Heating works but no hot water?

[markymarcdiy]

I have a Saunier Duval boiler in a pumped, gravity gas central heating system. Having started using the heating again I've noticed that our hot water is going cold if hot water and heating run at the same time. If only the hot water is running then all is fine.

I don't really know where to go now as i've had a plumber out (he was looking for a heating / hot water valve but there isn't one, it's an old gravity system) and he's been no help.

Could there be an airlock in the heating system that would adversely affect the hot water?

Would a build up of sediment cause problems? Again, had a plumber out to flush the system year before last but some of the radiators are cooler at the bottom than they are at the top.

I'm a bit confused and would really appreciate a pointer!

Thanks,

Marc

 

[Agile]

You are confused!

Most S-Ds are combi boilers and its most likely that your boiler is a combi !

Its also not likely to be either open vented or gravity.

There are S-D heat only boilers but they are very rare indeed thank goodness. I have also seen one which was a conventional flue !

If its a combi there is no hot water cylinder !!! Do you really have one? They are big and hot !

If its a combi then someone posted about that fault on here recently but there was no suggestion of the cause as they are a most unpopular boiler and its very unlikely that you will find anyone to come to repair it.

Start to come to terms with replacing it.

Tony

 

[markymarcdiy]

Hi,

It really is an SD but definitely not a combi (have had a combi in a previous house)! Retro-fitted a few years ago now to a much older (and needlessly complicated) gravity system. Hot water tank in airing cupboard, header in loft all present and correct!

Any thoughts?

 

[Agile]

In that case the boiler is fine but the fault is in the heating/hot water circuit or controls.

I suspect the header is dry or blocked.

OPlease inspect the F&E tanks and report back.

See FAQ for help.

Tony

 

[markymarcdiy]

Will do, thanks for the pointer!

 

[markymarcdiy]

Right, have inspected the F&E tank. Filled with water, stopcock working, no apparent blockages. There was some fine rusty sludge, a little like algae in a pond but the water was generally clear. It appears that the water level has been going up and down by a couple of inches judging by the 'tide marks'.

I'm not sure if this helps?

Could it be possible that there's an airlock in the system anywhere? We occasionally get a noise like air in the system when the central heating switches on.

Thanks for your help so far!

Marc.