Reviewed by the Quel Tap product team. Last updated on: August 2026.
It’s 7 am. Someone’s already run the cold tap over their toothbrush, the toast is halfway up, and you just want a cup of tea before anyone asks you anything. With a kettle, that means filling it, waiting, and probably overfilling it because guessing the right amount of water for one mug is usually a lot harder than it should be. With a boiling water tap, it means turning a handle and pouring.
That convenience raises an obvious question: where does the boiling water actually come from, and is it safe to have sitting there, ready, all day long? The answer is a small, well-insulated system that works quietly under your sink, and once you understand the three components that make it up, the whole thing stops feeling like a mystery and starts to feel like straightforward engineering.
Quick answer
A boiling water tap keeps a small reserve of water, usually around 2.5 to 4 litres, heated up to 98°C in an insulated boiler unit under the sink. A thermostat will constantly monitor the temperature of the water in the boiler and reheat it as water is drawn off, so boiling water is available on demand rather than needing to be boiled fresh each and every time
| Component | What it does |
|---|---|
| Boiler | Holds a small reserve of heated water in a vented, non-pressurised tank |
| Thermostat | Monitors the temperature of the water continuously and switches the heating element on/off to maintain it |
| Insulation | Reduces heat loss between uses, keeping standby energy use down |
| Filter | Treats the water that is feeding the boiling, filtered, chilled and sparkling functions and is separate from the tap’s standard hot and cold supply |
None of this is exotic engineering. It’s a scaled-down, purpose-built version of principles used in immersion heaters and hot water cylinders for decades. What’s changed is the precision. Modern thermostats and insulation are now good enough that the boiler tank can sit ready at all times without needing constant reheating. Here’s how each part actually does its job.
1. The boiler: a small reserve, not a bathroom-sized cylinder
The boiler on a boiling water tap is compact. Quel Tap’s own units, for example, come in 2.4-litre and 4-litre capacities. The 2.4-litre delivers up to 54 cups of boiling water an hour, while the 4-litre model stretches to around 92 cups an hour, making it better suited to larger households or office kitchens. They’re deliberately small rather than a limitation: the boiler only needs to hold enough water for a few cups or a saucepan top-up between refills, because as soon as you draw water off, cold mains water is then pulled in behind it and reheated.
Inside the boiler is a stainless steel inner tank designed to hold water at up to 98°C without corroding or affecting the taste. A heating element, similar in principle to the one you would find in a kettle, sits inside the boiler tank and brings incoming water back up to temperature after each draw.
Quel Tap boilers use a vented, non-pressurised system. This means the heated water stored inside the boiler is not held under mains water pressure. Instead, the boiler and tap work together as an open-vented system, allowing the natural expansion of water as it heats to be relieved through the tap rather than contained within a sealed pressurised tank.
One advantage of this design is that the inner tank itself is not continuously subjected to mains water pressure. In the Quel Tap system, it also helps provide a smooth, controlled flow of boiling water from the spout, making it easier to fill a cup, jug or saucepan with greater control.
The result is a compact point-of-use system designed specifically to store and deliver boiling water at the kitchen tap without keeping the boiling-water tank under mains pressure.
2. The thermostat: the part actually doing the thinking
The thermostat is what makes the boiler reliable, not just hot. It constantly monitors the water temperature inside the boiler and switches the heating element on when the temperature drops, and off once it’s back where it should be.
This is also where energy efficiency lives. Because the boiler is small and well-insulated, the thermostat only needs to top up the temperature periodically rather than running constantly, which is part of why standby energy use on a boiling water tap is modest compared to what people often assume.
It’s worth being honest about the trade-off here, too: unlike a kettle, which draws no power when it’s not switched on, a boiling water tap draws a small amount of standby energy around the clock to keep the reserve hot. For households that frequently overfill a kettle, some of this standby use may be offset by avoiding the energy used to repeatedly boil more water than needed.
3. Insulation: the unsung part of the system
The insulation around the boiler is what makes the standby energy use manageable in the first place. Without it, the thermostat would be fighting constant heat loss and cycling the heating element far more often. With it, the boiler holds its temperature between uses far longer, so the element only needs to nudge things back into range occasionally.
Where filtration fits into the loop
Alongside the boiler, a boiling water tap still does the job of an everyday kitchen tap. Standard hot and cold water, for washing up, rinsing, and cleaning, will come straight from your home’s existing plumbing, exactly as it would do from any other tap, and isn’t filtered or heated by the boiler at all.
The filtration system sits on a separate line. Cold mains water destined for boiling, filtered, chilled, or sparkling functions will pass through a filter cartridge first, then will split off toward the boiler, the chiller, or straight out through the spout as filtered cold water, depending on which functions your model has. This is the same across our whole range; every Quel Tap filters water the same way, at the same point in the plumbing. What changes between taps is simply what filtering systems are on offer: boiling and filtered cold on a 4-in-1, with chilled and sparkling water added on the 5-in-1 and 6-in-1 models. Filtration has no impact on how the boiling side works; it is simply an earlier step on a separate line.
