Replacing a Standard Boiler with a Gas Condensing Boiler

Replacing an old constant- or low-temperature boiler with a modern condensing boiler that recovers flue gas heat.

How does a gas condensing boiler work?

A gas condensing boiler uses not only the heat of combustion but also the latent heat of the water vapour in the flue gas. In standard and low-temperature boilers, this vapour escapes unused through the chimney at 120 to 180 °C. In a condensing boiler, the flue gas is cooled in the heat exchanger to the point where the vapour condenses and releases its latent heat to the heating water. This raises the annual efficiency, based on net calorific value, from around 78 % to as much as 98 %. Condensation requires a low return temperature, ideally below 50 °C. The resulting condensate is slightly acidic and is discharged into the sewer via a drain; the chimney must be moisture-resistant, usually by means of a plastic liner.

Which buildings are suitable for switching to a condensing boiler?

The replacement is suitable for existing buildings with a gas connection whose boiler is older than 15 to 20 years: detached houses and apartment buildings up to around 20 dwellings. Requirements are an acid-resistant chimney or chimney refurbishment with a PP liner, plus a condensate drain. The lower the flow temperature of the heating system, the greater the condensing benefit; hydraulic balancing and an adjusted heating curve get the most out of it. Important for context: the condensing boiler remains a fossil technology and should only be seen as a transitional solution. If an oil boiler replacement is due anyway, switching directly to a heat pump, biomass or district heating is preferable. Even with gas, it should be checked whether a renewable alternative is economically viable.

What does a condensing boiler actually deliver?

Natural gas consumption falls by 15 to 25 %. A detached house with 22,000 kWh of useful heat saves around 5,500 kWh of natural gas and about 1.1 t of CO2 per year. The investment is 5,000 to 9,000 € for a detached house, with a service life of 18 years. The replacement therefore pays back in 5 to 9 years – short enough to bridge the period until a later switch to a renewable system economically.

Energy carrier

Erdgas → Erdgas

Savings potential

15-25 %

Worked example

A detached house with 180 m² of living space has an annual useful heat demand of 22,000 kWh for heating and hot water. The existing standard boiler operates at an annual efficiency of 0.78 and consumes 28,205 kWh of natural gas per year. A new condensing boiler with an efficiency of 0.97 needs only 22,680 kWh for the same amount of heat. The saving is 22,000 × (1/0.78 − 1/0.97) = 22,000 × (1.282 − 1.031) = 22,000 × 0.251 = 5,523 kWh/a. With the natural gas emission factor of 0.201 kg CO2/kWh, this corresponds to a CO2 reduction of about 1.1 t per year. With investment costs of 5,000 to 9,000 € and a service life of 18 years, the replacement pays back in 5 to 9 years.

Investment & payback

5.000-9.000 € (EFH) · Service life: 18 a · Payback: 5-9 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much does a gas condensing boiler cost including installation?

A gas condensing boiler for a detached house costs 5,000 to 9,000 € including installation and hydraulic integration. Additional costs may arise for chimney refurbishment with an acid-resistant plastic liner and for the condensate drain. With a natural gas saving of 15 to 25 %, the investment pays back in 5 to 9 years. The service life is around 18 years.

Does a condensing boiler need a new chimney?

A condensing boiler needs a moisture-resistant and acid-resistant flue because the flue gas is cool and the condensate slightly acidic. A brick chimney therefore usually has to be refurbished with a plastic or stainless steel liner. A complete rebuild of the chimney is rarely necessary. Sizing follows ÖNORM EN 13384; the chimney sweep checks suitability before installation.

What is the difference between a low-temperature boiler and a condensing boiler?

A low-temperature boiler lowers the boiler temperature according to demand but discharges the flue gas unused at 120 to 180 °C. A condensing boiler additionally cools the flue gas to the point where the water vapour it contains condenses and releases its heat to the heating water. This raises the annual efficiency from about 80 to 85 % to 95 to 98 %. A low return temperature below about 50 °C is required.

Is a condensing boiler still worthwhile given the planned phase-out of gas?

A condensing boiler is economically worthwhile if an old gas boiler has to be replaced at short notice and a switch to a heat pump, biomass or district heating is not currently possible. With a payback of 5 to 9 years, it is justifiable as a transitional solution. In the long term, however, it is not a decarbonisation measure, as the energy source remains fossil. If an oil boiler replacement is due, the switch should be made directly to a renewable system.

What flow temperature is allowed with condensing technology?

The flow temperature with condensing technology should ideally be below 60 °C, but the return temperature is decisive: only if it is below the dew point of the flue gas, around 55 °C, does the water vapour condense and the condensing effect occur. The lower the system temperatures, the higher the efficiency. Hydraulic balancing and a flat heating curve setting improve condensation noticeably.

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Standards & sources

Metering points in the EDM Toolbox

Empfohlen: Wärmemengenzähler + Gaszähler (M-Bus) zur Berechnung des realen Jahresnutzungsgrades. Vorlauftemperatur monitoren - hohe VL-Temperaturen reduzieren Brennwertnutzen.

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