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    • Cardew
    • By Cardew 14th Dec 11, 12:56 PM
    • 25,074 Posts
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    Cardew
    • #2
    • 14th Dec 11, 12:56 PM
    • #2
    • 14th Dec 11, 12:56 PM
    Not all combis have a pre-heat facility, and others have a switch on the front panel.

    It hardly wastes a 'massive' amount of gas. Well insulated tanks lose very little heat - A 100+ litre hot water tank with water @65C will only lose approx 2kWh in 24 hours. In any case where does the lost heat go? it warms the house.

    Another factor for those of us with water meters is that the cost of the water wasted while waiting for warm water to emerge is probably greater than the 'wasted' gas. Don't forget that water(+sewerage) costs a minumum of a penny for every 5 litres(over 2p in some areas)

    Probably a sensible compromise is to switch it off over night or when away from the property - then the noise of the boiler switching on for a few seconds won't disturb.
  • tony4563
    • #3
    • 14th Dec 11, 1:03 PM
    • #3
    • 14th Dec 11, 1:03 PM
    Combi boilers don't have tanks, do they?
    • Cardew
    • By Cardew 14th Dec 11, 1:22 PM
    • 25,074 Posts
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    Cardew
    • #4
    • 14th Dec 11, 1:22 PM
    • #4
    • 14th Dec 11, 1:22 PM
    Combi boilers don't have tanks, do they?
    Originally posted by tony4563
    Some have a small internal tank which holds pre-heated water.

    The idea being that when water is demanded at a tap, there is a shorter delay before the hot water emerges - and hence less cold water to run off.

    I was making the point that if a 100+ litre HW tank only loses about 2kWh worth of heat in 24 hours(say 6p), how much would a little tank in a combi lose?
    • rogerblack
    • By rogerblack 14th Dec 11, 9:34 PM
    • 9,287 Posts
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    rogerblack
    • #5
    • 14th Dec 11, 9:34 PM
    • #5
    • 14th Dec 11, 9:34 PM
    Small tanks will use more to keep hot than you might think.
    A 10l tank has a third of the surface area of a 100l one.
    In addition, there is likely to be thinner insulation on it, meaning the loss may well be half that of the big tank, even though it's a tenth of the volume.
  • SYNERGY
    • #6
    • 14th Dec 11, 10:28 PM
    • #6
    • 14th Dec 11, 10:28 PM
    Small tanks will use more to keep hot than you might think.
    A 10l tank has a third of the surface area of a 100l one.
    In addition, there is likely to be thinner insulation on it, meaning the loss may well be half that of the big tank, even though it's a tenth of the volume.
    Originally posted by rogerblack
    Small tanks will use more to keep hot than you might think.

    Also in some pre-heat tanks, the water isn't kept hot by the boiler firing, but with a small immersion heater at ??? per KWh

  • gas4you
    • #7
    • 15th Dec 11, 8:26 AM
    • #7
    • 15th Dec 11, 8:26 AM
    Most pre-heat systems just keep the water in the boiler and plate up to temp.

    I always advise keeping pre-heat turned off as well.
  • Dave save
    • #8
    • 15th Dec 11, 8:52 PM
    • #8
    • 15th Dec 11, 8:52 PM
    Some of the Worcester boilers allow you to set the operation of preheat (via the internal timer) to periods when you're likely to need hot water. For me this is the best compromise
  • renegade
    • #9
    • 15th Dec 11, 8:59 PM
    • #9
    • 15th Dec 11, 8:59 PM
    Some of the Worcester boilers allow you to set the operation of preheat (via the internal timer) to periods when you're likely to need hot water. For me this is the best compromise
    Originally posted by Dave save
    I have a Worcester and mine does not have that facility, when the heating is on the water is on, can't have heating and no hot water... can you?
    You live..You learn.
  • aelitaman
    First off the pre-heat option is not used for instant hot water, they are used to provide high flow rates. The water from the pre-heat tank is added to the normal mains water hot water flow to get a greater flow rate so that these boilers can run more than one tap/shower at a time.

    I measured mine with the pre heat on 24/7 and then off. The preheat tank 18 lites in my case used 30kwh per week so 1560Kwh per year or about 50 quid a year at current prices.
  • TwinDaddy
    On balance it sounds like I should be using it before busy times in the house and keeping it switched off the rest of the time. Thanks for all the feedback!
  • SYNERGY
    First off the pre-heat option is not used for instant hot water, they are used to provide high flow rates. The water from the pre-heat tank is added to the normal mains water hot water flow to get a greater flow rate so that these boilers can run more than one tap/shower at a time.

    I measured mine with the pre heat on 24/7 and then off. The preheat tank 18 lites in my case used 30kwh per week so 1560Kwh per year or about 50 quid a year at current prices.
    Originally posted by aelitaman
    the pre-heat option is not used for instant hot water

    Afraid you're wrong, pre heat in a combi boiler keeps up to approx. 3ltr of water hot in one of two ways.

    One method is that gas is used intermittently to keep the the boiler and the water within, hot.

    The second method is that the boiler has, within the casing, a miniature version of a hot water cylinder, complete with a small electrical heating element, when the hot tap is turned on water from the boiler passes through this ' small cylinder ' then on to the taps. When there is no call to the boiler for domestic hot water the heating element maintains the water temperature in the ' small cylinder' constantly at approx. 55d eg - 60 deg.

