Bath mixer at different temp from other hot water?

Discussion in 'Plumbers' Talk' started by Offshore, Apr 30, 2015.

  1. Offshore

    Offshore Active Member

    I've just moved into a new build house and struggling to understand my central heating system so far. It is a Worcester Greenstar boiler and cylinder, dual zone control system. The hot water is pressurised. First question is, there is only one motorised valve in the system yet it seems to operate when both zone thermostats are operated - my understanding was that if the thermostat switches off then the desired temperature has been reached, the radiators are no longer required and the motorised valve shuts and hot water is diverted away from the radiators. Both thermostats seem to do this via the same valve so what happens if one of the zones is satisfied but the other isn't?
    Secondly, although the hot water at the taps and showers is sufficiently hot, the hot water at the bath mixer is much cooler, just above lukewarm I would say. I have tried various settings on the boiler and the cylinder thermostat and the water is so hot that you can hardly put your hand under it at the taps and showers but still significantly cooler at the bath mixer. I have returned the controls to more sensible levels but this illustrates the point. As far as I can see the mixer is not thermostatic (Bristan Nero range).
    It' s annoying me because my background is in industrial controls and I should be able to work this out.
    Thanks for any advice.
     
  2. Dave does Gas

    Dave does Gas Screwfix Select

    If it's a new build get the building company back, under no cuircumstances should you touch a thing. You have a warranty with the building company, if you fiddle you may invalidate the warranty.
    There may be a thermostatic mixer under the bath which has not been set up correctly.
     
  3. Hi Offshore.

    As Dave says - don't meddle too much!

    I suspect he is right about the mixer being thermostatic - very possibly a requirement now in new homes? (Possibly not actually a 'requirement', I don't know, but highly recommended I'd have thought to prevent accidental scalds in the bath - there have been some horrific cases...)

    If you have two heating zones, then I'm pretty sure you must also have two zone valves. The theory is that whichever zone control calls for heating, this will open that zone valve and then operate the boiler.

    Both zones calling = both valves open.

    It could be that the second zone valve is located close to where the thermostat is for that zone - upstairs bedroom eaves, for example?
     
  4. Offshore

    Offshore Active Member

    Thanks for the replies. Don't worry, no intentions of meddling other than trying a few different settings on the boiler or cylinder thermostat. The builder's plumber is due back to replace a thermostatic mixer in one of the showers so i'll ask him then.
    Regarding the motorised valves, I found them in a cupboard below the HW cylinder. I could hear them winding shut when I operated the stat and assumed it was coming from the valve next to the cylinder but expect it was just the noise being transmitted along the pipes making it sound closer. One mystery solved at least. Cheers.
     
  5. As the bath mixer is very likely to be thermostatic - or with a thermo unit mounted elsewhere - turning up the boiler DHW temp won't change anything.

    Yes - I guess there will be three valves in total - I forgot about the DHW one :)
     
  6. G&W Plumbing & Heating

    G&W Plumbing & Heating Active Member

    Is it a new build? Regs require a blending valve to be fitted under the bath unless your tap is thermostatic
     
  7. Offshore

    Offshore Active Member

    Yep, found the blending valve behind the bath panel. Gave it a wee tweak and water now at an acceptable temperature. So much for not meddling! The plumber is still replacing the shower though, it is definitely rubber ducked - steam or ice, nothing in between.
    Cheers.
     
  8. Plumberbish

    Plumberbish Active Member

    I don't get why we have to use blending valves on new builds...I appreciate why you'd use them in care homes etc but not in a regular persons house...let natural selection do its job or we'll have a country full of cotton wool wrapped numb nuts
     
  9. G&W Plumbing & Heating

    G&W Plumbing & Heating Active Member

    I had this with NHBC cos he didn't like me not fitting them on thermostat bath mixers, there was no allowance for it in the regs, I believe it's been ammended now, it's for legionares development according to NHBC
     

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