SDHW overheat diversion [RE-wrenches]

[Darryl Thayer]

Thanks Brad
I think I will try something on my next install,
however I have several systems over 20 years with the
same antifreeze, many of these are painted absorbers. 
The one system I had a lot of problems with was a
double glazed selective surface, The stagnation
temperature was very high, and the system would boil
antifreeze at over 45 PSIG. 

I think a radiator in the attic might surfice. 
DT 

--- bbassett at rockisland.com wrote:
> 
> 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. 
> 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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