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ok, sorry about this, i'm not doing it on purpose i promise. so basically, for example, the room stat say takes an hour to heat up the room to temp, but during this hour, the boiler stat will reach temp and shut down/main solenoid in gas valve close, etc and re-open quite afew times...

that's what i meant anyway, same thing really though. Can i just say though: when the room stat is making and calling for heat, the boiler stat at that time is close/making too, and when the room stat (if set at he usualy 21 etc, breaks (is satisfied) the boiler stat is still closed...

ok thanks for your help. sorry for sounding annoying. when water boils though in an o/v system it does usually pump over, a classic sign, and then the cold feed replaces lost water, and in 5 mins or so, the process repeats itself in a cycle.

i know that :) i meant what agile was saying about having the pressure set higher at the burner pressure than was required, range rated boilers.

my first point was this, please explain as i am trying to understand !! - say the stat has just failed of its own accord, no scale, etc - how would it get to 100 and pump over, wouldn't the room stat break the circuit before boiler stat temp anyway, hence the point of a room stat??

i'm sorry, i get it now. what you sare saying is: if the stat is in a wet pocket waterway and gets scaled up, then the water temp will rise due to this, higher than it should, and thus an open vent ejection as it reaches boiling point/boils. most boiler stat failures (ie leading pumping...

yes, so as my earlier post- other than a wrongly/higher set gas valve (pressure) - a boiler stat failure usually always means another fault is present in the system, as you say scaled up, etc if the boiler was scaled, and overheated, then it is not a faulty boiler stat at fault is it...

they are not always in the waterways. but yes if scaled up would make false readings, but then again the symptom would be lower tempeartue water at your rads (most noticeably) and h/w not as hot either. and boiler cycling, not pumping over.

seco, as agile has just said (what i am trying to kind of say)! : the room stat takes the temp of the room, say at 21, and the water in the oipe at this time is about 80ish. and the boiler stat is slightly higher, say 85ish, so boiler stat temp never reached anyway, so even if it had failed...

seco, if that were true then the room stat would not be doing anything ever. and the boiler stat would be constantly be cycling on and off. thus the reason for the room stat!!! bengasman: explain how all but 1 rad closed or a scaled/sludged up boiler / failing pumo would cause the boiler...

bengasman, ok fair enough i never thought of it that way. So basically, if a boiler stat fails, it is more than likely that another system fault is present and needs looking in to? don't you mean manual bypass too far closed??

Ok tell me how a boiler being scaled up can cause a boiler stat to fail?? Yes the boiler may cycle on and off quicker and the water not get to temp, but the scale will not affect the stat's workings. If a room stat is set to 21 as the norm (80 plus in the pipes), and the boiler stat a little...

boiler stat failure is only lijkely to happen really if there is no room thermostat installed, as with a room and cylinder stat the temperate shouldn't and won't get up to boiler stat temperature anyway, it will shut off before that. the water in the pipe will get to about 80ish and the...

why would low water pressure and not high pressure cause the gas valve not to moudlate down?? OP are you in a hard water area?

use a magnet to detect any sludge in copper pipework, it it sticks there is sludge inside.

I suspect no lights on the boiler (fault lights) show up when this happens?

If the plate heat exchanger was blocked it would affect all your hot taps fed from the boiler, not just 1? As said though, there's no plate heat exchanger. A blocked filter/flow regulator would also affect all hot taps though ......?

Gas pressure being set low would make water temp generally not very hot surely?, but OP said it was hot as normal and then just goes cold. Could be scale? Is it a hard water area? IS there a reducer fitted before the boiler? The diaphragm may be failing possibly inside the diverter valve or...

I want to replace my showerhead / hose as they seem scaled up and are giving a poor flow from my mixer shower. It is an aqualisa shoer, about 10-12 years old. Can I buy any type from B & Q, etc or won't any type fit? WHat size thread should I go for ??? Is it better to get a direct...

the vent pipe should not pump over if a mixer fitting is letting by, as desribed. as water always levels up, so the water level in the cold water tank will be the same as that in the vent pipe, and vice versa..... so a mixer fitting being fauly shouldn't allow your vent pipe to discahrge...