Yes, a BTU is 1 F for 1 lb of water; heat input. You must measure the rate of heat loss from a particular set-up ; it is irrational to try to calculate it.
I am down to my last 2 Vis-atherms out of about a dozen( in about 5yr of service); both are stuck on ,so I use them as pond heaters ( always on) or connect them through my 60yr old Es Es thermostat to control temperature in an aquarium.
I am down to my last 2 Vis-atherms out of about a dozen( in about 5yr of service); both are stuck on ,so I use them as pond heaters ( always on) or connect them through my 60yr old Es Es thermostat to control temperature in an aquarium.


