Solar Plumbing issues onlyThis area is for qualified plumbers only
All advice is informal and is not guaranteed by The Smart Hot Water Company to be accurate
This informal advice is aimed at open loop solar hot water systems
Cylinder Rumbling;Usually rumbling in a hot water cylinder (that is part of a solar hot water system) is caused by steam. This steam comes from the solar plumbing loop.
Note: At sea level water boils at 100°C but at mains water pressure water boils at around 140°C -170°C.
There are two causes of this rumbling (sometimes banging).
1. Faulty Air Relief Valve
Steam or vapour will tend to build up in many functioning solar loops due to pressure differentials across the pump, sharp angle bends and such. Especially if the temperature in the loop starts to exceed 100°C but even at lower temperatures this can happen. A functional Air Relief Valve will allow this vapour to escape as a matter of course. A faulty or poorly installed Air Relief Valve can lead to some rumbling but is usually not excessive. However if the loop becomes full of vapour and the pump has trouble restarting (due to little water to push) then this problem can lead to unintentional stagnation and the symptom below now becomes significant.
2. Stagnated solar loop
When water is no longer pumped in a solar collector loop, such as when the controller is in 'Topout' the water in the loop (around the collector) is subjected to an uncontrolled heat source (the sun). There is only a few litres of water so it heats up until it boils, this will usually be manageable for a functioning Air Relief Valve. However if the water is at or close to boiling point (e.g. 160°C at mains pressure) a sudden drop in water pressure, such as when a tap is turned on, will cause spontaneous steam generation that will overwhelm the Air Relief Valve and make it's way down into the cylinder. The cylinder water is much lower in temperature and the steam bubbles now violently collapse (at a ratio of 1700:1) such that they make a rumbling or banging noise inside the cylinder
Cylinder Rumbling Solution;1. Replace Faulty Air Relief Valve
2. Correct Faulty Air Relief Valve installation error
The Air Relief Valve MUST be at the highest point in the loop, even a dip or 1 or 2 cm will render it ineffective
3. Add a heat dumping system
An effective option is to use a pumped heat dumping loop to a radiator that cools the cylinder if it heats close to maximum temperature. This way 'Topout' temperature (Tmax) is never attained so the loop is always being cooled by the circulating water.
This will require a SolaStat-ET2, a pump and a potable water radiator.
4. Reduce collector surface area in summer
A) Angle collectors for high collection in Winter and low in Summer. This will mean a higher pitch angle with say latitude + 20° (Auckland NZ = 57° from horizontal). If the problem is excessive then perhaps latitude + 30°
B) Cover part of the collector surface area for summer with a sturdy heat resistant covering (non burning), an evacuated tube collector in stagnation can reach 240°C.
Secure well against high winds and the elements.