HPAC Magazine

Pellets To PEX

October 1, 2014 | By JOHN SIEGENTHALER

Combi-systems supplied by modern pellet-fueled boilers.

European countries such as Austria, Germany, Sweden, and Switzerland, have embraced high tech boilers fueled by wood pellets as part of a responsible renewable energy portfolio. This attitude has led to committed research and development efforts that have significantly advanced wood fueled heating technology. The best-in-class technology currently available to burn wood pellets has leapfrogged what was available in the 70s and 80s.

Today, residential and commercial size pellet-fired boilers can operate fully automatically for weeks. Depending on climate, these boilers may only require two or three ash removals per heating season. 

Many of these modern pellet-fueled boilers have integrated web-accessible controls that can run the boiler, as well as several peripheral devices such as pellet transport systems, circulators and mixing valves. Owners and service providers can monitor the entire system using smart phones or tablets.


Designers and installers should not look at a modern pellet-fired boiler as simply another “box” that burns fuel and produces heated water. These boilers have unique operating characteristics that must be respected if the system is to perform to its maximum potential.

First, nearly all the pellet-fired boilers currently available in North America are intended to operate without sustained flue gas condensation. To prevent this condensation, the minimum sustained inlet temperature to these boilers needs to be maintained in the range of 130F.

There are several ways to provide this protection. The most common include three-way thermostatic mixing valves, three-way motorized mixing valves and “loading stations.”  The latter was discussed, in the context of wood gasification boilers, in HPAC March 2012 (Lower Limits, p28–find it in the archives at www.hpacmag.com). Figure 1 shows how to pipe each of these options in between the pellet-fired boiler and thermal storage tank.

Secondly, unlike a conventional boiler, which delivers heat almost immediately after being turned on, pellet-fired boilers require 10 to 30 minutes from when they are called to operate, to when they can deliver heat to loads at the desired water temperatures. This operating characteristic implies that thermal storage is highly desirable.

Thermal storage can lengthen the on-time of the boiler once it is fired. Doing so allows it to operate with reduced emissions and higher thermal efficiency. It also helps smooth out heat delivery to the load, especially if the load happens to be a highly zoned hydronic heating system. A properly piped thermal storage tank can also provide a thermal reserve for domestic water heating and serve as a hydraulic separator between multiple simultaneously operating circulators.

A low temperature hydronic distribution system complements this thermal storage. The objective is to heat the thermal storage tank to a relatively high water temperature, such as 170 or 180F, and then “drain” heat from it to the lowest possible temperature at which the load can still be served. The lower the water temperature at which the heating distribution can operate, the better.

My suggestion is to design every new distribution system so that it can deliver design load output using a supply water temperature no higher than 120F. Even lower supply water temperatures are possible using radiant panels in well-insulated buildings. Use of outdoor reset control for the mixing assembly between the thermal storage tank and distribution system further extends the working temperature range of the tank under partial load conditions.


Figure 2 shows a system that uses a single pellet-fired boiler to supply space heating and domestic hot water.

Both loads are supplied from the thermal storage tank. Think of this tank as the thermal battery and the pellet-fired boiler as the battery charger. 

The pellet-fired boiler is turned on when the temperature at the upper tank sensor (S1) approaches a minimal value at which the tank can no longer supply the load. Since domestic water is heated on-demand through the external heat exchanger, the minimum acceptable temperature at sensor (S1) is typically 120 to 125F.

Once the pellet-fired boiler is operating, it remains on until the hot water reaches down to the lower temperature sensor (S2). This logic may be built into the controller supplied with the pellet-fired boiler. If not, it can be created using either two one-stage temperature setpoint controllers, or a single two-stage temperature setpoint controller.

This control logic “stacks” the tank with hot water, so that when the pellet-fired boiler is finally turned off, it can remain off for some time. Longer and fewer boiler operating cycles are preferred over more frequent and shorter cycles.

Space heating is provided by a highly zoned low temperature radiant panel system. A motorized three-way mixing valve, operated based on outdoor reset control, supplies warm water to the radiant panels at the minimum temperature required to heat the building. The variable speed, pressure-regulated circulator automatically adjusts pumping power as the zones open and close. This reduces electrical energy use under partial load conditions.

Domestic hot water is heated “on-demand” as it flows through the secondary side of a stainless steel brazed plate heat exchanger. Hot water from the upper portion of the thermal storage tank is pushed through the primary side of this heat exchanger by a small circulator whenever there is a demand for domestic hot water of 0.6 gpm or more. This control action is handled by a domestic water flow switch, which is similar to those used in most tankless water heaters. The switch contacts activate a small relay that passes 120 VAC to the circulator. An anti-scald rated thermostatic mixing valve ensures a safe domestic hot water delivery temperature to fixtures.


