Best case, a condensing heater uses 20% less gas than a conventional one. But there are some caveats...in order to condense the exhaust fumes, the returning wat

Best case, a condensing heater uses 20% less gas than a conventional one. But there are some caveats...in order to condense the exhaust fumes, the returning water has to be cold enough. Low temperature radiators or underfloor heating are best for that. If you have huge, old, cast iron radiators that get super hot and stay hot for a long time, there probably won't be much condensing going on. Basically any time you see white smoking coming out the flue, it's not condensing.