The discovery of mould and spider beetle in the book collection held within the Long Gallery at Lanhydrock House in Cornwall prompted an investigation into, amongst other things, our environmental control systems and heating services. A resultant action plan recognised that controlling levels of relative humidity to a set point with a Victorian heating system can be problematical. In making this assessment special consideration had to be given to the size of the architectural space, any constraints on the historic building fabric, the efficiency of the heating system and the Cornish climate. These factors, when related to the results of environmental monitoring carried out over a three-year period, draw us to an inevitable conclusion that the underperformance of our heating system contributes, in part, towards a higher risk of microbiological growth and insect pest infestation within the collections.