It is also possible to use a pellet-fired boiler in combination with an auxiliary heat source, such as a small mod/con boiler fueled by propane. Doing so allows the pellet-fired boiler to be sized significantly smaller than design space heating load, while still supplying the majority of the total seasonal space heating energy.

Studies for buildings in relatively cold climates have shown that sizing a pellet-fired boiler to 60 per cent of design space heating load allows it to supply about 84 per cent of the total seasonal heating energy. If the pellet-fired boiler were sized to 75 per cent of design load, it would supply about 96 per cent of the total seasonal heating energy. The auxiliary boiler turns on only during peak demand periods, or in a situation where the pellet-fired boiler may be inoperable.

Figure 3 shows a slight modification of the system in Figure 2. A mod/con boiler has been added as the auxiliary heat source. It is piped such that it interacts with the upper 20 per cent of the water in the thermal storage tank. This provides adequate thermal buffering to prevent short-cycling under minimal loads, but does not require the auxiliary boiler to heat the full volume of the thermal storage tank.

If the load exceeds the rate of heat production from the pellet-fired boiler, cooler water returning from the distribution system will eventually move up within the tank until the temperature at sensor (
S3) in the upper portion of the tank drops to some value at which auxiliary heat input is deemed necessary. The auxiliary boiler is then fired.

Once the combined heat output of both boilers exceeds load, the temperature at the top of the tank will increase. Hot water will start “building” downward from the upper to lower portions of the tank. The auxiliary boiler can be turned off when the temperature at sensor (S3) has risen through some reasonable differential such as 15 to 30F.  

To lengthen the operating cycle of the pellet-fired boiler, it should continue operating until most of the thermal storage tank is filled with hot water. This condition can be detected by sensor (S2) near the bottom of the tank. The pellet-fired boiler is turned off when this sensor reaches a reasonable upper limit such as 170 to 180F.

Figure 4 shows a thermal storage tank with multiple temperature sensors located at different heights to facilitate this operating logic. The position of sensor (S1) relative to sensor (S3) is based on the warm-up time of the pellet-fired boiler. The idea is to allow the hot water volume between these sensors to supply the load as the pellet-fired boiler warms up to the point where it is delivering heat to the tank.


The control logic required to regulate the temperature stacking process can be created using a two-stage temperature setpoint controller along with a one-stage temperature setpoint controller. These controllers are readily available from several suppliers. Figure 5 shows a ladder diagram that combines these controllers with relays to provide the necessary control logic.

With this configuration, a demand for space heating comes from thermostat (T1). This supplies 24VAC to the coil of relay (R1). Contact (R1-1) closes to power on the two-stage setpoint controller, as well as the one-stage setpoint controller.

The two-stage controller examines the temperatures at sensors (S1) and (S2). If the temperature at the sensor (S1) in the upper portion of the tank is less than or equal to 120F, the stage 1 contact in the two-stage controller closes. This passes 24VAC to the coil of relay (R2). Contact (R2-2) closes to complete a circuit between the (T T) terminals on the pellet boiler, enabling it to fire. Another contact, (R2-1) also closes to pass 24VAC to one side of the stage 2 contact. This contact will be closed because the temperature at the bottom of the tank, at this point, is much lower than 170F (i.e., because the top of the tank is only 120F or less). If the temperature at sensor (S1) increases to 130F or higher, the stage 1 contact in the two-stage controller opens. However, there is still a path for 24VAC through (R2-1) and the stage 2 contact to keep the coil of relay (R2) energized. This keeps the pellet-fired boiler operating. When the temperature detected by sensor (S2) in the lower portion of the tank reaches 180F, the stage 2 contact in the two-stage controller opens and the pellet-fired boiler turns off.

If the pellet-fired boiler is unable to maintain the temperature in the upper portion of the tank above 115F, the one-stage setpoint controller turns on the auxiliary boiler to supplement the heat output of the pellet-fired boiler. The auxiliary boiler remains on until the temperature detected at sensor (S3) in the upper portion of the tank reaches 130F.

When using this scenario, it is important that both boilers are solely under the control of the controllers shown in Figure 5. They should not be operating based on outdoor reset control, which could interfere with the objective of the external setpoint controllers.

If your future installations include pellet-fired boilers, this combination of hardware and control logic provides a unique and stable combi-system. It is yet another example of how modern hydronics technology can enhance the performance of renewable energy heat sources. <>

John Siegenthaler, P.E., is a mechanical engineering graduate of Rensselaer Polytechnic Institute and a licensed professional engineer. He is currently teaching an online course on Hydronic-Based Biomass Heating Systems (http://bit.ly/btecbiomass). 



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