Hot water tank only getting to 46deg even with boiler on constant at 75deg??

[-FRED-]

As per title. British Gas 430i boiler feeding into a Santon PP300B hot water tank, it's literally on 24/7 but is very poor at actually heating the tank. Initially thought it was a faulty thermostat that wasn't calling for heat, but even with the boiler on constant it has the same issue.

It's been like this for a while, not something that's happened suddenly, which would obviously point to a component failure.

The 46deg is taken using an IR thermometer from the three nearest taps to the tank, they are all mixer taps. If there's a better way to test the tank temp, let me know and I'll do so! The 75deg is just what's displayed on the boiler.

Any thoughts? Thanks in advance.

 

[Stuckinarut]

I would use a digital metal probe thermometer. Is there a thermostatic mixer off the cylinder? What is the cylinder thermostat set to?

 

[-FRED-]

Probe thermo - don't own one but will purchase if necessary to get to the bottom of it
Thermostatic mixer - I'm almost positive there isn't one, I'll double check in about an hour when I'm home
Cylinder thermostat - unfortunately it doesn't have degree markings on it, it's just marked 1 through to 5. It was set to 4 this morning, I moved it up to 5 and a couple of hours later there was no difference to the tap temp. However, the hot water is on constant so I don't think the thermostat should be part of the equation here? Correct me if I'm wrong, I'm by no means a professional!

 

[-FRED-]

Oh and thanks for the reply by the way @Stuckinarut

 

[Stuckinarut]

Yes the thermostat absolutely matters, what it would do if set to constantly on is maintain the temperature set by the thermostat, so if 45 degrees then that’s all it will heat to, whether on for an hour or all day. Make sure the cylinder thermostat is making contact with the metal part of the cylinder or all the way in if unvented.

 

[Madrab]

If it's the HS Premier Plus 300L then it has a large coil rated @ ~ 18Kw so it shouldn't be that, only other thing that would cause a problem like that would be the stat, setting or faulty.

Run the HW till it's as hot as it gets then fill a cup and use a thermometer, an IR will struggle with running water.

 

[-FRED-]

Thanks for the replies. Can confirm there is no thermostatic mixer anywhere on the system. Also tried running a cup of water and measuring with a conventional thermometer, it actually comes out just under 40 - I can definitely feel it's colder than straight out the tap so I'm assuming the 46 is accurate and the discrepancy is just heat loss into the cup/air etc.

So I'm currently thinking it's a faulty thermostat. One other thing I noticed on another forum is to slowly turn it down to a lower temp and see at what point it clicks off - I've tried this and it's at about 2.5. It clicks back on at about 3ish. Now if the thermostat was doing it's job correctly, would it not click off just slightly shy of whatever it was set at? Hopefully that makes sense, not sure how to explain my logic...

I'm realising I know less about CH systems than I thought

 

[Harry Bloomfield]

I would use a digital metal probe thermometer. Is there a thermostatic mixer off the cylinder? What is the cylinder thermostat set to?

Place your hand, on the pipe, coming out of the very top of the cylinder, whilst someone runs the hot water. You should find that is very hot, if that cylinder is getting up to temperature.

A mixer, is a device which blends hot and cold water, so that water is delivered out of the taps, at a lower/safe temperature. It will have three pipes - hot in, cold in, then the output, with an adjuster to set the output temperature.

 

[-FRED-]

Place your hand, on the pipe, coming out of the very top of the cylinder, whilst someone runs the hot water. You should find that is very hot, if that cylinder is getting up to temperature.

Already done that, and it's pretty warm but by no means hot. You can keep your hand on it as long as you like without it getting uncomfortable.

 

[-FRED-]

Ok so this morning's update, water is now at about 30deg. I'm now wondering if turning the stat anticlockwise from 4 to 5 makes it colder, so I've turned it clockwise round to 2 to see what happens...will report back when I get home from work

 

[kidgreen61]

Turn the cylinder stat right up and see what happens

 

[-FRED-]

Turn the cylinder stat right up and see what happens

That was what I did first, by turning it from 4 to 5. That is anticlockwise, and you can see from the manual that the higher number equates to a higher temperature, even though it's anticlockwise. However as that appears to have made it colder I wonder if the stat has been wired wrong or something, so I've turned it clockwise round to 2 to see if that makes it hotter or not

 

[kidgreen61]

You can check the thermostat by watching the zone valve as you alter the stat, it should close when stat turned down (assuming cylinder is warm), and open when turned up. There's every chance it's been wired wrongly too.

 

[Madrab]

Can I suggest you might want to get someone in that is qualified to work on unvented cylinders? The stat needs properly tested and if not that then potentially another safety component. The temp controls are one of the cylinders safety features.

There isn't really anything on an unvented that is DIY'able, apart from re-charging the bubble on some units.

 

[-FRED-]

so I've turned it clockwise round to 2 to see if that makes it hotter or not

This resulted in it going back up to 49, so I then turned it all the way round to 1, and that basically made no difference. I'm pretty definite now that it's a faulty thermostat, unless anyone has any other comments or ideas?