Mixer tap temperature fluctuates

[Donsboig]

Hi all,

I recently moved house and the bath/shower mixer tap constantly fluctuates in temperature. The swings aren't huge but definitely noticeable. It's the same whether using the bath tap or combined shower head.

The adjacent sink mixer doesn't have the same issue so I'm assuming it's something specific to the bath mixer.

We're on a combi boiler with good water pressure. The incoming hot and cold pipes both have basic flow regulators fitted and restricting flow. Playing around with these didn't seem to improve the fluctuations.

One final clue (possibly): the hot and cold levers are very sensitive. They turn 90 degrees in total but once you've gone from 0 (off) to about 20 degrees they are basically fully open.

Any help much appreciated! I'm hoping I can understand or fix the issue without replacing the unit as the bath and everything is tiled in so fitting a deck mounted thermostatic mixer would be a pain.

 

[ericmark]

You can get thermostaticly controlled mixer units, but not cheap.

 

[Donsboig]

Yeah I'd like to avoid that if possible due to the difficulty of accessing underneath the bath. And I'm not totally sure it would solve the problem?

 

[Johntheo5]

Your HW is fed from the combi from mains cold, is the cold to the bath mixer from the mains as well and not gravity fed?.

 

[Donsboig]

Hi, yes that's right, the cold into the mixer comes from the mains too

 

[Johntheo5]

If you measure the flowrates with mixer to fully cold and then to fully hot, are, or do they seem similar?

 

[Donsboig]

If you measure the flowrates with mixer to fully cold and then to fully hot, are, or do they seem similar?

With the isolating/restricting valves as they were, I got:

Cold: 4 litres per minute
Hot: 6 litres per minute

Then I opened them both fully and got:

Cold: 16 litres per minute
Hot: 12 litres per minute

Both times I allowed the hot to run for a minute first so the boiler was in a steady state.

Do you think matching the flow rates exactly would help? Thanks for your input

 

[Johntheo5]

Forgot to ask you to measure these flowrates with/through the showerhead, maybe you did, as that will further reduce the flowrates but significient only with the restricted flowrates.
Most combis have a minimum flowswitch set to 2 to 2.5LPM, if the HW flow is less than this then the burner will stop firing and the water will run cold then hot again when flowswitch resets, probably not your problem but worth checking out.

My mains water temp is 12C just now so if the combi is set to 60C then you need 1.43LPM of cold water mixing with 2.0 LPM of HW to give you ~ 3.5LPM at 40C, (to avoid the burner cutting out) probably not a problem even with restricted flowrates but can you try showering with the unrestriced flowrates, throttled with the individual hot & cold taps to give you a reasonable flowrate of at least 5/6LPM at ~ 40C or at your comfortable showering temperature and see how that works out.

 

[kidgreen61]

Those thing never work particularly well as they are so sensitive to water pressure fluctuations, it may be something as simple as your neighbours flushing their loos at the same time causing a small pressure drop, which in turn will affect your shower.
If it were me, I'd fit a thermostatic mixing valve under the bath.

 

[Donsboig]

Forgot to ask you to measure these flowrates with/through the showerhead, maybe you did, as that will further reduce the flowrates but significient only with the restricted flowrates.
Most combis have a minimum flowswitch set to 2 to 2.5LPM, if the HW flow is less than this then the burner will stop firing and the water will run cold then hot again when flowswitch resets, probably not your problem but worth checking out.

My mains water temp is 12C just now so if the combi is set to 60C then you need 1.43LPM of cold water mixing with 2.0 LPM of HW to give you ~ 3.5LPM at 40C, (to avoid the burner cutting out) probably not a problem even with restricted flowrates but can you try showering with the unrestriced flowrates, throttled with the individual hot & cold taps to give you a reasonable flowrate of at least 5/6LPM at ~ 40C or at your comfortable showering temperature and see how that works out.

Thank you. I didn't measure it through the showerhead, but the temperature fluctuation happens through the main tap as well so I don't think that's the issue. Also in the restricted setting I always had the hot lever fully open, so probably was getting more than 2.5 LPM (combi HW set to 50).

I just tried showering with the valves fully unrestricted, and the hot lever very open with the cold lever throttled to get a comfortable temp. It still fluctuated in the same way.

It feels like there's an extra slug of cold water being introduced every 1-2 seconds. I checked the boiler with the shower running and it made a very constant noise, and the app also reported a consistent HW temp.

Is there anything else you can think of? Appreciate your help

 

[Donsboig]

Those thing never work particularly well as they are so sensitive to water pressure fluctuations, it may be something as simple as your neighbours flushing their loos at the same time causing a small pressure drop, which in turn will affect your shower.
If it were me, I'd fit a thermostatic mixing valve under the bath.

The temperature fluctuation is too rapid and consistent for that I think. It fluctuates every few seconds, all the time. If I can't figure it out then thermostatic mixer is the backup solution, but some numpty tiled the bath in so it will be a bigger job than I would like to pay for!

 

[Johntheo5]

Jusr set the combi HW temperature to say 35 to 40C, whatever showering temp you use, use no cold and see does it remain steady as a temporary measure.

 

[Donsboig]

Jusr set the combi HW temperature to say 35 to 40C, whatever showering temp you use, use no cold and see does it remain steady as a temporary measure.

yeah, it does stay steady in that setup

 

[Johntheo5]

Just looking at the flow swings from a thermal point of view, if you were having a substantial shower of say 11.69LPM using 8.0PM at 50C + 3.69LPM at 12C cold to give a showering temp of 38C then the cold flow will have to fall to 2.48LPM to increase the showering temperature to 41C (easily noticeable), thats a fairly substantial fall, from 3.69LPM to 2.48LPM, or 67.2%. Are you happy that there is no (faulty) pressure reducing valve on the cold but not on the cold supply to the combi?. Obviously, if only showering at a flowrate of 4.38LPM at the above conditions then the cold flow must only fall from 1.38LPM to 0.93LPM, but still a fall to 67%. I presume a TMV would control to 0.5C of setpoint?.

 

[Donsboig]

Just looking at the flow swings from a thermal point of view, if you were having a substantial shower of say 11.69LPM using 8.0PM at 50C + 3.69LPM at 12C cold to give a showering temp of 38C then the cold flow will have to fall to 2.48LPM to increase the showering temperature to 41C (easily noticeable), thats a fairly substantial fall, from 3.69LPM to 2.48LPM, or 67.2%. Are you happy that there is no (faulty) pressure reducing valve on the cold but not on the cold supply to the combi?. Obviously, if only showering at a flowrate of 4.38LPM at the above conditions then the cold flow must only fall from 1.38LPM to 0.93LPM, but still a fall to 67%. I presume a TMV would control to 0.5C of setpoint?.

I'm not aware of any pressure reducing valves, I've had a good look and can't see any. Could it perhaps be an issue with turbulence inside the mixer tap? So that the hot and the cold are getting slugged in in uneven spurts?

My main concern with the thermostatic mixer is that if the flows coming in are for some reason fluctuating in this way, then can the mixer control them with enough precision and speed to maintain a constant temp?