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Discuss Thermal Store “Boiler-Pump” query in the Central Heating Forum area at

  1. Retiredbri

    Retiredbri New Member

    Hi, I'm a newbie and need some advice please.
    My 4 bed house has gas central heating with the Regular boiler (BG 330 set to 18Kw output) located in the garage. The boiler is connected directly to the header tank (vented system) and is also connected via a dedicated system pump to a thermal store all located in the upstairs airing cupboard. The “boiler-thermal store circuit” is controlled by a timer and thermal store thermostat.

    The boiler temperature is set to maximum (80deg) and thermal store stat temperature is set to 75deg. The store pump is controlled by the boiler circuit to deal with the overrun. No bye-pass valve is needed because there a never any restrictions to the flow. The “boiler-pump” work fine but the boiler is 10 years old and “creaking” (fan probably stiff) and the boiler-store pump is noisy.
    A separate pump controlled by the room stat and timer uses the thermal store water for the central heating (13 rads).
    ALL hot water is provided by mains pressure water being fed into the store heat exchanger (coil internal to the store) with the output mixed with cold water to achieve the safe temperature for the 2 bathrooms plus shower and all basins and downstairs sinks.
    I want to replace the boiler and pump (next summer if the present system keeps going) and before I do, can you guide me please (as a householder):
    I’m reasonably happy to use a new Greenstar 18Ri regular boiler.
    I would like to use an Alpha L pump as being a quieter and more efficient than a standard pump. If so, what would it be the setting to get 18kw peak out of the boiler but only 6-8kw for most or the winter?
    I am very pleased with system, the house is quick to warm up, the hot water is plentiful the heating costs lower than our previous 3 bedded house so want to stay with the store.
    So, why I hear you ask do I not seek local advice? Well, over the 5 years we have lived here, one engineer said “never seen that type before, it can’t possible work”. Two engineers said that they would fit a Combi. One said that thermal store should not have been fitted in the first place when the house was built in 1991! Only one (who has moved away) understood the design and sizing of the parts.
    Looking forward to your advice please, regards Retiredbri
  2. gmartine

    gmartine GSR

    Pictures would be very helpful and out of curiosity.

    You need to look at the pump curves to decide what speed but dont forget modern condensing boilers can modulate and the WB Ri series modulates from memory at 3:1.
  3. Retiredbri

    Retiredbri New Member

    Thanks. I've attached a schematic (acknowldegement Gedhill). Note: "15" is the Store Timer (the pump overun timer is part of the WB boiler). Pump "9" is the one in question. Vessel "3" is integral. schematic heating.jpg . Regarding the pump curves, this is where I'm stumped. What flow is needed to remove the 18kw from a WB boiler?

    Last edited: Dec 16, 2018
  4. gmartine

    gmartine GSR

    Whoever installs the boiler should be able to look at your chosen pump curves assess your system's performance and decide.
  5. Retiredbri

    Retiredbri New Member

    Yes, but!
    It's the chicken and egg situation. I don't yet know who to go to (and from previous experience), there are not many Thermal Store systems around and, it seems, a dislike for ALPHA 2 pumps).
    My neighbour has a similar system in the same size house and has an "ALPHA 2 L15-60 130" with setting "fixed speed 2" and although it is a lot quieter than my old pump, it is not very quiet (low frequency vibration). I do not know if this is the right pump and right setting and as quiet as it can be . Hence the question "what pump type/size to be able to get the flow rate for 18kw" with a WB 18Ri boiler, 22mm pipework (about 16 metre total length) and a MagnaClean filter .
    Hope you can help.
  6. gmartine

    gmartine GSR

    Flow rate of 18kw?? You need alot more information than that and of the correct sort.

    Ask your neighbour for their engineer's details.
    Last edited: Dec 16, 2018
  7. chris watkins

    chris watkins Plumber

    The flow rate for 18kw is around 0.215 kg/sec based on a temperature difference of 20deg between the Flow & Return, this is what the new boiler will be looking for.
    As there are no valves opening or closing on this circuit you can use the fix speed closest to this flow rate.

    Nothing wrong with Thermal Stores despite what some may say & not rocket science, you need to find a better quality of engineer.
    Last edited: Dec 16, 2018
  8. Retiredbri

    Retiredbri New Member

    I can see that is about 750 l/hr and is well within the range of the ALPHA 2 L15-60 130 pump. I need to get a slot booked after the winter rush with next door's engineer.
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