While it's a long horizontal run, I suspect it might just be possible to have the flow go up, across within the floor, and down to the tank. With the return going down from the tank, across within the ground floor, and up to the back boiler.
Once the water is flowing round, the drop back down to the tank would be balanced by the extra rise from the back boiler. You'd need to consult someone who knows what they are talking about though !
The hardest part would be getting a slight gradient in the horizontal pipework so as to guarantee that air can escape to suitably placed vent pipes.
Alternatively, I do know that there are combined pump/valve units (or it should be possible to assemble the required parts) that allow for a pumped circuit with automatic failover to a gravity heat dump if required.
ISTR a thread on here where the heat dump was an upstairs radiator where the hot water from the (in that case, I think) WBS going into one of the top tappings on the rad, while the normal CH flow was via a TRV into one of the bottom tappings.
On that basis, how about this ...
Back boiler has pumped circuit to tank. For heat dump, tap into flow and return and take them up to an upstairs rad. The tee into the bottom of the back boiler also doubles as the return for the rad during normal operation. Also for normal operation, the rad has a TRV off the CH flow circuit.
For the controlling the flow from the tee into the back boiler upper connection into the top connection of the rad, I see three ways of doing it ...
1) Use a flap valve so that the pump will pull it shut while running, but it will drop open without the pump running.
2) Use a spring return to open electric valve.
3) Use a thermostatic valve with a remote sensor bulb stuck on the upper pipe (or better still, within the back boiler) so it opens when the back boiler exceeds a certain temperature.
Once the water is flowing round, the drop back down to the tank would be balanced by the extra rise from the back boiler. You'd need to consult someone who knows what they are talking about though !
The hardest part would be getting a slight gradient in the horizontal pipework so as to guarantee that air can escape to suitably placed vent pipes.
Alternatively, I do know that there are combined pump/valve units (or it should be possible to assemble the required parts) that allow for a pumped circuit with automatic failover to a gravity heat dump if required.
ISTR a thread on here where the heat dump was an upstairs radiator where the hot water from the (in that case, I think) WBS going into one of the top tappings on the rad, while the normal CH flow was via a TRV into one of the bottom tappings.
On that basis, how about this ...
Back boiler has pumped circuit to tank. For heat dump, tap into flow and return and take them up to an upstairs rad. The tee into the bottom of the back boiler also doubles as the return for the rad during normal operation. Also for normal operation, the rad has a TRV off the CH flow circuit.
For the controlling the flow from the tee into the back boiler upper connection into the top connection of the rad, I see three ways of doing it ...
1) Use a flap valve so that the pump will pull it shut while running, but it will drop open without the pump running.
2) Use a spring return to open electric valve.
3) Use a thermostatic valve with a remote sensor bulb stuck on the upper pipe (or better still, within the back boiler) so it opens when the back boiler exceeds a certain temperature.
