Residential hot water plumbing systems, layouts, and things to consider about flow rates and hot water recirculation and delivery.
Residential Hot Water Plumbing System Basics
We will be dealing with the hot water distribution plumbing and for our purposes the kind of water heater used is irrelevant.
Hot water distribution systems can waste both energy and water. They can also be irritating and stressful.
Standing at the shower door, holding your hand in the shower, while waiting for the hot water to arrive in the cold morning air isn’t the most fun way of starting the day.
Water Flow and Pipe Size
Often future home owners want to use one inch diameter pipe whenever possible because they think larger pipes will provide a higher flow rate. Not true.
The Federal Energy Policy Act of 1992 required all faucet / shower fixtures made the USA to have a flow rate of no more than 2.2 GPM at 60 PSI. Before 1992, older fixtures used much more.
Prior to 1992 fixtures could deliver 5, 6, even as high as 8 gallons per minute or more. With those kinds of flow rates sometimes larger pipe would be needed if several fixtures were running at once. A half-inch diameter copper pipe will typically be able to deliver over twenty gallons per minute at 60 psi.
The larger the pipe diameter the slower the water will travel for a given flow rate.
As an example, a 1/2″ diameter copper pipe, type K, holds .012 gallons per foot. That works out to approximately 80 feet of pipe per gallon. One hundred feet of the 1/2″ copper pipe would hold 1.2 gallons. A fixture flowing at 2 gallons/minute will take 36 seconds for the hot water to reach the fixture.
A 100 foot long piece of 3/4″ type K copper pipe holds 2-1/2 gallons. At a flow rate of 2 gallons per minute it will take one minute and 15 seconds.
A one hundred foot long 1″ diameter pipe would hold 4.3 gallons and the wait for the water would be over 2 minutes.
It actually takes more than the stated amounts of water for hot water to reach the fixtures because as the water travels through the cold pipe some of the heat is pulled out of the water. It can take up to 40% more volume than the pipe holds to get the hot water to the fixture.
The more slowly the water travels through the piping the more heat it gives up on the way. That means more hot water running water down the drain.
For the quickest hot water delivery and the least amount of wasted water use half inch diameter pipe instead of 3/4″ or 1″ pipe, but be sure to follow the local plumbing codes.
Hot Water Plumbing
Residential housing can be plumbed in a variety of methods. Some homes have branched plumbing where there is one or two main runs with short (or sometimes long) branch lines running to the fixtures. Some homes have looped plumbing where hot water pipe goes from fixture to fixture in series, and others have home run piping or manifold type plumbing.
Let’s take a brief look at the differences between the different kinds of residential plumbing layouts.
Looped Plumbing System
Looped plumbing is where the hot water pipes connect from the water heater to the first fixture, and from there connects to the next fixture, and so on with the fixtures in series. As you might suspect, this can lead to pretty long pipe runs.
Looped plumbing is mainly used when there is a hot water circulating system being used. A pump with a low flow running constantly can provide hot water instantly at every fixture. It will also lose a lot of heat energy and increase your utility bill.
This type of plumbing saves the maximum amount of water since you never have to run water down the drain to get your hot water, if you have a circ system of some type. If no pump is being used it will be a big water waster due to the long run of pipe.
Branched plumbing is when you run a main hot water line to the furthest fixture and tap off of the main line with short branch lines to each fixture. This is a very common method of plumbing. Using a recirculating pump to circulate the hot water will only fill the main pipe with hot water, so to get hot water at the other fixtures requires running the water contained in the branch line down the drain. If the branch line is short it’s not a problem, but for longer branch runs you can waste a lot of water.
Home Run Plumbing
Home run plumbing is where each fixture has its own separate pipe leading back to the water heater. Home run plumbing is the most difficult type of system to use with hot water circulating systems for obvious reasons. Home run plumbing wastes water.
Manifold plumbing systems
Manifold plumbing layouts usually use PEX plastic pipe. I can’t think of any advantage to a manifold system other than cost of installation. You usually end up with a 3/4 inch main run and probably 3/8 inch runs from the manifold to the fixtures. If the branch tubing from the manifold to the fixture is long it will save water because it holds much less water. But that makes it also more of a home run type system. A circulating system can be created by putting a return line and pump at the manifold, but that only works well with short branch lines.
Point of Use Hot Water
Point of use means you design your home with a floor plan that has all of the water usage areas located so that no pipe run from the heater to the fixture is more than about 8 feet. If you can design your home with such a floor plan then it is the greenest possible plumbing system.
Locating a small tankless water heater at each location such as the kitchen and bathrooms is also considered point of use, but can get a little pricey. This is a common application for electric tankless water heaters.
Pipe insulation is important! Pipe insulation obviously reduces the amount of heat lost from the pipe while the hot water is being used. In most cases pipe insulation will reduce the amount of time it takes to get hot water, reducing the amount of wasted water. Since the insulation will keep the pipe warmer, less heat is sucked out of the water as it travels to the fixture, thus reducing the time it takes for hot water to reach the faucet.
Pipe insulation can also reduce or eliminate thermo siphoning effects, further reducing the energy required to keep the hot water hot.
Common Sense Plumbing Layouts
Layout your residential plumbing system based on your floor plan and use common sense when planning the system.
Try to use the shortest possible pipe runs and use the smallest diameter pipe you can. If a fixture will rarely be used it might be better to run a branch to it and loop the rest of the fixtures. Maybe a branched system with one long branch would be better, maybe not. Each plumbing layout needs to be custom fit for your floor plan and usage patterns. At least that would be the ideal.
Hot Water Circulation Systems
There are a number of hot water systems on the market, but one thing they all have in common is that they work best on a looped plumbing system. With branched, home run, and some manifold systems you need two or more pumps to cover all the fixtures.
This is true whether the pump mounts under the sink or at the water heater.
You should now have a good basic understanding of how to design a green energy efficient and water conserving hot water plumbing layout.
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