Azinat

I have a pretty standard chauffe-eau setup.  Mains water supply has stopped in the house (frozen pipes outside) so no cold running water.  However, no hot either (the ballon was full).  Basically no hot will come out if there is no cold supply in (some sort of gravity system?).  In normal use we need to keep hot tap open full and cold about 10% to have very warm water.  If we add any more cold, it will draw the supply away from the chauffe-eau and the hot water supply will reduce considerably. Is there some form of valve that can be added to the system? Thanks