Composting toilets are the most misunderstood element of an ecologically friendly building. There’s no need to be scared!

This introduction to composting toilet basics is adapted from the book Making Better Buildings by Chris Magwood:

Composting Toilet Basics

Composting toilets collect urine and feces — referred to as humanure in the rest of this chapter — and treat it completely on-site, until it is transformed into useful compost or humus.

This category of treatment system does not include common pit outhouses, which do not provide ideal conditions for the conversion of humanure to compost, though given enough time the material in a pit toilet can undergo this transformation.

There are three common types of composting toilet:

Bucket toilet — This low-tech version of the composting toilet uses a bucket or similar portable receptacle placed under a seat/container to receive humanure deposits. Sawdust, wood shavings, chopped straw or another form of cellulose material is used to cover each deposit in the toilet, helping to reduce odor, absorb urine and provide aeration. Once full, the bucket is emptied into an outdoor compost heap. Here the material is layered and mixed and covered with more cellulose material, providing the right conditions for the natural conversion to compost/humus.

The indoor toilet construction is usually provided with passive or active ventilation, but no water connection or flushing action is used.

Self-contained toilet — These units provide a seat over an integral composting tray in a single, self-contained structure. Humanure deposits are received in the tray and provided with the appropriate conditions for composting action within the unit. These toilets all use some form of mechanical ventilation to reduce odor. Excess urine may require a separate handling system, or heat may be used to speed evaporation. Due to limited storage capacity, these toilets normally use some form of mechanical action and/or acceleration for the composting process and are only suitable for low numbers of users or for intermittent use.

The compost tray is removed from the unit when processing is complete or when the tray is full. It is often necessary to have an outdoor compost heap to receive material from these units, as it can prove difficult to complete the composting process within the unit.

Some models of self-contained toilet use chemicals or high heat to “cook” the humanure into a benign state. The material from these toilets is not useful compost, as the biological activity that creates rich, useful soil has been killed off.

Remote chamber toilet — A toilet (dry chute or low-water flush) sits above a large, enclosed chamber that receives humanure. The chamber is of sufficient capacity and design to contain and process a high volume of completed compost.

Units handle humanure in various ways. Some use heat and/or evaporation to rid the chamber of excess urine and water and speed the composting process, while others retain and process all material. Mixing or stirring capabilities, misting sprayers and rotating trays are options offered by certain manufacturers. Vacuum flush, allowing the toilet to be level with or below the height of the chamber, is also available.

Some units gather excess urine after it has passed through the bulk material in the chamber and retain this liquid as a high-quality fertilizer. This makes best use of the potential value of all material entering the toilet, as up to 80 percent of the nutrient value in toilet waste is in the urine. Once transformed into nitrites and nitrates after passing through the biologically active compost solids, the liquid can be a safe and low-odor fertilizer.

All chamber-style toilets provide humanure with enough time and adequate conditions to fully convert to compost before being removed from the unit. These are the only units that do not require additional outdoor composting capacity.

Types of waste handled

– Black water (though most systems are waterless)

Tips for successful installation

1. Understand the maintenance requirements of any type of composting toilet before committing to installation. All require some maintenance, and dealing with humanure is not for everybody. Some units require infrequent maintenance, others daily.
2. Check local codes before planning for a composting toilet. They are an accepted solution in some codes but not in others.
3. Check local codes for the legal status of composted humanure. Though a good deal of documentation exists to show the material is biologically benign, some jurisdictions require compost to be treated as hazardous waste.
4. Some types of composting toilets require specific layout arrangements that must become part of the home design.
5. Mechanical ventilation is part of most composting toilets, requiring an exit tube that passes through the roof of the building with as straight a run as possible.
6. Plan for an easily accessible route from the point of removal to the outdoors, to facilitate emptying of the toilet or chamber.
7. Be sure there is sufficient provision on the property for units requiring outdoor composting facilities, and that the process of finishing humanure compost outdoors is well understood.
8. When using commercially produced units, follow the manufacturer’s instructions for successful installation.

Pros and cons

Environmental impacts

Composting toilets are the only form of toilet that does not treat human excrement as waste, and rather as a potentially regenerative material for amending soils and fertilizing plants. A large environmental problem is thereby transformed into a solution to soil depletion, creating more robust growing environments.

