Failsafe systems for solar hot water

As I mentioned in earlier blog entries, my system has a number of failsafe features to maintain system heat and pressure within tolerable boundaries. In the summer, the system dumps excess heat into the swimming pool, and in the winter it dumps the excess heat into a radiant water heater in the basement.

This spring I got to see what would happen if I didn’t have these alternate heat sinks and I found out how robust the failsafe systems are in my installation.

Here’s what happened. To change my secondary (failsafe) heat sink from the basement radiant water heater to the swimming pool for the swimming season I need to open or close 3 manual valves. This spring, due to a brain malfunction, I only switched two of the three valves which meant that I had shut off the alternate heat sinks entirely. When I then manually switched the compter controlled valve (that determines whether to send water to the primary or the secondary heat sink) to use the “secondary” heat sink to dump heat, I had inadvertently gotten rid of all the system’s sinks. Effectively, I was just taking hot water from the solar collectors and circulating it back to the collectors without dumping any heat.

The first thing I noticed was that the pressure in the system built up (quickly) to 80 psi, from its normal 40 psi. This immediately got my attention and I looked at the collector temperature and found that it was 120 C – yes, that is degrees celsius, which means that the system was now boiling. I looked at the valve settings and realized my mistake and adjusted the valve settings appropriately, but by that time, it was too late … the failsafe features of the system had kicked in. My system had begun dumping antifreeze into a five gallon drum to reduce the pressure in the system. By the time I had realized this, the system was almost empty and the pressure had dropped to about 60 psi (by this time almost all vapour).

When I called my installer, he explained to me what had happened and suggested hooking the cold water from the city up to the antifreeze loop and filling the loop with fresh cold water. This had the effect of reducing the temperature and pressure immediately and everything and everyone calmed down immediately.

So what did I learn. Well…first and most important, even if you understand your solar system, you should not take it for granted. Keep checklists that help you with seasonal conversions and follow them. Second…the failsafe systems work. Even if I had not noticed the problem, the system would have emptied and the worst that would have happened is that my pump would have burned out. Finally, I learned how important it is to have your installer (or other expert’s) number close at hand…in my case the installer’s cell phone number was dymo-taped onto my controller and I was able to get hold of him – even though he was up at the cottage.

Does this make me less happy with my solar hot water system? On the contrary, I now know that even if I screw up, the family will still be safe, and I will be able to fix up the problems without spending a lot of money.


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