A gas boiler is a type of heating system that heats water to provide warmth, usually distributed through radiators or a hydronic heating setup. It differs from a furnace, which heats air and distributes it through ducts and vents. A gas boiler is an integral part of many heating systems, particularly in homes and buildings that use hydronic floor heating.
There are various types of heating systems, including electric baseboard heaters, heat pumps, and furnaces powered by gas, electricity, or oil. In systems with a combi gas boiler, it not only provides heated water for space heating but also supplies hot water for domestic use such as showers and sinks.
This guide covers how a gas boiler works, the three main types available, what determines its real efficiency as opposed to its label figure, how it integrates with underfloor heating, and how to think about the eventual conversion to a heat pump.
How Does a Gas Boiler Work?
The clearest way to picture a boiler system is as a closed loop. Water circulates continuously around a circuit, and the boiler sits at one point on that loop reheating it each time it passes.
Cooled water returning from the building is pumped into the boiler, where a gas burner heats it. Once it reaches the target temperature, the pump sends it out through the pipework to the emitters, whether radiators or underfloor loops. Those emitters release the heat into the rooms, the water cools as it gives up that heat, and it returns to the boiler to be reheated. The same water circulates indefinitely rather than being consumed.
Gas enters the boiler either from a mains connection or from bottled LPG. The whole process is governed by a thermostat, which decides when the boiler fires and what temperature the system holds.
Where the hot water goes depends on the boiler type
This is the point that most often confuses people, and the answer is straightforward once separated.
In a heat-only or system boiler arrangement, space heating and domestic hot water are handled separately. The boiler heats the circuit for space heating, and it also heats a cylinder that stores hot water for taps and showers. The cylinder acts as a reservoir, so multiple outlets can draw at once.
In a combi boiler, there is no cylinder. When a hot tap opens, the boiler diverts to heating water on demand and sends it directly to the outlet. Space heating pauses briefly while this happens. Combi boilers therefore perform two functions from one appliance, acting as both room heater and hot water supplier, without needing storage space.
Three Different Kinds of Gas Boilers
Boilers are usually categorised by how much heat they recover from the combustion process, and the differences are substantial.
Condensing boilers
This is the current standard and, in many markets, the only type now permitted in new installations. A condensing boiler adds a secondary heat exchanger that captures heat from the exhaust gases which a conventional boiler would send up the flue. Because water vapour in those gases condenses and releases its latent heat, considerably more energy is extracted from the same amount of fuel.
Modern condensing boilers are typically quoted at 90 to 95% efficiency. You will occasionally see figures above 100%, which arise from a measurement convention rather than from breaking any physical law: efficiency can be expressed against the net calorific value of the fuel, which excludes the latent heat the condensing boiler recovers. Against gross calorific value, the meaningful comparison for a household, the practical ceiling is around 95%.
Low-temperature boilers
These are designed to operate at reduced flow temperatures, generally around 50 to 70°C rather than the 70 to 80°C a conventional system uses. Running cooler improves efficiency and reduces heat loss from the pipework.
The important qualification is compatibility. Emitters must be able to deliver the required output at the lower temperature, which usually means underfloor heating or oversized radiators. Fitting a low-temperature boiler to a system with standard radiators sized for 75°C water will leave rooms underheated. Checking compatibility before specifying is essential rather than optional.
Conventional or non-condensing boilers
The older generation, which vents exhaust gases without recovering their residual heat. Efficiency typically sits in the 70 to 80% range, meaning a fifth or more of the fuel energy leaves through the flue. Purchase price is lower, but the running cost difference over a fifteen-year service life usually outweighs that saving substantially. In many markets these are no longer permitted in new installations.
| Type | Typical efficiency | Flow temperature | Best suited to |
|---|---|---|---|
| Condensing | 90–95% | Works best with cool returns below 55°C | Almost all new installations |
| Low-temperature | High, when emitters suit | 50–70°C | Underfloor heating, oversized radiators |
| Conventional | 70–80% | 70–80°C | Legacy systems only |
Why a Condensing Boiler Often Does Not Condense
This deserves its own section because it is the most consequential thing a boiler owner can learn, and very few are told it.
