Most new and recently installed domestic gas boilers in the UK use balanced flues not chimneys and are condensing boilers. The exhaust is fan assisted and the

Most new and recently installed domestic gas boilers in the UK use balanced flues not chimneys and are condensing boilers. The exhaust is fan assisted and the temperature is very low. See https://en.wikipedia.org/wiki/Condensing_boiler#Exhaust https://en.wikipedia.org/wiki/Condensing_boiler#Exhaust