... heat efficiency. 4. Suitable for many heat source such as electric, gas, vapor, fuel and so on, convenience and widely adapted. 5 ... heat efficiency. 4. Suitable for many heat source such as electric, gas, vapor, fuel and so on, convenience and widely adapted. 5 ... heat efficiency. 4. Suitable for many heat source such as electric, gas, vapor, fuel and so on, convenience and widely adapted. 5 ...