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    • abssorb
    • By abssorb 15th May 18, 9:55 AM
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    abssorb
    Heating and Hot water. Same time or separate?
    • #1
    • 15th May 18, 9:55 AM
    Heating and Hot water. Same time or separate? 15th May 18 at 9:55 AM
    Hi,

    I've always had combi boilers in the past, but now we've moved to a house with an oil fired system boiler with a hot water tank.

    In general, when is it most efficient (in terms of fuel use) to time the hot water? At the same time as the heating, so it's sharing the same energy, or separately?
Page 1
    • macman
    • By macman 15th May 18, 10:19 AM
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    macman
    • #2
    • 15th May 18, 10:19 AM
    • #2
    • 15th May 18, 10:19 AM
    It doesn't 'share the energy'. If you want to space heat and hot water heat at the same time, then it has to burn more oil to satisfy the increased demand.
    It's true that any lost heat from the hot tank serves to warm the house too, but this is a marginal efficiency.
    Most people set it up to ensure some hot water in the morning and evening, but if the house is occupied during the day, then you may want something different.
    With a combi, you cannot heat rads and hot water simultaneously.
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    • AndyPK
    • By AndyPK 15th May 18, 12:19 PM
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    AndyPK
    • #3
    • 15th May 18, 12:19 PM
    • #3
    • 15th May 18, 12:19 PM
    you can put the hot water on 20 - 30 minutes before the heating, this gives a good compromise. (allows time for the water tank to warm up quickly)


    But if there aren't many people in the house, and want to save pennies then try putting them on together in winter
    • molerat
    • By molerat 15th May 18, 1:41 PM
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    molerat
    • #4
    • 15th May 18, 1:41 PM
    • #4
    • 15th May 18, 1:41 PM
    My only rule, as above, is don't have them both start from cold at the same time as it takes a bit longer to get the benefit of either if you do. Outside of that rule anything that suits your particular requirements.
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    • tonycottee
    • By tonycottee 16th May 18, 10:28 AM
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    tonycottee
    • #5
    • 16th May 18, 10:28 AM
    • #5
    • 16th May 18, 10:28 AM
    I used a smart meter to measure how much it cost me to leave the hot water tank on all day compared to just turning it on when needed. It only cost around 2 extra.
    • Alter ego
    • By Alter ego 16th May 18, 10:34 AM
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    Alter ego
    • #6
    • 16th May 18, 10:34 AM
    • #6
    • 16th May 18, 10:34 AM
    [QUOTE=tonycottee;74291745 It only cost around 2 extra.[/QUOTE]

    Is that daily?
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    • tonycottee
    • By tonycottee 18th May 18, 11:41 AM
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    tonycottee
    • #7
    • 18th May 18, 11:41 AM
    • #7
    • 18th May 18, 11:41 AM
    Is that daily?
    Originally posted by Alter ego
    Sorry, that was per month.

    There's some more about leaving hot water on constant.
    Last edited by tonycottee; 25-05-2018 at 10:54 AM.
    • Hengus
    • By Hengus 18th May 18, 2:23 PM
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    Hengus
    • #8
    • 18th May 18, 2:23 PM
    • #8
    • 18th May 18, 2:23 PM
    I used a smart meter to measure how much it cost me to leave the hot water tank on all day compared to just turning it on when needed. It only cost around 2 extra.
    Originally posted by tonycottee
    There is no one size fits all here. I have a 250l Oso unvented cylinder that is on from 5.30 to 9am as our system is configured for hot water priority. The cylinder temperature is managed in the range 60 to 55C during the 4 hour on time. With CH off, my cylinder usually takes about 40 minutes to re-heat and uses an average of 6kWhs of gas per day.

    The problem with constant re-heating is that most modern boilers are profiled to go to max flow temperature for hot water re-heating. If they didn't then on a warm day like today, the cylinder would be re-heating the HW boiler flow. Constant boiler on/off each time there is a HW demand is inefficient according to my 15 minute optical spot gas meter reader. We find that it is perfectly possible with a large cylinder to bath and shower in the evening without a water re-heat. That said, no one size suits all.
    • Cardew
    • By Cardew 18th May 18, 5:36 PM
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    Cardew
    • #9
    • 18th May 18, 5:36 PM
    • #9
    • 18th May 18, 5:36 PM
    I used a smart meter to measure how much it cost me to leave the hot water tank on all day compared to just turning it on when needed. It only cost around 2 extra(per month).
    Originally posted by tonycottee

    All modern tanks have their loss of heat, measured to a British standard, stamped somewhere on the body(normally where you can't read it!!) Typical losses are 1kWh to 1.5kWh a day with water in the tank held at 65C. So in practice losses will be smaller than that figure.

