Problem with Thermostatic Shower

Discussion in 'Plumbers' Talk' started by ThomWat, Mar 16, 2007.

  1. ThomWat

    ThomWat New Member

    I recently have had my bathroom fitted with a Thermostatic shower. I have a combi-boiler supplying hot water to it.

    When I turn the shower on, with the flow set to full, temperature hot, water flows to the shower head, but then the flow fluctuates, caused by the boiler automatically switching the hot water on and off...

    I used to have a shower on mixer taps, and if I was to turn on the hot tap, water would come from the boiler, hot and it would not switch on and off as it does at the moment.

    I also have a new basin, with a mixer tap, which you can use to that seems to work OK.

    It seems that the shower cannot take hot water in a constant flow, and mediate the temperature of the water in anway.

    Does amyone know why this is happening and what can be done to resolve the problem?
     
  2. jdlplumb

    jdlplumb New Member

    seems to me that the valve is reducing the flow of hot water due to the set heat temp of the valve in turn shutting down the boiler(like a tap turning off). Look in the installation instructions for ways to adjust pre set temp of valve.Most are set at 43degs to prevent scalding. Check also the incoming temp of water from the boiler isn.t too high as this will have the same effect on the valve. The idea of the valve is to mix a proportion of hot to cold to a satisfactory user temp avoiding the risk of scalding. So the set up of these valves requires balanced pressures on both feeds and temps on hot feed to within manufacturers specs.
     
  3. > Does amyone know why this is happening




    ...yeah...that mixer valve aint compatible with a combi




    > and what can be done to resolve the problem?




    ...fit one that is compatible
     
  4. ThomWat

    ThomWat New Member

    The supplier spec says it is combi compatible.

    Perhaps it's not with mine... T482 are you confident that's the case? is it common for suppliers to say they are when they are in fact not.

    Architect Avus Thermostatic Bar Shower Mixer... anyone familiar with these?
     
  5. ThomWat

    ThomWat New Member

    JDL- cheers for your input...
     
  6. tgs

    tgs New Member

    Combis and thermo mixers are a potentially fraught combination.

    First, is the mixer installed correctly? It is vital that the hot and cold connections are each on the correct side. Usually left for the hot.

    Second, it is possible that the temperature of the hot water from the combi is either too high or too low, so try experimenting with the hot water thermostat control on the boiler.

    Third, are there any conditions when the mixer does work? For example if you turn down the flow. You might be trying to draw hot water from the combi more quickly than it can supply it. Did you get any little plastic flow limiters with the mixer?

    Fourth, the control action of the mixer may be too sudden for the commbi to cope with (your particular model). Mixer manufacturers quite often have different wax capsules for different applications. Is there a helpline number for your mixer?

    Fifth, it may be that your combi may not be working to spec. Check the hot water outflow from a hot tap. Normal operation from a combi is that you get an initial splurge of very hot water (particularly when the heating is on) followed by a cool flow followed by a gradual rise to temperature which then feels steady to the hand. If this is not the case it might be worthwhile ringing the boiler's helpline number.
     
  7. Jimbo

    Jimbo Screwfix Select

    The thermo mixer I've just got (and not plumbed in yet :-() says it is only compatible with fully modulating combis.
     
  8. tgs

    tgs New Member

    only compatible with fully modulating combis.

    Often the case. Check point 5 above, you just need your hand to feel the temperature, though if you have a meter with a temp function even better.

    If the boiler produces a steady temperature (within a degree variation) then it will probably be OK. If it is more or you can definitely feel the water temperature varying with your hand then it may be a problem.

    A lot of (so called) thermo mixers specially for combis are actually pressure balancing mixers with no thermo function. They rely, for their temperature output, on the steadiness of the hot water temperature. (just like a manual mixer) Hence the requirement.

    If you can't feel the hot water temperature varying with your hand it is likely (but not guaranteed) to be OK with the shower.
     
  9. plumbob squarepants

    plumbob squarepants New Member

    ThomWat
    I've seen this in the past, I think you,ll find the pipes have not not been flushed. Turn water off, undo nuts and check the filters remove any **** and put back togeather.
     
  10. ThomWat

    ThomWat New Member

    OK, thanks all for your help with this, will look into it further and report on what the problem turned out to be.
     
  11. DrainCock

    DrainCock New Member

    tqs,

    Very interesting read there. I was looking for a new mixer shower myself with a combi setup. All the triton showers I saw had a cavait that the water temp must not fluctuate more than 1.5 degrees. Now I understand why?

