Domestic hot water too hot

[Marnhullman]

My old mother-in-law's old indirect oil fired boiler is set at 75c and the cylinder stat is set at 55c but the DHW is nearly very, nearly dangerously, hot. I assume that suggest the cylinder thermostat is shot? Reluctant to set the cylinder stat below 50c because of risks of Legionnaire's Disease although at present it's actually much hotter than it's currently set.

 

[JohnD]

tell us what controls there are and what happens when you turn the cylinder stat does down/up until it clicks.

Is the cylinder gravity feed or pumped?

 

[Marnhullman]

Programmer, thermostat on boiler, thermostat on cylinder and room stat. The cylinder stat does click when turned up and down. Cylinder is gravity fed.

 

[JohnD]

when the programmer is set to "HW on, CH off" and the cylinder stat is satisfied, does the boiler fire to keep itself hot?

(when the cyl stat clicks hot it should turn the boiler off)

has it always done that? or has it recently gone wrong?

No zone valve, I take it?

Have you got a multimeter to do some tests? If not, invest £10 in one.