    Both these systems are used simply to provide almost instant hot water at the taps, nothing more.

    The preheat tank 18 lites in my case used............

    I would be interested to know which boiler has an 18ltr pre heat tank.

    The water from the pre-heat tank is added to the normal mains water hot water flow to get a greater flow rate so that these boilers can run more than one tap/shower at a time.

    The thermal store system is designed around this principal, however, unless the thermal store has a reasonably substantial reserve, more than 18ltr, as soon as the reserve is used it will revert to the same flow as a conventional combi.

    Are you getting mixed up between pre-heat and an external thermal store to supply the boiler such as produced by Alpha?
    • Cardew
    • By Cardew 22nd Dec 11, 3:14 PM
    • 25,074 Posts
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    Cardew
    I think some of the combi's have internal tanks even bigger than 18 litres. The Worcester Greenstar Hi Flo 550 combi has I understand a 50 litre internal tank!(this was discussed in a thread recently) which enables it to advertise a flow rate of 25 litres per minute - until that tank is empty at least!

    The technical specs say 41kW but this Highflow boiler although they call it a combi boiler it's not really a combi boiler. It's really a system boiler with a built in 50 litre cylinder of pre-heated hot water. It will run out of water eventually but it would take quite an effort. You could easily fill 2 baths with hot water and have the washing machine and dishwasher all going at once and it won't run out as it will catch up very quickly.
    • roddydogs
    • By roddydogs 22nd Dec 11, 3:25 PM
    • 5,563 Posts
    • 2,291 Thanks
    roddydogs
    Well ive never heard of it as well, the WB junior certainly dosent have one.
    What i try to do is make the boiler fire by increasing the temp for a few minuits before showering, then the water is almost instantly hot.
  • SYNERGY
    I think some of the combi's have internal tanks even bigger than 18 litres. The Worcester Greenstar Hi Flo 550 combi has I understand a 50 litre internal tank!(this was discussed in a thread recently) which enables it to advertise a flow rate of 25 litres per minute - until that tank is empty at least!
    Originally posted by Cardew
    Ahhhhh yes, storage combi boilers.

    Been retired a good few years now.

    Though I do try to keep up to date with everything new, I occasionally forget .

    Now what did I come upstairs for.
    • Cardew
    • By Cardew 22nd Dec 11, 4:08 PM
    • 25,074 Posts
    • 11,956 Thanks
    Cardew
    Now what did I come upstairs for.
    Originally posted by SYNERGY
    To have a shower or clean the car?
    • macman
    • By macman 22nd Dec 11, 5:05 PM
    • 39,716 Posts
    • 16,072 Thanks
    macman
    I have a Worcester and mine does not have that facility, when the heating is on the water is on, can't have heating and no hot water... can you?
    Originally posted by renegade
    It's not a combi then. With a combi, the CH cuts out when there is a demand for hot water.
    No free lunch, and no free laptop
  • Mr Ted
    It is a combi, its just not an instantaneous combi for the hot water supply

    The reason for preheat is to prevent thermal shock which could damage the heat exchanger!!!!!!!!

    Which is best save a few pence or buy a new boiler?

    Armchair engineering can be dangerous!!!!!!!!!!!!!!
  • aelitaman
    I was technically incorrect to say the water from the tank (called a heat bank in WB speak) was added to the mains waterflow, what I should have said is the heat stored in the heatbank is added to the mains cold water before it reaches the heat exchanger, because the flow of water through the heatbank is via pipes. So it is a pre-heat method but it is used to get the high flow, one aside may be instant hot water (but this depends on how long a heat exchanger takes to get to temp without a heatbank). Also the other reason that it is not used primarily for instant hot water is because as you said you only need a 3ltr tank for that. I was wrong when I said mine was 18ltrs it is in fact 60litres and according to the specs designed to provide the highflow rate of 18ltrs per minute for 10 mins.

    I just read the technical specs for my WB highflow and what happens in practice is that with the heat bank up to the temp that the hot water temp gauge is set up. When hot water is demanded the mains cold flows through the heatbank (via coils) and then to the heat exchanger which is operating at max. Because the mains cold has been "pre heated" more water can flow through the heat exchanger than if it was at mains water temp, hence the highflow rates.

    Yes you are correct that when the heatbank is at the temp of the mains cold water the divertor valve kicks in and the mains cold goes directly to the heat exchanger and the flow rate of hot water drops.
  • SYNERGY
    It is a combi, its just not an instantaneous combi for the hot water supply

    The reason for preheat is to prevent thermal shock which could damage the heat exchanger!!!!!!!!

    Which is best save a few pence or buy a new boiler?

    Armchair engineering can be dangerous!!!!!!!!!!!!!!
    Originally posted by Mr Ted
    The reason for preheat is to prevent thermal shock which could damage the heat exchanger!!!!!!!!

    Not correct Ted, the pre-heat, in a combi, is nothing at all to do with preventing thermal shock !

    Don't believe me, do some research, such as:

    some combi boilers (Glow worm and Ideal Boilers) have a "pre-heat" facility that heats a small quantity of water ready for your use, this helps to eliminate running the tap for a number of minutes waiting for the hot water to come through.

    If what you say is true, why don't the hundreds of thousands, or more, of boilers without the pre-heat function, constantly suffer from thermal shock induced failure?

    They don't do they ?
    This is taking into account the fact that in comparison, there are far fewer boilers produced with pre-heat.

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