But the answer here is pretty clear: Don't put automotive antifreeze in a hydronic system.
Those products are designed to protect an engine cooling system, so they have additives that are not necessarily friendly to hydronic pumps and seals. Along with freeze protection, you would get a load of problems.
If you must use antifreeze, make sure it's designed for hydronic systems, that it has the proper concentration of inhibitors, and that it's well mixed before you put it in the system.
That brings up another issue. Small systems are filled with water through a pressure-reducing valve from the city water supply. City water pressure usually is high enough to fill the system and establish a little extra pressure at the top to prevent boiling and allow venting. To fill a small system with antifreeze solution, you would need a mixing tank and pump, but people generally don't have access to that equipment. It might be a great idea to add antifreeze to a larger hydronic system, because coils can freeze if they are exposed to cold ventilating air. But in a large building with a large system, there are maintenance people and equipment to do it right.
Most small systems don't need antifreeze and additives for corrosion protection, anyway.
Unlike a larger system, for which renovations, system changes, and maintenance actions are probable, a small system rarely, if ever, is opened up and drained. A system that is tight - as it should be - does not require any significant amount of make-up water. That means the oxygen in the system is rapidly "locked up" in forming metallic oxides, and the system as a whole becomes very non-aggressive. In short, corrosion stops.
We routinely see small systems that have been in operation for decades with very little corrosion, simply because there's little loss of water. So there is no need to add new water, with its load of dissolved oxygen.
Given those drawbacks, most people simply set the thermostat down a bit and let the system maintain a high enough temperature to avoid damage. There are a few things you should double check:
Reprinted with permission from the Bell & Gossett newsletter CounterPointâ„¢, volume 10, issue 2, December 2003. Originally published in the November/December 2003 issue of the association magazine Plumbing Systems & Design. For more information, visit the Bell & Gossett Web site at www.bellgossett.com.
Publication date: 12/27/2004