My 89 yr old mother has a hot water tank that is connected to her multi fuel stove. I want to add an immersion heater so she can have hot water without lighting the stove. The tank has no immersion boss. I understand that a Howden DH501 will do the job. The tank is installed in a very tight corner in her kitchen. Can the Howden unit be installed from the outside of the tank without access to the interior?