Thermal Store and Condensing Boiler

Discussion in 'Plumbers' Talk' started by Ballcocks, Jan 9, 2005.

  1. Ballcocks

    Ballcocks Member

    Can anyone give any guidance as to how a Condensing boiler will work with a Gledhill thermal store? As far as I can see it will hardly ever be in condensing mode as the store needs to be at approx 80C and so the return will in practice never be lower than say 65C.

    Gledhill say any boiler will work with their system. Worcester and Vaillant both tell me that their boilers are efficient even when not condensing.
  2. ezecool

    ezecool New Member

    What the manufacturers tell you is true.
    What you have understood about the reduced efficiency is also true. Its unlikely that the boiler will ever effectively be running in condensing mode.
  3. devils advocate

    devils advocate New Member

    Condensing boilers will probably be very <u>slightly</u> more efficient than 'standard' models even when in non-condensing mode due to their effective , finely-matrixed exchangers.

    However, if they will always be running in non-condensing mode I suspect any advantage will be <u>marginal</u> - is the added cost, and likelyhood of reduced reliability, really worth it in this case?
  4. Water Systems

    Water Systems New Member

    Condensing boilers will probably be very
    <u>slightly</u> more efficient than 'standard' models
    even when in non-condensing mode due to their
    effective , finely-matrixed exchangers.

    However, if they will always be running in
    non-condensing mode I suspect any advantage will be
    <u>marginal</u> - is the added cost, and likelyhood
    of reduced reliability, really worth it in this case?

    This is not the case. Modern thermal stores are run at approx 75C. Stratification means that the bottom of the cylinder is at a much lower temperature than the top. Using two cylinder stats, one top and one bottom means that the bottom of the cylidner will be below 50C before the boiler is switched in. The two stats also eliminate inefficient boiler cycling. For a reasonable percentage of the time the boiler return will be at a low temperature promoting high condensing efficiency.

    Using an integrated thermal store, DHW & CH, from the same cylinder, it is possible to have the top at a high temperature and bottom at a low variable temperature controlled by an outside weather compensator. This means the boiler is operating at very low high efficient temperatures for most of the heating running time. DHW will always be at a high temperature as DHW is always stored at high temperatures anyhow.

    The AVC UK Heatmaster floor mounted combi has an integral thermal store that operates at two distinct temperatures.
    http://www.acv-uk.com/
  5. changename-12753

    changename-12753 New Member

    Using two cylinder stats, one top and one bottom means that the bottom of the cylidner will be below 50C before the boiler is switched in.

    2 stats. One turns on the boiler when temp drops below 50c. What does the other one do?. I am just curious as the lower stat should acheive boiler lockout until the temp drops below 50c.
    Regards r2d2.
  6. Water Systems

    Water Systems New Member

    Using two cylinder stats, one top and one bottom
    means that the bottom of the cylidner will be below
    50C before the boiler is switched in.


    2 stats. One turns on the boiler when temp
    drops below 50c. What does the other one do?.
    . I am just curious as the lower stat should
    acheive boiler lockout until the temp drops
    s below 50c.
    Regards r2d2.

    Top stat say set to 55C, the bottom say set to 75C. From cold.... The cylinder heats up, the bottom stat cuts out the boiler at 75C. fully heated The temperature in the bottom of store drops below 75C, the boiler does not cut in, no boiler cycling as the temperature drops. Only when the top is below 55C will the boiler cut in reheating the store in one long efficient burn taking all the boilers ouput so reheating ASAP. The water above the top stat will be higher than 50C so always usful water available at the taps.

    See, I think the DPS site at http://www.heatweb.com, explains it well.

    The two stats is quite common and used on normal cyldiners with quick recovery coils too. High rated boioers to ensure quick reciovery is esential. The great thing about thermal stores is that the boiler can be any size and not cycle using two stats.

