Indoor Wood Boilers vs. Outdoor Boilers: What's the Difference?

If you live in a rural area (you're reading this so you likely do), you are probably familiar with outdoor wood boilers, otherwise known as outdoor water stoves, outdoor wood furnaces, OWB, or hydronic heaters. Outdoor wood boilers are small, outhouse-sized, metal-sided shanties often painted dark blue, green, brown, or black with a short smokestack.

Along with mullet haircuts, outdoor wood boilers became mainstream in the 1990s. Due to their use of smoke signals and prominent positions in yards, outdoor wood boilers (sadly) became the public face of the wood-burning industry.

Meanwhile, tens of thousands of clean-burning indoor wood boilers are installed in homes around the country, but they generally go unnoticed, which in many ways is a good thing. Like grandma dotes after her grandkid who stays out of trouble, we are pleased that indoor wood boilers avoided notoriety for the wrong reasons.

What is an Outdoor Wood Boiler?

Outdoor boilers are generally crude steel fireboxes surrounded by water then covered with insulation and metal siding. Our testing of a better quality outdoor wood boiler model found about 4" of spray foam insulation on the top of the boiler, but only 1-2" of foam on other faces.

Often, outdoor wood boilers contain several hundred gallons of water, which is open to the atmosphere through a vent and requires treatment to prevent corrosion and freezing. Some outdoor wood boilers have had better combustion designs in recent years, but most are essentially a barrel of fire with another barrel of water surrounding the barrel of fire.

Outdoor wood boilers often have very large fireboxes and doors designed to encourage burning large pieces of wood. They generally cycle on and off as demand requires, which can lead to very smoky burns.

What Is an Indoor Wood Boiler?

Though indoor boilers are carefully jacketed and insulated, they are not weather-tight. That means indoor boilers can't sit outside. Though, they can be located in remote buildings. Remote indoor boiler buildings require 4 walls, a roof, insulation, power, and water.

Indoor boilers are compact compared to outdoor boilers and contain relatively small volumes of water. The water is pressurized and usually is pure, untreated water. Indoor boilers are designed for use with stove-sized dry wood. Most indoor boilers, even those that existed prior to EPA regulation, use two-stage combustion designs also known as wood gasification. For more about wood gasification visit this page.

Today's indoor boilers use thermal storage to enable hot and fast fires. They only need to be tended every 12 -24 hours. Here is a video that shows what operating a Fröling indoor boiler is like.

The Pros and Cons of Each

Indoor and outdoor boilers have different features, and there are defined pros and cons for each. Between access to wood, the potential for smoke, required time, safety, and cost, there are several reasons why an indoor boiler may be a better option.

Think about the implications of each location. While keeping your boiler outside may save indoor space, you'll have to contend with smoke around your house, smoke on your clothing, and contending with bad weather, all of which come with having an appliance outside. Understanding these pros and cons can help you make a decision for your boiler purchase.

Labor vs. Comfort

Bringing the boiler outdoors to the woodpile reduces labor slightly, but now requires the operator to go outside to tend the fire in the cold. It's like going shopping and then putting the groceries in a pantry beside the driveway rather than bringing them indoors; you might as well put the refrigerator outdoors too.

Our indoor boiler customers bring their wood inside a couple of times a week and then enjoy loading their boilers in stocking feet, out of the rain, wind, and cold. Some of our customers dawdle as they load their boilers, enjoy a beer, and enjoy the wonders of wood gasification.

Labor vs. Time

Having huge fireboxes and doors allows less processing of wood before burning. Yet, using large, un-split wood almost guarantees that the wood will not be dry enough for good combustion. When wood is kept beside the outdoor boiler in freezing temperatures, it sure isn't going to burn very well when first placed in the boiler. Even dry wood placed into an outdoor boiler firebox could contain 20% ice.

Imagine a 150-pound load of wood at 20% moisture content. It would contain 30 pounds of ice, which is more than 4 gallons. How do you think it will burn? With wood and wood-burning appliances, every bit of temperature that can be added improves combustion. Poor wood quality wastes much of the energy in wood straight up the chimney as smoke.

Because much of the fuel is wasted up the chimney, 2 to 3 times as much wood is needed to make the same heat that smaller, drier pieces of wood could have yielded in an EPA-approved indoor wood boiler. All of that smoke "wasted" by outdoor wood boilers becomes a nuisance or worse a health hazard.

Cleanliness and Smoke

Placing the boiler outside ensures that smoke and debris are kept outside of the house. That's fine until the person tending the fire has to lean into the blast of smoke billowing out of the loading door of an outdoor boiler. Sure, the smoke isn't in the house, but it's in the operator's face and the neighbor's yard.

By contrast, the indoor boilers handled by Tarm Biomass are practically smoke-free. Not only are they batch burned, which means that the boiler isn't re-loaded on a half-burned bed of wood, but the boilers are built with smoke hoods, which pull escaping smoke back into the boiler during loading.

Firewood isn't dirt-free and we can't argue that a broom or vacuum is handy around the boiler loading zone. On the other hand, it is arguable that the appearance of a soot-stained boiler in the side yard and mud tracked into the house after filling a boiler aren't much to look at either.

