Long Story - ready?
Back in the early days I decided to buy a Fogwash - basically the same as yours but with a flue. I connected it all up with the pump before the heater. At that time I also had a Gardiners Claber reel, they were great - cheap as chips and really lightweight. Around the same time Gardiners brought out some new hose I can't remember much about it but it wasn't microbore as we know it but some quite thin walled stuff I also had a crappy Varistream digital controller. These factors added together were a disaster waiting to happen (although I didn't know it at the time).
First job I ever did with hot water and the thin walled hose expanded with the heat - so much that it jammed the reel and crushed the core, the problem being that the dead end setting on the controller didn't pick up that the hose was growing and wouldn't shut the pump down.
New hose and new reel and off I went again, backed the pressure down on the calibration and started work of course now the heater wouldn't fire up because the DE kept shutting the pump off
Re-callibrate and off we go ....................
Same thing happened - the hose started to expand - unfortunately you can't alter the pressure with a digital controller without it being switched on which meant by the time I managed to faff about with it the reel was wrecked (that's two now in two days). Whilst all this was going on the pressure relief valve under the boiler blew and squirted water out of the bottom.
I thought the problem lay with the controller and bought another ........ exactly the same.
It dawned on me that all this pressure inside a boiler probably wasn't a good idea
So I changed it around.
I now have a metal reel, proper hose, analogue controller and no trouble at all!
And hardly ever use the heater