    It is possible to have two boilers conected directly to the store, so that you always have backup. The two boilers can be switched in simultaneously and re-heat within a few minutes - and no cycling

    The domestic meter can supply about 62kW of energy, most only use a fraction available to them. The average combi only uses less than half what the meter can supply. Use it all, it is there, and have smaller cylinders and small cased backup boilers.
  7. changename-12753

    changename-12753 New Member

    What type of cylinder stats are these WT ?.
    A normal cylinder stat will have a 8 degree switching differential.
    When the lower stat turns of the boiler at 75C, what breaks the power to this stat to stop it re-energising the boiler.
  8. Water Systems

    Water Systems New Member

    What type of cylinder stats are these WT ?.
    A normal cylinder stat will have a 8 degree
    switching differential.

    8C? must be cheapo, or just about to pack up. The thing about using thermal store is that the stored water temperature does not have to be very accurate. Normal good qualoty Honeywells stat would do. There are more accurate electrionic stat available that have tighter switching differentials.

    When the lower stat turns of the boiler at
    75C, what breaks the power to this stat to
    o stop it re-energising the boiler.

    A latching relay. DPS have the wiring for it, if you look at their design software. The relay and base would cost around £4-5 from maplin and a plastic box abouit £1-2.

    Highly effective and elimianetes boiler cyling.

    Another great point about thermal stores/heat banks when the CH is run from them, a simple non-modulating boiler can be used. Most condensers are modulating these days, some have a link in pcb that disables the modulation, but a simple, cheap reliable boiler can be fitted. No need for expensive boilers to modulate down to 3kW. No auto by-passes. The rad circuit, run from the store can have just a Grundfoss Alpha modulating pump and TRVs on all rads. When all rads are closed the pump winds down, when they open up it winds up. No electric room stats, which always turn off the heating when some rooms are cold, and spoil the wall decor. No radiators open all the time. Simple, reliable and effective.
  9. mike h

    mike h New Member

    Water Systems,

    I&#146;m encouraged by your comments on thermal stores particularly the full direct type as they offer the only practical way of managing heat. For fixed output units most supposed efficiencies (TRV&#146;s, Zoning, reduced heat losses etc etc) actually work against condensing technology as they simply increase the return temperature i.e. unused heat, thereby increasing the likelihood of short cycling. When variable weather demands are added in it&#146;s a bit like driving everywhere foot down in 3rd gear. Rapid full modulation at the heat source to demand changes is the only fuel efficient way, but of course this is either expensive or not really possible as in the case of fixed output PJ oil boilers which have limited downturn. Variable output spray burners are in the next order of prices up from the usual domestic Stirling/Riello units.

    I agree your use of the Alpha variable pump, but an alternative to the single variable pump is a smaller unit in each circuit zone; this is what Viessmann do from a common low loss header.

    As to the combi question - I have used a combi for many years and it is fine as it matched my lifestyle in that it provided sufficient DHW on a purely as and when basis. The main problem I have with them is that to get a high flow rate, a DHW heat supply that is disproportionate to the CH heat load is required. Typically the combi would require 20kw to heat instantaneously, but heating load is likely to be under 10kw if fully insulated and controlled, and given weather patterns the full heating capacity of the boiler is only required for a small percentage of the average year anyway.

    So with variable DHW needs and a larger but variable heating load I&#146;m considering a full blown heat bank approach to buffer and iron out these fluctuations. I&#146;m not sure why people seem to be so scared of thermal stores, because any cylinder vented/unvented is generically a thermal store, the only differences is that the heat is stored on the potable side. The key to the DPS (I don&#146;t work for them either) heat bank is the mixing valve on the boiler return which ensures the boiler only ever sees a 75/55 deg load (except on commissioning start up) or any other, if set to return at 20 deg less than delivery. Not only does this agree with the condensing theory to force the return down to approx 55 deg, but also ensures that regardless of heat taken out, only water at 75 deg is passed back into the cylinder. The heating load is then metered out from this with out contact with the boiler, as it is the store thermostat that controls the boiler. The other advantage with any heat bank is that also enables heat to be collected from other sources such as solar or stoves.

