WJcandee
Wise One
Reach is on to something. It's not definitive either way from what you have told us, but if you do not have an open system, then one normally needs an expansion tank. An expansion tank isn't going to solve the problem if you have a malfunctioning pressure regulation valve at the main, but in a closed system without an expansion tank, the pressure can go pretty darn high when the water heater is on. The key to whether you need it is whether there is a backflow prevention valve between the water main in the house plumbing. Sometimes recently, such a valve is part of the water meter unit. The EPA has been wanting municipal water systems to insist that their customers have a backflow prevention set up, to prevent any contamination of the household system from flowing back into the main and into other people's houses. As you may know, in an open system, in other words one without a backflow preventer, the main picks up the expansion; that is, your hot water heater will cause an expansion of the water in the pipes, and pipes not being flexible, will result in an increase in volume throughout your house water system, which, when the system is open, that extra pressure simply pushes out into the water main and water system, where it is essentially absorbed, because it's small potatoes compared to the rest of the municipal system. Where the system is closed, the limited amount of piping in your home will simply contain the increased volume of the water, as it cannot flow back into the main, and thus the pressure within the pipes will rise dramatically. An expansion tank takes up that extra expanding water, leaving the water pressure relatively steady. But if in your rural area, they have not installed any valves that would make your system a closed system, then you probably found the culprit and would not need an expansion tank. Many PR V's have the effect of closing the system, so if you in the future find that your water pressure is rising, then an expansion tank would be in order. A lot of people got away with not having expansion tanks for a long time, because they did not realize that the weakest valve in the house was functioning as a relief valve. In other words, when the hot water heater was on, a hose bib outside or a toilet tank fill valve would simply start flowing slowly when the pressure reached the valve's resistence, and the homeowner was none the wiser.
