SDHW overheat diversion [RE-wrenches]

[bbassett at rockisland.com]

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I believe that I sense a movement toward protection from overheating in 
solar domestic hot water systems, probably due to potential liability 
issues. I've noticed that there are some installers that have decided to 
make their antifreeze system capable of being left in a stagnant 
condition. This will indeed prevent overheating of the water in the 
solar storage tank. This is accomplished by having a huge expansion tank 
that can absorb the large increase in volume when the fluid boils in the 
collectors, or in one potential case by having a semi drainback 
antifreeze system that shuts down and drains the collector when a high 
temperature is reached in the storage tank. 

Regular, water only, drain back systems have been made to do this all 
along, and I see no problem with it, other than maybe being hard on the 
collector, both from being very hot and from being hit with water while 
it's very hot. I'm not sure if this is a problem for longevity of the 
collector or not.

If anyone reading here is thinking along these lines, I would like to 
say that I think this is a REALLY BAD IDEA. When any of the glycol type 
antifreezes (propylene or ethylene) are exposed to stagnation 
temperatures reached in a collector, at up to 350 F, they will degrade 
and become acidic. This acidic fluid will then corrode the copper 
collector and all of the balance of the plumbing. Premature failure of 
the system is almost certain. Even if the fluid is no longer present in 
the collector in bulk, there is always a film left on the inside of the 
pipe when drained, that will then degrade. We do not need failing  
system problems in our industry!

I have now installed in my own system, a method to prevent overheating 
of the solar storage tank. In the solar loop I have installed a high 
temperature range anti-scald valve as a diversion valve. Most are OK'd 
by the manufacturer for this use in heating systems. This diverts solar 
fluid to a heat exchanger where it can dump the excess heat safely. This 
keeps the entire system from overheating. In my case the diversion 
radiator is a few parallel copper pipes in the crawl space under my 
house. It could also consist of a stretch of baseboard heater under the 
eave, or other innovative setups. I'm sure some of you can come up with 
good ideas for this. So far mine has worked flawlessly and clamps the 
temperature quite accurately. 

One thing I have found to be aware of is that these valves have fairly 
high resistance to flow, with Cv around 3.9 for the best ones, and worse 
for others. My PV direct pumped system dropped from about 2 gpm to 1.6 
gpm flow with the addition of this valve. Not a problem for mine, but if 
your system is marginal for flow it would be a consideration. I have 
found a valve made for the heating industry where the accuracy of an 
anti-scald valve is not required and would present much less resistance 
to flow, but I've hesitated so far because it is only available in cast 
iron. I don't like iron in my systems, though it's supposedly OK with a 
closed loop. 

If you try this on any of your systems, please let us wrenches know how 
it turns out. Any thoughts?

Brad
AEE Solar

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