    There are many many other suppliers of thermal stores and heat banks out there but the principals are they same and they are all willing to discus things. So I definitely recommend that people accurately calculate what heat loads they require, and over various ranges of conditions and with different usage patterns to produce a heating load schedule and then design with that in mind to deliver controllable and variable heat exactly to the patterns they want, when they want it. THEN discuss it with the manufacturers &#150; they will help. Zoning etc merely changes the demands, not the supply, so it is the controlled rationing out of heat that is needed.

    The price of fuel is only going to continue going up and up that is a certainty. Using the minimum amount of fuel to heat your home & water to the minimum you can get away with will soon be most critical part of any design. Not comfort or convenience. Somewhere in this thread somebody questioned how often 2 baths would be run at the same time, also with the wash machine on, and with the kitchen sink running etc etc. This is true, why design for unlikely peak demands, what are you going to do with all that spare capacity (apart from save it in a thermal store). You will soon be paying through the nose for it. I&#146;d make them wait for the bath, I can&#146;t afford to waste fuel just for other peoples convenience (or mine), and in olden days when it was a bit chilly we had a thing called a jumper (someone else has already mentioned putting on jumpers on here as well &#150; the best approach is always to use less).

    Anyway that&#146;s my rant for today.

    MH
  10. Water Systems

    Water Systems New Member

    Water Systems,

    I&#146;m encouraged by your comments on thermal stores
    particularly the full direct type as they offer the
    only practical way of managing heat. For fixed output
    units most supposed efficiencies (TRV&#146;s, Zoning,
    reduced heat losses etc etc) actually work against
    condensing technology as they simply increase the
    return temperature i.e. unused heat, thereby
    increasing the likelihood of short cycling.

    True to a point. A bunch of Myson fan convectors work on high flows to get the place warm. And they operate on a Deta T of 10C.

    When variable weather demands are added in it&#146;s a bit like
    driving everywhere foot down in 3rd gear. Rapid full
    modulation at the heat source to demand changes is
    the only fuel efficient way, but of course this is
    either expensive or not really possible as in the
    case of fixed output PJ oil boilers which have
    limited downturn. Variable output spray burners are
    in the next order of prices up from the usual
    domestic Stirling/Riello units.

    Thermal stores are ideal for fixed rate burners. The cycling is eliminated using two cyl stats.

    I agree your use of the Alpha variable pump, but an
    alternative to the single variable pump is a smaller
    unit in each circuit zone; this is what Viessmann do
    from a common low loss header.

    I agree and I thought I mentioned that but never. Have the flow and return of each zone off the store, with rads of 60C flow and 40C return (standard in Scandinavia) and TRVs on all rads and simplicity itself, with no electric wall stats. The store is a great neutral point. With solar easily incorporated, that will even be used to heat the CH too.

    The store can have a lower section that is weather compensated and runs very cool. Even in summer the DHW can promote condensing efficiency. You just engineer it have low return temps, ACV did withy their Heatmaster. It even condenses when on DGHW only for virtually all of run time, by passively engineering the store to have it cold at the bottom. My plate heat exchanger sends the return water back into the bottom of store at 20-30C, way below condensing dew point.

    As to the combi question - I have used a combi for
    many years and it is fine as it matched my lifestyle
    in that it provided sufficient DHW on a purely as and
    when basis. The main problem I have with them is that
    to get a high flow rate, a DHW heat supply that is
    disproportionate to the CH heat load is required.
    Typically the combi would require 20kw to heat
    instantaneously, but heating load is likely to be
    under 10kw if fully insulated and controlled, and
    given weather patterns the full heating capacity of
    the boiler is only required for a small percentage of
    the average year anyway.

    So with variable DHW needs and a larger but variable
    heating load I&#146;m considering a full blown heat bank
    approach to buffer and iron out these fluctuations.

    The Alpha CD50, wall mounted stored water combi, can fill bath very quickly and the CH modulates down to about 6kW. Many floor mounted job have very high flow rats and modulate low too. You may have an old combi which is not so flexible.