A condensing boiler only achieves its rated efficiency when the water returning from the heating circuit is cool enough for the flue gases to condense. That threshold sits at roughly 55°C on the return. Above it, condensation does not occur, the secondary heat exchanger does nothing useful, and the boiler operates several percentage points below its label figure.
In practice, a great many condensing boilers are installed on systems running at 75 or 80°C flow with correspondingly hot returns, and they spend most of their working lives never condensing at all. The homeowner paid for the technology and receives conventional-boiler efficiency.
The practical takeaway. Lowering the flow temperature improves efficiency on a gas system, not only on a heat pump. If your emitters can deliver enough output at 55 or 60°C rather than 75°C, reducing the boiler setting costs nothing and recovers efficiency you have already paid for. This is also why underfloor heating pairs so well with a condensing boiler: it operates at temperatures that guarantee condensing conditions.
Gas Boilers and Underfloor Heating
A gas boiler can serve an underfloor heating system very effectively, but it cannot do so without one additional component, and omitting it causes real damage.
A boiler produces water at 60 to 80°C. An underfloor heating floor requires 30 to 45°C, depending on the screed construction and the floor covering. Sending boiler-temperature water directly into floor loops wastes energy, makes the floor surface uncomfortably hot, and risks damaging temperature-sensitive coverings such as engineered wood, which can cup or gap.
The mixing valve requirement
The solution is a mixing valve installed between the boiler and the manifold. It blends hot supply water with cooler water returning from the floor until the mixed flow reaches the target temperature. Because the return water is already part of the circuit, nothing is wasted in the process; the valve simply recirculates a proportion of it.
Two approaches are available. A thermostatic mixing valve holds a fixed set point, adjusted once at commissioning, using a wax thermostatic element that senses the blended temperature and moves the valve accordingly. It needs no electricity and is the simpler, more common choice. A weather-compensated motorised valve adjusts the flow temperature according to outdoor conditions, reducing it in mild weather so the floor delivers exactly the output required. This improves both efficiency and comfort but adds a controller and sensor.
The rest of the assembly
Beyond the mixing valve, a boiler-fed underfloor system needs the same components as any hydronic installation. A manifold distributes the mixed water between individual floor heating pipe loops. Each loop carries a thermal actuator that opens or closes it in response to a room thermostat, giving room-by-room control on top of the system-level flow temperature.
Underfloor circuits also generally need their own circulation pump, because their long narrow loops present more resistance than a radiator circuit and a single boiler pump serving both will usually satisfy neither properly. Our guide to the UFH manifold with a pump covers this assembly in detail.
Mixed systems with radiators and underfloor heating
Many homes run underfloor heating downstairs and radiators upstairs from the same boiler. This works well, and it is a common arrangement, but it requires the two circuits to operate at different temperatures: the radiators want 70 to 80°C while the floor wants 35 to 45°C.
The boiler produces the higher temperature for the radiator circuit, and the mixing valve at the underfloor manifold reduces it for the floor. Each circuit then behaves independently, and the underfloor side gets what it needs without the radiators being starved.
Pros and Cons of a Gas Boiler
Before deciding to install anything, it is worth understanding both sides. Here are the points that matter most.
Advantages
Fast, high-temperature output. A boiler reaches operating temperature quickly and can produce water hot enough for any emitter type, which makes it forgiving in older buildings with high heat demand and standard radiators.
Compact wall-mounted options. Where a floor-standing unit creates layout difficulties, a wall-mounted boiler solves them. It occupies little space, fits inside a cupboard, and typically costs less to install.
Long service life. With normal use and good maintenance, a gas boiler lasts 10 to 20 years. Maintenance requirements are modest, generally an annual service with occasional component replacement that is unlikely to cost a great deal.
Straightforward installation. In both renovation and new build, a boiler is comparatively simple to install where a gas connection exists, and it works with pipework and emitters that may already be in place.
Lower upfront cost. Purchase and installation cost less than a heat pump of equivalent capacity, which matters where budget is the binding constraint.
Disadvantages
Fossil fuel dependence. A gas boiler burns fuel and emits carbon dioxide at the point of use. Its emissions remain constant over its service life, unlike an electric heat source whose emissions fall as the electricity grid decarbonises.
LPG storage and supply. For propane users, the supply must be refilled when it runs out, and storage requires care with regard to temperature and humidity. LPG is also expensive relative to mains gas, so managing consumption matters.
