Buffer Tank Piping
Drawings on this page are from my online or live seminar

Buffer Tank - A Different Idea

All the piping diagrams on this website are simplified and do not include any devices required for actual installation. Please apply standard installation practices.

For decades, buffer tanks were piped using the four tapings, two for the boiler and two for the system. Recently, that has started to change. Instead of using all four tapings, many jobs now use two, three, or all four. You may also not want to maintain the temperature in the tank, but instead let it heat according to zone demands. If you pipe a two- or three-pipe tank application, I usually wouldn't maintain the tank temperature, depending on the application. This allows the use of a buffer tank for an alternate system temperature instead of using a mixing valve. For instance, if you have a high-temperature system and a small radiant load, the tank could maintain the lower temperature for the radiant in the tank and eliminate the mixing valve. The advantage of this would be that it has no moving parts. See the drawing at the bottom of this page. On a four-pipe application, I would consider maintaining the temperature in the tank. When you apply a buffer tank to a system, be aware of how to pipe the IWH into the system.

4-Pipe Connections
Buffer tanks normally have four pipe tapings, two designated to the system and two designated to the boiler flow.

2-Pipe Connections
Here is a thought: you use only two of the buffer tank tapings and pipe the system supply and return pipes to the boiler supply and return pipes. Why? You will get heat to the heating system faster and save fuel by not maintaining the tank temperature, unlike some people. When doing this, it is important to keep the supply tees close to the tank tapings to create hydraulic separation between the tank and the tees.

3-Pipe Connections
Another idea is using a 3-pipe application. This will keep the boiler running with cooler water temperature longer, and the boiler efficiency will be higher. When applying a buffer tank to a high-efficiency boiler or air-to-water heat pump, this is my go-to piping choice.

Flow in a buffer tank
This is what happens when the flow in the system is lower than the flow through the boiler. Notice the boiler water goes directly to the zone, but some water passes through the tank, cooling off the return water, causing the boiler to run at a higher efficiency.

When another zone opens in this example, the flow to the system is equal to the required flow in the system, and no water flows through the tank.

Next, when all the system zones are in demand, the system flow rate will exceed the boiler flow rate. As you can see by the text showing GPM flow in purple, the water temperature is lower than the boiler supply temperature. We will be adding a cooler return temperature through the tank; however, as the supply temperature increases, the return temperature also increases. Another advantage is that during most of the heating season, your demand will not be in all the zones, so that the tank temperature will store hotter water than a cold start.

While we are thinking out of the box, here is another way of using buffer tanks

Another scenario is that there is some low-temperature radiant in a home that also has high-temperature heating, like copper tube baseboard and/or hydro-air. The radiant could be added to the system with an aquastat to limit the tank temperature to approximately 120F. We would not want the radiant supply pipe before the tank, like the examples above, because the water would be too hot for the low-temperature radiant. Use all four tapings on the tank, two for the boiler loop and two to the radiant heat loop(s).
Using this application, all the radiants on the tank would have to be able to heat effectively, using the same water temperature. Since the tank temperature can be set using the tank aquastat, there would be no need to mix the radiant. I would use the aquastat as a limit control to a maximum setting for the application of the radiant heat. The setting would be dependent on the radiant heat application. Wire the aquastat in series between the zone control for the radiant and the boiler demand terminals. Most high-efficiency boilers today have more than one heat input terminal, and some even allow multiple temperature outputs. If not, piggyback the single heat demand terminals. Do not piggyback on DHW terminals, as we do not want this as a priority. If you do not have an IWH, you could use the DHW terminals, but make sure you turn priority off. Applying this piping as any other zone, it is critical to size the circulator properly for the radiant buffer tank. Of course, as with any piping application, you may have to change the piping slightly. Do not. This may also require a pump setting change on the pump parameter.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc and its personnel accept no responsibility for improper information, application, damage to property, or bodily injury from applied information found on this website.