HOT WATER PIPING EFFICIENCY
The distribution of hot water has three phases with each phase known as an event. The first event is the initial delivery of the hot water. The second event is the use period of the water, and the third event is the cool down period of the water.
The first event, or delivery of the water, is dependent on a number of factors including the flow rate, the volume of water in the pipes between the water heater and the faucet, the distance between the water heater and faucet, if the pipes are insulated, and the initial temperature of the water in the pipes compared to the temperate of the water in the water heater.
Cold water is "thicker" or more viscous than hot water. The amount or volume of cold water that comes out of a faucet before the warm water begins to flow is higher than that of the warm water. In long runs of pipe, the actual volume of cold water "wasted" is larger than the volume of water that is used if only hot water was in the pipes. This amount varies between 10% to 40% between the cold and hot water amounts. In low flow devices, the percentage of cold water wasted is higher than in high flow devices. (Low flow is generally considered to be less than 1 gallon per minute.) A quick shower using a low flow head could actually use more hot water than a normal showerhead. A long shower using a low flow head would save hot water.
The second event, or use stage, has several factors that affect the outcome of the delivered water. The lower the flow rate, the larger the temperature loss of the water is. When flow rates are less than one gallon per minute, the temperature loss of the water is significant. The size of the pipe contributes to the drop in water temperate. The heat loss at a given flow rate is less in a 1/2" pipe than it is in a 3/4" pipe.
The type of pipe plays a factor in the water temperature loss. Un-insulated Pex-A1-Pex pipe has a higher heat loss than copper pipe. This is due to that the Pex-A1-Pex pipes have a larger surface area than the copper pipes so that when the pipes are hot there is more pipe, or surface area, to keep hot. Insulating the pipes reduces the difference between the copper and Pex-A1-Pex piping. Insulating the water pipes helps to decrease the temperature loss at comparable flow rates.
The third event, or the cool down stage is a common problem in residential construction. The obvious note is that the longer the time that the pipes sit after hot has been run through them, the more the pipes will cool down. The larger the size of pipe, the longer it takes for the useful hot water to cool down. Insulation extends the amount of time it takes for the pipes to cool down. With un-insulated copper pipes, it takes an average of about 10 minutes for the water to cool down. When those pipes are insulated the cool down time is increased to about 30 minutes. In residential use where hot water may be ran at 10 minute to 30-minute intervals, the use of pipe insulation reduces the amount of water used in the delivery stage.