Should it be a fire hazard concern to mount a point-of-use tankless water heater on a plywood wall?

I am just thinking to install a shower in my shed which has plywood walls. The water heater I am going to get is a 5.5kw tankless from Home Depot. The manual says the heater must be installed in a place away from flammable objects. I don't think plywood is flammable, but it is uncoated wood that can be burnt.... show more I am just thinking to install a shower in my shed which has plywood walls. The water heater I am going to get is a 5.5kw tankless from Home Depot. The manual says the heater must be installed in a place away from flammable objects. I don't think plywood is flammable, but it is uncoated wood that can be burnt. So I just want to know if it is necessary to get a metal sheet or something in between the heater and the plywood wall. Thank you very much!
Don
8 answers 8