Oil Fired Combi Boilers

[Johntheo5]

Often seen posted on various websites re above running out of hot DHW rapidly, and ditto with Gas fired Combis. Oil fired combis have stored primary water which I suppose is a must since they don't modulate and would result in the DHW running hot & cold due to the boiler cycling if the DHW demand is below the thermal output of the boiler.
I've built a spreadsheet below based on a 21kW Firebird Combi which I believe has a 35/40L store (I've used 35L) with a stored temperature of 75C, the calcs (if correct) show, assuming DHW temp of 45C, that in winter with a pretty low mains of 5C, dT (45-5), 40C that the maximum continuous flowrate is 7.4LPM but should sustain a flowrate of 13.7LPM for 10 minutes before the DHW temp starts falling off. At a present mains temp of 18C then those numbers should be 11.2LPM continuous and 18.5LPM for 10 minutes.
Has anyone monitored the flowrates of their oil fired boilers which might reflect the above, the spreadsheet numbers can easily be changed to reflect any oil fired combi, output, store vol and stored temperature, DHW setpoint temperature etc.

 

[ericmark]

All I can say is, thank goodness I did not go for an oil combi. I must say, I am not sure about the whole idea of heating DHW with a boiler, when the central heating is not running. When the central heating is running, the cylinder of hot water will keep hot without firing the boiler to just heat DHW, and any hot un-lagged pipes, are heating the house, so heat not wasted.

But summer, I have a lot of pipework, boiler to cylinder which I really don't want to be hot, and really don't want the boiler hot either, it is a large lump of metal which will cool down before wanted again, so the losses must be huge.

I have C Plan, so DHW hot coil is not pumped, and I find my 20 kW boiler will run for around 20 minutes before the return water temperature turns the boiler off. There is no cylinder thermostat connected to the boiler, so the only control is time, there is a lot of energy stored in the cylinder, so working out how many times to run the boiler was not easy, at ½ hour per day, (only actually runs for 20 minutes) the water was hotter than required, and running 4 times per week at ½ hour each time, it was a little too cool, and a little worried about legionnaires, I did consider a combination run boiler then run the immersion heater so sure it was hot enough, but the immersion was only 9" long.

So 4 x 20 x 20 / 60 = 27 kWh and 7 instead of 4 = 47 kWh, so would say needed somewhere around 35 kWh to heat the DHW per week with oil. Since I now have an iboost+ to control a 27" immersion rather than 7", I can read on the display what I am using, which is around the 14 kWh per week. So need 2.5 times more to heat with oil, electric cost between 8.5 to 15p/kWh depending on time of day used, so it just seems pointless to use oil in the summer.

It does not seem worthwhile to alter what I have, but if we get a kitchen refit, I would consider an under sink 5 litre hot water heater, and turn off the immersion heater, as only used to wash hands.

I think back to my mother's gas combi boiler, and how many times a day it fired up, and I question the whole idea of a combi boiler. I did think of asking my son how much gas used in summer months, but his is a family of 4, and the shower comes from the combi, where we have electric showers, and only 2 of us.

Mother's gas combi had two options eco on or off, one used a small reservoir of hot water, but when using the shower it would run out before the boiler could heat the replacement, so shower would go cold then hot again, and with other option could not turn on taps with a dribble of hot water or boiler would not fire up, one had to turn taps full on.

She got rid of cylinder and fitted combi as cylinder leaking, and sons house airing cupboard turned into an office, and a cylinder does take up a lot of room.

I as said did look at under sink water heaters, but 5 sinks in the house, if starting from scratch, then a combi with 12 mm pipe work well lagged would seem a good compromise, but when fitted in an existing house so often we have 32 mm 25 mm and 15 mm pipe work with no lagging and no real way to access to lag it, so the losses are huge.