Boiler Pump Overrun - Where does the Heat Go ?

Discussion in 'Plumbers' Talk' started by MikeM2, Aug 24, 2008.

  1. MikeM2

    MikeM2 New Member

    My ancient cast iron boiler is being replaced with a shiny new Worcester Greenstar 24Ri. It will drive an S-Plan system with separate valves for Heating and Hot water. My CORGI Installer has correctly advised me that an automatic bypass valve should be fitted to the system.

    I understand that this bypass valve together with the in-built pump overrun allows boiler hot water to continue to be be circulated and thus cooled when both Hot Water & Heating valves are closed after the Demand signal is removed.

    My question is where does the heat get dissipated as it is no longer diverted to either the radiators or the
    hot water coil ? I assume it just circulates in the pipework between the boiler the and pump ?

    It occurs to me that it would be better to perhaps usefully 'dump' this excess heat into the hot water coil ?

    A simple bit of 'electronics' could detect pump overun, open the HW valve and also remove the Demand signal until the boiler has cooled sufficiently.

    Does anyone see any 'snags' in this scheme ? It will save me the cost and labour of installing a bypass valve
    and more importantly use this precious heat.
  2. ripitout

    ripitout New Member

    i understand where your coming from mike,but the boiler must have an escape path to lose its residual heat once it has shut down,the heat is lost through the flow&return
    pipework between the boiler and bypass.


    part l specifies that the primary pipework between the boiler and cylinder must also be lagged,so your not really wasting any energy.

    i cant see the point of dumping residual heat into lagged pipework myself,but who knows im sure i will be corrected on this one.
  3. rome60

    rome60 New Member

    Mike,the pump overrun is to protect the boiler heat exchanger & must have a automatic bye pass or separate curcuit to a towel rail in the bathroom.Without either the boiler will lock out on the high limit stat.;)
  4. Ryluer

    Ryluer Member

    The simplest method is to fit a differential by-pass around the zone valve you want to dump the heat into.
    No need for electrical modifications.
  5. ripitout

    ripitout New Member

    Mike,the pump overrun is to protect the boiler heat exchanger & must have a automatic bye pass or separate curcuit to a towel rail in the bathroom.Without either the boiler will lock out on the high limit stat.;)
    _

    the towel rail in my bathroom is normally draped with towels,ie its lagged!! it also is fitted with a trv,i wouldn't call that an effective bypass.
  6. ripitout

    ripitout New Member

    No need for electrical modifications.

    <u>_________________________________</u>

    yes good point,but if its on the hw zone valve and the cylinder return has a ls gatevalve on it screwed right down to achieve system balance it wouldn't be a effective bypass.
  7. Ryluer

    Ryluer Member

    Then remove the gate valve !.
  8. ripitout

    ripitout New Member

    Then remove the gate valve !.
    <u>_________________________________</u>

    yeah fine,no problem..... and lose the flow to the rads in the winter when the hw is calling!!!!
  9. Ryluer

    Ryluer Member

    Thats no big deal.
    Its called hot water PRIORTY !.
  10. ripitout

    ripitout New Member

    Its called hot water PRIORITY !.
    <u>_________________________________</u>


    the priority in my house is, that i don't freeze mineballsoff when the kids & mrs are in the bathroom for hours on end.
  11. Our system has two towel rails, both fed before the zone valves which effectively serves the same purpose as a bypass as long as at least one of the towel rails has lockshield valves fitted (which one of ours has).

    At least with this setup any surplus heat is more likely to be doing some good by drying the towels.
  12. simon@HWCH

    simon@HWCH New Member

    This is a good question and the answer is, in general, the heat is wasted.

    The sedbuk testing routines do not measure this accurately enough in my view.

    Some enlightened boiler manufacturers do not need pump overrun. Buderus, for instance, with their 24R. But this has just been dropped by....Worcester in the UK, who handled the Buderus domestic range in the UK; Buderus are the largest boiler manufacturer in the Bosch Group.

    The Kidd VHE condensing boiler (launched in 1981) also has no pump overrun, being designed to waste as little energy as possible.

