Unless you’re a plumber, it’s often hard to know where the blockage is and who owns which bit of the pipework. Is it time to replace your gas boiler? call. Which says: 'the grant vortex pro combi 26 oil-fired combination boiler was far better than the others we tested it loses little heat from its casing and emits low nox [nitrogen oxide, which is emitted by all boilers].'. Get in touch with us now to arrange your repair. In the end, choosing between an electric.