This is a cold water pipe just in front of the hot water heater it is feeding. Why is it shaped that way? What is its function?
It is highly likely that that piping arrangement is made that way to facilitate the installation of a water softener. A water softener will have a need to accept a pipe with incoming hard water and another for the outflow of conditioned water to the rest of the residence.
When a water softener is installed the U shaped fittings at the bottom of those two pipes would be removed and replaced with a new setup that usually consists of three or four valves. The valve setup allows for bypassing the softener, allowing water into the softener and also a configuration to backflush the unit. These days the whole valve system is often all integrated into a control unit with solenoid valves so that the controller can manage the flushing under electronic control.
Looks to me like a heat trap, intended to keep hot water from flowing back through the cold water inlet.
A connection for a water softener would be arranged so that all water to the house (with possible exception of the outdoor faucets and the kitchen cold) could be routed through the softener. Softening just the hot water makes little sense.
Without knowing the rest of the layout, I can't be 100% certain, but there a few possibilities:
- As others have mentioned, it could have been placed that way because the original owner intended to fit a "softener" or "filter".
- Since the house is over 20 years old, its possible there was previously some other device fitted that has subsequently been removed.
- It's possible it is designed to limit backflow, for example when the inlet pressure drops, or supply is turned off for maintenance.
- Similarly, it could be designed to stop air bubble making their way into the heater and becoming trapped.
@Michael Karas has it right. This website has a DIY that explains things rather well, complete with diagram and YouTube video. They recommend 10ft of pipe between the water softener and water heater to prevent backflow of hot water into the softener, similar to @Hot Licks heat trap, but I personally would use a check valve, which allows flow in only one direction (e.g. into the water heater.)
If you eliminate the outdoor faucets and kitchen cold from water softener, as both @Hot Licks and the above website recommend, there isn't that much water usage left unheated, 2 or 3 toilets, the cold on 2 or 3 sinks and 2 or 3 tubs/showers, maybe an ice maker in the fridge and maybe a dishwasher (hot and cold), perhaps a clothes washing machine (hot and cold).
So, @Hot Licks, since we don't know anything about the provisioning for softened water to the remaining cold usage points (ice maker, toilets, etc.), I would bet there's an easy way disconnect them from the incoming supply and connect them to the softener.
Regardless, softening just the hot water still can make sense. The hard water in areas with limestone and chalk, dissolved by weakly acidic rain containing carbonic acid, gets its hardness from calcium hydrogencarbonate (CaHCO3)2. When heated, calcium hydrogencarbonate breaks down into CO2(gas), H2O (liquid, obviously), and calcium carbonate (CaCO3, solid), also called limescale in this scenario. This can clog pipes and encrust the heating elements of a water heater, causing poor heating efficiency and hastening the burnout of electric heating elements. In areas with hard enough water, by keeping energy usage down and preserving the life of the water heater, a water softener just for the water heater can pay for itself long before you pay off the mortgage.