Options for multiple boiler plants

One of the main goals when installing multiple boilers in an application is to increase operating efficiency by being able to more accurately match the boiler plant’s output to the load of the building. Another resulting benefit includes gained redundancy. If one boiler is down for maintenance purposes, the remaining boiler(s) should still be able to maintain the building load for the majority of the heating season, since the overall plant is sized for the coldest day of the year.
While for the majority of the season, only a portion of the boiler(s) in a multiple boiler installation is needed, the addition of equal run-time rotation through implementation of a control system can further extend the life cycle of each of the boilers installed.
There are typically four different piping options you may see deployed in the field. Those options are commonly referred to as direct return, reverse return, primary/secondary and parallel primary/secondary. A review of the features and benefits of each will help you to determine the most appropriate choice depending on the circumstances.
The direct return piping system shown in Figure 1 mainly lends itself to high mass boiler applications. The supply and return from each boiler are piped into the supply and return of the common and main system/building loop. All boilers being equal, without the means of balancing, the single primary loop pump would provide unbalanced flow through each boiler.
Since water takes the path of least resistance, the first boiler being the one closest to the primary pump would naturally see the highest flow, whereas the last boiler would see the least amount of flow. It is therefore important to install balancing or globe valves at each boiler so that the flow through each can be balanced to achieve equal flow. Equal flow will also impact the deltaT across the boiler and ensure the best operating efficiency.
The benefits of this piping arrangement include that it is conceivably the easiest to install, it is cost effective with the least amount of pipe used and all boilers see the same system return water temperature. The negative aspect of this piping arrangement is that the operating efficiency can be compromised if the flow is not properly balanced through each boiler. This piping also results in the biggest stand-by losses with water flowing through boilers that are sitting idle, incurring additional stand-by losses through the boilers chimney(s) and jackets.
The reverse return piping system also, shown in Figure 2, lends itself to high mass boilers. Here, each boiler’s supply and return are piped into the supply and return of the common and main system/building loop in a fashion that ensures equal amounts of piping for each. The result is that the first boilers supply will end up being the last return going into the main building loop. Since the piping is equal length and the distance the water will be forced to travel through all boilers is equal, this piping method inherently self-balances the flow through each of the boilers without the added cost and proper adjustment of balancing valves through each. Equal flow will impact the deltaT across the boiler and therefore ensure best operating efficiency.
While it utilizes more piping than that of direct return, the benefits of this piping arrangement include that it is still conceivably easy to install and all boilers see the same system return water temperature. This piping also results in bigger stand-by losses with water flowing through boilers that are sitting idle, incurring additional stand-by losses through the boilers chimney(s) and jackets that are unnecessary when compared to the remaining two piping options.
Before discussing primary/secondary systems, the only reason this particular boiler piping arrangement is included here is because you will see it occasionally in the field. While it is in many ways a favourite piping arrangement for heat distribution piping systems, it is in no way the piping arrangement of choice for me in near boiler piping systems. It has one very critical downfall that will be discussed below.
The primary/secondary piping arrangement lends itself to both, high and low mass boilers. It has all boilers connected separately to the primary loop through means of hydraulic separation. Hydraulic separation in this example can be achieved through engineered fittings or sets of closely spaced tees to eliminate the pumping in series of the boiler pump(s) and the primary pump. Closely spaced tees are shown in the piping example in Figure 3.
The benefits of this system include that it is easy to pipe boilers in this fashion. The installation of a dedicated boiler pump for each boiler ensures adequate flow through it at any time it is needed. It also allows the stand-by losses to be minimized in boilers that are not firing by not running the warm system water through boilers that are off, therefore eliminating losses through the boiler’s jacket or chimney.
Most low mass and condensing boilers these days come with their own controls that integrate pre- and post-purging of the boilers with the use of the boiler’s pumps. Some even control the speed of the pumps based on deltaT while the boiler is firing. Many boilers won’t fire unless they have been through a pre-fire process using their own onboard controls.
The downfall of this particular installation method is that each boiler sees a different return water temperature that can greatly impact the efficiency of the boilers downstream, especially if the system uses condensing boilers. To summarize again, while this piping method successfully hydraulically isolates each boiler from the primary loop, most condensing boiler applications would suffer from the inherent downfall of the cascading return water temperature.
Parallel primary/secondary is often the most effective and beneficial multiple boiler piping option. It combines the parallel piping for each boiler to ensure equal boiler return water temperature to each boiler, dedicated boiler pumping and then the integration into the primary or system loop via means of a common hydraulic separation. This again, eliminates the potential for pumping in series between the boiler pump(s) and the primary system or loop pump. Just as before, hydraulic separation can be achieved through specifically designed engineered fittings, or a set of closely spaced tees or a hydraulic separator as shown in Figure 4. Hydraulic separators are often designed to include features such as air and dirt elimination, which are a crucial addition to any closed loop heating system.
As is the case with the primary/secondary benefits stated above, this piping method also increases operating efficiency of the boilers, and minimizes stand-by losses through the boiler’s chimney or jackets of the boilers that are not firing.
While any of these piping options can be deployed for various applications and for various reasons, the most dominant piping method I see in the field is the parallel primary/secondary. Overall, it provides optimal equipment performance, minimizes stand-by losses, and is the easiest to add boilers to down the road, if required.

Mike Miller is chair of the Canadian Hydronics Council (CHC) and director of sales, building services with Taco Canada Ltd. He can be reached at hydronicsmike@taco-hvac.com.