Unlike the surface condenser often used on a steam turbine or marine steam engine, the condensing apparatus on a steam locomotive does not normally increase the power output, rather it decreases due to a reduction of airflow to the firebox that heats the steam boiler. In fact it may reduce it considerably. Condensing the steam from a high volume gas to a low volume liquid causes a significant pressure drop at the exhaust, which usually would add additional power in most steam engines. Whilst more power is potentially available by expanding down to a vacuum, the power output is actually greatly reduced compared to a conventional steam locomotive on account of the lower air flow through the firebox, as there is now no waste steam to eject into the firebox exhaust in order to pull more air into the firebox air intake. In order to produce similar power, air to the firebox must be provided by a steam driven or mechanically driven fan. This often cancels out any improvement in efficiency.