Changed radiator CH is fine but no Hot water from Taps

Hi All,

We have an unvented central heating system (house built in 1996)

Changed a bathroom upstairs radiator on Sunday afternoon. As I had to cut the pipes (to replace 8mm micro bore with 15mm) I partially drained the central heating system by turning the off the stop valve on the small header/expansion tank in the loft and opening a radiator drain valve on a downstairs radiator with a piece of hose attached etc.

Fitted the radiator all OK . To refill the central heating system I just opened the stop valve on the small tank in the loft, as the level dropped the ball cock did it’s job and allowed new water into the system etc.

I bled all the radiators upstairs and downstairs (all upstairs radiators had air in them, downstairs seemed to be ok). Fired up the boiler and turned on the central heating, all radiators including the new one all working OK.

On Sunday evening hot water seemed OK but I guess we were just using the hot water already stored in the hot water cylinder.

Then this morning (Tuesday) no hot water ? all hot water taps are flowing but with cold water. I guess it's an air lock but don't understand how I can introduce air into the hot water system when I was working on the central heating system.

I turned on the boiler for hot water only but after about 45 second the boiler cuts out. Boiler sounds like a kettle and is boiling itself dry ??

The central heating system on the other hand seems fine i.e. boiler does not cut out all radiators are hot.

There are 2 motorised valves in the airing cupboard next to the hot water storage cylinder. I guess one is for hot water and one is for central heating.

I'm not sure what to do next to try and fix etc.

Thanks in advance

you seem to have an air lock in the HW heating coil.

There may be a manual bleed valve by the cylinder.

Try piutting hot water only and set the pump to "3" which might clear it.

Have you bled the pump ?

All these have lots of help in the FAQ on this site.

Tony

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vent

Guys thanks for the quick response.

Agile - no I haven't bled the pump, I didn't know you could do that. I'll take a good look around the FAQ.

Kevplumb - vent. Sorry I know what vent means, just not sure how to do it. Will there be a vent on the cylinder somewhere (just like bleeding a radiator ?) There is a drain point at the bottom of the cylinder. I'll have a good look around the FAQ.

I'm just surprised I managed to screw-up the HW by draining the CH ?

Thanks
Tony

did you lock the motorised valves open when filling

vent is usually as in pic will be a screw on the top of the pipe

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An update

The hot water tank is 180 litres on an unvented system. There are 4 pipes going into the top of the cylinder. There doesn’t seem to be any pipes coming into from the side or bottom of the tank.

Last night was able to identify the HW (2 way) motorised valve and the CH (2 way) motorised valve. Fired up the boiler for HW only but it only stayed on for about minute (before I think it boiled itself dry) due to airlock.

The pipe coming from the motorised HW valve is one of the 4 pipes going into the top of the hot water tank and has a bleed screw just before it enters the tank. I bled this, expecting lots of air but unfortunately only a tiny amount of air before dirty water.

Pump is definitely working ok it’s on position 3. I removed the centre screw and again only a tiny amount of air before dirty water.

Next I connected a hose on the cold tap in the bathroom & attached the other end to the vent pipe (inverted U bend) of the header/expansion tank in the loft. Turned on the cold tap feeding cold water at mains pressure through the vent pipe. It worked its way through the system and as expected started to fill-up the header tank accompanied by lots of air bubbling up into the header tank (but was this air from the hose and vent pipe ?? or air from hose and vent pipe and air lock ??).

I assumed this had removed the air lock, switched the boiler on, HW only, but it only stayed on for about 2.5 minutes ? (even though the 180 litre tank is cold).

At this point I gave-up and turned on the immersion heater overnight for 6 hours. This morning I turned off the immersion heater and 4 of us had showers plenty of hot water etc.

With the immersion heater now off, I fired-up the boiler HW only and this time it stayed on for about 4.5 minutes then off for few minutes and back on for a couple of minutes etc. I think this is normal as the hot water tank is already up to temperature from the immersion heating ?

So I’m not sure if it’s fixed or not, I guess I’ll find out tonight

All thanks for your advice yesterday.

I think this is none of those dreadful blue tanks!

That are so old that DIA is probably to only one still alive who remembers them!

Tony