is off the list until a plumber runs a hot pipe to it.

Three-function taps are more flexible. A 3-in-1 with a mixing valve will cover a cold fill kitchen, and the same tap on hot fill in a conventional kitchen gives you the fast, unlimited version. This is the main reason the cheaper end of the market is often the more practical one in an awkward kitchen.

Terminology varies between manufacturers, and so does what is included in the box. Treat the installation sheet for your specific model as the authority, and ask your installer to confirm which fill type your quote is priced for before any money changes hands.

The cupboard is the other constraint

Pipework is half the job. The tank has to physically fit, and under-sink cabinets are already busy with a waste trap, a bin and sometimes a softener or a waste disposer.

Measure before you buy, with the waste pipe in place rather than on a clear floor. A cylinder can be tucked behind a trap that a wide box will not clear, which is one of the reasons a multifunction tap is a bigger commitment than its price alone suggests.

Two more numbers from the spec sheet decide whether the tap fits above the worktop. Franke's Maris and Mythos taps need a 35 mm tap hole, which is the standard size for a monobloc mixer, and they take a worktop up to 50 mm thick. A thick stone or laminated worktop can exceed that, so measure the thickness at the tap hole and tell your supplier the figure. Standard mains pressure matters too: the Franke Omni, for example, is specified between 1.5 and 5 bar, and at the low end filtered models noticeably slow down.

Power, and the socket nobody plans for

Every tank needs a correctly earthed 13 amp socket inside the cabinet. Whether that socket already exists is the single biggest factor in what your installation costs, and adding one is notifiable work under Part P in England and Wales. Our installation guide has the price ranges.

If the cupboard has one socket and it is already taken by a waste disposer or a softener, there is a tidier answer than an extension lead. Franke sells a load balancing switch that lets three appliances share a socket by switching between them so they never draw at once. It is a niche accessory, and it is the sort of thing worth knowing exists before an electrician quotes you for a second circuit.

The checklist to run before you order

1. Look under the sink and count the pipes. One feed or two. 2. Time the hot tap with a stopwatch. Over 30 seconds points to a mixing valve. 3. Measure the free floor area of the cabinet with the waste trap in place. 4. Measure the worktop thickness at the tap hole and check the hole diameter. 5. Find the socket, and check what is already plugged into it. 6. Ask your installer which fill type the quote assumes and whether a mixing valve is included.

Answer those six and the rest of the choice is about tank size, filtration and how much you want to spend. Get one of them wrong and you find out on fitting day, when the alternatives are an extra visit or a tap that does less than you paid for.

Figures here come from Franke's published specifications for the Maris and Mythos Water Hub systems in its 2026-2027 catalogue, from our own model database, and from UK wiring rules on under-sink sockets. Other manufacturers publish their own fill requirements and capacities, and they do differ.