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    • CommentAuthorjamesingram
    • CommentTimeJan 4th 2010 edited
    from this forum ,http://www.imeasure.org.uk/carbontalk/index.php/topic,76.0.html
    thought it of interest to post here

    The figures he quote show approx £407+ in gas waisted every year with the HW preheat popping on through out the night!??
    Not sure about his original system sounds like a standard heat only boiler with HW cylinder set up

    "Turn of your condensing boiler when you sleep "
    "Here's a little tip for you all, we recently upgraded our old combi boiler to an all singing dancing Condensing boiler, A-rated super efficient etc etc, only to find its worse than our OLD combi.
    Here's what drew me to this conclusion
    Older combi Boilers have a hot water tank that used through the day as your main source of hot water, its kept hot with the running of your normal central heating, so when heating on Hot water is heated. Heating off, no hot water only what's stored.
    Condensing boilers don't use a external hot water storage tank, they have an small internal boiler, that's only heated when you turn on the taps. The boiler diverts heating away from the central heating system and for the duration the tap is turned on its given priority. Sounds good, no having to heat a big tank and keep loads of water heated up that just slowly cools off. Well every boiler manufacturer uses a similar design, they have this Big heat exchanger plate that is heated to minimise the delay between turning the tap on and hot water coming out.
    Now this is where the problem arises... When your in during the day and are using hot water its not an issue as your heatings normally on, so the boilers kept warm all day long. its when you go to bed there's a problem. Our heating goes off at 10:00pm and comes on again at 0:9:00am. it normally takes around 3-4 hours for the house to cool down to the point you need to switch it back on again, at which time I'm asleep in bed.

    So while I'm sleeping the boiler is "pre-heating" this heat exchanger in case I might want hot water when I'm sleeping :/ So I decided to check how much Gas it burns when I'm sleeping, and this is when i got a shock !

    Over a period of 6 months I did the following :-
    I took a reading from meter at 10:05pm after heating had gone off
    In morning before heating comes back on Take another reading
    Subtract the values, and find out how much was used.

    On average it burns 2.8 units a night, and on cold winter nights its burning 3.5 uints (Cubic meters) This didn't sound like a lot till I converted it into How much does it cost on money.

    If my maths is right :
    1 Cubic meter = 10.86Kwh
    Our Gas Co charges 0.03675p per Kwh (Flat rate)
    365 days in a year.

    so 2.8 x 10.86 x 0.03675 x 365(day) = £407.89 that's just under a 1/3 of the bill for the entire year!

    If you can check my maths is right then i think everyone should switch off condensing boilers at the mains before they go to sleep, and save 1/3 on your bills

    I spoke to the manufactures, the installers and they both say that the usage sounds about right from their tests in the labs, and from other installations they done. I asked if its possible to remove or turn this preheat off when the heatings off, as a "sleep feature" they said no, but its something they might add in future boilers. As you know your selves a boiler firing up every hour makes a bit of noise and can disturb your sleep pattern. So since I worked it out I have my boiler now on a timer plug that kills it when we sleep."
    posted by deeco
    cheers Jim
    • CommentAuthorTuna
    • CommentTimeJan 4th 2010 edited
    That can't be right? His boiler produces 30Kwh heat overnight?? That's (roughly) a 3kW heater on constantly all night. Now our nice shiny new combi does much the same, and I'm sure as hell that the cupboard isn't that warm.

    Maybe the 2.8 units per night aren't cubic metres but are at the rate the gas co charges - ie. omit the 10.86 in the calculation above.
    • CommentAuthorjamesingram
    • CommentTimeJan 4th 2010 edited
    might be cubic feet ? or decimal point confusion in reading the meter ?
    • CommentAuthorRobinB
    • CommentTimeJan 4th 2010
    James I couldn't quite believe your initial figures either. I always imagined the pilot light was all that was on when no hot water was on demand - there's definitely no great energy consumption overnight here.
    • CommentAuthorTuna
    • CommentTimeJan 4th 2010
    Our combi has a small store of water pre-heated exactly as he describes. The boiler comes on a couple of times overnight for maybe thirty seconds a go to keep the store up to temperature. As we're running on LPG, we'd notice pretty quickly if 30cu.m of gas were going each night.
    If the combi is a heat store type then there is a small primary store that then heats the the dhw through the plate heat exchanger. Not all use this some just go from boiler jacket to heat exchanger. the idea is that you get more instant hot water whilst the boiler fires ups (will be triggered by flow switch). This is more normal with oil which is slower to fire up.
    The small thermal store has a thermostat on it, when the programmer is set to go then this will fire the boiler as and when the store temp falls to the set point. When the heating is on then usually the hot water is on, but when the heating is off, then the other prog channel for DWH should control whether the boiler is on or off. So you should ahve two channels 1 for heat, 1 for DHW. The prog / clock should for each have on/off/timed/all day so when in ON its 24 hrs, timed just at the timed points and all day is between the earliest start and the latest stop. It may be permantly set in DHW = on.
    However, having spoken to the manfacturer it does sound typically like one of the dumb things they do supposedly for your benefit but actually more of a problem.