    I&#146;m not sure why people seem to be so scared of
    thermal stores, because any cylinder vented/unvented
    is generically a thermal store, the only differences
    is that the heat is stored on the potable side. The
    key to the DPS (I don&#146;t work for them either) heat
    bank is the mixing valve on the boiler return which
    ensures the boiler only ever sees a 75/55 deg load
    (except on commissioning start up) or any other, if
    set to return at 20 deg less than delivery. Not only
    does this agree with the condensing theory to force
    the return down to approx 55 deg, but also ensures
    that regardless of heat taken out, only water at 75
    deg is passed back into the cylinder.

    This is brilliant for stratification. Only full temp water is pumped into the top of the store. A long thin cylinder really makes matters better, as the temp difference from top to bottom can be very wide indeed. After a minute or two from cold start there is enough high temp hot water in the top of the store to run off, through the plate heat exchanger for sinks and the likes. The store is heated top down with 75C water at the top gradually moving down. Excellent. No slow warm up of all the agitated store

    The heating
    load is then metered out from this without contact
    with the boiler, as it is the store thermostat that
    controls the boiler.

    The boiler can be a cheap, simple non-modulating model. A great saver and advantage. The elimination of boiler cycling, promotes boiler longevity as the controls are not clicking in every few minutes. The boiler works more efficiently with a long efficient burn.

    The other advantage with any
    heat bank is that also enables heat to be collected
    from other sources such as solar or stoves.

    Yep, which can be used for CH & DHW.

    There are many many other suppliers of thermal stores
    and heat banks out there but the principals are they
    same and they are all willing to discus things. So I
    definitely recommend that people accurately calculate
    what heat loads they require, and over various ranges
    of conditions and with different usage patterns to
    produce a heating load schedule and then design with
    that in mind to deliver controllable and variable
    heat exactly to the patterns they want, when they
    want it. THEN discuss it with the manufacturers &#150;
    they will help. Zoning etc merely changes the
    demands, not the supply, so it is the controlled
    rationing out of heat that is needed.

    Yep. Know what you want beforehand and present it to them, otherwise they send you to their standard off the shelf models &#150; they may actually be what you needed.

    The price of fuel is only going to continue going up
    and up that is a certainty. Using the minimum amount
    of fuel to heat your home & water to the minimum you
    can get away with will soon be most critical part of
    any design. Not comfort or convenience. Somewhere in
    this thread somebody questioned how often 2 baths
    would be run at the same time, also with the wash
    machine on, and with the kitchen sink running etc
    etc. This is true, why design for unlikely peak
    demands, what are you going to do with all that spare
    capacity (apart from save it in a thermal store).

    Each 13 amp socket can run a 3kW heater. Try and plug a heater in all in your house and see what happens. They design the system to realistic usage. In water it appears they design for worse case scenario.

    You
    will soon be paying through the nose for it. I&#146;d make
    them wait for the bath,

    Wait for a bath? I think not. Say a combi can do one bath an fill in 5 minutes. Waiting 5 minutes for bath is no great thing. Another point about slow bath filling combi&#146;s. Most people want the bath to be full before they step in. Those with sense ¼ fill, step in and keep the water going. So obvious, yet most people don&#146;t think of it.

    I can&#146;t afford to waste fuel
    just for other peoples convenience (or mine), and in
    olden days when it was a bit chilly we had a thing
    called a jumper (someone else has already mentioned
    putting on jumpers on here as well &#150; the best
    approach is always to use less).

    You can also add cavity wall insulation, eliminate air leakages in the house, triple glaze, have 12&#148; of insulation in the loft, which in this weather keep heat of loft out of the bedrooms, keeping the cool.

    Good post MH. There is far too much ignorance around thermal stores/heat banks.
  11. Scousemouse

    Scousemouse New Member

    The majority of times I hear about heat banks/stores is when people complain that their HW goes too cool when the CH comes on - the rads take a slug of heat quicker than the boiler can make it up. This is often annoying, but "put up with" as a shortcoming of the system. It can be seen on new-build installations as well as individual systems. My customers do not want to tolerate this sort of behaviour to save some trifling sum, they want their bath, or two or three simultaneous baths, when it suits them! Secondary circulation where the connection point is at the cylinder CW input is appropriate for these people - the whole of the stored water is at nearly the same temperature.

