Solar Hot Water Systems - PV vs delta T control [RE-wrenches]

[Allan Sindelar]

    We routinely use PV-direct on the solar thermal input source loop, with
a ~50/50 glycol mix. This uses the same mix that circulates through a heat
exchanger and even through PEX tubing in infloor systems. We approach it
either of two ways: 1) module > pump; end of story. As the PV wakes up
before the panel does, you cycle a little cold fluid. 2) add a low-limit
snap switch (Grainger part #4E116), mounted on the collector supply loop
just off the top of the collector header, that turns on the pump when
threshold temperature is reached.
     The latter option is more thermally efficient, but adds one more piece
of hardware and attendant wire, etc. that could fail. Failure of the pump to
work when needed leads to stagnation, loss of pressure at the PRV, and a
service call. Some would prefer maximum reliability over maximum thermal
performance. And by the same rationale, if your differential t-stat is
AC-powered, will a utility failure mean a service call?
    Incidentally, we had one callback where snow didn't slide off the PV
module as quickly as it slid-or-melted off the thermal panel, and the system
blew off pressure. We moved the PV module to fix the problem.
Allan at Pos E

----- Original Message -----
From: "David Blittersdorf" <davidb at nrgsystems.com>
I am looking at the trade-offs of going to the PV powered pump
> system vs the delta T control. I like the temperature sensor system as it
will not turn on
> the pump when the stored hot water is warmer than the collector
temperature.

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