It all depends on check valves Mike. My system, which is a boiler and does not have a hot water heater...it has a hot water storage tank instead. Big difference!
Originally Posted by Mike Henderson
My boiler is a closed circuit heating system. The only reason it is tied in with my water system at all, is to make sure the water inside the internal boiler loop,does not evaporate the water away during operation. It is tied into my water system via ½" line that injects the water into the boiler as needed. This passes through a check valve that isolates the boiler water from my domestic hot water.
On the storage tank end, my boiler water never comes in contact with my domestic hot water. It circulates through a coil which in turn transfers that heat to water in the tank. For safety reasons, any loss of pressure in the internal boiler circuit, shuts off the boiler. That is to prevent nasty internal boiler loop water from getting into potable domestic water.
Now in a water heater you are indeed right, thermal expansion of water, upon being heated, could flow back into the pipes and expand harmlessly...as long as the plumbing system is long enough that is. If the water heater is plumbed to heat say just one facet not far from the heater, then problems could result, especially when using plastic pipe. There would not be enough room for the expansion to take place.
Another area that is often overlooked, but also against code (at least here in Maine) is in using a water heater as a heat source for radiant floor heating. In that case you are using the water heater as a domestic heat living source and while not permitted, is often done. Not using a thermal expansion tank in this case could prove to be catastrophic because of the changing thermal dynamics that the system is constantly undergoing.
Keep in mind too Mike that the principal role of a thermal expansion tank is to control variances in pressure, not just thermal differences that create differences in pressure. There are other instances that cause pressure spikes.
After installing my system I was amazed at how often the expansion tank does its job. Just about every time the boiler comes on (modulates) the expansion tank comes into play. I am sure this happens more on my system because my boiler drops to 80º before rising back up to 180º, but a hot water heater cycles as well, just not so often or to such temp extremes. Still I can see why some municipalities require them now. For 60 bucks, its cheap insurance.
I have no intention of traveling from birth to the grave in a manicured and well preserved body; but rather I will skid in sideways, totally beat up, completely worn out, utterly exhausted and jump off my tractor and loudly yell, "Wow, this is what it took to feed a nation!"