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Last month, we discussed using a non-pressurized thermal storage tank, in combination with brazed plate heat exchangers in a system supplied by a cordwood gasification boiler. The brazed plate heat exchangers were presented as an alternate to copper coil heat exchangers immersed within the tank.

The system presented in last month’s column is a complete system for those committed to operating the cordwood gasification boiler whenever the thermal storage tank has cooled to the point where it cannot maintain comfort in the building. Wood burning “purists” would be fine with this system. However, others may want to enhance its ability to maintain building comfort at times when the wood-burning boiler is not active and thermal storage is depleted.

Perhaps the owner wants to go on a winter vacation and no one is available to keep the home fires burning. Or maybe there are times when the owner just doesn’t want to bother firing the boiler. Whatever the reason, the versatility of modern hydronics technology makes it easy to add an auxiliary boiler.

Respecting thermodynamics

Some of you may be thinking no big deal, just pipe an auxiliary boiler in parallel with the cordwood gasification boiler, and let it heat the thermal storage tank when the cordwood boiler is off. Although this could be done, it would force all heat to pass through thermal storage on its way to the load. One might argue that the thermal storage could act as a giant buffer tank. That’s true, but the multi-hundred gallon tanks used with cordwood gasification boilers have a lot of surface area, and thus, the potential for relatively high standby heat loss. That approach would also be “offensive” to the second law of thermodynamics because it converts high grade energy, stored as fuel, into low grade energy (e.g., heat) before it’s needed by the load.

Figure 1 shows a mod/con boiler connected to the load side of the system using a pair of closely spaced tees. In rural settings where cordwood gasification boilers are more common, the mod/con boiler would likely be fired by propane.

If the wood-fired portion of the system is not operating, circulator (P2) would be off. Flow returning from the zones passes through the secondary side of the heat exchanger, and then absorbs heat at the closely spaced tees beneath the mod/con boiler. The remainder of the distribution system uses zone valves and a variable-speed pressure-regulated circulator.

Necessary controls

Whenever an auxiliary boiler is added to a system with a thermal storage tank supplied by a renewable heat source, it’s important to prevent heat generated by the auxiliary boiler from entering that tank. Although this situation, if allowed to occur, wouldn’t damage the system, it again violates the thermodynamic principle of not converting high grade energy into low grade energy until it is needed by the load.

Fortunately it’s easy to add this control functionality to the system. Just install a differential temperature controller that compares the temperature of the water in the upper portion of the thermal storage tank to the temperature returning from the distribution system. This controller is designated at (T156) in Figure 1.

This temperature comparison should be initiated and maintained whenever one or more zones are calling for heat. As long as the tank temperature is say 10° F above the temperature returning from the distribution system, heat exchanger (HX2) can make a positive energy contribution to the flow stream returning from the load. Circulator (P2) would remain on under such conditions. The auxiliary boiler would also fire — when necessary — to boost the supply water temperature to the load. That supply water temperature would be based on outdoor reset control.

When the temperature in the upper portion of thermal storage dropped to say 7° or less above the temperature of water returning from the zones, circulator (P2) would be turned off. At that point, the auxiliary boiler would the sole heat source for the system. Its firing rate would continue to be varied as necessary to maintain a suitable supply water temperature to the active zones. When the biomass boiler brings the temperature of the thermal storage tank back up, circulator (P2) would be re-enabled to inject heat to the load through heat exchanger (HX2).

The on and off differentials suggested above, (e.g., on when S3 ≥ S4+10°, and OFF when S3 ≤ S4+7°) are only suggested values. They are based on an assumed approach temperature of 5° across the upper side of the heat exchanger, and some allowance for sensor tolerances. Modern differential temperature controllers allow these differentials to be adjusted over a wide range.

Quick recap

I’ll close out with a summary of important details needed in this system:

• Be sure the circulators handling water from the non-pressurized tank are stainless steel and mounted low relative to the water level in the tank;
   
• Keep the head loss in the piping leading to these circulators as low as possible;
   
• Install purging flanges on each side of these circulators to allow the air in the piping to be blown out when the system is commissioned;
   
• Be sure there are no air vents, valves or other components in the ‘“gooseneck” piping that’s above the water level in the tank.
   
• Make sure both closed-loop portions of the system have expansion tanks, pressure relief valves and air separators; and
   
• Install diffuser tees inside the thermal storage tank, as shown in Figure 1, to reduce vertical mixing currents and maintain beneficial temperature stratification.

Follow these details and you’ll create a smoothly functioning renewable energy system that will automatically ensure comfort is never compromised.