    What make is it??
    Mike up north
    • CommentAuthorSigaldry
    • CommentTimeJan 4th 2010
    "so 2.8 x 10.86 x 0.03675 x 365(day) = £407.89 that's just under a 1/3 of the bill for the entire year!"

    "0.03675p per Kwh"

    Try dividing the total by a hundred to change the pence to pounds...

    = £4.0789 per year
    must be £0.03675 kw/h if its UK gas prices, so not that
    Don't forget that all that "wasted" heat from the preheat is not wasted at all - it's going into your house so that when the heating eventually does come back on, the house is not quite as cold as it would have been if there had been no preheat.

    Paul in Montreal.
    Yes , but surely only in a well insulated home ( most UK home aren't ) in the heating season .Also in the summer its just going to waste
    Posted By: jamesingramYes , but surely only in a well insulated home ( most UK home aren't ) in the heating season .Also in the summer its just going to waste
    Whether the house is insulated or not makes no difference in this case as the house would be colder without the "waste" heat from the combi. But it's definitely a waste outside of the heating season. What would be interesting would be to compare the standing losses of a hot water tank in summer versus the combi's preheat and see which is lower.

    Paul in Montreal.
    • CommentAuthorjamesingram
    • CommentTimeJan 4th 2010 edited
    Yes , heat only verse combi , would be interesting , hard to find any info about these things
    I was lead to believe combi's are generallly less efficient than a heat only as a heat source , due to over sizing issues and short run times for direct HW ,even without the preheat function ( I've got a condensing combi with no preheat , works fine for me , preheats a bit of a unecessary luxury in my mind.) , but as you say standing loses
    may well be higher for HW storage.

    I was trying to find info on condensing boilers, energy payback , Life cycle analysis etc. , to help persuade a friend to update from a older cast iron boiler ,
    Lots of organisation in the UK and Europe ( AECB , EST etc) saying time to upgrade to a condensing boiler ,
    90% efficiency compaired 70% for non-condensing , seems straight forward
    but what about the embodied energy of the new system etc. would the savings cover this quickly ( 1-2 years )?
    I presumed they would , but cant find any data on it.

    cheers Jim
    ps. if there's no heating on or required at night as in the case above its still "wasted heat" to me

    after this xmas I really need to do something about my "waist"
    • CommentAuthorjamesingram
    • CommentTimeJan 4th 2010 edited
    • CommentAuthorjamesingram
    • CommentTimeJan 8th 2010 edited
    I've copied this reply from Deeco ,
    Might the high usage be because its up in loft ? asked him for his summer usage as i'm interested find out why it so high , manifacture said this was average , the boiler mentioned is one of the most fitted for the warm front scheme .

    "HI James ive had a read over at the forums you posted, some think i may be confusing the decimal or its in cubic feet,

    but its new Gas meter approx 2yrs old with CUBIC M3 stamped at the side has , with 5 black number and 3 red numbers being the decimal.
    The heating runs normally and is using approx 8-10units a day on gas, and 2.8-3.5 a night.

    House is extremely insulated, and all rooms are thermo valved, set at approx 18-20c cut offs the main digital room stat (set at 20c) is kept in the living room so that the heating can knock off quicker as we spend most our time in there. The living room rad stat is set a 4 otherwise the temperature would never be high enough to knock the room stat off.

    The boiler maker is ISAAR, its a He30 with analogue controls for the DHW and CH ( must also say this boiler is one of the nosiest things you'll ever hear, the boiler is situated in the loft space and when its running its sounds like a your vacuum upstairs

    I was asking for people to check my figures are right,

    my gas supplier is ebico EquiGas costs 3.5p per kWh excluding VAT (3.675p per kWh including VAT) effective from 30th March 2009."
    • CommentAuthorDarylP
    • CommentTimeJan 10th 2010
    Unless I have missed something.... all the modern combis I have seen with the pre-heat function have timers that control this function. Perhaps the OP has not been shown how to use his/her boiler properly.

    Posted By: jamesingramOur heating goes off at 10:00pm and comes on again at 0:9:00am.
    So while I'm sleeping the boiler is "pre-heating" this heat exchanger in case I might want hot water when I'm sleeping.
    On average it burns 2.8 units a night, and on cold winter nights its burning 3.5 uints (Cubic meters) This didn't sound like a lot till I converted it into How much does it cost on money.
    so 2.8 x 10.86 x 0.03675 x 365(day) = £407.89 that's just under a 1/3 of the bill for the entire year!

    If a third of all gas consumed is being used to keep water pre-heated for 11 hours overnight then it seems likely that another third is being used to pre-heat water for the other 13 hours of the day. This leaves just one third for all heating and hot water actually consumed.

    The water consumed in the day will need to be heated regardless of the pre-heat function & in between these events the heat exchanger will continue to lose heat at the same rate. So it is analogous to static losses from a hot water tank & will depend mainly on the level of insulation around the heat exchanger & the temperature in the loft.

    How well insulated is the heat exchanger? Is the boiler designed for installation in an unheated space? Is automatic frost protection playing a part here?

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