Gas boiler or instant heater? The real difference before you buy

Both heat water with natural gas, and the difference between them is not "which is better" — it is the size of the job each one is designed for. An instant heater warms water the moment it passes through and serves a limited number of outlets at once. A boiler is designed around a whole home, and it can run a heating system from the same source.

So the right question is not "which is better?" but "what does my home need?" That is what this article settles.

The instant heater — how it works and when it fits

An instant gas water heater has no tank. Water passes through it, is heated at that moment, and comes out of the tap. The practical result: no waiting for a tank to warm up, and no standby heat loss from stored water cooling and reheating all day.

It suits:

• Flats and homes with one or two bathrooms used in sequence rather than simultaneously.
• Intermittent use — hot water needed at defined times rather than around the clock.
• Tight spaces, since there is no tank taking up room.
• Budgets that want a simpler entry point into gas-fired hot water.

Its real limitation: it is bound by the number of simultaneous outlets. Once you exceed its limit, flow drops or the water cools. That is not a fault — it is the boundary it was designed to.

16 L or 20 L? The difference is outlets, not quality

The two approved models differ in how many outlets you can run together:

• 16 L model — 30 kW output, covering up to 3 outlets at once.
• 20 L model — 37 kW output, covering up to 4 outlets at once.

Both offer temperature control from 35 to 65°C, operate at pressures from 0.1 to 10 bar, and include automatic ignition and overheat protection.

The simple rule: if you have more than one bathroom used together, take the larger one. The price gap between the two models is far smaller than the cost of discovering after installation that the unit cannot keep up.

The boiler — a whole home, and heating from one heart

A boiler heats water with natural gas and also runs heating systems, wall or underfloor, from the same source. It is not a larger version of an instant heater — it is the heart of a system.

It suits:

• Wanting continuous hot water at more than one point simultaneously.
• Running a heating system too — now or within a couple of years.
• A villa, building or project with high demand.
• No roof, or not enough sun to rely on solar.

The trade-off: a higher entry cost, a more involved installation (connections, flue and ventilation to specification), and an output that must be calculated properly — covered in the right boiler capacity.

The difference, one line per point

Storage: the instant heater has no tank — a boiler works within a system that may include storage depending on the design. Simultaneous outlets: the instant heater is bound to 3 or 4 depending on model — a boiler is sized to the whole household's consumption. Heating: the instant heater cannot run heating — a boiler runs it from the same source. Space: the instant heater is a small wall unit — a boiler needs space, connections and a flue. Installation: the instant heater is simpler — a boiler is an engineered install. Best for: instant heaters for flats and limited use — boilers for larger homes, villas, and anyone adding heating.

The mistake that costs money

The most common one we see: someone takes an instant heater to save money, then a year later adds heating or an extra bathroom, and finds themselves buying a second system from scratch. The fair comparison is not "which is cheaper today" — it is which will cover your home in two years.

The reverse is equally true: taking a boiler for a one-bathroom flat that will never have heating means paying for output and connections that will never be used.

The real cost is not the unit price

When comparing the two on cost, do not compare the price of the appliance alone — that is the part that varies least. Five things determine what it actually costs you:

• The capacity or output required — derived from your simultaneous outlets, not from preference.
• Consumption — how much hot water you genuinely use per day, which drives the monthly running cost rather than the purchase price.
• Purchase timing — the buying season within the year; cost is not flat across the calendar.
• Accessories and connections — flue, ventilation, pipework, valves, and a pump where needed. These vary between installations more than the appliance does.
• Extras — additional controls, thermostats, distribution across more than one floor.

That is why we do not publish price figures: this is an intelligent heating system, not an off-the-shelf commodity, and any published number would be wrong for most readers. The accurate figure comes out of the survey once those five things are measured at your home.

What we ask at the survey

To settle it we ask four things: how many hot-water outlets you have · how many genuinely run at the same moment · whether heating is in your plan even later · and whether the natural-gas network reaches you.

Those answers decide it in minutes. Book a free inspection and we will tell you what fits, even when that is the cheaper option — and if you are still comparing everything, the smart advisor gives an initial recommendation, and you can see all water-heating solutions side by side.