CH is hot, hot water is not - sometimes!

[hc]

Help! Sometimes we cannot get hot water, although we still have GCH.
We have a gas boiler in the kitchen, and a hot water cylinder in the loft. We can select HW only, or HW and CH.
The problem seems to be triggered by running a bath.... so is it an air-lock issue?
The boiler fires up, CH is on, the radiators all get hot as expected, but then we have no HW. (Water does come out of the hot tap, but it is cold.)
If we select HW only, the boiler goes off (my partner thinks it fires up briefly before going off, but I'm not certain). We can only get it back on by selecting the CH and HW option. But still no hot water. It usually sorts itself out sometime the following day, either during the timed period the following morning, or the timed period the following evening. I have heard very loud gurgles coming from the loft and then the water does get hot.
Where is the (proposed) air-lock? What can we do to remove it, and how do we prevent it happening again?
Or is it a sticky valve somewhere? If so where? Or an intermittently faulty switch?
We haven't called a heating engineer, because we know by the time he comes, there will be nothing to see!
Any help gratefully received,
H.

 

[chrishutt]

Presumably you have gravity circulation to the hot water cylinder and there are no motorised valves (can you confirm?). In which case the problem is likely to be lack of water in the gravity circuit. Check the header tank to see if it has water in it. If so, drain a little out of the CH system and see if this is replaced by the tank/float valve.

 

[hc]

Being an utter novice thanks for any help on this.

Have inspected the loft more closely, yes it is a gravity feed to the hot water cylinder. There are no motorised valves.

If so, drain a little out of the CH system and see if this is replaced by the tank/float valve

By this do you mean out of the radiators eg through a bleed valve? How would this help the intermittent cold instead of hot (tap) water problem? Aren't the two water circuits separate?

 

[ItsOverFlo]

Is it is a fortec type cylinder in the loft? ( combined tank and cylinder. Or is it separate tank and cylinder?)
There might be a bleed point on the side of the cylinder or there might be an expansion pipe coming off the primary flow leading over the top of the separate tank???
Sometimes, if no expansion pipe or bleed vent. You may have to crank the primary flow cylinder union to allow air to disperse. When water comes out, re tighten cylinder union and see how you go.