Our Hills Esteem Solar HWS consists of a 250L stainless steel tank and 22 tubes, located in Adelaide hills. House is single storey and tank is directly beneath the tubes so pipe run is 5-ish metres each way.
This system has been experiencing the following symptoms lately and I'd like to know what others think and if these are common -
1. Late in the day the circulation pump has been activated and operating but zero flow was indicated until I turned on a hot water tap inside the house. The flow rate was originally set to the optimum of 0.8L/min. I increased it to maximum which reduced the occurrence of zero flow symptom and now for much of the time the flow indicator jumps about between 1 and 2L/min.
2. Also late in the day, once per day I would hear two bangs in quick succession from the solar manifold.
I believe the above two symptoms are being caused by an air lock. It is normal for gas, mainly hydrogen, to separate and bubble out of water as it is heated. Typical methods for preventing this are to -
1. Raise pressure (see Henry's Law in Wikipedia). Our inlet water pressure is 400kPa. According to Apricus's US installation manual, to avoid stagnation it's recommended to ensure inlet pressures are above 350kPa. If correct this would obviously apply to any solar HWS.
2. Install an automatic air vent. Our system has a Purg-O-mat 150 installed on the manifold outlet but I do *not* ever hear it steam. I have checked the cap is not done up and if I remove the cap and carefully insert a thin piece of wire to push down on the check valve I *do* get a jet of water shoot out.
3. Reduce the system's summer solar performance by -
a. Increasing the flow rate by changing the circulation pump setting or reducing the flow restrictor. We have a single speed Salmson pump, model SB04-15, so all I can do is reduce the flow restrictor to maximise flow. I now have a flow rate little above 2L/min.
b. Tilt the tubes to maximise winter solar gain and therefore minimise summer. Our tubes are 45 deg relative to horizontal (A frame set to maximum of 67deg on a 22 deg reverse pitch). There's no shading. Tubes face exactly north.
c. Cover the tubes. I've now installed 70% shade cloth over 20 of the 22 tubes and secured it to the tilt frame using cable ties.
So for the meantime the shade cloth will prevent the symptoms re-occuring but are our HWS' symptoms normal? What's everyone else doing? Not noticing the symptoms or just ignoring them? Ignoring stagnation when it does occur? (i.e pump switches off due to tank reaching max temp).
Would be funny to see all the evacuated tube HWSs in China being covered by shade cloth around the time of the summer solstice.
This is annoying because the system over performs in the summer and under performs in the winter causing us to need electric boost for almost half the time. I suspect in the future I'd simply install an electric only HWS and install sufficient solar PV to compensate because our average electricity consumption is only 4kWh/day. Obviously if one's consumption is 15+kWh/day then that would be impractical.
