I have a combi boiler and all my rads have TRVs installed. If the heating is on but all the TRVs are set to 0, the boiler makes some horrible noises and then shuts down. What is the best solution? I was wondering if I could modify a TRV so that it isn't able to completely shut off one radiator (a stack of washers around the valve's pin?), or should I replace one TRV with a lockshield valve?