The composting of humanure is not without issues, and untreated or partially treated material can be contaminated with pathogens that are potentially dangerous to humans and animals and can contaminate soil and ground water. There is a growing body of evidence that complete composting of humanure is relatively easy to accomplish reliably, but the correct conditions must be understood and created.

Material costs

Simple bucket toilets and appropriate outdoor composters can be built for as little as a hundred dollars. Complete remote chamber toilet systems can cost between four and eight thousand dollars.

Labor input

Depending on the type of composting toilet, labor input can vary greatly. Other toilets do not require direct ventilation, and even the simplest composting toilet has more components and longer installation times than a conventional flush toilet.

Skill level required for the homeowner

Installation — Moderate to Difficult. Multiple components and connections can complicate installation.

Use — Easy.

Maintenance — Moderate to Difficult. Some form of regular maintenance is inevitable with composting toilets. Bucket toilets can require daily maintenance to transfer full buckets to the compost pile. Chamber units may only need monthly inspections and annual emptying.

Sourcing/availability

There are many commercially available self-contained and remote chamber toilets. These are typically sold directly from the manufacturer or in specialty shops. Bucket toilets are homemade, with plans readily available online or in books.

Plumbing for any composting toilet system are standard components available through any plumbing supply outlet.

Durability

The simpler the toilet system, the greater the durability. Units with heaters and moving parts are more prone to durability issues. Consider the accessibility of parts that may need repair or replacement; if they are in difficult locations (especially if they require emptying of the toilet’s contents) they will be unpleasant to service.

Future development

Interest in composting toilets is just beginning to grow, and the technology is likely to develop rapidly in coming decades. There has been a significant shift in understanding about humanure, from a sense of revulsion and the certainty of contamination and illness to an appreciation of the simplicity and value of composting. It will be some time before this shift affects a broad constituency of builders and homeowners, but the research and experience currently being gained in this field by early adopters will be valuable contributions to a technology that is potentially transformative. There is little else in home-building practice that could so radically improve the environmental impacts of our homes.

Resilience

Build and operating a composting toilet system in a low- or no-energy scenario is straightforward. The bucket toilet is an excellent example of resilient technology, as it not only replaces an energy- and resource-intensive practice, but does so in a way that gives back valuable nutrients to the ecosystem.

Resources for further research

Jenkins, Joseph C. The Humanure Handbook: A Guide to Composting Human Manure. Grove City, PA: Joseph Jenkins, 2005. Print.

Del Porto, David, and Carol Steinfeld. The Composting Toilet System Book: A Practical Guide to Choosing, Planning and Maintaining Composting Toilet Systems, an Alternative to Septic Systems and Sewers. Concord, MA: Center for Ecological Pollution Prevention, 2007. Print.

Darby, Dave. Compost Toilets: A Practical DIY Guide. Winslow, UK: Low-Impact Living Initiative, 2012. Print.

Envirolet vacuum flush toilets

Phoenix composting toilets

Residential Heating System Basics

The ability to employ mechanical systems to automatically modulate the temperature (and often humidity levels) of our homes is a radical change from the previous centuries of human habitation. Our heating and cooling systems are often complex and high performance devices that give us fingertip control over indoor climate that would have been unthinkable less than a century ago. Until quite recently, the devices we used to achieve stable temperatures were functional but quite inefficient, using large quantities of fuel to meet our thermostat settings. A lot of development has gone into increasing efficiency, and in many cases this has come with increased complexity and cost. The following residential heating systems basics will help you understand your options.

Though most heating devices are intricate systems, it is quite easy to understand the basic technology behind each of them. It is worthwhile as a homeowner to understand these systems, and not leave it to company reps or installers to provide selling points.

It is easiest to think of heating and cooling systems as falling into categories of means of heat production and means of heat delivery. From this understanding, it is possible to narrow down the pool of options to those that suit the needs of a project.

Means of Heat Production

Despite all the competing products in the heating and cooling market, there are just four kinds of heat production. Details for each system are provided individually later in the chapter.

1) Solar Heat

In effect, all sources of heat are based on solar energy, as the fuels used in every heating system are the result of captured and stored solar energy. However, this classification of heating systems is based on direct harvesting of solar energy in real time. Heat from the sun can be collected (and sometimes concentrated) in, on or near the building and distributed for use throughout the building.

There are three basic types of solar heat, which may be used in any combination.

