Combi, system and heat-only boilers, flue types and condensing

Boilers are classified in two ways: by how they provide hot water (combi, system or heat-only) and by how they get combustion air and discharge products (open-flued or room-sealed). Condensing is a separate feature again.

Combi, system or heat-only

Where the hot-water and system components sit decides the layout, the controls and how you fault-find.

Combi (instantaneous combination) boiler

• Heats mains water as it is drawn off, with no hot-water cylinder. Its usual sealed heating circuit needs no feed and expansion cistern.
• In a common design, flow detection at a tap creates the hot-water demand and a diverter sends heated primary water through a separate plate heat exchanger. Check the actual layout before assuming those parts are fitted.
• Simultaneous hot-water outlets share the boiler’s available hot-water capacity and the incoming water supply. If demand exceeds either limit, flow or delivery temperature can fall; an instantaneous combi has no stored reserve.
• A storage combi is a different arrangement, with an integrated hot-water store.

System boiler

• Carries the circulating pump and expansion vessel inside its casing, for a sealed primary circuit.
• Heats a separate cylinder when it supplies stored hot water.

Heat-only (regular) boiler

• Leaves the pump and the expansion arrangement outside its casing.
• On a traditional open-vented primary system it needs a feed and expansion cistern with an unobstructed open vent, plus a separate cylinder for stored hot water. It can also serve a sealed primary system with the specified external expansion, relief and filling components.
• Used only for space heating, it needs no hot-water cylinder.

Pumped primary circulation

In England, Approved Document L 2021 calls for fully pumped primary circulation for space heating and domestic hot water where this is compatible with the heat generator. Primary water carries the boiler's heat to the radiators or cylinder coil. A pump serving only the radiator branch does not make a gravity-fed cylinder circuit fully pumped.

This concerns the circulation arrangement. A system boiler can have its pump built in, while a heat-only boiler uses an external pump. For timing, room controls and interlock, see boiler controls and the interlock principle.

Open-flued or room-sealed

• Open-flued: takes combustion air from the room and discharges products outside through a flue, so it depends on adequate room ventilation. Blocked or missing vents can starve combustion and cause spillage.
• Room-sealed: takes air from outside and discharges outside through a circuit sealed from the room. While the case and flue seals are intact, products cannot readily spill indoors, which is why it is generally considered safer.
• Fanned flue: the fan drives combustion air in and products out, and fan proving is often required before the burner fires. The fan reduces dependence on natural buoyancy, allowing the routes the appliance is designed for. Its permitted length, diameter and bend allowances still limit the flue layout.
• Only a room-sealed appliance may be installed in a bath or shower room.

Condensing

A condensing boiler has a larger heat exchanger that cools the flue gases below their dew point, so water vapour condenses and gives up latent heat. It works best with return water broadly below about 55 °C. The condensate is mildly acidic and must drain safely, with external pipework protected from frost. Under current Building Regulations guidance in England, a new domestic gas boiler is normally a condensing type. Approved Document L 2021 sets a minimum seasonal efficiency of 92% (as defined in ErP) for new and replacement gas boilers; a lower SEDBUK 2009 figure (78% natural gas, 80% LPG) is allowed only in exceptional circumstances established by the Condensing Boiler Installation Assessment Procedure. A combi should be chosen to modulate down to the home's typical heating load, and a replacement that switches fuel must not raise CO2 emissions or primary energy per kWh of heat.

The return water is what cools the flue gases. Their dew point, around 55 °C for natural gas, is set by the combustion products and does not move with the water temperature, so the cooler the return, the more vapour condenses. A higher flow setting may raise the return temperature, but the emitter load and water flow also determine how much the water cools before it returns. Measure the return at the relevant load before deciding the boiler has stopped condensing.

Flueless water heaters

Approved Document J allows a flueless instantaneous water heater only up to 11 kW net input, and not in a room smaller than 5 m³. Any instantaneous water heater must be room-sealed or fitted with a device that shuts it down before combustion products build up.

What a combi's hot-water rating means

Compare hot-water figures at the same temperature rise and with an adequate incoming supply. The October 2023 Ideal Logic+ Combi2 table gives 9.9 litres/minute for C24, 12.4 for C30 and 14.5 for C35, all at a 35 °C rise. A larger rise needs more heat per litre, so the available flow falls. The model's minimum hot-water delivery is 2 litres/minute; this is not its full-flow rating.

The C24's rated maximum hot-water output is 24.2 kW, despite the rounded model name. A heat-balance check uses water's heat capacity of about 4.2 kJ/(kg·K): divide 24.2 by (4.2 × 35) to obtain about 0.165 kg/s, then multiply by 60 to get about 9.9 litres/minute, taking 1 kg of water as approximately 1 litre. Use heat output to the water, rather than fuel input, in this calculation.

The maximum domestic hot-water inlet pressure is 10 bar. The separate 0.5 bar minimum-to-operate figure is quoted at 0.6 bar heating-system pressure; it does not promise full rated flow at 0.5 bar. The table's full-flow supply requirements are higher. Check available flow as well as pressure when selecting or diagnosing a combi.

Compartment air and the surrounding environment

The 2021 Vaillant ecoTEC plus VU/VUW ..6/5-5 manual says compartment ventilation is unnecessary with its approved concentric, room-sealed air/flue arrangement, because little heat is lost through the casing. This conclusion depends on the specified arrangement and does not cover an open-flued installation.

Being room-sealed does not lift every restriction on where it goes. For salons, painting or joinery workshops and similar premises, that manual specifies a separate installation room whose air is technically free of chemicals. Keep chemicals away from the installation and prevent sprays, solvents, ammonia, chlorinated cleaners and similar contamination reaching the combustion air; they can cause corrosion in the appliance and flue.

Exam cues

• Instantaneous combi: mains hot water during draw-off, with no separate cylinder. A storage combi has an integrated store; check its layout.
• System boiler: pump and expansion vessel built in.
• Regular boiler on an open-vented system: feed and expansion cistern plus a cylinder.
• Do not call every condensing boiler room-sealed just because most modern ones are.