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Heating and cooling: shower valve compliance for solar tank variation?

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      CommentAuthorJustin
    • CommentTimeMay 2nd 2010
     
    Hi.
    My new solar thermal is working brilliantly but is (of course) providing a wide range of tank temperatures

    Whilst figuring out (as I ought to) the fitting of various TMV's round the house, I was hoping that my existing thermostatic shower valve would suffice at least for the shower. There seems to be the problem.

    Its a Hudson-Reed brand. It does not seem to be able to cope with the wide temperature range available now from the hot side. When the tank is coolish (say 46C, ie adequate for a shower), the shower valve lets through too much diluting cold. This may be because both hot and cold are pumped. I can starve the pump on the cold side in these conditions by turning down the supply valve. This allows the shower valve to work within it's range and provide a hot shower, but the pump complains, it is obviously labouring, and eventually tends to runs dry on the cold side.

    I have a low head (header tank in loft), perhaps only 1M, and it was clear when I installed my shower pump some years ago (before solar thermal) that the pump auto-start mechanism only just had sufficient pressure to switch the pump on via the shower valve as it is. I fear that adding a TMV ahead the shower thermostatic valve would reduce the head too much to allow the shower pump to start.

    So is it that my shower valve (HR brand) just doesn't have enough compliance to cope with a wide range -say 42..70C on the hot side? Are there better shower valves which can? Do I instead try to find a pump which will switch on with only 1M head and a TMV and thermostatic shower valve?

    Has anyone else had this? Is there a better solution?
    • CommentAuthorJulian
    • CommentTimeMay 2nd 2010
     
    Justin
    I have an ESBE mixing valve (fitted with solar thermal) on the hot from the solar cylinder and reccomend these. It means the cylinder can go up to 80 degrees but no danger of scalding. It is set at <60 degrees so still hot enough for washing up. I think these ought to be fitted as standard with solar thermal installations.
    • CommentAuthortony
    • CommentTimeMay 2nd 2010
     
    Definitely fit a good mixing valve to the outlet of the hot cylinder (after the vent) rack it down to 50C or less

    Biggest danger with solar hot water is scalding
    • CommentAuthorwookey
    • CommentTimeMay 2nd 2010
     
    The horne valves are highly recommended on the Navitron site (and the Horne tech person answers queries there from time to time). Useful discussion on limitations of TMVs:
    http://www.navitron.org.uk/forum/index.php/topic,9570.0.html
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      CommentAuthorJustin
    • CommentTimeMay 3rd 2010
     
    Thanks I'll take a look at those valves.
    My problem is the pressure loss introduced by their NRV's would subtract from my already (barely adequate) water head which is the on/off switching mechanism for my shower pump. I might end up with perfect water temp but shower pump unable to turn on..
    • CommentAuthorowlman
    • CommentTimeMay 3rd 2010
     
    Would not a negative head pump work? As tony and Julian suggested you really need to be looking at a good quality, well engineered, thermostatic valve, they are available but pricey when compared to bog standard stuff. I think I paid over £120 for one on my plate heat exchanger, some are even dearer. But what price safety.
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      CommentAuthorJustin
    • CommentTimeMay 3rd 2010
     
    Hi. I checked out the Navitron forum on that and the complaint seems to be that TMV's in general require to have ~12C across them, ie they REQUIRE to mix in order to work. Perhaps this explains the performance of my own shower valve which won't allow a hot shower (unless I mechanically starve the pump of cold), even though there is adequate nice hot water in the solar tank.

    The Horne H25 seems to have 5C minimum differential (Their technical data). I guess this is a huge improvement on 12C, but is there anything which will operate down to even lower differential? Perhaps I need to do this anyway, put up with 5C, and maybe then do away with my thermostatic shower valve in favour of a manual control.

    Negative head pump - now there's another thing to look at - thanks Owlman.
    • CommentAuthorwookey
    • CommentTimeMay 3rd 2010
     
    No, there was a long discussion about the minimum temp differential where Hannah from Horne tried to explain to us why it wasn't possible (due to the mechanical nature of TMVs) to build a TMV that closed the cold side entirely when the hot was barely hot enough. Personally I still didn't see whay it couldn't be done, but it definately isn't in anything you can actually buy. This is a huge disadvantage when you have a solar thermal system which often gets your water just about hot enough, but not 5-10C hotter then that. This is the main reason we have no TMV here. Perhaps electronic control is the only way to solve this (or more simply a manual bypass for borderline times).
    • CommentAuthorchuckey
    • CommentTimeMay 3rd 2010
     
    Justin it is bad practice to cock down the input to any pump, always cock down the output otherwise the pump will cavitate causing premature seal failure. If there is a possibility of the pump over pressurising itself or a fitting then a pressure relief valve should be installed across it.
    I have got a "head" of about two feet above my shower head and the flow is not enough to get the pump flow sensors working. dropping the shower head into the bath, then causes enough flow so the pump gets going, now as the head is about 7 feet (water level to pump inlet) every thing works great. I am presently building a really low flow detector. Prototype worked but I then broke it .:cry:
    Frank
    • CommentAuthorGavin_A
    • CommentTimeMay 4th 2010
     
    Posted By: wookeyNo, there was a long discussion about the minimum temp differential where Hannah from Horne tried to explain to us why it wasn't possible (due to the mechanical nature of TMVs) to build a TMV that closed the cold side entirely when the hot was barely hot enough. Personally I still didn't see whay it couldn't be done, but it definately isn't in anything you can actually buy. This is a huge disadvantage when you have a solar thermal system which often gets your water just about hot enough, but not 5-10C hotter then that. This is the main reason we have no TMV here. Perhaps electronic control is the only way to solve this (or more simply a manual bypass for borderline times).

    such things do exist, they're just called boiler TMV's, and not advertised for use within the actual hot water systems. definately not to be used as the only tmv on a solar system as they aren't fail safe, and could result in scalding water being fed through before the TMV kicked in properly. On a system where there is already a master TMV on the top of the tank, but someone wanted an additional tmv on a shower / bath or whatever, then this shouldn't be any more dangerous than, and realistically much safer than standard none tmv boiler/shower set ups.

    advanced water systems sell them from watts industries for about £30 or so, and IIRC, they do 2 temp ranges - 49-79deg, and 39-something.

    can't vouch for how they'd work as shower tmv's, but I've been using a couple for other purposes for a few months now and they seem to work fine, coped ok with being fed boiling water for a few minutes to test their robustness etc.
    •  
      CommentAuthorJustin
    • CommentTimeMay 5th 2010
     
    I'll check out those boiler TMV's then.
    Alternatively I'll consider fitting an ordinary TMV (say the Horne) at the tank with a manual bypass tap. Not very clever if someone leaves the tap open and we have a very sunny day?.. Hmm.

    And Chuckey_ good point about the pump input. Yes it is cavitating when I overdo it. Restricting the output gives a lukewarm shower though. - Can't win!

    Rgds
    •  
      CommentAuthorJustin
    • CommentTimeMay 24th 2010
     
    I removed the input restriction and added a output side restrictor (screwdriver gate valve) to the pump output on the cold side. So far have managed to leave it in just one setting for tank variation between ~45 and 76C. I seem to be winning.Thanks.

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