Using pressurized thermal storage tanks to buffer wood-fired boilers.

Figure 1.


In the first part of this column (“From Wood To Water, Part 1,” October 2009), we discussed high-efficiency wood-gasification boilers, and how they should be piped and controlled, along with an auxiliary boiler, in systems that supply space heating and domestic hot water.

The system concepts discussed in Part 1 all involved unpressurized thermal storage tanks to buffer the wood-fired boiler. One of the piping schematics we discussed is shown in Figure 1.

Although most of the water in this system is held in the unpressurized thermal storage tank, that water is not part of the closed piping circuits that make up the rest of the system. The water in the thermal storage tank is isolated from these circuits by suspended coil heat exchangers. This allows the unfilled upper portion of the thermal storage tank to serve as an expansion space for the water in the tank. The closed piping circuits each have their own expansion tanks, sized to the specific expansion requirements of those circuits.

This month we’re going to look at system designs using pressurized thermal storage tanks. An example of one such system is shown in Figure 2.

Unlike the system shown in Figure 1, the entire water content of this system is held within a single, closed piping assembly. The water flowing through the wood-fired boiler can also flow through the thermal storage tank, the auxiliary boiler and the distribution system. A single suspended coil heat exchanger is used to separate domestic water from system water.

This approach eliminates the heat exchanger between the wood-fired boiler and thermal storage tank, as well as the heat exchanger between the thermal storage tank and the distribution system. This obviously reduces cost - at least as far as heat exchangers. It also does away with the temperature differential between the storage tank water and distribution system water.

That differential was necessary in systems with unpressurized tanks to drive heat across the heat exchanger. The lack of a heat exchanger lowers the temperature below which the thermal storage tank can no longer supply adequate heat to the distribution system. Lowering the useful tank temperature delays the need to fire the auxiliary boiler.

Pressurized storage and a closed piping system also greatly reduce evaporation water loss. I can’t state that such systems eliminate water loss, because every closed-loop hydronic system experiences minor water losses over time.

A properly designed closed system is also far less prone to any corrosion reactions involving oxygen.

Not All Positive

Using a pressurized thermal storage tank does have some drawbacks. One is that pressure-rated tanks almost always cost more than nonpressure-rated tanks on a dollar/gallon basis. This is especially true if the tank must be ASME-certified. Check your local mechanical codes to see if this is the case. Most mechanical codes require thermal storage tanks used in pressurized heating systems to be “listed and labeled” by an independent rating organization. The ASME section VIII, division 1 listing applies to pressure vessels designed to operate at pressures over 15 psi.

Another consideration is size and weight. Whereas many unpressurized tanks are shipped in a “knocked-down” configuration and assembled on site, pressure-rated tanks are delivered as a finished product. This increases shipping cost and necessitates handling potentially large, heavy tanks on site. Be sure that any such tank cannot only be maneuvered into its allotted space, but that it could also be removed from that space if required at some future date - preferably without using cutting torches!

Figure 2.

Voluptuous

Another issue associated with a pressurized thermal storage tank is the need of a larger expansion tank - substantially larger than that required for an isolated unpressurized buffer tank. With a pressurized tank, the entire system volume is a single closed circuit. The expansion tank must absorb the increased volume of heated water in system piping and the volume increase within the thermal storage tank.

A residential system using a wood-gasification boiler may need a storage tank containing several hundred gallons of water. That water could vary from room temperature to perhaps 200 degrees F. This combination makes for a substantial expansion volume and requires a large floor-mounted expansion tank or perhaps multiple tanks.

Here’s an example: Assume a system uses a 500-gallon pressurized buffer tank that could cycle between temperature extremes of 50 degrees and 200 degrees F. That same temperature range applies to the wood-fired boiler and its piping, which adds another 35 gallons of volume to the system. The remainder of the system is low-temperature floor heating, which adds 50 gallons of water cycled through a temperature range of 50 to 115  degrees F.

Also assume the top of the system is 20 feet above the pressure-relief valve/expansion tank inlet and the relief valve is rated at 30 psi. A minimum pressure of 3 psi will be maintained at the top of the system for venting. How large does the expansion tank have to be?

Because different portions of the system undergo different temperature swings, each portion can be treated as if it was a separate system: one containing 535 gallons cycled from 50 to 200 degrees F and the other with 50 gallons cycled from 50 to 115 degrees F. Size an expansion tank for each portion, then add the two volumes together.

I used the expansion tank sizer module in the “Hydronics Design Toolkit” software to make the calculations. The larger volume requires an expansion tank of 63.6 gallons. The smaller volume requires an expansion tank of only 1.7 gallons. Thus, the minimum total expansion tank volume is 65.3 gallons. An expansion tank larger than this would be acceptable. So would two smaller expansion tanks that add to the same total volume and are piped in parallel. In this system, the air-side pressure in the expansion tank (or multiple tank configuration) needs to be set to 11.7 psi.

In situations like this, I prefer two tanks. If one tank ever fails, the other provides partial expansion compensation until a repair or replacement can be made.

A hydraulic separator is shown as an alternative to an air separator and closely spaced tees at the interface to the distribution system.

Figure 3.

How It Operates

When it comes to control, we’re going to keep things simple with the following assumption: The only way the wood-fired boiler heats domestic water is as that water passes through the suspended coil in the storage tank. In other words, the buffer tank cannot serve as a heat source for the coil of the indirect water heater. The auxiliary boiler can still boost domestic water temperature within the indirect tank when necessary. It can also supply the space-heating load when the buffer tank cannot.

