This information was taken from Rick Mayo, Western product and application instructor at Taco Comfort Solutions, presentation on “DHW Re-Circulation Systems” on April 24.
Is there anything better than turning on your shower and immediately feeling hot water wash over your body? I think it’s safe to say that most Canadians can attest to this sweet luxury’s appeal, especially during our colder months.
Installing a recirculation system is one method for ensuring that the overall efficiency of the plumbing system is reaching your customers’ expectations. At its core, it helps minimize water waste and provide the desired temperature quickly.
The beauty behind recirculation systems is that there isn’t just one solution — various configurations can be utilized depending on the user’s needs. To start, there are traditional recirculation systems. These consist of a dedicated return line that continuously circulates hot water from the water heater to each fixture in the building. It should be noted that these types of systems can lead to increased energy consumption due to the constant circulation of hot water. Ensuring the system is appropriately sized can help mitigate this concern.
Another option is to design a partial circulation system. This system employs a pump at the water heater and a temperature-controlled valve at the furthest fixture. The pump circulates water when the temperature at the farthest fixture drops below the set temperature, reducing energy consumption compared to the traditional method.
On-demand recirculation systems utilize a thermal sensor valve that opens to allow hot water into the cold water line when the temperature drops. The reason is that the thermal sensor valve allows instant hot water delivery without needing a dedicated return line.
Get your pump on
The hot water circulation pump is like the heart of the recirculation system. It helps circulate hot water continuously, keeping the water in the pipes hot and ready to be used instantly at each fixture. The sole responsibility of the circulator is to have water at the fixture.
There are several ways to achieve instantaneous hot water depending on the layout of the building. In new construction projects, you will have full control over the design and layout of the building. However, in retrofit projects, you are dealing with a layout already created.
In Figure 1, you can see the hot water tank’s energy delivery through the system. In that delivery, the energy is taken and directed to close proximity of everything the hot water serves. To make this concept more efficient and effective, it is crucial to think of the system as an energy delivery system that puts the source right next to where you want to use it. This can be referred to as fixture groups.
If it’s broken, fix it
Now, let’s look at what happens when a system fails. A system can fail due to various factors, such as sprays or leaks. The reasons why a system fails vary, but there are four common reasons: elevated water temperatures, fluid velocity moving through the pipe, added chlorine, and continuous circulation.
It is worth noting that the velocity of the fluid can be controlled when moving through the pipe. There are ways to slow the water down. A key area of focus is ensuring that the circulating pump is not oversized and that the return piping is not undersized, as both are common occurrences in installed piping systems. This is crucial in controlling the velocity of fluid moving through the pipe. You can see an example of how to properly size the pipe for the GPM maximum flow in Figure 2.
Circulator basics
Two things need to be determined when properly sizing the circulator — finding out what the GPM is and establishing the feet of the head.
There are six steps or considerations you can use to determine what the GPM is. The first step would be figuring out the line or heat losses of the supply portion of the system to the hot water fixture. If done correctly, and the numbers are accurate, that will provide you with the minimum flow required to overcome the line losses. This will become a continual GPM requirement, resulting in the pump running continuously. Most people will typically use
this consideration as the main source of determining the GPM.
Secondly, how quickly do you purge the lines? If the circulator is shut off and turned back on, how quickly do we need to evacuate all that tepid water and bring the hot water system back up to a usable temperature? The third consideration is understanding the maximum velocity limitation based on the material inside the system.
Another consideration is the minimum flow through mixing valves.
If you are using tankless units, you will also need to consider the minimum flow requirements. You will also need to figure out a strategy to control the pump.
These six considerations will drive the GPM requirement. However, as long as you are insulating the hot water piping system and the dedicated returns, the heat losses will generally be the lowest GPM number, and you could only use that as your criteria number. Once you have the GPM, you can determine the accurate number of feet of head.