The market for high-performance wood-fired boiler systems is growing, especially in the Northeast. This makes sense: It gets cold here and trees are plentiful. There’s also a significant rural population that often doesn’t have access to natural gas pipelines. Fuel choices are often narrowed to fuel oil, propane and electricity.

With the dollars per MMBtu price of heat delivered from pellets and cordwood currently well below that for heat supplied by these fuels, the financial incentive to use wood for heating is being “stoked.” Other, more altruistic motivations come from wood being a carbon-neutral fuel, the desire to keep dollars spent on heating fuel in local economies and viewpoints that correlate fuel choice with national security.

Not your granddaddy’s woodstove

Although several ways are available to burn wood and use the heat it generates to warm water, the residential and light commercial market is trending toward high-efficiency/low-emission cordwood or pellet-fueled boilers. Both rely on two-stage combustion where wood is heated in a primary chamber to a point where a chemical reaction called pyrolysis occurs. The pyrolitic gases are then channeled into a secondary combustion chamber where they mix with superheated air. The resulting combustion resembles a blow torch, both in appearance and sound.

Steady-state combustion efficiencies in the mid-80% range are possible. This is almost double the steady-state combustion efficiency of single-stage wood burners, especially when the latter are operated in a “smolder” mode with inadequate air supply.

The most common types of high-efficiency/low-emission wood burners designed to supply hydronic systems are wood-gasification boilers and pellet-fueled boilers.

The operating characteristics of these boilers is significantly different from conventional boilers. Neither can simply turn on instantly, come up to operating temperature in three or four min. and repeat this several times an hour if necessary.

Instead, the best results are attained when these boilers operate continuously for several hours. In nearly all cases, this requires the use of water-based thermal storage to absorb the excess heat produced over that required by the load.

When the fuel load of a wood-gasification boiler burns out, the load draws heat from thermal storage, often for several hours before the boiler has to be refired. Similarly, in systems with pellet-fired boilers, when the thermal storage tank temperature reaches a high-limit setting and the boiler stops firing, a properly sized and piped thermal storage tank should be able to supply the load for several hours.

Lessons learned

Over the last two years, I’ve been working with the New York State Energy Research & Development Authority to train heating professionals on best practices when using high-performance wood-fired boilers. I’m also involved in reviewing system designs submitted to NYSERDA for funding under the Renewable Heat NY program.

One of the trends I’ve noticed, especially in retrofit applications, is matching these boilers up with hydronic distribution systems that were originally designed around high water temperatures. The most common scenario is fin-tube baseboard sized for supply water temperatures of 180° F or higher under design load conditions.

Wood-gasification boilers and pellet-fired boilers are capable of producing water at these temperatures, but they can’t operate like a conventional boiler that just turns itself on and off based on a fairly narrow temperature differential. They’re intended to turn on, run for several hours to fully charge a thermal storage tank and turn off for several hours while the load draws from storage.

During design load or near design load conditions, the problem that develops is insufficient heat output from the high-temperature heat emitters as the thermal storage tank supplying them drops more than 10° to perhaps 20° below the design load supply water temperature.

Perhaps you’re thinking, just set up the controls so the thermal storage tank can’t drop more than 10° to 20° below design water temperature. Unfortunately, this greatly limits the ability to store and release sensible heat. The amount of sensible heat stored or released is directly proportional to the change in the tank’s average water temperature.

One also could argue that supply water temperatures such as 180° for fin-tube baseboard are only necessary at design load, which usually only occurs about 2% of the year. During other times, the water temperature required by the heat emitters is less.

This is true. For example, a distribution system that requires a supply water temperature of 180° to maintain a 70° interior temperature when the outdoor design temperature is 0° only requires a supply water temperature of 125° at half load and about 153° at 3/4 design load.

Running the numbers

I made some calculations to evaluate how biomass boiler cycling is affected by the supply water temperature at design load, as well as under partial load conditions. These calculations are based on a sizing criteria and control method often applied in systems with state-of-the-art pellet-fired boilers.

The boiler was assumed to be sized for 60% of design load. This is a mandated criteria within the NYSERDA Renewable Heat NY program for pellet-fired boilers with rated outputs over 300,000 Btu/hr. It biases the heat output of the pellet-fired boiler toward baseload vs. peak load. This reduces boiler-cycling, resulting in higher cycle efficiency and lower emissions. This sizing criteria also can be used in smaller systems that include an auxiliary boiler, which is fairly common, especially in retrofit applications.