    Do all the stores have secondary circulation connections where stratification would not occur - obviously it would stir the contents?

    My gut feel is that your critical stratification might be OK on paper but in practice something would make it different. I would never advocate relying on something over which you have no direct control.

    I'm not impressed by the general approach - suggesting 300mm insulation in a loft in this country! WOrk out the saving on the middle 100mm, let alone the top 100. How many people use the loft for storage meaning it stops at 100mm.
    Well worth the little it costs, plus the saving in installation.
  12. FX

    FX New Member

    A latching relay. DPS have the wiring for it, if you look at their design software. The relay and base would cost around £4-5 from maplin and a plastic box abouit £1-2. Please can you provide a link to the page Water Systems.Ta
  13. Water Systems

    Water Systems New Member

    The majority of times I hear about heat banks/stores
    is when people complain that their HW goes too cool
    when the CH comes on - the rads take a slug of heat
    quicker than the boiler can make it up.

    That is apoor thermal store. The cH circuits are typically taken off the bottom section of the cyoidner and the DHW at the top. The water above the CH flow and returns should not be sucked into the CH circuits at all.

    My customers do not want
    to tolerate this sort of behaviour to save some
    trifling sum, they want their bath, or two or three
    simultaneous baths, when it suits them!

    I don't blame them as it is poorly designed. This problem is easy to get rid of. Have TRVs on all rads and a Grundfoss Alpha auto variable speed pump on the CH circuit - no central controlling electric wall stat. Then each room is individually controlled which is better for them. The TRVs will gradually open and large slug of water will not be dragged out of the store. A large slug will be pumped out on morning fireup, which gives instant heat at the rads. Most stores only take minutes to recover.

    Secondary
    circulation where the connection point is at the
    cylinder CW input is appropriate for these people -
    the whole of the stored water is at nearly the same
    temperature.

    Do all the stores have secondary circulation
    connections where stratification would not occur -
    obviously it would stir the contents?

    A secondary circulation loop can be made using a pump, pipe stat and two check valves. The pipe stat is near the return to the store, which will be set to approx 40-45C. When cool the pump pumps water through the coil or plate, and when at 45C the pump is off. As with any loop, it should be well insulated.

    You can have the pumps continually running and the blending valve keeping the loop temperature down.

    My gut feel is that your critical stratification
    might be OK on paper but in practice something would
    make it different. I would never advocate relying on
    something over which you have no direct control.

    The stratification is nor that critical, but certainly helps a lot.

    I'm not impressed by the general approach -
    suggesting 300mm insulation in a loft in this
    country! WOrk out the saving on the middle 100mm, let
    alone the top 100.

    Saving in heating bills? Energy has just gone up about 15%. It will pay for itself quite quickly. The more the fuel hype the quicker the clawback. Many housing associations are installing 300mm to keep the poor out of fuel poverty.

    How many people use the loft for
    storage meaning it stops at 100mm.

    Counter joists can be installed raising the centre of the loft to add more insulation. Easy to do. Also high performing Kingspan insulation can be used &#150; very thin and high performing. Not cheap though.

    Well worth the little it costs, plus the saving in
    installation.
  14. Water Systems

    Water Systems New Member

    A latching relay. DPS have the wiring for it, if
    you look at their design software. The relay and base
    would cost around £4-5 from maplin and a plastic box
    abouit £1-2.


    Please can you provide a link to the
    page Water Systems.Ta

    Go to:
    http://www.heatweb.com and down to Standard Hot Water Cylinder Options. Click on Design, This is a design tool, and select twin cylinder stats on the right. Then the wiring button top right. It gives you wiring diagram. It needs a small cheap relay to pperate properly - £5 inc base from Maplin.

