I have been asked to fit a twin coil solar cylinder for a contractor, went to look at the job today, current cyl is heated by a Janus circulator on gravity. Can anyone think of a reason not to fit an unvented cylinder in this situation? Do you think that fitting a 2-port valve in the gravity circuit presents a safety issue? My concern is that the circulator has no electrical control, just a mechanical gas valve. While the 2-port valve will satisfy G3, is it acceptable to prevent the circulator flow?