Combi Boiler gas consumption confusion

matelodave

TBH it really doesn't matter what sort of boiler you've got nor even the size of the coil in the tank. 200 litres of water heated from 10 degrees to 60 degrees will take 11.6kwh of energy (plus a bit more if you are heating by gas taking boiler efficience and pipe losses into account) You can either deliver it with a 3kw heater in four hours, a 12kw in 1 hour or a 24kw in half an hour. If the house requires a certain amount of heat as well then it's down to the effective output power of the boiler and how much heat gets dissipated either into the water tank as to how long either the tank or the house take to heat - it will stiil use the same amount of heat and cost the same.
If the tank has a big coil and dissipates all the heat into the hotwater then there wont be much left for heating the house.

In the whole great scheme of things, all the OP can do is to try it out and see how he gets on although it's now the wrong time of year now that the heating season has nearly finished.

coffeehound

60BOWENS said:

Our Combiboiler apart from providing central heating is also set up to provide hot water by the 300L indirect unvented cylinder.

At present boiler powers on twice a day to provide hot water and again twice a day to provide central heating-but at different times.

I was wondering if I could get boiler timings synchronised to provide central heating AND hot water for the cylinder at the same time.

As mentioned upthread, something you can do to minimise gas use is to use the minimum boiler water temperature you can.  In your case this is determined by your megaflo since the boiler water temperature must be at least as high as the thermostat setting on the megaflo.

Megaflo's are supplied with the thermostat set to 60℃.  Assuming yours is still at this setting, then you could reduce the boiler flow temperature to 65℃ and allow the boiler an hour to heat the water each time.  Your radiators may need longer to heat the house at this temperature, but it will still be cheaper in terms of gas usage to run the central heating for longer at a lower temperature.

Regarding the scheduling of hot water vs. radiators, I tend to overlap the transition by 15 minutes on the programmer (since that is usually the minimum time interval you can use).  So say water heating on from 05:00 to 06:00.  Central heating on from 05:45 to 15:15.  Water heating on from 15:00 to 16:00, etc.  I don't know if it makes much difference but at least it should be the most efficient transition from one mode to the other.