I might add that I had similar problems - the cure for me was to use car radiator hose to the hose reel, thermobore hose on the reel and to remove all restrictions like the barbed connectors in all the push fit connections. I also got rid of as many hoselock connectors as possible, re plumbed the heater to suck water through it and used braided steel hoses to and from the heater. I also dumped my Varistream controller and fitted an analogue Spring controller.
Most of the above problems as I mentioned before could quite possibly be fixed with a bypass system providing a pressure release.
My problems were so bad - I wrecked two Gardiners hosereels and a band new 100m reel of hose. Filled the back of the van with water whilst stopped at traffic lights (connector blew off) and blew the connector off in the reel center several times.
I don't have much hair left anymore.
