Most hydronic heating systems that have renewable energy heat sources, such as solar thermal collectors, an air-to-water or geothermal heat pump or a biomass boiler also have an auxiliary heat source. In most cases, that heat source is a boiler, especially when an existing system is being retrofitted with a renewable heat source. 

Many of these systems also have one or more thermal storage tank that absorbs excess heat when it’s available from the renewable heat source, and park it for later use. The thermal storage tanks used in systems with solar thermal collectors and biomass boilers often contain several hundred of gallons of water. 

The auxiliary heat source can be treated as a backup to the renewable energy heat source. If the renewable energy heat source is unable to provide any useful heat to the system, the auxiliary heat source provides all the heat the system requires. This implies that the auxiliary heat source must be sized to handle the system’s full design load.

The auxiliary heat source can also be treated as a supplemental heat source. This refers to a situation where the renewable energy heat source is providing some of the heat the system requires, while the auxiliary heat source provides the remainder. In this capacity, the auxiliary heat source would not necessarily be sized to handle the system’s design load.

In either of these roles, it’s important that the auxiliary heat source is only operated when necessary. It’s also important that heat produced by the auxiliary heat source flows directly to the load rather than into thermal storage.  

The rationale for the latter is based on the second law of thermodynamics. Fuels such as natural gas, fuel oil or electricity are “high grade” energy. It’s easy to store high grade energy for long periods of time without degradation. However, when high grade energy is converted into heat, which at the temperatures needed for space heating is a relatively low grade energy, storing it for more than a few hours is difficult and expensive. 


Think about this

If a 500 gallon tank contained water at 150° F, and was located in a room at 70° F, how long could the heat it contains be stored? 

Regardless of how well insulated the tank was, heat would immediately begin “leaking” from it. The insulation system on the tank would affect the rate of heat loss, but no insulation system can totally stop heat transfer as long as there’s a temperature difference between the inside and outside of the tank. The moral: Don’t use high grade energy to keep a thermal storage tank at an elevated temperature while waiting for a load that eventually needs that energy.


This does happen 

One of the undesirable characteristics that I’ve witnessed on several systems with renewable heat sources is an inadvertent set of circumstances where heat created by the auxiliary boiler ends up in the thermal storage tank. 

In one system that I had opportunity to visit, the thermometer on a 700 gallon thermal storage tank associated with a pellet boiler showed an internal temperature of 145°. That, in itself, isn’t a problem. However, it became a problem when the person responsible for the system told me that the pellet boiler had not operated in over a month due to a maintenance issue. 

So how was the tank maintaining that elevated temperature when the boiler intended to heat it had been off for a month? 

The answer was a 3-phase electric auxiliary boiler. To add salt to the wound, this system was in a location where electricity was provided from large diesel-powered stationary generators, and thus it was quite expensive. 

This system had building automation controls that staged the pellet boiler and auxiliary electric boiler, but those controls didn’t “understand” that the pellet boiler was not operating due to some fault condition. The control system kept the circulator between the thermal storage tank and distribution system operating as it supplied heat to the system from the electric boiler, which was treated as a second stage heat source. The water returning from the distribution system flowed in and out of thermal storage, and thus unnecessarily maintained it at an elevated temperature. While it’s arguable that this doesn’t damage the system, it certainly adds to uncontrolled heat loss into the mechanical room. 

This inadvertent condition is usually the result of controls that treat the renewable heat source portion of the system as fixed first stage heat input, and the auxiliary boiler as the second stage heat input. That’s how a tandem boiler system would typically be controlled, and thus it’s how many control systems would be routinely configured. 

In most cases, the controls just assume that both stages of heat input are available to contribute heat to the system whenever necessary. The controls don’t necessarily verify if this “assumption” is valid. If the first stage heat source can’t provide the necessary supply water temperature to the distribution system, the controls activate the second stage heat source. The first stage heat source remains active (e.g., the “go” signal to the first stage heat source is on, and so is the circulator that moves water through that heat source).

This is not a “big deal” when two identical boilers serve as the first and second stage heat inputs. However, when a large thermal storage tank is involved, there is much greater potential for uncontrolled heat loss due to its high surface area and large thermal mass. Furthermore, if the circulator between that tank and a biomass boiler remains on when that boiler is not producing heat, the boiler’s jacket and air flow through the combustion chamber further increase uncontrolled and undesirable heat loss. There is no automatic flue damper in a biomass boiler to limit off-cycle stack losses.


This doesn’t need to happen 

There’s a relatively easy and inexpensive way to prevent this undesirable condition. To understand it, consider a system that uses a biomass boiler and its associated thermal storage tank as the first stage heat input. 