Passive Solar – A building may be designed with sufficient glazing on the sunny side of the building to allow for a measurable increase of indoor temperature when the sun is shining.

Active Solar Air – Collector units are used to gather and concentrate the sun’s heat in a flow of air that is supplied to a heat exchanger or directly to the building.

Active Solar Water – Collector units are used to gather and concentrate the sun’s heat in a flow of liquid that is supplied to a heat exchanger or directly to the building.

This category of heating devices does not include photovoltaic cells, which use solar energy to generate electrical current, and not directly to produce heat. Heat created by solar electric current is considered in the category of electric resistance heating.

Solar energy systems may appear to have low efficiency rates, with figures ranging from 10-70%, depending on ambient temperatures and type of collector, among other factors. These figures represent the percentage of available potential energy from the sun: approximately 1000 Watts per meter squared (W/m²) for a surface perpendicular to the sun’s rays at sea level on a clear day. A reduced figure of 800 W/m² is often used in generating comparative figures for solar devices. While it is beneficial to increase efficiency rates to produce more heat from less collector area, the efficiency rates aren’t directly comparable to those of combustion devices as no sunlight is actually “wasted” and no harmful byproducts are generated by the solar energy that is not absorbed by the collector.

Solar heating systems do not generate emissions, fuel extraction and transportation impacts or air pollution, with the exception of those systems that use non-solar energy to drive small pumps or fans.

2) Combustion Devices

This category of equipment is by far the most prevalent. Regardless of the type of system and fuel used, all combustion devices burn a fuel and extract heat from the flame. All these devices rely on a supply of oxygen to react with the fuel and create the flame, an exhaust to allow spent combustion gasses to exit the unit and the building and a heat exchanger that passes the heat from the flame to the delivery system that supplies heat to the home. There are two broad categories of combustion devices:

Gas/Liquid Fuel Combustion – These are the dominant players in the market, and include all the various forms of fossil fuel such as natural gas, propane and oil, as well as bio-fuels like biodiesel and vegetable oil.

Solid Fuel Combustion – This group includes wood burning devices, as well as those that use other forms of biomass such as compressed pellets.

Combustion devices have efficiencies that range from 50% for some wood burning devices to 98% for some new gas burning devices. This means that less than all of the heat potential of the fuel is captured and used to supply the building.

Exhaust gasses from combustion devices differ depending on the fuel used and the combustion efficiency and conditions, but all devices release CO2 and a host of other byproducts with environmental effects.

3) Heat Pumps

This category of equipment is widely used in the form of air conditioners, and has started to capture a larger portion of the heating market. These systems use the refrigerant cycle to transfer latent heat from a source and deliver it to a destination or heat sink. It can be difficult to understand exactly how a heat pump works, but it is worth figuring out the principle at work in order to decipher manufacturer claims. Heat pumps can seem like they magically make heat from no heat if the refrigerant cycle is unclear.

Heat pump explained

Mechanical energy (usually in the form of an electric motor) is used to cycle a volatile refrigerant that is chemically designed to boil and condense in the expected operating temperature range of the heat pump. The refrigerant is in its liquid state when it absorbs latent heat from a source (air, ground or water for most residential purposes). This heat does not need to be in a temperature range that feels warm or hot to the touch, as the refrigerant’s chemical properties will ensure its boiling point is at or near to the source temperature. Once the refrigerant has passed through the heat collection exchanger, the electric compressor pressurizes the warmed refrigerant, which is at or close to its boiling point. The pressurization causes the refrigerant to become a hot vapour. This hot vapour passes through a heat exchanger where the heat generated from pressurization is dispersed. The refrigerant condenses as the heat is removed. Condensed refrigerant now passes through an expansion valve where the pressure is released and the refrigerant returns to its liquid state and returns again to the exchanger and repeats the cycle until a thermostat indicates the proper room temperature has been reached.

It is the process of boiling and condensing a refrigerant under different pressures that creates the heat exchange. Useful heat exists in this boiling/condensing cycle, regardless of the actual boiling point of the refrigerant. As long as heat collection side is above absolute zero, there will be heat to extract. Consider the home refrigerator: cold is not being generated in the freezer, rather heat is being extracted from the freezer and released via the heat coils on the back of the fridge.