Is it safe to have boiling water sitting there all day?
This is the question most people actually want answered, and it’s a fair one. A few design features exist specifically to address it:
- A two-stage handle or button action, so the tap can’t dispense boiling water from a single accidental knock or a child’s curious push
- An insulated spout, which stays cool to the touch even while boiling water passes through it
- A child-lock function on most models, which needs to be engaged before boiling water will flow at all
None of these features replaces ordinary household care around hot water and children. The NHS lists hot drinks and hot water among the more common causes of burns and scalds at home, and a kettle, a pan on the hob, or a boiling water tap all deserve the same basic awareness. But the two-stage action and insulated spout are specifically there to reduce the risk of accidental contact with genuinely boiling water, in a way a kettle’s exposed spout or an unguarded pan simply doesn’t offer. For full NHS guidance on preventing and treating scalds at home, see the NHS burns and scalds page.
What installation actually involves
Because a boiling water tap connects to your mains cold supply and the electricity supply for the tank, it needs to be fitted correctly, but it isn’t a specialist plumbing job on the scale of, say, fitting a new boiler. Most customers fit their own Quel Tap in around 1 to 2 hours, following our detailed, step-by-step installation guides, provided there’s a spare socket and standard under-sink access.
Whether you fit it yourself or bring in a plumber, any tap connected to your mains water supply needs to meet the UK’s water fittings regulations. Products that carry approval from the Water Regulations Approval Scheme (WRAS) have already been independently tested against those standards, which is one less thing to check yourself during installation.
So is it actually cheaper to run than a kettle?
It heavily depends on how you use it, and it’s worth answering honestly rather than with a flat “yes.” The Energy Saving Trust notes that avoiding overfilling the kettle can save around £10 a year on electricity bills, which gives a sense of how much energy typically goes to waste boiling more water than a single cup actually needs.
A boiling water tap avoids that particular waste, since you only dispense the amount you actually need rather than a full kettle-load. However, it does draw a small amount of standby energy continuously to keep the stored water at temperature, which a kettle does not.
Households that currently boil and frequently overfill a kettle several times a day may therefore have more opportunity to avoid wasted energy than very occasional users. The exact running-cost difference will depend on the particular system and how it is used.
| Factor | Kettle | Boiling water tap |
|---|---|---|
| Standby power when idle | None | Small, continuous (keeps tank near 100°C) |
| Energy per use | Depends on how much you fill and often more than needed | Dispenses only the amount required, while also using energy to maintain the tank temperature |
| Common source of waste | Overfilling costs the average household around £10/year, per the Energy Saving Trust | Standby use on households that boil water only rarely |
| Best fit | Occasional, low-frequency use | Frequent use, making several hot drinks or top-ups a day |
In practice, the difference shows up in the everyday habit rather than the spec sheet: fill a kettle for one mug, and you’ll usually see far more than a mugful left over once it’s boiled, which is exactly the habit the Energy Saving Trust’s figures are pointing at.
The everyday result
A boiler, a thermostat, and some insulation, working quietly under the sink, so that when you want a cup of tea or to fill the saucepan, the water is already there. That’s the idea behind Quel Tap’s range: water, beautifully controlled.
Frequently Asked Questions (FAQs)
How hot is the water from a boiling water tap?
Quel Tap boilers can dispense filtered boiling water at temperatures up to 98°C, with the temperature adjustable to suit your preference.
Are boiling water taps safe to have in a house with children?
They’re carefully designed with safety precautions in mind: an insulated spout that stays cool to the touch, a two-stage handle or button action, and a child lock on most models. As with any source of boiling water in a kitchen, normal supervision around children still applies.
Do boiling water taps use a lot of electricity?
They use a small amount of standby energy continuously to keep the tank hot, and then a short burst to reheat water after each draw. Actual electricity use depends on the boiler and household usage. One advantage is that you dispense only the amount required, avoiding the common waste caused by repeatedly overfilling a kettle.
Can I install a boiling water tap myself?
Many people do, typically in around 1 to 2 hours, provided there’s a nearby socket and standard under-sink access. It’s still worth checking your installation meets UK water regulations, and a WRAS-approved product makes that easier to confirm.
Will the boiler ever run out of hot water?
The boiler refills and reheats as you draw water off, so for normal day-to-day use, such as making cups of tea or topping up a pan, it keeps pace. It isn’t designed to replace a full-sized hot water cylinder for large-volume uses, such as filling a bath.
Does the boiler need any maintenance?
Like any water-heating appliance, occasional filter changes (on filtered models) and following the manufacturer’s care guidance will keep performance consistent over time.