    It is also a good idea to fit two stats when using a quick recovery coil cylinder of any sort, especially when you have a largish boiler to re-heat it fast.
  15. changename-12753

    changename-12753 New Member

    A secondary circulation loop can be made using a pump, pipe stat and two check valves. The pipe stat is near the return to the store, which will be set to approx 40-45C. When cool the pump pumps water through the coil or plate, and when at 45C the pump is off. As with any loop, it should be well insulated.

    This will be operating in tandem with the store, plate heat - exchanger pump then!!.
  16. FX

    FX New Member

    It needs a small cheap relay to pperate properly - £5 inc base from Maplin. Which relay are you suggesting?
  17. Water Systems

    Water Systems New Member

    It needs a small cheap relay to pperate properly -
    £5 inc base from Maplin.


    Which relay are you suggesting?

    Go to Maplin. Ask for a 230V relay that ccan take 5 amp at the contacts. I think thety have a 13 amp one there. All for around £5. I forget the numbers. I usually get two relays and have spare one in the plastic housing box. Any problems, plug one out and one in, in seconds.
  18. Scousemouse

    Scousemouse New Member

  19. changename-12753

    changename-12753 New Member

    RS do a latching relay for about £12 covering all the voltage ranges. Would it not be better to use one of these.
    In the summer time when DHW only on , the store temperture will be controlled by the latching relays . In the winter time when CH is on or at any point at which CH is turned on ,the boiler will need to be turned on via a separate thermostat , placed on the lower section , because heating circuit is pulling hot water out of the lower section of the store . So CH on , will be creating all the DHW. I think I have got this right. I think we lose the one long efficient burn from top of store to the bottom at this point but with the cooler water being returned to a condensing boiler controlled with a TMV , I think it will make up for this loss .
    I am getting more impressed with this system the more I study it.

    Lets say we fit a compensator , say the Honeywell Aquatrol 2000 as an example
    and a 3 port, floating control valve , controlling temperture to the store.
    The load on the system as far as the compensator and boiler are concerned is the thermal store. We have 3 separate heating loads , lets say, underfloor heating on the ground and 1st floor level and radiators on the 2nd level of new property.
    The system has just came out of latching relay DHW mode, so lets say 75 degrees water is in the entire store. The central heating is turned on, compensator mode kicks in, the outdoor sensor takes a reading and the indoor sensor takes a reading and sets the flow temperture to the store accordingly. . The Alpha kicks in and delivers 75 degrees water to the system.
    The underfloor heating will not like this temperture so we fit a manual thermostatic mixing device to both underfloor systems to deliver the high temperture at a preset safe level, say 55 degrees. Problem solved, I think !!. With system operating on mild days the delivered temperture may be only lets say , 30 degrees , the manual mixer will be calling for 55 but this will be irrelevant as it will be overuled by the compensator control. We could use 2 separate circuits, 2 Alpha pumps tapped of the Thermal store . One supplying the underfloor circuits and the other supplying the radiator circuit.
    Hmmm . ( Thinking ) One compensator, cheaper installation, underfloor circuit and radiator circuit. But the indoor sensor will be taking the indoor reading for the temperture flow to the underfloor system. The radiator circuit will receive this water temperture also. !!!. If we add heated towel rails to this, they will receive a compensated water temperture also. But I think there are electrically boosted versions available.

    Also.The top of the store will be at 75 degrees so the water delivered to the store on a mild day from a compensator could be only 30 degrees thus cooling the stored DHW fom the top down. Not very good. Hhmmm!
    So we deliver the water to the store via tappings on the lower section of the store , using the extra tappings on the boiler. One set of boiler tappings for the DHW and one set for compensated circuit.
    DHW delivered to top of store and compensated CH water delivered to bottom section.

    Compensators will be setup according to their terminal units ( ie, Underfloor heating, Fancoil , Fin tube convector, radiator,s etc).
    I think it would be a lot simpler to compensate the circuits directly after the store. Would it not cut out a lot of complications ??,. Sorry for the Essay.
  20. doitall

    doitall New Member

    Would you care to cost that little lot r2d2, and don't forget to add a couple of grand for the electrics

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