Keston C36 DHW - suspect faulty thermostatic mixer valve.

[Adam Abdullah]

Hi All

Apologies in advance but I will try and be thorough as it's my first post

Need some advice, I have a Keston c36 which has been regularly maintained over the last 10 years since moving into a 6 bedroom Victorian terrace. Yes, many parts replaced and maintain spares just in case Recently DHW has been fluctuating from Hot then lukewarm in all taps (closest is a bathroom (s) and furthest being kitchen) Boiler temps do rise by 2-3 degrees however boiler isn't cutting out or showing any errors.

Heating runs great, no cold spots, all balanced. Engineer asked me to get hold of 3 thermistors DHW, return and flow replaced but no change. I have 2 thermostatic concealed showers (one right next to the boiler and the other in the basement) Having had sludge in the system when I first moved in resulted in a blocked Plate HE which was replaced then subsequently cleaned again after a complete power flush but this isn't the case this time around.

In the last month, the following was performed.
Recently I had some extension work carried out, rads moved etc so the engineer decided to mains flush ( High mains pressure where I am) the entire heating system several times (F3 & other chemicals run in the system before purging, all rads prior to this were also individually removed and flushed in the garden)
The boiler was flushed including the secondary Plate heat exchanger and is clean with plenty of inhibitor.

* I decided to try and isolate the problem as the temps were fluctuating by a few degrees by isolating both cold and HW via the isolation valves to the shower closest to the boiler, which to be fair was out of use due to the construction works.
DHW is now working as expected so could this be a faulty / scaled up/blocked thermostatic cartridge?

 

[ianmcd]

Yes does sound like it, isolate the cold water inlet at the boiler ( in effect stopping the HW) then turn the shower to max hot and open up if you get water coming out of the shower or the hot tap at the basin then you have a faulty cartridge

 

[Adam Abdullah]

Thanks

Will try that I think I need to remove it first and inspect but it's been several days now and DHW throughout the house is running spot on. The larger isolation valve was a bugger to isolate initially as it would only turn partially, cold one sprung a leak so changed that over quickly.

Will report back for others to benefit from.

 

[Adam Abdullah]

Update:

So I removed the cartridge today, cleaned it out and replaced one damaged O ring. Soaked in Viakal and vinegar until I could feel the mechanism move freely so must have had limescale buildup internally. After reinstalling and testing the cold water now works fine (previously it would only run lukewarm regardless of the thermostat position and trigger the boiler to supply hot water) but Hot is still lukewarm at best.

I only cleaned the brass body of the valve with a rag and some Viakal and used the cold water side to flush, I'm guessing I should have used the HW side also as it has improved the function as mentioned above?

Thoughts / Opinions as the cartridge (brass with 2 large mesh filters) looks to be in good condition.