AC condensing gas unit heaters operate by condensation, meaning the unit extracts additional heat from water vapour in flue gases before they exit the stack. Instantaneous efficiency up to 105 %, seasonal 89–96 %. Conventional (non-condensing) unit heaters reach seasonal 82–86 %. The 6–14 percentage-point difference translates directly to annual gas savings on the bill.
Four LCC scenarios
Assumptions: gas €0.40/kWh, electricity €0.18/kWh, 12-year service life, condensing unit installed cost €12 000, conventional €8 000 (€4 000 difference).
Scenario A: Warehouse, short operating hours
Hall 4 000 m², 6 months/year, 50 h/week = 1 300 seasonal load hours.
- Annual gas (conventional, ηs 84 %): 38 000 kWh, cost €15 200
- Annual gas (condensing, ηs 94 %): 33 950 kWh, cost €13 580
- Annual saving: €1 620
- Payback: 4 000 / 1 620 = 2.5 years
Scenario B: Production, single-shift
Hall 6 000 m², year-round, 40 h/week = 2 080 hours. Mostly full load.
- Annual gas (conventional): 142 000 kWh × 0.40 = €56 800
- Annual gas (condensing): 126 800 kWh × 0.40 = €50 720
- Annual saving: €6 080
- Payback: 4 000 / 6 080 = 0.7 years = 8 months
Scenario C: Seasonal activity (sports hall)
Hall 8 000 m², 7 months (September–March), 60 h/week, mixed regime 60 % full + 40 % modulated to 30 %.
- 30 % modulation is the point where condensing equipment dominates most clearly (conventional drops to η 78 %, condensing holds at 92 %)
- Annual saving: €4 200
- Payback: 0.95 years = 11 months
Scenario D: Distribution centre, multi-shift
Hall 12 000 m², 24/7 with night setback (16 °C day, 12 °C night), year-round.
- Annual saving at ηs 96 % vs 86 %: €9 200
- Payback: 5 months
When NOT to choose condensing
Condensing systems have requirements conventional ones don't:
- Condensate drain: 1–3 L of water per m³ of consumed gas. Must be routed to sewer with pH neutralisation (condensate pH 4–5). Without a nearby drain, retrofit cost is €800–2 500.
- Polypropylene or stainless flue: standard galvanised flue is destroyed within 2–3 years. Condensing flue cost: €1 200–4 000.
- Modulating control is mandatory. Without modulation the unit never enters the condensing regime and ηs drops to 85 %.
Below 800 operating hours per year, and without space for a condensate neutraliser, a conventional unit is the economically rational choice.
Quick threshold
Rule of thumb: at annual gas consumption above 30 000 kWh (≈ 4 000 m³ at G20) condensing technology typically pays back in < 4 years. Below that threshold the comparison depends on regime (modulation, season, night cycles).
Related: ErP class and seasonal efficiency ηₛ explains why a condensing unit holds 92 % efficiency at 30 % load while a conventional one drops to 78 %.