http://www.ebay.co.uk/itm/250953679066?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649just set this up on your system to come on every hour for 15 minutes.
If I was going down this route, I would use one of these http://www.garden4less.co.uk/hozelock_water_timer.aspwhen the timer opens the valve the fogwash would fire and heat the water, I would have the hot going back to the pump di and back to heater. And have it do so every couple of hours for 5mins.Roy