Heat Transfer Fluid

A good solar collector should last the life of the building.

The heat transfer fluid (HTF) transports heat energy from solar collectors to the heat exchangers where it is converted to domestic hot water and/or space heat. When we select a HTF, we will be concerned with a) freezing point, b) boiling point, c) corrosion inhibition, and d) toxicity, e) service life, and f) compatibility with everything else in the system. Radiantec systems will use an antifreeze solution as the HTF for the reasons below.

FREEZING POINT - A solution of antifreeze and water can be used to prevent damage from freezing conditions. A 30% solution is actually enough to prevent freezing damage down to extremely low temperature because of the particular way that antifreeze solutions freeze. As the temperature drops, crystals of pure ice precipitate out of the antifreeze solution. When this happens, the concentration of the remaining solution increases. The result in a non-rigid slush that will not harm the solar collectors.

BOILING POINT - It is desirable to specify a HTF which has a boiling temperature that is higher than the maximum temperature and pressure that the collectors can reach under stagnation at so that the pressure relief valve does not operate and vent HTF.

CORROSION INHIBITION - Glycol type antifreezes tend to become acidic as they age because a degradation product of glycol is glycolic acid. Acidity is not good for heating systems because it promotes corrosion. An antifreeze solution should have buffering compounds that will neutralize any build-up of acids. When these buffering compounds are used up, acidity will increase and the HTF should be replaced.

SERVICE LIFE - Your local automotive shop or automotive supply store will likely have test strips that will test for temperature protection, acidity and reserve alkalinity. I would test at 1 week, 1 year, and every 5 years thereafter. If the antifreeze solution is not exposed to oxygen, the solution might last more than 20 years, but test to be sure. This one maintenance item will result in a solar system that lasts for the life of the building.

TOXICITY - There are two types of glycol antifreeze solutions commonly available; ethylene glycol and propylene glycol. Ethylene glycol has a lower freezing point and a higher boiling point than propylene glycol Ethylene glycol is also moderately poisonous whereas propylene glycol is used as a food additive and is commonly found in ice cream. Either material is biodegradable and can be disposed of in a municipal or private sewage disposal system. Either material will kill plants if applied directly. Ethylene glycol might harm pets or wildlife if improperly disposed of. Either material may cause bacterial bloom if disposed of in a body of water. It is our opinion that ethylene glycol should not be used unless the chance of contamination of the potable water is reduced to zero because the heating system relief valve is set lower than the building water pressure.

COMPATIBILITY - The HTF must be compatible with all other components in the system. In particular, check the expansion tank bladder, any pump seals and pump flange seals, seals in the air eliminator cap, etc. All Radiantec materials are compatible.

HEAT CARRYING CAPACITY - Glycols have a slightly lower heat capacity than water. It is a common misconception that this fact requires the selection of a larger pump to move the fluid faster as compensation. In fact, the difference is miniscule. Bear in mind that the fluid will be 70% water anyway.

LUBRICATION - Glycol has an oily consistency, and its use lubricates the pump for even longer service life.

WHY NOT USE WATER AS THE HEAT TRANSFER FLUID? - Many solar designs do this, particularly in milder climates. There are some cost savings, but in our opinion they are not worth it. Some designs use mechanicals to drain down the collectors whenever freezing conditions occur. These mechanicals have to work flawlessly for over 40 years and sad experience shows that they do not.

Another serious problem can occur with drain downs if power fails in the middle of the day. Under stagnation, the solar collectors go well above 212 degrees F. When the power comes back on, a steam explosion can do considerable damage and present a safety problem as well. It may not happen often, but it will happen someday.

Please remember that a solar heating system will only pay itself back and benefit the environment if it lasts a good long time. If something can go wrong with a mechanical device, it surely will given enough time; and it only has to happen once. You have to consider everything and you just cannot gloss over inconvenient details. Whenever possible design your solar system so that nothing can go wrong.