Glow-worm Fuelsaver 'F' boiler unable to turn up

12 Nov 2005
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United Kingdom
It's a boiler fitted in 1986 by prev. owner boiler type is a Glow-worm fuelsaver 'F', 8 rads are fed from it aswell as tap hot water. the boiler stat has a range of 1-5, if I try and turn up boiler stat from it's current setting of 2 upto 3+ then the rads start to go cold and tap hot water heat is increased to unbearable temp. Do not understand why as boiler turns on and off correctly and supplies the water correctly at setting 2. The hot water tank has an independant stat that is new. Any ideas what could be causing this problem as when weather gets colder aren't able to increase rad water temp? Thanks oldboiler.
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Turn CH off, HW on, tank stat down as low as it'll go; does the pump run and boiler light?
Now turn the tank stat high, does it click? Does the pump stop?

That should answer the "is it a wiring fault" question. Then have it wired correctly.
My HW works ok when no CH is on. and even when boiler is on low it still works and so does the pump, the zone valves close and open appropriately. Any other ideas, I have even switch off all rads then starting with the problem one switch them on. Gradually the 1st rad starts to diminish if the boiler is above 2 on the stat. If it is left on 2 then it stays on turn it up to 3 then you start to loose the 1st rad? Any other ideas? (Admittedly this system is in a town house (3 levels) so doing balancing keeps hubby fit.) Room stat is new and ok.
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Boiler stat simply controls the temperature of the water in the system.
The cylinder stat senses water heated up by the system.

My theory is that when you have the boiler stat on 2 that the cylinder is calling for heat longer thereby allowing water to flow to the rads as well ie. You have found a 'sweet spot' where you get HW and some CH

A system wiring fault probably to do with your cylinder stat is therefore my biggest suspect

You asked.

Any other ideas?

Er No
oldboiler said:
... the zone valves close and open appropriately....Any other ideas... Room stat is new and ok.

So you have separate zone valves, check that the tank 'stat opens the DHW zone valve when turned up !!
Check the room 'stat opens the CH zone valve.

Then correct the wiring to the tank 'stat terminals, you will probably have found that the connections are the wrong way around causing your valve to "close" when the hot water is cool, though it is probably misbehaving (which is why you changed the 'stat perhaps) and not shutting off completely, so you get DHW at boiler temperature. Turning up the stat causes the valve to open when it should be closing (cyl 'stat now satisfied), and the balance of the system is so poor that almost all the heat goes to the hot water cylinder, starving the rads.

Consider cleaning/replacing the pump, cleaning the whole system then rebalancing, adding a balancing valve (gate valve) to the cylinder return pipe, replacing the DHW zone valve.
Hi ya

Thanks for your comments and possible solutions, this may take me a while to try out but will let you know if it solves the problem.


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