The refrigerant cycle can happen in either direction, and some types of heat pumps are designed to work as both heating and air conditioning units by reversing the direction of flow of the refrigerant.

There are three broad categories of heat pumps for residential use:

Ground Source Heat Pumps (GSHP) – The source of heat is the stable temperature of the ground (below the frost line in cold climates). Base ground temperature is very reliable and steady, and the ground provides a large surface area and capacity for heat exchange. Most GSHPs are reversible and can provide heat and cooling.

Air Source Heat Pumps (ASHP) – The source of heat is the ambient air temperature outside the building. As this temperature can be quite variable, different refrigerants and/or pumps are used to continuously extract useful heat from changing air temperature. Most ASHPs are reversible and can provide heat and cooling.

Air Conditioners (AC) – The source of heat is the uncomfortably warm air that is affecting comfort. These units provide cooling only.

Efficiencies for heat pumps far exceed those of combustion devices. The amount of energy input to provide compression of the refrigerant is significantly lower than the amount of heat energy that is extracted from the process and this how manufacturers claim efficiencies ranging from 200-500%. The heat is not “free” as some claim, but for every one unit of electrical energy applied to the system, 2-4 units of heat are returned. The systems don’t work without the electric motors, but they are much more efficient than combustion devices.

While there is no combustion and therefore no exhaust gasses from heat pumps, environmental impacts will vary depending on the source of electricity used to power the system. The refrigerants used can also be powerful greenhouse gasses, though stricter regulations are resulting in less damaging formulations.

4) Electric Resistance Heat

Electrical current can be passed through a resistive conductor to produce heat. This type of heat production is known as resistive, Joule or ohmic heating. The heat produced is proportional to the square of the current multiplied by the electrical resistance of the wire or element. The amount of current supplied can be adjusted to vary the heat output. Heat energy may be supplied through convection and/or infrared radiation depending on the kind of heating element used.

Efficiency of electrical resistance heating is considered to be 100%, as all the potential energy in the current is converted to heat. However, many sources of electrical power are less than 100% efficient, so overall system performance must take into account the type of generation used to supply the electricity.

The type of generation will also determine the environmental impacts, which can range from high for coal-fired power plants (with delivered power efficiencies as low as 35%) to negligible for renewable energy streams like solar, wind and microhydro.

Means of Heat Delivery

Heat flow always moves from a warmer object to a colder one. Heat flow can occur in three ways:

Conduction – Heat energy is transferred from a warmer object to a colder one by direct contact.

Convection – Heat energy is transferred from a warmer object to the air surrounding the object and then to cooler objects in contact with the warmed air. Warmed air become less dense and rises, creating convection currents that affect objects in the path of the current.

Radiation – Heat energy is transferred from a warmer object to a cooler object by electromagnetic waves, caused by energy released by excited atoms.

Heat transfer

These neat categorizations do not adequately describe heat delivery systems in buildings, as any heating system will warm a space in all three ways to varying degrees. Consider the element on an electric range: anything that touches the hot element will be heated by conduction. At the same time, air near the element will be warmed, become buoyant and warm objects near the range by convection and the heated element will radiate heat to nearby cooler objects, warming the surface of the range and nearby utensils that are not touching the element nor in the path of heated air.

Certain heating systems will rely on one of these methods of heat flow more than others, but cannot be categorized by a particular kind of heat flow.

Instead, it is more useful to consider heating systems in regard to the medium of delivery, of which there are two. Almost any kind of heat production can be twinned with any kind of delivery system.

1) Air Delivery

Passive Air Delivery – These systems rely only on natural convection currents to move heat from a source to the desired locations in the building. No fans or ducts are used to direct warmed air.

Active Air Delivery – These systems use some form of mechanical energy to force air movement in a desired direction. Ductwork is often used to deliver a concentrated stream of warmed air to a particular location.

Air can be an effective medium for delivering heat in some circumstances. It is not very dense, and it is therefore easy to change the temperature of a large volume of air quite quickly. The energy required to move air from one location to another is low, as it flows and changes direction easily and large volumes can be moved quickly. Natural convection loops of rising warmed air and falling cool air can be exploited to good effect to contribute some or all of the required flow.

Occupants in buildings will feel heated air against their skin and have an immediate awareness of warmth and perceived comfort.

