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Discuss Can you solve the mystery of the 70 degree hot water?? (unvented system) in the Plumbing Forum area at

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  1. Matt Lucas

    Matt Lucas New Member


    We have just had a complete overhaul of our heating and hot water.

    We previous had a conventional (?) system (two tanks in the loft, copper tank in the airing cupboard) heated by a Worcester Bosch 18ri. Controlled by a 3rd gen Nest.

    We have changed to an unvented tank (Kingspan) system heated by the very same worcester bosch 18ri. We have the same nest 3rd gen running it.

    The plumber took the job with full disclosure of what was there and that we still wanted the nest to run things but when it got to the final stage of wiring it all up he began giving lots of excuses as to why he couldnt incorporate the nest, mainly that he didnt know how to?? He eventually did and it calls for hot water and heating as it should do BUT...

    Since the change we get scolding hot water. The plumber has been back a few times and checked a few things but to no avail, I really don't want another unsuccessful visit!

    Can you help arm me with what it might before he comes back tomorrow??

    Here is what has been checked/replaced so far...

    Most obvious thing was tank stat so he replaced it... and plumber assures me it is "closing and opening valves as it should" (it is set at 55)

    He mentioned that the immersion may be constantly on (even when switched off at the spur) and interfering. I have turned the boiler off at the spur, run down the hot until cold and waited for 2 hours (with the immersion switched off) and no hot water came through so it isnt the immersion. (I also switched the immersion on after to test and it worked fine)

    With the immersion off I turned the boiler on (made sure heating and hot water was switched off on the Nest) to see if it is heating up the water without being asked. it isnt.

    I turned the heating on without hot water to see if that is interfering with the hot water. It isnt.

    It is literally just when we turn the hot water on.

    Help! Thank you in advance :)
  2. Chuck

    Chuck Active Member

    Once it's all working, the tank stat should be set to 60°C not 55°C and you should leave the electric immersion heater off and use gas to heat water unless you have a boiler breakdown. (Electricity costs five or six times what gas does to heat water.)

    If the system is correctly installed there are two possibilities. Either (a) the thermostat is faulty or incorrectly wired and is not allowing the zone valve that feeds the cylinder coil to close. Or (b) the zone valve that feeds the cylinder coil is faulty. They do sometimes stick open. Some models have manual overrides that lock them open, which is useful when filling a new system, and that people sometimes forget to release.

    Can you provide some photos of the setup?

    A competent person should be able to diagnose the problem in about 15 minutes flat from first sight. If your installer wasn't confident about including Nest, which is not exactly rocket science, it might be prudent to get an electrician to check their wiring and controls and get it sorted properly.

    If it's true that the thermostat is 'closing and opening valves as it should' then it's likely that it's not operating at 55°C. You can remove it and test it with a multimeter, thermometer and a jug of hot water. I'm not sure why the tank stat would open 'valves' (plural), which makes me suspicious about the whole installation.

    I'm pretty hard nosed about this sort of thing. A competent tradesman may make a mistake occasionally, but it shouldn't be a dangerous mistake and they should only need one go to fix it. If they need more than one try to fix something as simple as a domestic hot water system, assume you're dealing with a cowboy and get someone else in.
    Last edited: May 22, 2017
  3. justlead1

    justlead1 Trusted Plumber GSR

    It worth mentioning that the position the thermostat is fitted on the cylinder can have an effect. The stratification that takes place when heating a cylinder of water may be the cause of the over heat. (it hotter at top than bottom)
    Take the temp of cylinder at the point the stat is fitted. Then at the top of cylinder (draw off point) this may identify reason for difference.
  4. Chalked

    Chalked Plumber GSR

    Do you have a three port motorised valve near the cylinder or two port.
    Would be easier if you could post a picture of the cylinder.
  5. cr0ft

    cr0ft Trusted Plumber GSR

    Nest works fine with a typical system boiler/unvented cylinder setup. We've installed a few systems like that with either Nest/Hive.
  6. king of pipes

    king of pipes Trusted Plumber GSR

    I have had overheat problems with the kingspan cylinders the thermostat pocket is rather large and the bulb of the stat is a a bit small for the size of the pocket causing the tempreture to creep i wedged a piece of copper strip to keep the bulb pushed against the wall of the thermostat pocket and turned stat to 45 degrees c just try turning the stat lower run a bath or have a shower then test the water tempreture again after reheating has finished , quick fix would be turn your boiller tempreture down to 60 degrees c till you find the problem cheers kop
  7. Jones82

    Jones82 GSR

    I've had one recently where the cylinder stat wires were the wrong way round meaning the stat was constantly calling for heat. This can be tested with a multi meter as can the immersion heater.
  8. Matt Lucas

    Matt Lucas New Member

    Thank you so much for the replies! I am creating a checklist of things for him to tick off today based on your suggestions. Here are some pics 20170523_080532.jpg 20170523_080452.jpg 20170523_080417.jpg 20170523_080402.jpg
  9. cr0ft

    cr0ft Trusted Plumber GSR

    Doesn't look terrible to be honest although I don't like seeing electrical boxes such as the NEST controller underneath pipework really.

    It just needs someone with a good understanding of heating controls electrics (not something all heating engineers have sadly) and how to faultfind them.

    Shouldn't be a hard one to faultfind though for a competent heating installer.
  10. Matt Lucas

    Matt Lucas New Member

    That's what makes me think it is a wiring issue. He was very proud of the installation and I am impressed with how neat it was and him personally but as soon as we got to the wiring part it all went wrong and the confidence left the building.
  11. Chuck

    Chuck Active Member

    From the photos, it looks as though the data label on the tank hasn't been filled in. That is not a good sign IMO. Is your plumber G3 qualified?
  12. cr0ft

    cr0ft Trusted Plumber GSR

    Is the pipework being insulated by him or yourself?
  13. Matt Lucas

    Matt Lucas New Member

    I take he should be filling it in then? Is it a statutory requirement to fill it in and be G3 certified (just so I know when mentioning it) Should they be insulated then? I'll get on to him
  14. Chuck

    Chuck Active Member

    Only people with a current G3 competency qualification can work on unvented systems. Competency is normally demonstrated membership of a recognised scheme (Gas Safe is the best-known but there are others) and a scheme-issued ID card that explicitly confirms the type of work covered.

    Filling in the data label is an installation requirement. Without it I think you'll have difficulty making warranty claims or proving the system was installed by a competent person.

    Legal responsibility for the safety of such systems and their registration with the local authority building control dept rests with the owner, but this is normally discharged if the system was installed by and has been maintained by an appropriate scheme member.

    As others have commented, the pipework looks okay, but a job isn't finished until it's been proved to be working correctly and the paperwork is done. Your installer probably just needs to get the electrician they normally work with in to sort the controls out.
    Last edited: May 23, 2017
  15. cr0ft

    cr0ft Trusted Plumber GSR

    It looks like the install has been fitted in an uninsulated space, i.e. a loft. If this is the case then all pipes containing water must be insulated yes. Did they discuss this with you before hand? We often do with our customers as some people are happy to fit the installation themselves to save them being charged plumbing rates for what's a very easy job.

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