The assumed control strategy is called temperature stacking. Once the pellet-fired boiler is turned on, it operates until most of the water in the thermal storage tank has reached a relatively high temperature — such as 180°. This “boiler off” criteria is maintained even if the space-heating demand is satisfied before the water in the thermal storage tank is fully heated. This control method also helps lengthen the boiler on-cycle.

Here’s what the calculations indicate:

Case No. 1: A pellet-fired boiler sized to 60% of design load is coupled to a storage tank that provides 2 gal. of water per 1,000 Btu/hr. of peak boiler output. The distribution system requires 180° supply water at design load. The boiler’s operating controller keeps the boiler on until the tank is fully stacked with 180° water.

At 60% design load, the boiler runs continuously.

At 50% design load, the boiler is on 5.5 hrs. at full output to supply load and charge storage. Storage alone supplies load for 0.92 hrs.

At 30% design load, the boiler is on 2.6 hrs. at full output to supply load and charge storage. Storage alone supplies load for 2.6 hrs.

At 10% design load, the boiler is on 2 hrs. at full output to supply load and charge storage. Storage alone supplies load for 9.9 hrs.

Keep in mind these boiler on-times assume that the boiler fully charges the storage tank to 180°, top to bottom, before the boiler shuts off, even if the heating load is no longer active. Notice how the storage tank is able to supply the heating load, with the boiler completely off, for several hours under low-load conditions.

Case No. 2: Next, I turned the supply water temperature requirement down to 120° at design load. This is easily achievable using several types of low-temperature heat emitters such as radiant panels, high output fin-tube convectors and appropriately sized panel radiators. I kept all other assumptions the same. Here are the results:

At 60% design load, the boiler runs continuously.

At 50% design load, the boiler is on 8.5 hrs. at full output to supply load and charge storage. Storage alone supplies load for 1.7 hrs.

At 30% design load, the boiler is on 3.2 hrs. at full output to supply load and charge storage. Storage alone supplies load for 3.2 hrs.

At 10% design load, the boiler is on 2.1 hrs. at full output to supply load and charge storage. Storage alone supplies load for 10.5 hrs.

Notice the significant increase in both the boiler on-time and off-time at 50% load conditions. This happens because the water temperature range of the buffer tank increases when lower-temperature heat emitters are used.

The difference in on-times and off-times decreases as the systems operate at progressively lower percentages of design load. This happens because the difference in required water temperature between the high-temperature and low-temperature heat distribution systems gets smaller as the load get smaller.

Longer boiler cycles allow a higher percentage of the combustion to occur at preferrable steady-state conditions. It’s like driving your car for more highway miles and less city mileage. Overall fuel use is reduced for a given desired outcome (e.g., total heat delivered or total miles driven).

Don’t waste your money

Water-based thermal storage is expensive. It’s not uncommon for residential wood-gasification boilers to require 500 to 800 gal. of storage. If the storage is provided as pressure-rated closed tanks, it’s likely going to cost more than $10 per gal.

A thermal storage tank that operates with minimal temperature cycling is little more than a very expensive wide spot in the piping.

In systems using wood-gasification or pellet-fired boilers, I suggest use of controls and low-temperature heat emitters that allow the thermal storage tank to cycle through at least 60° between “boiler off” and “boiler on” under design load conditions. Even larger temperature cycling ranges are preferred when possible.

In retrofit applications, this often means existing distribution systems that were designed around high water temperatures need to be enhanced by adding more heat emitter surface area. The greater the total heat emitter surface area, the lower the supply water temperature can be for a given rate of heat delivery.

I can attest this advice often generates pushback from contractors and building owners who weren’t anticipating any need to make changes upstairs just because a shiny new biomass boiler is being installed in the basement. It’s obviously less expensive to just connect the new boiler to whatever happens to be in place for a hydronic distribution system and call it a day.

This has already resulted in installations where the biomass boiler can’t deliver what was expected regarding fuel use or low emissions. Although the boiler typically gets the blame, it’s usually the overall system and the lack of sufficient heat emitter surface in particular that constrains performance.

My suggestion is to ensure your heat emitters and distribution piping can deliver design load output using a supply water temperature no higher than 120°. This ensures the storage tank will be well exercised and the biomass boilers will operate at optimal conditions.