Imagine the schenario. Boiler controls pack up and/or SF boiler is allowed to boil over, as some numpty tries to get it going fast. Steaming hot water gushes into your CH header tank and straight out of the overflow. This causes two hazards. Firstly the steaming hot water will cause a normal plastic overflow to warp and secondly, anyone underneath the overflow will get scalded with hot water. It seems to me that the problem is linked to the fact that the overflow is at the TOP of the tank, and not the bottom. Now, before you all think I'm mad and have lost the plot in not understanding what the overflow does - read on. If you take water out of the bottom, you'll get colder water - kind of reverse of a copper HW cylinder really. Hot water in - cold water out, until you empty all the cold water! This would be achieved by employing an inverted U bend on the overflow - the top of which would be level with the desired level in the tank. Copy this idea to the DHW header tank, in case you have a dodgy thermostat on the cylinder!
Polypropolene pushfit can withstand high blow off temperatures. Your method would need to be WRAS approved.
OK that deals with one issue. Whether or not my approach would have to be WRAS approved, from a technical standpoint, would it not at least help to reduce the risk of scalding water coming out?
it sounds like your proposing a syphon for the tank , is this correct ?? plus with the tank being small i cant see the water temp being much lower, the water level rises as the heating system warms up anyway
1. The vent pipe MUST be above the waterline - otherwise the HWC could implode. How are you going to satisfy this 2. Since there's a gap between water level and ovrflow, usually the tank would get hotter and hottr before overflow, so no change 3. The "warning pipe" should be so placed so that its discharge is not dangerous. Seen more people killed in storms by falling trees. I think you're barking up the wrong pipe.
coming out? I can see where you're coming from but it's a no no idea Cornish. Ever since the part L requirement was passed for a permanently open zone valve to be fitted on solid fuel systems I've been to loads of boil overs, mainly because the supposed "heat leak" simply doesn't circulate properly. What are you supposed do when the government passes a regulation that all plumbers should know is badly flawed? We simply stopped fitting solid fuel CH!
OK, some interesting points - thanks for that! As Imran says, the warning pipe should be fitted so it can't do any harm to anyone. I have a combined gas and SF system. Each boiler has its own vent pipe to the CH header tank. Controlling the gas one is easy but the SF one needs to be brought up slowly. The temptation is to light the fire and have all the air intakes open full. However I've since learned that turning the flu vent down to 1 as opposed to 4 (max) and letting it reach temperature slowly works very well. The problem seemed to be twofold. One was that there was no bypass between the flow and return for the heat exchanger. A CH engineer advised that this was needed to get circulation going, so I fitted one. The other problem that couldn't be helped was that the flow rises then falls about 8 inches just behind the boiler. I have fitted an AAV at the crest of this pipe work, so that any air locks that occurr should be discharged though that. Though I do get kettling if it heats up too quickly and that's when the problem occurs.
if its getting hot then are you relying on gravity or a pump ?? if its a pump your pipe stat might need to be moved closer to the appliance