Bath fluctuating hot water

[robbo83]

Hi,

We've got a problem of fluctuating hot water from our bath mixer where the water is hot and then a minute or so later will go cold and then few mins later will go hot for a bit and then cold again. No other taps in the house have this issue.

I took the mixer apart and checked the cartridge and it was all clean. The bathroom is less than a year old so wasn't expecting it to be that.

We had a plumber come out today to look at it. While he was running his tests on the boiler he had the central heating on and when we then checked the bath it was fine, no fluctuating hot water so he assumed he had fixed it (even though he couldn't really explain how).

Checked it again this evening and the problem returned. So I turned the CH on as a test and the problem didn't happen, hot water from the bath is constant.

Can anyone advise why it would be fine when the CH is on but if only the DHW is on then it fluctuates?

Thanks!

 

[Madrab]

Sound like it could be the same issue that happens sometimes, when a shower will run hot and cold. Does the boiler cycle on and off when it's doing it and the CH isn't on?

 

[Johntheo5]

If its a combi boiler then there should only be cold water when the CH is on, if from a cylinder via a system boiler then CH on should have no effect.

 

[robbo83]

Sound like it could be the same issue that happens sometimes, when a shower will run hot and cold. Does the boiler cycle on and off when it's doing it and the CH isn't on?

Well to my amazement I have tried multiple times today and haven't experienced the issue at all. Really confused. From what I remember the boiler was cutting out when the issue was happening. If I recall correctly the flow temp in the heat exchanger was getting above 90° and would then drop down. But I wish I could double check this. I am not confident the issue is permanently resolved so will report back if it starts again.

Still trying to figure how having the CH would have had an effect. Is it possible there was crap in the boiler side of the plate heat exchanger and having the CH on has helped flush it through?

If its a combi boiler then there should only be cold water when the CH is on, if from a cylinder via a system boiler then CH on should have no effect.

Sorry not sure what you mean. On a combi boiler if the CH is on and you turn a hot tap on you will get hot water as DHW has priority over CH.

 

[Johntheo5]

Correct, with both CH and HW OFF open a hot tap to give a small flow of say 1 LPM but not enough to fire the boiler, then fire the boiler with a CH demand and see if the hot tap still runs cold.

 

[Johntheo5]

Still trying to figure how having the CH would have had an effect. Is it possible there was crap in the boiler side of the plate heat exchanger and having the CH on has helped flush it through?

At least some Vaillant boilers like the Ecotec 831 have a filter on the primary inlet to the PHex

 

[robbo83]

Correct, with both CH and HW OFF open a hot tap to give a small flow of say 1 LPM but not enough to fire the boiler, then fire the boiler with a CH demand and see if the hot tap still runs cold.

With the CH heating on the hot tap is fine. The issue was when the CH was off, then the temperature of the hot tap would fluctuate.

 

[robbo83]

So the issue has started again and I managed to observe what the boiler does while the mrs checks the temp at the bath tap.

I recorded it with a video showing option i9 from the service menu which is the temp from the main heat exchanger and uploaded it to you youtube below:

The water is hot at the start of the video and then at 33secs in you can hear the boiler click off and the waters stop circulating inside but the light on the front is still on. It then clicks on and off a few times over the following 30secs and then stays on but the temp drops from 88° down to 56°. The temp then increases again up to 78°-80° and the water is hot again at the end of the video. You can hear the mrs in the background saying when the water is hot or cold.

If anyone can help me figure out what this might be it would be greatly appreciated!

 

[terryplumb]

Are you certain that no other taps suffer the same as bath tap ?
When the bath tap is running with the problem ,try other hot taps at that time. If they are
Ok and bath tap isnt ,that would be indicative of an issue with the bath tap. Is it a thermostatic mixer tap ? Show us a pic of it.

 

[robbo83]

Below is a photo of the mixer. It is a Grohe 33963000 Concealed Mixer. It's not thermostatic. I took it all apart and there wasn't much crap in there just a few tiny bits of black stuff.

 

[Johntheo5]

If you have a HW tap only, (no mixer) can you open that and see if the boiler behaves differently?

 

[robbo83]

All the taps in the house are mixers unfortunately.

If I run the basin tap (mixer) in the same bathroom it's fine, the boiler remains on throughout and the temp in the heat exchanger remains constant between 67°-68°.

Below is a video of the boiler with basin mixer on hot. Apologies at the start the display wasn't lit up.

 

[Johntheo5]

Does the concealed mixer do both the bath and shower. since its non thermostatic then possibly the cold water pressure is higher than boiler HW pressure and possibly reducing its flow rate although it would have to restrict the HW flowrate to a very unlikely < 1.5LPM to cause the DHW temp to rise to 80C assuming the boiler is modulating down to say 6kw??, if for some reason the boiler isn't modulating then the flowrate would only have to fall to 8LPM to avoid reaching 80C.
There should be non return valves installed at the mixer hot & cold supplies.

 

[terryplumb]

If you run both basin hot and The mixer at the same time ,does boiler then give constant temperature ?

 

[robbo83]

Does the concealed mixer do both the bath and shower. since its non thermostatic then possibly the cold water pressure is higher than boiler HW pressure and possibly reducing its flow rate although it would have to restrict the HW flowrate to a very unlikely < 1.5LPM to cause the DHW temp to rise to 80C assuming the boiler is modulating down to say 6kw??, if for some reason the boiler isn't modulating then the flowrate would only have to fall to 8LPM to avoid reaching 80C.
There should be non return valves installed at the mixer hot & cold supplies.

The concealed mixer does the bath only, there is a separate thermostatic mixer at the other end of the bath for the shower. We do have high mains pressure (nearly 8 bar if I recall) but we had a PRV fitted which is set to 3 bar which I believe is normal? This is higher than the 1bar on the boiler but it was working fine in the past.

If you run both basin hot and The mixer at the same time ,does boiler then give constant temperature ?

I just did a test and with both basin hot and mixer at the same time and got the same issue, the water went cold on both after a min or so and the boiler behaves as it does when only the bath mixer is on i.e. gets to 88° and then starts cycling on and off and then temp drops to 56°.