Can you solve the mystery of the 70 degree hot water?? (unvented system)

[Matt Lucas]

Hello,

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

 

[Chuck]

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.

 

[justlead1]

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.

 

[Chalked]

Hi
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.

 

[cr0ft]

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

 

[king of pipes]

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

 

[Jones82]

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.

 

[Matt Lucas]

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

 

[cr0ft]

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.

 

[Matt Lucas]

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.

 

[Chuck]

Thank you so much for the replies! I am creating a checklist of things for him to tick off today based on your suggestions.

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?

 

[cr0ft]

Is the pipework being insulated by him or yourself?

 

[Matt Lucas]

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

 

[Chuck]

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

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.

 

[cr0ft]

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

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.

Keiran.

 

[king of pipes]

All the hot and cold and heating pipework needs insulating the data lable on the cylinder needs filling in and the benchmark section in the installation manual also, ask to see his gas safe or reconised registration on the reverse it will see if he is qualified to fit unvented on the whole looks tidy enough could of put the filling loop and a gauge somewhere so you dont need to keep getting into the loft space to check your system pressure and top it up cheers kop.

 

[ShaunCorbs]

Also is there a wall below the cylinder as personally I wouldn't want 210-230 kg on just two rafters (regs agree with me also)

 

[muggles]

I agree - that tank stand is far from adequate considering there's a quarter of a tonne on it

 

[Welder]

BTW as the insulation has to perform two functions, reduce heat loss, and reduce heat loss to prevent freezing, it needs to be nitrile based not the cheap grey Polyethylene stuff.

It is physically impossible for that to provide frost protection unless prefitted at factory. The insulation must be glued with no gaps whatsoever.

 

[Zippymark]

This may be a bit of a daft question but have you actually measured the temperature of the hot water? Is it definitely much higher than 55, as that's still hotter than you'd want to keep your hand under.