Bath fluctuating hot water

[terryplumb]

Sounds like an issue with the bath mixer valve. If you turn the handle on it to hot only ,no cold mixing, what's the result ?

 

[Johntheo5]

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.



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°.

There must be another PRV set to 1bar at the boiler then except you are looking at the system pressure which would be quite normal at 1.0 to 1.5bar with a fully hot CH system. If it is mains reduced 1bar then there is really no advantage in this as any mixing will still take place at 1bar but ideally both pressures should be equal to help prevent the sort of problems you have, it would be more normal IMO to have one PRV set to 3bar supplying all the water needs.

 

[robbo83]

Sounds like an issue with the bath mixer valve. If you turn the handle on it to hot only ,no cold mixing, what's the result ?

All the tests I have done are with the bath mixer on full hot, no cold. When I took the mixer apart and checked the cartridge it all seemed fine, it was closing the cold inlet when on full hot and vice versa, can't really see what could go wrong with it. Anything else I can check on the mixer?

There must be another PRV set to 1bar at the boiler then except you are looking at the system pressure which would be quite normal at 1.0 to 1.5bar with a fully hot CH system. If it is mains reduced 1bar then there is really no advantage in this as any mixing will still take place at 1bar but ideally both pressures should be equal to help prevent the sort of problems you have, it would be more normal IMO to have one PRV set to 3bar supplying all the water needs.

Sorry just to clarify the PRV is installed where the mains water enters the house just after the stopcock. So all mains fed water in the house is at 3bar. I believe 2-4 bar is normal for mains.

The 1bar is the reading on the front of the boiler for its system pressure.

 

[Johntheo5]

All the tests I have done are with the bath mixer on full hot, no cold. When I took the mixer apart and checked the cartridge it all seemed fine, it was closing the cold inlet when on full hot and vice versa, can't really see what could go wrong with it. Anything else I can check on the mixer

You could have a problem with any of the mixers even when off causing the problem.
You could measure the flowrate from the bath mixer by running it fully hot into a container for exactly 30 secs then measure this into a 1 liter container and x2 to give the flowrate in LPM, this will provide some useful info.

 

[terryplumb]

If the problem was with another mixer ,and not the one in the bathroom,then the issue with hot water temperature fluctuating would surely be prevelant with other hot tap outlets and not just confined to the one in the bathroom.

 

[robbo83]

I just measured the flowrate for 30s and got 4.8l so 9.6LPM.

 

[Johntheo5]

My mains temperature is 20.0C just now so if the mixer outlet temp is 67C then the boiler is firing at 31.47kw. If the temp rises to 80C then the flowrate only has to fall to 8.6LPM IF the boiler is NOT modulating and running at its max output of 36kw. You might measure the flowrate from the basin tap on fully hot. If you have a household thermometer take a few measurements.

The minimum output of this boiler is ~ 6kw and the flow switch trips the burner at 2.0LPM so the flowrate should be able to fall to 2.5 to 3 LPM without any fear of the temperature rising above 67C or the burner cutting out. a few more measurements of different flow and temperature will confirm this.

 

[robbo83]

I've measured the flow rate at various hot water outlets with hot on fully:

Bath in main bathroom 9.6LPM
Basin in main bathroom 4.6LPM
Rain shower in main bathroom 6.6LPM
Handheld shower wand in main bathroom 8LPM
En-suite basin 9LPM
Downstairs toilet 8.8LPM
Kitchen sink 4.8LPM

I haven't a thermometer I can use it is one of those ones that you put in your ear.

Can we conclude anything from the above, I am not really following.

I am still puzzled by the fact that the issue doesn't happen when the CH is on, surely this rules out any issue with the bath mixer?

 

[Johntheo5]

Your last query first, the issue doesnt arise on CH only because the primary boiler (hot) water is circulating around the rads & HW cylinder coil so has nothing to do with the DHW, its as if you don't have a combi boiler in effect. A defective/sticking diverter valve (on a combi) can cause issues but don't think thats your problem.

The minimum HW flow above, is 4.6LPM on the bathroom basin, no problems, so the boiler output required to heat this from 20C to 67C is 15.1kw so the boiler must be modulating down at least this far, as stated above a boiler output of 31.5kw is required to heat the 9.6LPM of bathroom mixer from 20C to 67C so, logically it points to a mixer problem, ie the HW flow from the boiler is being restriced for whatever reason.
Can you feel the outlet from this mixer, it should feel only lukewarm and compare it with the HW "feel" from the bathroom basin.

 

[foxhole]

Have you tried running the boiler at 60degrees ?

 

[robbo83]

Sorry I am struggling to understand how the HW flow can be restricted if there is 9.6LPM from the bath, wouldn't this be a lot lower if it was being restricted? I was thinking the opposite, that the bath has the highest flow rate in the house and the boiler isn't able to heat that demand quick enough for some reason? That's why I was suspecting something wrong with the boiler?

When you say feel the outlet from this mixer do u mean on the body of the mixer where I have put a red arrow below or at the filler on the bath where the water is coming out?

 

[robbo83]

Have you tried running the boiler at 60degrees ?

The DHW and CH are both set to 55degrees on the boiler. Should I try it lower than this?

 

[Johntheo5]

Sorry I am struggling to understand how the HW flow can be restricted if there is 9.6LPM from the bath, wouldn't this be a lot lower if it was being restricted? I was thinking the opposite, that the bath has the highest flow rate in the house and the boiler isn't able to heat that demand quick enough for some reason? That's why I was suspecting something wrong with the boiler?

When you say feel the outlet from this mixer do u mean on the body of the mixer where I have put a red arrow below or at the filler on the bath where the water is coming out?




View attachment 311540


View attachment 311539

Measure where the water is flowing out.

Those temperatures of 67C and 90C or whatever, what (on display) temperatures are they?, are they the DHW temperatures?.

 

[robbo83]

So you want me to feel the water coming out of the bath filler compared with basin?

Those readings of 67C and 90C are from option i9 on the service menu, which the manual says is 'the actual temperature from the main heat exchanger displayed in real time.' So they're not the DHW flow temperature that is i11.

 

[Johntheo5]

Yes, just concentrate on the bath so, just feel the water flowing from the bath mixer and note the DHW temp, I 11 & the heat exchanger temp, I 9.
Just do the bath first.