Efficiency ceiling. Even the best condensing boiler is limited to around 95%, because it converts fuel into heat. A heat pump moves existing heat instead and therefore delivers three to four times the energy it consumes. This is a difference of category, not of quality.
Regulatory direction. Many European markets are progressively restricting new gas boiler installations as part of heating decarbonisation. A boiler fitted today will operate for fifteen years or more, so it is worth checking where local policy is heading.
Combustion safety requirements. A boiler needs a flue, adequate ventilation, and annual safety inspection. These are routine but they are obligations an electric heat source does not carry.
Gas Boiler or Heat Pump for Underfloor Heating?
Since underfloor heating works with either, this comparison comes up constantly, and the answer turns on a small number of factors.
| Factor | Gas boiler | Heat pump |
|---|---|---|
| Efficiency | 90–95% at best | 300–450% (COP 3–4.5) |
| Upfront cost | Lower | Higher |
| Mixing valve needed for UFH | Yes, essential | Often unnecessary |
| Also provides cooling | No | Yes, reversible units |
| Emissions over service life | Constant | Fall as the grid decarbonises |
| Requires flue and gas supply | Yes | No |
| Regulatory outlook | Increasingly restricted | Increasingly favoured |
Note the third row, because it is often overlooked. A boiler feeding underfloor heating requires a mixing valve to bring 75°C water down to 40°C. A heat pump already produces water in that range, so the temperature-reduction function becomes redundant, and specifying unnecessary mixing adds resistance and cost for no benefit.
Where a boiler still makes sense is in buildings with a serviceable gas connection, mixed radiator and underfloor circuits requiring high-temperature capability, tight upfront budgets, or high heat demand that insulation improvements cannot address in the near term. Where a heat pump makes sense is essentially everywhere else, and particularly in new builds or well-insulated properties running underfloor heating throughout. Our comparison of the pros and cons of heat pumps examines that side in detail.
Converting from a Boiler to a Heat Pump
For owners considering the eventual switch, the good news is that a boiler-fed underfloor heating system is already most of the way there.
The pipework, the manifold, the actuators, and the thermostats all carry over unchanged. Underfloor heating already runs at the low flow temperatures where a heat pump is most efficient, which removes the single biggest obstacle that radiator-based properties face. In many cases the conversion amounts to replacing the heat source and adjusting the controls.
Two things do need checking. The hot water arrangement changes, since a combi boiler produces hot water on demand while a heat pump generally requires a cylinder, so space for one must be found. And if the property has a mixed system with radiators as well, those radiators will demand high flow temperature and drag the heat pump into its inefficient range unless they are oversized to work at 45 to 55°C.
A hybrid arrangement, keeping the boiler alongside a heat pump and switching between them according to conditions, is a legitimate intermediate step where a full conversion is impractical. It allows the heat pump to be sized for typical rather than peak demand while the boiler covers the coldest days.
“The thing I wish more boiler owners knew is that lowering the flow temperature is free. People assume efficiency is fixed by the appliance they bought, but a condensing boiler only condenses when the return water is below about fifty-five degrees, and an enormous number of them are running at seventy-five and never condensing at all. That homeowner paid for a condensing boiler and is getting conventional efficiency. If their emitters can cope at a lower setting, turning the flow temperature down recovers something they have already paid for. And as a bonus, it is exactly the change that makes a future heat pump conversion straightforward rather than disappointing.”
— Maggie Shen, Director of Legom
Components That Work with Either Heat Source
One practical advantage of a wet heating system is that most of it is independent of what generates the heat. Legom manufactures the distribution and control components that serve a boiler and a heat pump equally: manifolds, floor heating pipe with oxygen barrier protection, thermal actuators, room thermostats, base stations, and the HVAC valves including the thermostatic mixing valves that a boiler-fed underfloor system requires.
Specifying these to a common standard means a system installed today around a boiler can accept a heat pump later without replacing the distribution. All are produced at our facility in Jiaxing, Zhejiang Province and supplied to partners in more than 90 countries, with OEM and ODM services available across the range.
Frequently Asked Questions
Can a gas boiler run underfloor heating?
