By Francesco Lo Presti
Information taken from Gilles Legault’s, certified hydronic designer and trainer at CB Supplies, presentation on “Optimizing Energy Performance: Hydronic Heating Systems and the Role of Buffer Tanks” on Feb. 7.
Buffer tanks aren’t new in the hydronics market. They have been around for quite some time and play a crucial role in optimizing the performance of a hydronic system, serving as a key component that enhances overall system performance and efficiency.
These tanks act as temporary storage units for hot water, allowing for balanced and controlled heat distribution throughout the system. Buffer tanks also help reduce wear and tear on system components, extend the lifespan of boilers, and facilitate better integration with renewable energy systems.
Looking at condensing boilers and renewable energy sources, buffer tanks allow return temperatures to be at their lowest possible. This action can serve as a way of future-proofing a system. The lower the return temperature, the higher your condensing boiler’s efficiency and the higher the coefficient of performance (COP) of your renewable energy source will be.
The buffer tank also plays an important role in hydraulic separation, which refers to the segregation of the boiler and the heating piping loops. By incorporating a buffer tank, the boiler can efficiently separate these loops and maintain proper flow dynamics. The buffer tank will act as a hydraulic separator between both circuits, which allows them to operate independently. By using a buffer tank instead of closely spaced tees for hydraulic separation, the installer can ensure the lowest return temperatures to the heat source.
Necessity
Hydraulic separation is necessary between the boiler and the heating loops because there could be a large difference between the loops’ necessary flow rates and a minimum flow rate which is not respected in the boiler loop. If not taken into consideration, it could result in the malfunction of the boiler.
For example, let’s say that a modulating condensing boiler requires eight GPMs in the boiler loop and four GPMs in the heating loop and that the hydraulic separation is done through closely spaced T’s. Well, what can happen is some of the heated water being set to the heating system will come back in that closely spaced T, which would make the boiler reach a higher temperature than desired. However, a buffer tank will ensure minimum return temperatures since the return fluid will be from the bottom of the tank and the minimum possible temperature.
Another key benefit of having buffer tanks in a hydronics system is that the buffer tank can help prevent boiler cycling.
This cycling can cause inefficiencies within a system, unnecessary wear and tear on the boiler, and increased energy consumption. However, by incorporating a buffer tank into the hydronics system, a heating demand might be satisfied by the energy stored in the buffer tank instead of necessitating the start-up of the heat source.
Stratification
Going one step further, the benefit of having a buffer tank is stratification, which naturally separates water into distinct levels of different temperatures. Your buffer tank will have stratification, and there will be different water temperature levels.
For example, in a buffer tank where we achieve a Delta-T of 20 F, there will be stratification ranging from 100 F at the bottom of the tank to 120 F at the top. The amount of energy will be the same as having a blended temperature of 110 F. By having the hotter water at the top, it will allow the buffer tank to have a higher level of supply temperature to the heating elements. Having a vertical buffer tank rather than a horizontal one will also increase the amount of stratification.
Additionally, since a heat pump can do both hot and cold water, using a buffer tank can allow for the reversal of stratification, especially during summer. With this configuration, your buffer tank can use water to supply an air handler to cool a home.
Determining energy loads
Now, one of the questions people will have is how much energy can be stored in a buffer tank. The answer is based on the specifics surrounding the water and its density. For example, the graph above shows that water at 60 F will weigh about 62.37 pounds per cubic foot or 8.3378 pounds per US gallon.
Now, if you have a buffer tank with 80 gallons of water, the weight would be approximately 668 lbs. Let’s also add in stratification. So, for example, if you have a buffer tank with stratification of 160 F at the top and 90 F at the bottom, you will have 70 degrees of energy within the tank and therefore 46,760 BTUs. The math behind this is taking that 70 degrees and multiplying it by the weight of the water, which will then give you your BTUs.
We’ve looked at the benefits of buffer tanks and how much energy you can store in them. The next important step is knowing how to size the tank. A standard formula for calculating the tank size can be seen on page 53.
This formula involves the volume of the buffer tank, the cycle time, the heat source capacity, the smallest load/zone on the tank, and the temperature rise in the tank or Delta-T of stratification.
Now that we understand how to size the tank, let’s focus on how to pipe the tank. Typically, you will see either a four-pipe or two-pipe configuration. A four-pipe configuration consists of feeding the buffer tank on one side and releasing the energy from the other. This configuration allows for good hydraulic separation as all energy must pass through the tank.
In comparison, a two-pipe configuration allows the system to bypass the buffer tank. The reason for doing that? Well, let’s say the system and buffer tank are requesting demand. Well, this configuration can allow it to go directly to the system. This configuration isn’t as flexible and requires that the pipe going into the buffer tank is properly sized allowing for hydraulic separation.
There is also a three pipe-configuration, which allows you to feed your system directly from either the buffer tank or your heat source. In this setup, it’s important to have the load supply T as close as possible to the tank to allow for hydraulic separation between the two circulators.
Domestic hot water
Adding a buffer tank can also allow for domestic hot water production. A two pipe strategy would be best for this scenario because by adding a buffer tank, you will now have two fluids. Within that buffer tank, there will be a coil that will deliver the exchange of energy to the domestic hot water supply.
In this scenario, you will need to have the water temperature in the buffer tank higher to permit the supply of higher domestic hot water. There will therefore need to be a mixing valve to lower the heating system supply temperature.
You can also use a combination of instantaneous water heaters and buffer tanks. In combination with the buffer tank, the instantaneous water heat would be preheated by the energy taken from the buffer tank. This option could potentially be more cost-effective than an electric water heater or a natural gas water heater.