Propylene glycol is classified as non-toxic, while ethylene glycol is toxic. This
difference is critical to occupant safety. Generally, a toxic solar fluid should only be
used whenever the heat exchanger is of double wall construction. If a single wall
exchanger is used, a non-toxic fluid should be used. This is particularly important
when the storage fluid is potable water, as with a DHW system.
Toxic fluids should also be stored and handled carefully. Access by children and
pets should be avoided.
Because the glycols eventually break down chemically, annual testing is required.
Piping systems to be filled with glycol mixtures can be pressure tested and flushed
with water before the glycol/water mixture is put in the system.- It is important to
remove "hung-up" water during the filling process to avoid diluting the fluid.
Pump gaskets and seals, expansion tank diaphragms, valve seals and seats and
other elastomers and plastics must be compatible with glycols. Typical acceptable
materials are EPDM, Hydrin, Viton and Teflon. Thread sealants must be Teflon-
based. See Appendix E for a complete listing of fluids and materials compatibility.
Glycol/water mixtures have a lower surface tension than water. This results in a
"leakier" fluid, making tight joints more important than usual.
The major advantage of synthetic oils is their almost unlimited lifetime.
Furthermore, unless the fluid leaks out of the piping system, virtually no
maintenance is required. Toxicity is low as well.
The specific heat and thermal conductivity of synthetic oils is considerably lower
than water. These factors increase pumping and heat exchanger requirements and
costs. Synthetic oils attack more materials than the glycols, and can damage
roofing materials if spilled. Materials resistant to degradation include Teflon, Viton
and Hydrin. Thread sealants must be Teflon-based. Appendix E contains a
complete compatibility listing.
The surface tension of synthetic oils is very low, resulting in a fluid which is even
more difficult to confine in piping.
Synthetic oils must never be mixed with water, and must be introduced into a
perfectly dry piping system. Water must never be used for pressure testing or
2.7 COMPONENT OPERATION