Condensing boilers don’t condense

When hydrocarbons like oil or gas are burnt they produce a fair bit of water in the form of steam and that steam contains energy that can be reclaimed if it can be cooled enough to condense. If your heating system returns water to the boiler at 55c or below then condensing happens in the boiler and that plume of steam outside disappears. The benefit to you, the boiler owner, is immense. The boiler that was sold to you as 97% efficient actually gets there instead of the mid eighties which is probably where it is now. On a gas bill of £3,000 a reduction of £300 – £450 makes this a topic worthy of some perusal. Plough on, it’s boring but not difficult.

There are different accepted design targets for the temperature drop across heat emitters (like radiators). A Dt of 20c or 11c. Both so widely apart to be pretty unhelpful. Maybe all of that is irrelevant as your system already exists and you can only move on by measuring what Dt you get.

So, say a radiator circuit might be set at 90c in and 70c out (Dt of 20) the desired return flow at 55c is far out of reach. Owners of oil boilers are a bit stuck here as the boilers don’t modulate which means they work on full power and are either on or off. Gas boilers, however, can have their output temperature turned down and this is the place to start but, unless you can tolerate a lot less heat in the house, it isn’t the full solution. The trouble is that the radiators don’t move as much heat at lower temperatures so they won’t achieve a Dt of 20 – maybe only 15 or less – so say you drop the input temperature down to 75c the return only drops to 60c. Getting close, but the heat to the house will be down to 12kW from 16kW for example, and we are still not condensing. Turning on more radiators will drop the return temperature and this where the juggling needs to begin. If you time the hot water heating to coincide with the heating that will help too. Looking at that standardised Dt of 11c and working back from a return of 55c the feed would have to be 66c which is quite low for radiators. The chances are your current set up is a bit above that.

Taping thermometer sensors to the feed and return pipes next to the boiler is essential for seeing what is going on.

DIY fan-coil unit

There is one tweak that could help reach down to those last few degrees and this will be particularly useful where oil boilers are concerned. Check out the DIY fan-coil unit featured here.

This works well at lower temperatures, such as the reduced return flow you are reaching for, so it will work just on the return flow pipe alone (via a diverter loop) anywhere towards the boiler end. It’s all about removing power to widen the Dt and it doesn’t make too much difference where this happens but removing heat from the return will leave the main circuit running hotter and with a higher Dt. So, when you have turned down the supply temperature as much as can be tolerated the return flow pipe can be tapped for energy and maybe you’ll reach the magic 55c.

Hardly anyone has a heat-bank based system but these have a return feed to the boiler drawn from the cooler bottom of the tank and that results in the boiler running in condensing mode most of the time and with much longer runs which is particularly good for oil boilers and their short cycling problems. If you are starting from scratch there are many other reasons why a heat-bank is the way to go. There’s a case study on one here.