I have a Vaillant combi boiler and recently i have been having a problem with the shower. Everything else is ok, the radiators are fine and the hot water when turned on in the downstairs and upstairs sinks runs forever. My son was having a shower yesterday and the hot water light on the boiler would light up for about 30 secs giving hot water, go off for 30 secs giving cold water, light up for 30 secs giving hot water and so on. I thought it must be a build up of scale so i took the shower head off and de-sclaed it for several hours before re-attaching it. However turning the shower on again earlier it behaved in the same manner as before so im not quite sure what the problem is. Any help would be appreciated. Thanks.