Yes, and it is a common arrangement, but it requires a mixing valve between the boiler and the manifold. A boiler produces water at 60 to 80°C while underfloor heating needs 30 to 45°C, and sending boiler-temperature water directly into the floor wastes energy, makes the surface uncomfortably hot, and can damage temperature-sensitive floor coverings. The mixing valve blends hot supply with cooler return water to deliver the correct flow temperature. An underfloor circuit also generally needs its own circulation pump because of the resistance in its long loops.
What efficiency does a gas boiler actually achieve?
Modern condensing boilers are quoted at 90 to 95%, but the figure achieved in service is frequently lower. A condensing boiler only reaches its rated efficiency when return water is cool enough for flue gases to condense, roughly below 55°C. Many are installed on systems running at 75 to 80°C and never condense at all, delivering conventional-boiler efficiency instead. Lowering the flow temperature, where the emitters allow it, recovers that efficiency at no cost. Conventional non-condensing boilers sit around 70 to 80%.
Why do some boilers claim over 100% efficiency?
It is a measurement convention rather than a physical impossibility. Efficiency can be expressed against the net calorific value of the fuel, which excludes the latent heat contained in water vapour in the exhaust. Since a condensing boiler recovers precisely that latent heat, the figure can exceed 100% on that basis. Measured against gross calorific value, which is the meaningful comparison for a household, the practical ceiling is around 95%. When comparing products, check which basis a quoted figure uses.
What is the difference between a combi and a system boiler?
A combi boiler heats water on demand with no storage cylinder, so it saves space and avoids standby losses, but its hot water output is limited by flow rate and space heating pauses briefly while a tap is running. A system or heat-only boiler works with a hot water cylinder, which handles simultaneous demand from several outlets far better. The cylinder also matters for anyone contemplating a future heat pump conversion, since heat pumps generally require one and the space must be found somewhere.
What is a low-temperature gas boiler?
One designed to operate at reduced flow temperatures, generally around 50 to 70°C rather than 70 to 80°C. Running cooler improves efficiency and reduces heat loss from pipework. The essential qualification is emitter compatibility: the radiators or floor loops must deliver the required output at that lower temperature, which usually means underfloor heating or oversized radiators. Fitting one to a system with standard radiators sized for 75°C water will leave rooms underheated, so compatibility must be confirmed before specifying.
How long does a gas boiler last?
With normal use and regular maintenance, 10 to 20 years is typical. Annual servicing extends working life and is required in many jurisdictions for safety reasons in any case, since a boiler involves combustion, a flue, and gas supply. Component replacement occurs periodically over that span but is generally not expensive. Water quality affects longevity as well, since scale and sludge in the circuit reduce heat exchanger performance and eventually cause failures.
Should I replace my gas boiler with a heat pump?
It depends less on the boiler than on the building. If the property runs underfloor heating or has emitters that work at 45 to 55°C, a heat pump will deliver substantially lower running costs and emissions, and the existing pipework, manifold, and controls carry over. If the property has standard radiators sized for 75°C water, a direct swap will disappoint unless those radiators are oversized or insulation is improved first. Get a heat loss calculation done and establish what flow temperature the system requires before deciding.
Can I keep both a boiler and a heat pump?
Yes, and this is what a hybrid system does. The heat pump covers most of the heating season efficiently while the boiler handles the coldest days when heat pump output falls, which allows the heat pump to be sized for typical rather than peak demand and reduces its cost. It suits retrofit projects where replacing all the emitters is impractical. The trade-off is that a fossil fuel appliance remains in the building with its flue, gas connection, and maintenance obligations, so in markets phasing out gas heating it is best regarded as a transitional arrangement.
What maintenance does a gas boiler need?
An annual service by a qualified engineer, covering combustion analysis, flue integrity, gas pressure, safety controls, and general condition. Beyond that, keep an eye on system pressure and top it up when it falls, bleed radiators when they develop cold spots at the top, and have the system water treated with inhibitor to prevent scale and corrosion in the circuit. Persistent pressure loss indicates a leak somewhere and should be investigated rather than repeatedly topped up.
Reviewed by Maggie Shen, Director at Legom, on August 4, 2026. This guide to gas boilers was reviewed for technical accuracy, including condensing efficiency figures, low-temperature boiler operating ranges, and the mixing valve requirement when feeding underfloor heating.