Running a thermostatic mixer shower off of my old combi boiler

Discussion in 'Plumbers' Talk' started by Neeb, Dec 8, 2018.

  1. Neeb

    Neeb New Member

    I'm in a small flat with an old Biasi combi boiler. Currently I have an ancient electric shower that is absolutely pathetic - I have measured the flow rate at around 2.8 l/m of warmish water..

    I'm wondering how much better a shower I could expect from a thermostatic mixer fed by the combi boiler.

    A possible issue is that due to overheating problems when the central heating is on, the effective output of the boiler has been dialed back a bit, so it's not running on all cylinders. The boiler engineer who adjusted it mumbled something about t*rd polishing.. ;-)

    When I run the hot tap in the bathroom sink at maximum the flow rate is about 7 l/m (the flow rate of the cold tap is about 13-14 l/m). The temperature of the hot water at 7 l/m is about 40C when the cold water is around 8C (so I guess the boiler output is 7 l/m for a 32C rise?).

    This might seem obvious, but can I assume that the output in l/m from a thermostatic mixer shower will be much the same as from the hot tap in the bathroom, at least if the hot shower tap is fully on? In which case it would be 2.5 times as good as my current electric shower at least?

    To me this would seem like a massive improvement, but reading around I get the impression that 7 l/m at 40C wouldn't be considered a good shower by most people.. And I'm worried that the flow rate from the shower might be less than from the tap for some reason I'm not aware of.

    Or would I be better off with a really good new electric shower?
  2. Jimbo

    Jimbo Active Member

    Look at the watts - electric shower will be at most 11kW, your Combi is probably at least 25kW.
  3. Neeb

    Neeb New Member

    Thanks - yes, the boiler will be more powerful in theory but I’m not sure it’s delivering all of that power. Due to the overheating issues with the central heating the boiler engineer had to adjust it (he dialled it back so the flame height is less now). Works fine but the water is not as hot as it used to be. Thought if I just looked at the flow rate and temperature of the water coming out of the hot tap in the bathroom sink that might give me the best idea of what I can expect from a shower.
  4. Wayners

    Wayners Well-Known Member

    Just replace the electric shower with something better. Cheap enough and if you are a bit handy do it yourself. Plenty of info on net
  5. Neeb

    Neeb New Member

    If I was sure that I’d get at least as good a shower even with my current dodgy boiler than with the most powerful electric shower I’d still go for it though, because the boiler will almost certainly need to be replaced in the next 2 or 3 years. So in the long term I’d have a much better shower and in the meantime a decent one.. Just want to be sure that a mixer is not actually going to be worse than a good electric shower with my cr*p boiler..
  6. Wayners

    Wayners Well-Known Member

    I have a mixer and it's great with combi . 12lt per minute of hot water the boiler delivers. You need a decent flow rate. I think some combi boilers are 9lt per minute
  7. Allsorts

    Allsorts Well-Known Member

    Hi Neeb.

    Without question a combi boiler - even a dialled-back small one like yours - will give a FAR better shower than even the most powerful leccy shower (10.8kW?)

    There will be no comparison; your current shower is 'pathetic' and the new shower will be thoroughly enjoyable.

    Something to check first - what's the max flow rate of cold mains water from your kitchen tap?

    Your Biasi probably has an adjustable temp or output control on the front, and that's what your plumber dialled down? It obviously won't be able to deliver its full potential (lol) but it'll still be miles better than what you currently have.

    And then at some point you will need to replace your boiler so you can then choose a slightly more powerful one if you wish. I have 30kW combi and it gives a superb shower - I need to reduce the shower flow in most instances.

    You obviously couldn;t fit a huge boiler - a 38kW jobbie - in your flat as chances are you might not have enough rads for it to perform as it should, and also a fair chance that a new larger gas pipe will need running to it, but - when it comes to that time - have a good chat with at least two plumbers about your options.

    Modern combis are 'modulating' - ie they can adjust their own outputs to suit demand - so even a huge 38kW jobbie will be ok in most flats as it can dial the CH output down to 6 or 7kW if that is the only demand it has (half a dozen rads) but that full 38kW is available for an impressive DHW flow if it's needed (that's like 15 or 16 lpm of hot water!). However, that could be complete overkill and I think you'll find a modern 28kW combi will give you great results, and I know that 30kW certainly will.

    (For all these you would need to know that your incoming mains is good enough).

    What exact model Biasi do you have at the mo'?
  8. Neeb

    Neeb New Member

    Allsorts - that's brilliant, thanks!

    <<Something to check first - what's the max flow rate of cold mains water from your kitchen tap?>>

    Slightly weird thing is that the cold water flow rate from the kitchen tap is quite a lot less than from the bathroom sink cold tap. Maybe there's some sort of flow regulator on the kitchen one? The bathroom sink cold tap produces about 13.5 l/m. The kitchen sink cold tap is about 8.5 l/m. And in both rooms the maximum hot water flow rate is around 7 l/m. I have a digital thermometer and measured the hot water temperature at around 41C on full flow when the cold water was at 8C.

