Quote:
Originally Posted by Resurgam  That is a real bore. The temp is very high indeed. The temptation is to extend the flue and wrap a hot water pipe round it just to increase the temp of water going into the boiler. |
In order to transfer heat from the exhaust gas to the circulating water you need a temperature driving force: in other words, even in the most efficient boilers, the exhaust gas is likely to run 5-10 degrees hotter than the temperature of the water you are returning to the boiler from your heating circuit.
If you have a combi boiler, the exhaust gas should theoretically be cooler when you are heating domestic water because the final exhaust is being cooled by contact with the incoming cold water supply. Do you notice a difference when the tap is running?
However, all of this assumes that the heat exchanger(s) are piped up for counter-current flow. If the flow is actually co-current the exhaust gas will be much hotter (5-10 degrees hotter than the water you are feeding to your radiators) and will cost you dear. Is your boiler piped up correctly?