Strange Combi Problem?

I have 10 Combi units working from a large Biomass Furnace and experiencing a strange problem. The system is an independent section of a large holiday resort installation and is supplied by its own flow & return as a long straight loft pipe run with the drop-down flow & return connecting to each individual accommodation COMBI unit.
When we check the flow connection at the base of the combi it is red hot as are the combi heat exchangers but when we turn a hot tap on it goes cold within a minute and the flow pipe at the base of the combi also goes stone-cold but the feed from the loft is red hot . This seems impossible as how is the flow into the base of the combi able to go cold when it is connected to a red hot feed from the loft.
can anyone shed light on this problem?