    In contrast, most modern euroboxes seem designed to waste heat, having no insulation in the casings and intentionally discharging latent heat every time they stop.

    To those of you arguing that no heat is lost in overrun mode, you are missing the point. The pump overrun is specifically designed to discharge latent heat from the boiler casing and prevent damage. If no heat was discharged there would be no reason for pump overrun.

    Whether or not you discharge the overrun into a cylinder is not the issue, the boiler had been stopped because the demand was satisfied.
  13. ripitout

    ripitout New Member

    To those of you arguing that no heat is lost in overrun mode, you are missing the point. The pump overrun is specifically designed to discharge latent heat from the boiler casing and prevent damage. If no heat was discharged there would be no reason for pump overrun.

    rubbish,its not the casing that starts to crackle and bang when the over run fails is it!!!!.
  14. rome60

    rome60 New Member

    If the overrun fails ie-pump failure boiler locks out on high limit stat.Also a differential bye pass as mentioned by myself & others is required on all modern systems.The use of a towel rail or small rad can be used as a bye pass,but would need two l/shield valves,this still would not effect the system interlock.;)
  15. simon@HWCH

    simon@HWCH New Member

    I am amused by the last two responses.

    If the pump overrun is not to prevent excessive heat build up in the boiler - what exactly is it for?

    Do Rome60 et al believe the boiler manufacturers fit pump overrun for fun? When you talk about bypasses do you know what they are for, you seem to be making assumptions without giving any justification?

    If you fitted domestic and commercial boilers for a living you would understand that not all boilers need pump overrun, only the more fragile mass market 'built down to a price, it must fit in a kitchen cupboard' stuff.

    And that is usually because the heat exchanger and related componentry inside the box would be damaged by the latent heat build up. The more robust condensing boilers are designed to cope with this and thus have no pump overrun requirement in the design.
  16. ripitout

    ripitout New Member

    And that is usually because the heat exchanger and related componentry inside the box would be damaged by the latent heat build up. The more robust condensing boilers are designed to cope with this and thus have no pump overrun requirement in the design.
    <u>_________________________________</u>



    and thats another load of clap trap,
    its the hex that requires protecting!not the components within the boiler casing,all of the vunerable components are situated outside the combustion chamber.
  17. rome60

    rome60 New Member

    Ripitout,agree with your last post,& the post we are discussing is about boilers WITH a pump overrun not boilers without one.When a fully pumped system shuts down on the timer the boiler may be at max temperature that water in the exchanger is dispersed through the bye pass & circulated until it cools enough for the stat in the boiler to shut down the pump.;)
  18. ripitout

    ripitout New Member

    Ripitout,agree with your last post,& the post we are discussing is about boilers WITH a pump overrun not boilers without one.When a fully pumped system shuts down on the timer the boiler may be at max temperature that water in the exchanger is dispersed through the bye pass & circulated until it cools enough for the stat in the boiler to shut down the pump.;)
    <u>_________________________________</u>

    EXACTLY!
  19. rome60

    rome60 New Member

    Simon,for your information don"t use the word IF,I have installed boilers from domestic,commercial,Industrial on all types of fuel.And can guarantee i know what i am doing,so i don"t like your comment.
  20. simon@HWCH

    simon@HWCH New Member

    Can I see the guarantee?

    The original post asked why there was a pump overrun and how the heat could be used that seemed to be intentionally wasted.

    I explained that the heat was intentionally wasted in boilers like the 24Ri (and many others) because the boiler internals could not be relied upon to survive reliably with the latent heat inside.

    Posting inane and ignorant posts that only the heat exchanger is being protected is rather silly. Most people with the most basic electrical knowledge will know that all electronic components are rated with a MTBF for certain temperature ranges. Outside the temperature range the components beome unstable and their values change. Ask Potterton about their Suprima.

    The pump overrun is protect all vulnerable components in the boiler, both hydraulic and electronic.

    If you buy a more robust design, it will not need pump overrun.

    Like most oil condensers, as I am sure the previous poster will now remember with his multifuel experience. And some gas condensers, I can provide an actual list rather than a 'guarantee'.

Share This Page