I have a vented 170 US gallon (650) litre gravity feed hot water storage tank in my ceiling with a 25 US gallon (100 litre) cold water feeder tank mounted about 12 inches further above the top of the hot water storage tank. The water is generally heated by a slow combustion stove. The hot water distribution pipe runs from the top of the storage tank, down an inside wall to underneath the house where it is distributed back up the walls to the various outlets. It is all done in 3/4 inch insulated copper pipe.
Everything runs fine in summer however in winter (40-50 Degrees F.) an air block develops which stops the hot water flowing and only allows a poor volume intermittent flow.
The following events seem to lead to the blockage.
1. Low outside temp (40-50 F.)
2. Low hot water usuage
3. High temperature hot water generated by the stove.
I do not think the water in the storage tank is boiling as I would hear the rumble or see the overflow from the exterior vent pipe. Also I have checked that the vent pipe is not blocked.I have tried leaving one or all hotwater taps turned on but I cannot get the water to flow freely after the air lock develops.
My "fix" is to climb into the roof cavity and loosen the hotwater take off pipe at the "Tee" on the top of storage tank which seems to eradicate an air block and the water then runs normally as I refasten the pipe fitting.
Seems really odd as the vent pipe is attached to the top of the "Tee" and should do the same thing as I am doing when I unscrew the fitting a little and allow the water to syphon down and draw air into the system!