Safety vs. Comfort

We find it strange that safety is a selling point people buy into when choosing outdoor boilers. It would be crazy to bring a toaster or cooking range outside for safety or to avoid the mess, so why put a wood boiler outside? With freeze and thaw cycles, the ground around an outdoor wood boiler can turn to mud or a sheet of ice. With mud and ice underfoot while carrying large chunks of firewood, the situation is not exactly safe.

The truth is that indoor boilers are safe. Fröling wood boilers are at most warm to the touch. The combustion area is surrounded by water and then heavily jacketed with insulation. The boilers are safety tested and listed, have redundant controls, and produce virtually no creosote in the chimney. The boilers are so well insulated that clearances to combustibles can be as close as 6".

Batch firing is smoke-free and clean. Induced draft fans keep smoke from leaking during combustion. Ash handling is infrequent and easy. Because our indoor boilers are batch loaded and there is no tending between fires, there is little chance for embers to escape.

Cost of Installation

Many outdoor wood furnace owners learn the hard way. A simple furnace kit with a pump, short chimney, and some insulated pipe sounds easy and financially attractive. Just plop it down and plug it in. There are several problems with that assumption.

First, a trench for the supply and return pipes should be below the frost line to reduce heat losses, but placing the insulated pipe in groundwater isn't good for heat loss either. Big trenches are invasive and expensive. Indoor wood boilers don't require expensive trenching and burying of insulated pipes.

Furthermore, because indoor piping is relatively inexpensive, it is usually right-sized, whereas many outdoor boilers are sold with under-sized 1" pipe regardless of the boiler output. Using a one size fits all approach with underground piping makes about as much sense as putting moped tires on a car. Realistically, 1" pipe should carry no more than about 100,000 Btu/hr. and then, not over 50' one way (100' for supply and return combined lengths).

A power wire will also need to be brought to the outdoor wood furnace. Once operational, many quickly realize that tending a fire in the cold rain and snow is less than ideal. Consequently, many people put outdoor boilers on a slab and then build a shelter over the outdoor wood boiler. The installation savings diminish quickly.

By contrast, when space is available indoors, indoor boilers are almost always less expensive to install. Plus, Fröling indoor boilers are designed like Army tanks. They will outlast outdoor wood boilers. Indoor wood boilers are pressurized just as most traditional boilers are. This means that there are no additional electricity-hogging pumps or expensive heat exchangers needed to connect to an existing system.

Because Fröling indoor boilers are pressurized, their water is also maintenance-free and normally does not require the use of corrosion inhibiting chemicals or anti-freeze. Though, if parts of a heating system are at risk of freezing, using anti-freeze is not a problem.

We understand that there are some heating applications with no space available for indoor wood boilers. For those applications, EPA-approved outdoor wood boilers, installed properly, and fired with dry wood may be a reasonable choice. However, due to low chimneys and smoky on/off cycles, proximity to neighbors should be carefully considered.

Outdoor boilers should never be fired in shoulder seasons or in summer if smoke could impact neighbors. Indoor boilers used with thermal storage are normally OK to use in all seasons, but even with clean burns, there is a wood smoke odor, which some neighbors may object to.

Maintenance

Indoor and outdoor boilers have varying maintenance requirements.

Outdoor boilers are unpressurized, which means that they must use chemicals to protect steel and iron from corroding. Without proof of annual water testing, many outdoor boiler manufactures will not honor the product warranty. Without oxygen and pH maintenance chemicals, outdoor boilers, circulators, and valves can clog with rust or leak. Indoor boilers generally require no water maintenance. When operating with thermal storage there is no sticky tar to foul heat exchange areas, which also means that maintenance is very easy. Furthermore, by avoiding corrosive tar in the firebox, batch-burned indoor boilers will last much longer.

While old-fashioned outdoor wood boilers were very simple and required little maintenance other than water chemistry, modern outdoor boilers contain many of the same combustion systems as indoor boilers, which means that maintenance on combustion components is similar. However, cycling boilers on and off with modern combustion designs leads to a build-up of soot, which reduces heat transfer and is difficult to clean. In the worst case, the accumulated soot in a boiler firebox can be very corrosive and will eat through steel surprisingly fast. This is one reason that Tarm USA requires thermal storage with its boilers.

Ash is formed from non-combustible minerals in wood. Therefore, the more efficient a boiler is, the less wood will be consumed and the less ash will be created. Indoor boilers are batch burned, which means that there are plenty of opportunities to clean the ash out without a hot fire burning. In most cases, ash will be removed from Froling wood boilers every 7-14 days depending upon wood consumption.

Indoor and outdoor boilers have heat exchange tubes to move heat from the smoke to the water. Keeping these tubes clean is essential to the efficient use of wood. Therefore, having easy-to-use built-in tube sweeping mechanisms is an important feature. Without built-in heat exchanger sweeping mechanisms, hand brushing with metal bristled brushes is required. Hand brushing is a labor-intensive and messy job, especially if there is a lot of tar in the heat exchange tubes. Before making any decision to purchase a boiler, learning how heat exchange surfaces must be cleaned is important.

Beyond self-cleaning features, Fröling also offers automatically fired, self-cleaning wood pellet boilers. Modern Froling pellet boilers like the PE1 require a lot less frequent maintenance due to low ash wood pellet fuel and self-cleaning automation.