    My large unvented tank loses heat at the rate 1.3kWh a day. Which would cost around 4p a day for gas if I kept the water at 65C but in practice I believe my losses are less than 1 a month.


    Also as said before the 'lost' heat isn't wasted as it warms the fabric of the house.
    Last edited by Cardew; 18-05-2018 at 5:39 PM.
    • matelodave
    • By matelodave 19th May 18, 6:22 PM
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    matelodave
    My heating unit will heat either the tank or the rads, but not both at the same time. many other systems are similar depending on how the timeswitch/programmer is configured and whether the system has a tank stat, room stat and motorised valves.

    We heat the tank for about an hour in the mornings before we get up - as it's a heat pump we use about 2kwh of leccy in the winter, a bit less in the summer. The hour a day gives us enough hot water for all our requirements (including a couple of showers in the mornings) and the water is still hot enough for two more showers if we want them. The secret being to minimise the time you stand in the shower and make sure that you control the flow rate.

    I'd suggest that you try heating the water for an hour or so before you get up to see if there's enough hot water for your morning ablutions and see how long it lasts. If you need a bit more for the evening then put in on for half an hour or so. Just adjust your timings to suit your lifestyle

    In the end there's no benefit in keeping the tank hot all day or even all night if you aren't using it but as Cardew says, a decently insulated modern tank doesn't lose all that much heat anyway so once it's hot is stays hot unless you use it all. You could always put another insulating jacket over it.

    If oil costs you 4p/kwh and you lose 1.5kwh a day = 6p/day = 22 a year.
    Last edited by matelodave; 19-05-2018 at 6:26 PM.
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    • abssorb
    • By abssorb 11th Sep 18, 2:03 PM
    • 90 Posts
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    abssorb
    Sorry for not returning to this thread sooner, I'm only an occasional user of this forum. Thanks for the replies, lots to think about

    It doesn't 'share the energy'.
    Originally posted by macman
    True - I worded it badly, I mean share the "same energy source" which in this case tops out at 30KW.

    The problem with oil of course is that being unmetered, there is no reliable way to measure actual consumption accurately enough to do my own tests.
    Also because insulation is yet to be improved (old house + inaccessible roof spaces), external temp and windchill add another variable.

    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.




    Then it's just about which will burn the least oil. This is probably going to be really hard to answer, but I'm interested in anyone's views
    Our system is configured such that it can do heat and water at the same time. Even given the losses through insulation, both the below scenarios result in a warm house and plenty of hot water:

    Scenario A
    Heating comes on at 7:00am to 8:30am
    Hot water comes on 7:00am to 8:00am

    Scenario B
    Heating comes on at 7:00am to 8:30am
    Hot water comes on 6:00am to 7:00am
    I've tried both over several weeks, but lack of accuracy in measuring consumption and a fluctuating windchill of up to -15degC make my results pretty meaningless.

    With your systems can you notice a distinct difference in fuel consumption between A and B? Or is it pretty much the same and down to comfort preference?
    Last edited by abssorb; 11-09-2018 at 2:09 PM. Reason: spelling
    • abssorb
    • By abssorb 11th Sep 18, 2:18 PM
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    abssorb
    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
    • By Cardew 11th Sep 18, 3:12 PM
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    Cardew
    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.



    Originally posted by abssorb

    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
    • By macman 11th Sep 18, 7:24 PM
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    macman
    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.
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    • abssorb
    • By abssorb 3rd Oct 18, 9:03 AM
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    abssorb
    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.
    Originally posted by Cardew
    Excellent to know this - thank you!


    Doing both at the same time doesn't provide any savings, except the most marginal ones, as I outlined in my original supply.
    Originally posted by macman
    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
    • By EachPenny 3rd Oct 18, 11:16 AM
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    EachPenny
    Excellent to know this - thank you!
    Originally posted by abssorb
    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.

    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.
    Originally posted by abssorb
    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.
    "In the future, everyone will be rich for 15 minutes"
    • Cardew
    • By Cardew 3rd Oct 18, 1:25 PM
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    Cardew
    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.

    Originally posted by EachPenny
    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
    • By EachPenny 3rd Oct 18, 2:41 PM
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    EachPenny
    Thanks, I should indeed have mentioned that very old boilers (30+ years??) did not modulate.
    Originally posted by Cardew
    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.
    "In the future, everyone will be rich for 15 minutes"
    • Cardew
    • By Cardew 3rd Oct 18, 7:00 PM
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    Cardew
    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.
    Originally posted by EachPenny

    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.
    Last edited by Cardew; 03-10-2018 at 7:03 PM.
    • EachPenny
    • By EachPenny 3rd Oct 18, 9:44 PM
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    EachPenny
    Thanks for the link, very interesting stuff. Especially the point about the additional cost (almost double ) 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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