Heating and Hot water. Same time or separate?

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  • abssorb
    abssorb Posts: 125 Forumite
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    I should also mention the worry isn't just the cost. Oil supply is challenging where we live, and being able to adjust settings to squeeze another few weeks out of the available fuel is sometimes necessary.
  • Cardew
    Cardew Posts: 29,037 Forumite
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    abssorb wrote: »
    Can I assume that the system boiler operates the same as a combi in that any demand for heat causes the boiler to come on flat out (to dial setting) until demand satisfied? I.e. it consumes oil at a fixed flow rate, and doesn't vary according to type of demand; space / water / both.
    That would make my understanding simpler. :)





    Neither a combi or system modern boiler works flat out; the output is modulated down(i.e. turned down) typically from 30kW output to 8kW output.


    There are hundreds of articles on-line explaining boiler modulation for experts to novices.


    The flow of water to radiators is relatively low and your 30kW boiler, if working at maximum output, would be switching on/off frequently as the water temperature reaches the set level. So the output is turned down(modulated) to cope with demand.
  • macman
    macman Posts: 53,098 Forumite
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    It takes x kWh of oil to heat your rads to y, and x kWh to heat your hot water to y. Doing both at the same time doesn't provide any savings, except the most marginal ones, as I outlined in my original supply.
    No free lunch, and no free laptop ;)
  • abssorb
    abssorb Posts: 125 Forumite
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    Cardew wrote: »
    the output is modulated down(i.e. turned down) typically from 30kW output to 8kW output.


    There are hundreds of articles on-line explaining boiler modulation for experts to novices.
    Excellent to know this - thank you!

    macman wrote: »
    Doing both at the same time doesn't provide any savings, except the most marginal ones, as I outlined in my original supply.
    That's what I needed to understand, as it's so difficult to measure consumption of oil it meant much of our experiments had wildly different results.

    Thanks all :)
  • EachPenny
    EachPenny Posts: 12,239 Forumite
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    abssorb wrote: »
    Excellent to know this - thank you!
    Cardew is absolutely correct in what he says, but this applies to more modern boilers. I don't think you mentioned how old yours was?

    Older oil fired boilers were built to last, so it is quite possible you have one made before modulation was introduced into the domestic oil boiler market. You really need to get hold of a copy of the manufacturers installation and operation instructions to find out it's exact capabilities and any recommendations regarding settings to maximise efficiency. Don't assume yours has been left set up correctly, or that advice applicable to modern boilers works for yours. :)
    abssorb wrote: »
    I should also mention the worry isn't just the cost. Oil supply is challenging where we live, and being able to adjust settings to squeeze another few weeks out of the available fuel is sometimes necessary.
    Have you considered the possibility of getting a larger tank, or a secondary tank? Not necessarily a cheap option, but could reduce the stress of running out of oil.

    Also, check (if you haven't already) if you have an immersion heater on the hot water cylinder and whether it works. If not, think about getting one as a back up in case the boiler breaks down or you run out of oil... this would mean you at least have hot water on tap, rather than having to boil kettles. :o
    "In the future, everyone will be rich for 15 minutes"
  • Cardew
    Cardew Posts: 29,037 Forumite
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    EachPenny wrote: »
    Cardew is absolutely correct in what he says, but this applies to more modern boilers. I don't think you mentioned how old yours was?

    Older oil fired boilers were built to last, so it is quite possible you have one made before modulation was introduced into the domestic oil boiler market. You really need to get hold of a copy of the manufacturers installation and operation instructions to find out it's exact capabilities and any recommendations regarding settings to maximise efficiency. Don't assume yours has been left set up correctly, or that advice applicable to modern boilers works for yours. :)

    Thanks, I should indeed have mentioned that very old boilers (30+ years??) did not modulate.


    The OP can determine if it does modulate by looking/listening to the flames and it will be apparent when 'the wick has been turned down'.


    Non-modulating boilers turn on/off frequently.
  • EachPenny
    EachPenny Posts: 12,239 Forumite
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    Cardew wrote: »
    Thanks, I should indeed have mentioned that very old boilers (30+ years??) did not modulate.
    I don't know the cut off date (if there is one) but my parent's boiler is under 20 years old and is non-modulating.

    As an aside, this was useful as it allowed me to fit a timer fed from the fuel pump relay (back in the days when it was legal to do such things ;)) which allows the fuel consumption to be reasonably accurately monitored ('x' minutes of fuel flow at a constant rate = litres of fuel consumed) Through that I could show them that in the Summer when the CH wasn't needed, it was cheaper to heat their hot water with the immersion (they already had E7) than to run the boiler twice-daily.
    :money:
    "In the future, everyone will be rich for 15 minutes"
  • Cardew
    Cardew Posts: 29,037 Forumite
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    edited 3 October 2018 at 7:03PM
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    EachPenny wrote: »
    I don't know the cut off date (if there is one) but my parent's boiler is under 20 years old and is non-modulating.


    Seems like I owe the OP and you an apology about when modulating oil boilers were introduced. My experience has been with gas boilers and I made the assumption that oil boilers technology was similar wrt modulation.


    However this 2011 article: https://www.worcester-bosch.co.uk/professional/news/erp-regulation-update
    shows this is not the case.

    The modulation of oil-fired boilers

    The major stumbling block is on the modulation of an oil-fired boiler. Generally speaking, certainly in the UK, a domestic oil-fired boiler typically has a fixed burner, a non-modulated burner.
    It simply cycles on and off when it reaches its required temperature.
    Teach me to make assumptions!


    Further 'research' on the internet indicates that there are now modulating oil burners being introduced.
  • EachPenny
    EachPenny Posts: 12,239 Forumite
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    Cardew wrote: »
    However this 2011 article: https://www.worcester-bosch.co.uk/professional/news/erp-regulation-update
    Thanks for the link, very interesting stuff. Especially the point about the additional cost (almost double :eek:) of a modulating burner unit (plus extra for the control systems) for virtually no increase in efficiency. :(
    "In the future, everyone will be rich for 15 minutes"
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