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Differential thermostats are available with high temperature protection and
low temperature (freeze) protection.  High temperature protection is
important, especially in evacuated tube collectors, in that it prevents a
very "hot" collector from suddenly receiving a supply of cold water thus
producing a thermal shock that could damage the collector components.
Another type of control called proportional control is available.  It is
similar to the ON/OFF differential controller in operation.  The difference
is that the proportional controller changes the threshold ON and OFF points
and controls the flow such that less than full flow can be achieved if the
sun is at less than full intensity.  The advantage is that the proportional
control can "turn on" the system when the other controller (the ON/OFF type)
is waiting for more sun to become available.  This is an advantage on cloudy
days and early morning start ups.  Overall system efficiency is increased
slightly with the proportional control.  These controls are more expensive
and one such experiment at NCEL has shown that proportional controls result
in considerably more cycling of the pump motor which could shorten pump life.
It is recommended that the control manufacturer be consulted on this point
before a proportional control is used.
As the building requires heat, other controls must direct pumps or blowers to
provide heat from the storage tank to the load.  This control is the
conventional thermostat.  The same room thermostat may control the auxiliary
heater; however, a delay timer or a two-step room thermostat must be
incorporated into the auxiliary heater control circuit so that the auxiliary
heat will not come on if heat is available from storage.  Ten minutes has
been suggested as a typical time delay before auxiliary heat comes on.  Some
manufacturers supply combination thermostat and solar system controls.
2.9 Piping, pumps, valves.
2.9.1 Pipe and header sizing.  Piping should be designed for low pressure
drop.  All exposed piping should be well insulated with approved weather-
resistant insulation.  Dielectric unions should be used at connections
between dissimilar metals.  Rubber or silicone hose used for connections must
be of a high temperature type.  The pipe ends should have ferrules to provide
a good seal with the hose.  In low pressure systems, spring type clamps are
preferred because they compensate for thermal expansion.  Copper pipe is
preferred to galvanized steel due to its longer life expectancy and relative
ease of installation.  Thermal expansion should be provided for all piping or
hard tubing.  Pipe sizing should be in accordance with recognized methods,
but for most installations the following estimates are reasonable:
For a single row of parallel collectors (see Figure 2-6) with "X"
number of branches, 0.5 gpm flow per collector, water or 50% glycol
as heat transfer fluid.
Up to 3 collectors
1/2-inch headers
4 to 7 collectors
3/4-inch headers
8 to 12 collectors
1-inch headers
13 to 18 collectors
1-1/4-inch headers
More than 19 collectors
1-1/2-inch or larger (size for each


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