no heating or water even though boiler and pump working

[andygg]

I have a 20+ year old baxi boiler. Recently I switched some of the radiators off so I can have hot water without having the heating on (both valves wired into timeswitch instead of being seperate- long story). This was working fine until a few days ago when I noticed the boiler was cutting out after only a minute or two. (I also switched the radiators back on).
The pipe to the pump is hot and the beyond the pump the pipes are also hot right into the top of the hot tank. However the water from the tap is luke warm and the radiators are cold. The pump is vibrating and seems to be working but it's making a bit of a noise. This has happened before with the radiators when they were switched off all summer and I had to get someone in to flush the system which was full of sludge.(according to plumber the system doesn't have a drain valve so he drained it from radiator?? even though it appears not to be a sealed system)

The pump was replaced 3-4 years ago and the themostast 2 years ago.
I've tried bleeding the pump and the radiators. I was thinking of replacing the pump and if that doesn't work I going to try and drain the system , as I found a valve under the hot water tanks which I think goes through to an outflow in the wall.

Any suggestions?

 

[kevplumb]

check the f&e tank has water in it ,usually in loft

 

[andygg]

checked f&e tank and it's ok. Bled top of hot tank too but only a tiny amount of air escaped and has made no difference. Suppose i'll have to change the pump and see if that makes any difference.

 

[Kristoflampie]

I am guessing here on some points but this sounds like a no flow situation throught the boiler so it thinks it is up to temperature and cuts out accordingly. If the timeclock has the facility to do both water and heating seperately then both should be wired to the timeclock . Like kev says check the F&E tank for water first . Not enough water in the system can make it noisy. The noise and no flow possibly could be a faulty pump, not the first time I have seen a pump detach its impeller.
Check the relevant valve is opening ( boiler and pump shouldnt run if valve does not open), then if sure valve is opening, stop system and check system for air, bleed the cylinder coil , pump and the radiators, still no joy,take the plug out of the end of the pump, can you see the shaft spinning when its on ??,if unsure or no plug fitted isolate and remove the head of the pump, switch it on to see if it is running.
Do not open the drain from the bottom of the cylinder as that is probably the cylinder drain for the hot water not the heating.
Hope these general points give you some idea.

 

[andygg]

Thanks for the advice i'll give it a go this evening and see what happens.

 

[kevplumb]

you could try opening the hot water zone valve they usually have a lever that says man/auto

 

[andygg]

I changed the pump as the old one was really stiff and covered in black gunk, however it's not made any difference. I've got another programmer so i can wire up the CH and HW valves so i can run them seperate again (present timeswitch has both wired into one so they can't be switched on seperate). I thought i'd then check if the hot water works on it's own.
The same thing happened a couple of years ago and I got someone to change the thermostat which did the trick, but I didn't think it would need changed again after only a couple of years (?).