why does my cylinder stat turn off too early?

7 Jan 2009
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United Kingdom
i have a honeywell L641A cylinder stat. i have it set to 60 on the scale, but when it turns off the boiler supposedly at 60, the measured water temp at an outlet less than 1 metre from the cylinder is 50 degrees (measured using a RTD probe)

if i adjust the control knob i can hear the switch operating. it clicks inline with 60 on the scale, so the stat thinks it is at 60!

my parents hot water feels considerably hotter and measures at 60 degress as per their cylinder stat set point.

obviously its easily sorted by turning the stat to 70, but i just wondered why this occurs? honeywell say they have never had an L641A sent back due to inaccuracy.

Glow worm space saver 40BR mk2
L641A cylinder stat
gravity hot water
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Bad contact with the cylinder perhaps :rolleyes:

Fitted in the wrong place perhaps. :rolleyes:

Water blending at the tap Perhaps. :rolleyes:

Try measuring the temperature at the top of the cylinder. :idea:
cylinder top temp measured with an infra red thermometer is 51.7 degrees c.

no tap blending, and stat is mounted as per instructions, between 1/4 and 1/3 of the way up, contact with cylinder is perfect...i cleaned and buffed the surface before fitting.
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bhm1712 said:
.i cleaned and buffed the surface before fitting.

that explains it its reflecting the heat
Not to worry, ignore what the indicator says, and simply turn it up 10 degrees.
Don't forget to check after a couple of days.

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