When water ceases to flow through a solar collector,stagnation occurs,resulting in potentially very high temperatures and steam. This can be caused by the following;
- Faulty pump.
- Blockage in the solar loop,including stuck non-return valve.
- Power failure.
- Controller switching off the pump when the water temperature in the hot water cylinder reaches a maximum,normally around 75 or 800 C. Some controllers such as the SR868C8 used by Go-Solar in our solar water heaters,manages the collector temperature to try and avoid stagnation but still has an upper tank temperature limit beyond which it will shut down circulation.
Once circulation through the collector stops,on a hot day the temperature in the collector can climb dramatically. At 400 kPa,the water in the collector will boil and produce steam at about 1400 C. If the collector is fitted with a solar vent,all or most of this steam should be released but if the pressure builds up too much,the vent may stop releasing steam. In addition,if a hot tap in the house is opened,or the expansion relief valve vents,the resulting drop in pressure results in an immediate drop in the boiling point and more steam. This steam can push through the return pipe from the collector to the hot water cylinder.
This results in a rumbling noise but more importantly,should the super-heated steam come directly into contact with the inner tank of the hot water cylinder,premature deterioration of the tank may occur. This potentially applies to all tank materials,whether stainless steel,PEX or enamel lined,fiber glass,or copper;super heated steam is very harsh on any material.
This can be minimized by avoiding the steam coming into direct contact with the tank using a return pipe that protrudes into the tank,rather than stopping at the inlet fitting. We supply all our systems with an adapted fitting to allow for the 15mm copper return pipe to be pushed 250 to 300mm into the tank. This means that the steam enters the tank into water rather than directly against the inner tank itself. Fig 1 shows a picture of the fitting. It basically consists of a 22mm copper direct to male or female (depending on the tank) Conex coupling with a 15mm reducing set. The inner ring of the reducer must be reamed out to allow the 15mm pipe to pass right though. Alternatively,a 15mm copper to 20mm male or female can be used and again,the 15mm flange reamed out to allow the pipe all the way through.
Fig 2 shows the layout of the Hills Solar system with the return pipe projecting about 300mm into the tank.