    Are there any thermostats that dont say this, and how do they work? I went to a shower store recently and they said that cifial low pressure thermostats are best?

    What I am trying to get at is that are there any thermostatic showers that dont worry about the temperature fluctuating and control it themselves. Or is there no such thing?
     
  12. tgs

    tgs New Member

    Some mixers are called 'pressure balancing', they have no thermostatic action and rely on the steadiness of the temperature produced by the combi.

    Other mixers such as the Peglar Mimo sold by SFD can be used with combis and I have successfully used this model with both modulating and non-modulating combis.

    There are three things to do:
    1. find a thermo mixer that specifically says it can be used with a combi.
    2. do the hand feel test on a hot tap
    3. ring the combi manufacturers helpline and ask if they recommend a thermo mixer with their combi.

    The basic problem with combis and thermo shower mixers is that you have two control systems in tandem. In some circumstances they react with each other to produce an effect callled 'hunting'. The output temperature from the shower varies widely from hot to cold and never settles.

    This problem is more likely to arise if the temperature produced by the combi varies more than a certain amount forcing the mixer to start altering the flow rates to keep the shower temperature constant. This alteration is fed back to the combi causing a bigger variation in temperature as the combi tries to counter the effect of the changed flow rate. This causes the mixer to produce an even greater change to compensate and so it goes on.
     
  13. DrainCock

    DrainCock New Member

    Thanks tqs.....

    So it looks like that if the Combi boiler cannot provide a constant temperature, you are in trouble. I will be getting the Vailiant 831 installed. Hopefully that is a decent enough boiler to deliver a constant temperature...

    Are the Cifial thermostats any good? Any advice on that?

    So if the thermostats say that it is Combi compatible and works on low pressure, it should be okay?
     
  14. tgs

    tgs New Member

    No experience of Cifial so can't say.
    One of the boilers I fitted the Mimo to was an older Vailiant and no problem.

    I think overall flow rate plays a part. If you are aiming for a shower of about 12 L/min then you need at least that from a tap (hot or cold). The boiler needs to have a flow rate at least around that. When you use the shower then you will have about half from the hot and the other half from the cold, but the mains ends up supplying both.

    Also, if you are getting a combi put in it needs priority access to the cold supply to get the best out of it. If the cold supply goes all round the house before meeting up with the combi then it will not give good results.

    I advised against using a combi for a shower more than once because of where it had been plumbed in on the cold feed. The successes (all I fitted) were all with the combi near the stop tap.
     
  15. DrainCock

    DrainCock New Member

    The cold water supply will go to the back of the house after it has gone past the kitchen sink. May have an issue with a combi boiler then if a shower was to be attached.

    One more question tqs, if you dont mind.

    Would it be much better to go for a Unvented Cyclinder. After looking at the diagrams (from Vailiant), it seems as though there is little to change with respect to the central heating pipes. I understand that the tanks upstairs will all go. Is this correct?

    This should give a much more constant supply to the shower correct? As the cold and hot water is mains pressure.
     
  16. DrainCock

    DrainCock New Member

    Also........

    At present have a vented cyclinder with a boiler downstairs. Do you think converting from this setup to a Vented Cyclinder would be cheaper than to a Combi Boiler. I'm not worried about the costs of the extra cyclinder, just the work that would be required from the plumber.....
     
  17. tgs

    tgs New Member

    Given the prices I've seen for unvented cylinders I don't think it will necessarily be cheaper.

    The thing to remember about using a combi or unvented or thermal store is that you are totally dependent on the mains pressure and flow rate for both the hot water and cold water supply. If this is found wanting then you will have poor hot and cold supplies.

    There is still something to be said for a couple of tankfuls of water in your house.

    Ultimately it is up to you and how much you want to pay. An unvented or thermal store setup is simpler from the shower perspective because the source of hot water is relatively constant and decent pressure. As long as you are not too ambitious with the shower there shouldn't be a problem even if the mains pressure is not brilliant.(just adequate)

    Having an unvented cylinder to replace a downstairs cylinder helps reduce a lot of problems which can be caused by the same setup in a vented cylinder. This can be quite difficult for showering.

    If you are thinking of an unvented cylinder then definitely look at thermal stores (Gledhill are a good place to start because they offer simple one box systems but there are lots of other firms to explore)

    The thermal store would have the advantage of making use of the simplest of boilers and would enhance the efficiency and performance of your heating system as well as providing hot water.

    Certainly, I find high pressure systems much easier to sort out than gravity. There is always an element of fingers crossed until you actually turn on the shower and it works as expected. But with combis I have found that as long as they pass my tests they work.
     

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