---
title: "Gas-air heater (GAH / GGH)"
description: "A gas-air heater (GAH) transfers heat from hot flue gas to cooler combustion air — functionally the same as the boiler air heater, with the term GAH used more often in cement-plant and metallurgical contexts. A gas-gas heater (GGH) transfers heat between two flue-gas streams, most commonly used in FGD installations to reheat scrubbed (cooled) flue gas before stack discharge so plume buoyancy and dispersion meet permit requirements."
canonical_url: "https://sylio.co/glossary/gas-air-heater-gah-ggh"
last_updated: "2026-06-28T02:29:33.842Z"
---

A **gas-air heater (GAH)** transfers heat from hot flue gas to cooler combustion air — functionally the same as the boiler [air heater](/glossary/air-heater), with the term GAH used more often in cement-plant and metallurgical contexts. A **gas-gas heater (GGH)** transfers heat between two flue-gas streams, most commonly used in FGD installations to reheat scrubbed (cooled) flue gas before stack discharge so plume buoyancy and dispersion meet permit requirements.

## Configurations

- **Regenerative GAH/GGH** — rotating matrix like a [Ljungström](/glossary/ljungstrom-air-preheater)
- **Recuperative GAH/GGH** — fixed tube bundle
- **Heat-pipe GAH/GGH** — sealed two-phase fluid in tubes, no moving parts

## Fouling and cleaning

GAH and GGH baskets and tubes foul with ash and (on units downstream of FGD) calcium-rich sulphite or sulphate deposits. Cleaning options follow the same pattern as for the boiler air heater: steam [sootblowing](/glossary/steam-sootblower), [sonic horns](/glossary/sonic-horn), and periodic water washing during major outages.

## Related terms

- [Air heater](/glossary/air-heater)
- [Ljungström air preheater](/glossary/ljungstrom-air-preheater)
- [Electrostatic precipitator](/glossary/electrostatic-precipitator)
- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)