Think of the biomass boiler and thermal storage tank as a single entity that we’ll call the “biomass heat source.” Heat supplied from this entity might come directly from the biomass boiler(s), or from thermal storage, or from both at the same time. It depends on the firing status of the biomass boiler(s) and the status of the load(s).

The “biomass heat source” is treated as a fixed lead heat source to the distribution system, in combination with and auxiliary boiler as the second stage heat source. This combination is shown in Figure 1.

Figure 1

The “biomass heat source” is connected to the distribution system using a pair of closely spaced tees. This provides hydraulic separation between circulator (P2) and the distribution circulator (P4). Circulator (P2) could be a fixed speed circulator or controlled as a variable speed injection circulator. The latter allows it to regulate the supply water temperature of the distribution system based on a desired set point or using outdoor reset.

The white rectangle labelled “anti-condensation details” represents some provision for maintaining the inlet temperature to the biomass boiler above dew point whenever possible. There are several hardware possibilities for this detail. The three commonly used hardware configurations for this detail include a 3-way thermostatic mixing valve, a motorized 3-way mixing valve or a “loading unit” which combines a thermostatic mixing valve with a circulator, and thus eliminates the need for circulator (P1).

The auxiliary boiler is also connected to the distribution system using a pair of closely spaced tees for hydraulic separation. These tees are located downstream of the tees that connect the biomass heat source to the system. This arrangement allows the thermal storage tank to contribute heat to the distribution system at lower temperatures than would be possible if the tees for the auxiliary boiler were upstream of those from the biomass heat source.

If the auxiliary heat source is a conventional boiler (e.g., one that is not designed to operate with sustained flue gas condensation), the distribution system will, at times, operate at low water temperatures, (typically below 130°), then an anti-condensation detail should also be included to protect the auxiliary boiler.


Positive contributions only

The key to preventing inadvertent transfer of heat produced by the auxiliary boiler into thermal storage is comparing the temperature of the water returning from the distribution system to that at the upper tank header. As long as the temperature at the upper tank header is a few degrees higher than the temperature of water returning from the distribution system, the biomass heat source can make a positive energy contribution to the space heating load. This control function is easily handled using a differential temperature controller. 

Figure 2

Figure 2 shows a differential temperature controller, labelled as (T156) comparing these two temperatures at sensors (S3) and (S4). Circulator (P2) is only allowed to operate if the temperature at the upper header of the thermal storage tank, at sensor (S3), is at least 5° above the temperature at sensor (S4) on the return side of the distribution system. This prevents heat generated by the auxiliary boiler, which might elevate the water temperature on the return side of the distribution system, from being inadvertently sent into thermal storage. It also prevents flow from what might be cool thermal storage into the distribution system. If the temperature at sensor (S3) drops to within 3° of the temperature at sensor (S4), circulator (P2) is not allowed to operate. 

The on/off temperature differentials of 5° and 3° are only suggested values. They include an allowance for temperature sensing accuracy. To minimize sensing error it’s best to use identical mounting techniques for both temperature sensors. 

The outdoor reset controller, labelled as (T256) in Figure 2, turns on the auxiliary boiler and circulator (P3) when and if the water temperature supplied to the distribution system at sensor (S2) falls slightly below the current “target” temperature so that it can maintain adequate heat delivery to the loads. Using an outdoor reset controller to “decide” when the auxiliary boiler needs to operate allows the biomass heat source to contribute heat to the lowest possible temperature that can still maintain building comfort. That, in turn, allows for longer biomass boiler burn cycles, which improve efficiency and reduce emissions.

The controllers shown in Figure 2 are not used to turn the biomass boiler on and off. That function is managed by a controller inside the biomass boiler (or in some cases an external controller) that measures the temperature in the upper and lower portions of the thermal storage tank. The biomass boiler and circulator (P1) are operated to maintain the temperature of the thermal storage tank within a specific range, regardless of whether the space heating load is on or off.

Figure 3 shows a simple electrical schematic that combines the differential temperature controller (T156) and outdoor reset controller (T256) to synergistically manage heat input to the distribution system. 

Figure 3

The differential temperature controller (T156) and outdoor reset controller (T256) are energized only when there’s a call for space heating. Together, they manage all heat input to the distribution system from the two available heat sources. Heat input from the biomass heat source takes priority whenever possible, but comfort is never compromised. It’s “guarded” by the outdoor reset controller and auxiliary boiler.

The combined logic provided by these two controllers, or their equivalent programming within a building automation system, is simple but effective. It prevents the system from “running blind” to a condition where heat from the auxiliary boiler is flowing into thermal storage.