Getting the best from your condensing boiler

Transition Cambridge have  been organising a question-and-answer session in Cambridge about 'managing your home heating' - little did I realise when we selected this topic quite how involved this can be. I have touched on this area before (see How much of your home do you heat?) but there is much more to it than choosing where to put the wall thermostat. I expect to learn more about zoned heating systems (required now for new houses though the potential savings are much greater in retrofit cases), smart controls, and, most importantly, what temperature to run your radiators at. One of the things I have learnt in preparing materials for this event is that condensing boilers are much more efficient when you run the radiators fairly cool - and this is why fancy controls like load compensation are important.


Condensing boilers are more efficient than non-condensing because they use a larger heat exchanger to get more of the heat from the flue gases into your heating water. Ideally, so much heat is extracted that water vapour in the flue gas  condenses -  condensing the water releases more than 5 times as much energy as cooling it by 100 degrees C - but to get effective condensation, you want the water being heated to start out fairly cool - below 55C. This is the temperature of the water that has returned from your radiators, so this means your radiators, at least the ones at the end of the heating circuit, will be running at only around 60C.

(The circuit that heats your water tank needs to be hotter than this because your water must be maintained warmer than 60C to avoid legionnaires disease)

For a non-condensing boiler it is very important that you don't have condensation in the flue because the acidic liquid can cause all sorts of problems; to avoid any risk non-condensing boilers run at a higher radiator return temperature - at least 60C. Therefore  less heat is recovered from the flue gases and they are less efficient. Condensing boilers have a drain for the condensate which is corrosion resistant.

Now radiators put out more heat when they are hotter, so if you are running your radiators cooler they need to be larger to give the same amount of heat. Radiators can run up to 90C -  although when they are this hot you can easily burn yourself on them - and at 60C they will need to be 30% - 50% bigger. However, most people don't want to replace their radiators and they don't want large ones anyway because they take a lot of space. In practice you can get around this problem because your radiators are sized to give you enough heat in the depths of winter and most of the time you don't need that much. Most of the time you can get away with running the radiators fairly cool.

This is what load compensation and weather compensation controls are about. The load compensator lowers the temperature in the radiator circuit when the room is nearly warm enough and less heat is needed. When the boiler comes on in the morning it runs at a high temperature to start with, but as the house warms up, the it runs more gently and efficiently with a lower radiator temperature. The weather compensator does a similar job based on the outside temperature - if it is cold outside it runs the radiators hotter because the whole house will be losing heat faster. The weather compensation system gives a more consistent internal temperature but both are good.

As it happens our controller does have a weather compensation function. When we got it I thought this was just a fancy bells and whistles gimmick feature. Now I know better.