    <<What exact model Biasi do you have at the mo'?>>

    It's a Biasi 24S (47 970 06). When the plumber dialed it down it wasn't with the dial on the front, it was some sort of control inside the boiler. The problem as he described it was that when the central heating was running the water was coming back into the boiler at too high a temperature, so the boiler was cycling on the overheat stat rather than on the thermister. *Possibly* something to do with the pipework or a "blocked return". It would cycle fairly rapidly for about an hour until the temperature crept over 92C and the automatic cutout kicked in (the red pop-out button), even if the radiator temperature was set really low with the dial on the front. Now it will cycle properly on the thermister as long as the dial on the front is kept low. If the dial is set higher I think maybe it's cycling more on the overheat stat, but the temperature still manages to just about stay below 90C so it never cuts out. There's no problem at all with it overheating when the hot water is running and the heating isn't on, in fact running the hot tap seems to be a good way to cool down the boiler.. But because the boiler was been dialed down internally to keep it from overheating when the central heating is on it's not providing full power for the hot water either. The boiler engineer really knew what he was doing I think, some sort of boiler guru they brought in because they hadn't managed to diagnose the problem on previous visits.. :)
    Allsorts likes this.
  9. Allsorts

    Allsorts Well-Known Member

    Yes, your boiler guy sounds switched on! The problem with these older boilers is that they don't 'modulate' - when they fire up they run at full chat, so if the heat is not removed quickly enough they trip the overheat stat. Modern - 'modulating' - boilers adjust themselves like the gas ring on your hob - the burner is automatically fired up and down to suit demand.

    I'm guessing your Biasi is 24kW which is pretty small for a combi, tho' perfectly adequate for your existing situation since it really only supplies hot taps at the mo'. It'll still give a much better shower than your leccy shower; these give only around 4-5lpm in winter time even with the most powerful 10.8kW jobbies (which would almost certainly require you to run a new supply cable.) Your Biasi would - I would guess - give you are shower flow of around 10lpm (there will be some cold added to that 7lpm hot), so hugely better than what you currently have. And when you then come to replace the boiler - when it finally goes 'pop' (tho' I bet the 'tard will go on for years yet...) a 28kW (or even 30kW if you want) will give you stunning showers.

    Yes, the tap in your kitchen is possibly an 'eco' type which limits the flow. If your bathroom tap gives you 13.5lpm then it's obvious that your actual cold mains supply is at least this.

    If I were you I'd go ahead with the new thermo shower asap - which you will love. When your Biasi finally goes kaput, then get some good advice from at least two - ideally three - plumbers who will test your mains flow and pressure before recommending suitable boilers.

    Please keep us posted, Neeb, as these sorts of threads are really useful for many folk.
  10. Neeb

    Neeb New Member

    Again, brilliant, thanks!

    Great to know that when I eventually need a new boiler it will be able to modulate to cope with the conflicting demands of the dodgy heating pipes and the hot water..

    The Biasi does seem to have a sort of moody stubborness, it's caused me all sorts of problems but it never wants to die.. ;-)

    Finally - does the brand and model of a thermo mixer shower make any difference? Some of them (e.g. MIra) claim to have special technology to increase the flow rate, but I can't see how that's really possible from a combi boiler.. Very approximately how much would you expect to pay to have a mixer shower fitted and plumbed in the the hot pipes (in this case they would be run up from the nearby sink piping through a partition), including removal of the old electric shower and a bit of tiling (just patching the holes with supplied matching tiles)?
  11. ajohn

    ajohn Active Member

    You'll get a much better shower via the combi but may have problems if some one turns on a hot or cold tap when it's in use. A thermostatic mixer valve will help in trying to avoid heat changes if this happens but can't do anything about flow dropping.

    It might be wise to check that the shower you choose can mix 40C down to some other temperature. I suspect some expect hotter water than that. Actually as water coming out of a shower cools pretty quickly there is a chance it might be ok direct.

    Your combi might just need a new pump.

  12. Neeb

    Neeb New Member

    Thanks John - not bothered about taps being turned on when the shower is in use as it's just me in the flat most of the time.

    I assume a mixer shower could provide a full range of temperatures between cold and 40C?

    <edit - looking at a Mira Agile EV>
  13. The Teach

    The Teach Well-Known Member

    Your boiler can be range rated to suit the heating load. if the radiator heating load is 9kw the boiler can be electronically range rated down to prevent overheating.That adjustment will not affect the hot water gas pressure setting although the gas pressure setting during hot water demand needs to be checked & correct.

    there are many shower mixer valves on the market some cheap (for a reason),some reasonably priced.
    That will be the mira agile shower mixer valve,its a very different valve to all other bar mixers and it does not restrict internal water flow.
    Personally my preference is sequential shower mixer valves (current product is the mira element) they also do not restrict water flow and offer very good showering experience.
  14. Neeb

    Neeb New Member

    Great, thanks - any particular reason for preferring the sequential mixer valve over the type in the agile? Looks like the agile has seperate levers for temperature and on/off/flow and it's cheaper than the element.
  15. The Teach

    The Teach Well-Known Member

    sequential shower mixer valves work well with combination gas boilers,the shower user cannot adjust the already limited water flow.

    The agile has an adjustable flow control, usually with combination boilers this will be used in the full on position only. Reducing the flow can deactivate the boiler hot water flow switch.

    Mains unvented & pumped water systems would benefit from the variable flow control :)

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