With this constraint in mind, here is a description of the control operation for the system shown in Figure 2.

1. The blower on the wood gasification boiler operates based on boiler limit controller. Boiler circulator (P1) operates whenever the master switch on the wood-fired boiler is on. If the water temperature exiting the three-way thermostatic valve is too low, most of the water coming from the boiler is shunted back to the boiler to avoid creosote formation. As the boiler temperature rises, this valve diverts water from the boiler into the buffer tank.

This action prevents the boiler from sustained operation at low water temperatures where the formation of creosote is likely.

2. Upon a call for space heating, the mixing valve controller (C2) calculates the necessary water supply temperature to the radiant panel circuits. Controller (C2) calls for heating by closing a set of contacts to power outdoor reset controller (C1), which monitors temperature at the top of the buffer tank. If this temperature is at or above the calculated target temperature minus half the control differential, the normally closed contacts in the relay adjacent to controller (C1) turn on the tank circulator (P2), allowing the tank to supply heat to the space-heating load.

If the tank is below this target temperature minus half the control differential, the normally open contacts in the relay close to turn on the boiler and boiler circulator (P3). In this case, the boiler’s internal reset controller regulates boiler temperature. A graph depicting a typical reset control function for low- temperature floor heating is shown in Figure 3. This would be the control action required of controller (C1).

3. Domestic water heating is treated as a priority load. When the aquastat on the indirect water heater calls for heat, the tank circulator (P2), if it’s currently running, is turned off. Circulator (P4) is turned on, as is the boiler and boiler circulator (P3). Boiler temperature is controlled based on its setting for DHW mode.

Keep in mind that all domestic water must pass through the suspended coil heat exchanger in the thermal storage tank before it enters the indirect water heater. If the water in the thermal storage tank is relatively warm, say 120 degrees F or higher, and the suspended coil is generously sized, the vast majority of domestic water heating temperature rise takes place within this coil.

The only limitation of this method is that wood-sourced energy cannot “top off” the water temperature in the indirect tank, assuming the temperature of the thermal storage tank was, at some point, high enough to do so. I don’t see this as a substantial negative, considering that it greatly simplifies control requirements relative to other methods.

Figure 4.

Instant DHW

Another variation of the basic schematic is shown in Figure 4. Here, an external brazed-plate heat exchanger replaces the internal coil for domestic water heating. A small circulator - probably 20 watts or less - creates flow from the top of the thermal storage tank through the hot side of this heat exchanger. This circulator is activated by a flow switch that senses flow of cold water into the other side of the heat exchanger. The very low mass and high heat transfer rate of a brazed-plate heat exchanger allows it to respond almost instantly.

After domestic water is “preheated” through this heat exchanger, it’s routed to a standard indirect tank, where its temperature is topped off as necessary by the auxiliary boiler. Doing away with the need of an internal heat exchanger coil allows more sourcing opportunities for the thermal storage tank. The external heat exchanger is easily serviced or replaced should this be necessary.

Figure 5.

Lights Out

Some code jurisdictions mandate that all wood-fired boilers have “heat dumps” that can dissipate the energy in the boiler during a power failure. The wood-gasification process is driven by a blower, which obviously stops during an outage. This quickly drops the intensity of the combustion process but doesn’t totally extinguish the fire.

Figure 5 shows a relatively simple heat-dump provision. It uses suspended copper fin-tube such as that found in hydronic baseboard. The fin tube is placed above the boiler to allow natural convection to create the circulation. A normally open zone valve, which remains closed whenever it’s powered, opens to all this natural convection through the fin tube.

In the absence of specific requirements from the boiler manufacturer, I suggest the fin tube be sized to dissipate the rated heat output of the boiler at a water temperature no higher than 200 degrees F. Be sure the heat-dump circuit doesn’t contain any valves or other restrictions that could inhibit the relatively weak natural convection effect.

Figure 6.

When The Sun Is Out

Given the nice, big thermal storage tank in these systems, it’s only natural that some owners will want to expand their system to include solar energy input. Free heat from the sun is certainly preferable to burning wood during warmer weather. If a couple of cloudy days are in the forecast, just light up the wood-fired boiler or simply relax and let the auxiliary boiler handle the situation automatically.

This is easy to accomplish. Just add another brazed-plate heat exchanger as the interface between a closed-loop, antifreeze-type solar subsystem and the storage tank as shown in Figure 6. Be sure to pipe it for counter-flow (streams flow in opposite directions through the heat exchanger) as shown in the schematic. Also be sure check valves are installed as shown.

A standard differential temperature controller operates the collector circulator whenever the collector temperature is a few degrees above the temperature at the bottom of the storage tank.

Because the solar collection subsystem is isolated from the remainder of the system, it requires its own expansion tank, air separator, pressure relief valve, check valve and purging valves.

Pressurized Or Unpressurized?

Like most things hydronic, both approaches have strengths and limitations. Each job requires scrutiny and cost comparisons before the answer will be clear. In either case, good hydronic details like thermostatic boiler protection, hydraulic separation and outdoor reset control enhance the high thermal performance of a wood-gasification boiler.

To see and read about what other advocates of wood-fired hydronic heating are up to, be sure to drop in on the “The Boiler Room” forum at www.hearth.com.

Wishing you and your families a blessed Christmas.

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