These advantages of air as a heating medium are also the disadvantages. The low density of air means that it loses its heat very quickly to more dense objects; raising the temperature of objects in a building via air flow can take a long time, and often objects and the surfaces of the building remain significantly cooler than the ambient air temperature. This can lead to discomfort as the warm occupants in the home will unwittingly be trying to heat the building’s surfaces radiantly, one of the reasons it’s possible to feel a “chill” even in a warm building.

The convection loops associated with air delivery ensure that the warmest area of the building is at the ceiling and the coolest is at the floor. Since occupants reside on the lower side of this balance, some heat is “wasted” by being concentrated outside the contact zone for occupants. Convection loops can also cause cool air to move against occupants in some areas of the building, causing the feeling of chilly drafts even in an air tight building.

In forced air systems with ductwork and fans, the layout must be done carefully if it is going to be efficient. Limiting the number of bends and restrictions increases flow, and proper positioning of outlets can reduce unwanted convection loops and create a relatively even distribution of heat.

Air heating systems will move a significant amount of dust and allergens as it flows. In forced air systems, inline filtration is highly recommended. For natural convection systems, an active filtration system is worth considering.

2) Hydronic Delivery

Using a fluid (typically water) to transfer heat from a source to a destination is a strategy with a long history. Water has a high capacity for absorbing and releasing heat in the range of temperatures used in buildings, and its high density means it can store a lot of heat.

Hydronic heating systems are often called “radiant” heating systems, especially when the heat is delivered to an entire floor area, but this is not an accurate description. Floor heat is conducted through feet, and a very even and useful form of convection accompanies the radiant transfer of heat.

Hydronic heat delivery can be achieved via a radiator that has a lot of surface area. Radiators can be the floor, walls and/or ceilings of the building, or purpose-built radiator systems. Water-to-air systems use a radiator inside forced air ductwork to create a hybrid system.

In all of these systems, produced heat is absorbed into the transfer fluid and moved through pipes through the action of a mechanical pump. The fluid is delivered through branch pipes to the point(s) of delivery where the heat is released, before being recirculated to the heat source again.

Hydronic heating systems will take a longer time to deliver perceptible heat, as the water requires more heat input to reach a perceptible temperature change. Once delivered to the radiator, the quantity of heat in the mass of the water and radiator creates much slower release times. The mass of air (and in some systems, even people) in the building will be lower than the heated mass, and all will rapidly be warmed to the radiator’s temperature without “draining” the radiator of its stored heat, resulting in longer but less frequent cycling of the heating system compared to air systems.

The design of hydronic systems needs to account for the surface area and distribution of the radiator(s), the length of piping and head, and the temperatures required to provide comfort based on the radiator layout. Systems can be quite simple, with a single pump and just a few radiator loops, or they can be complex with multiple pumps and valves responding to individual thermostats in each radiator zone.

Heating System Design: Heat Loss Calculation

Regardless of type of heat production and means of delivery, the design of an effective heating system starts with an accurate assessment of the expected heating needs for the building. This is achieve through the completion of a heat loss calculation. Many building codes now require such a calculation, and even if it is not a legal requirement it is recommended. Oversized or undersized heating systems are not efficient, and will cost a lot more than the calculation.

There are free, simple spreadsheets that can be used for heat loss calculations. These involve gathering dimensions for wall, floor, ceiling, window and door surface areas from the building plans and assigning each its expected heat loss rate (U-value or R-value). These figures are tallied and factored with the number of degree days and minimum anticipated temperature. The results include the hourly heat loss for the coldest expected day (expressed as BTU/hr) and total yearly heat loss (in millions of BTU).

More complex and comprehensive computer modeling programs will include more variables in the calculation, giving consideration to solar gain, thermal bridging in the building enclosure, anticipated air leakage rates and occupant behaviour among other factors. The more detailed the calculation, the more useful the resulting figures for sizing heating systems.

The hourly heat loss figure determines the maximum required output of the heating system, and the total yearly heat loss helps to anticipate energy requirements and costs. Figures from a good heat loss calculation also allow the design of the building to be tweaked for maximum efficiency at the design stage, as variables can be adjusted to determine ideal levels of insulation, window size and quality and air tightness.

With the parameters established by the results of the heat loss calculation, the particulars of the system can be designed to meet these needs in the most efficient and comfortable way.

This content is based on the book Making Better Buildings by Chris Magwood.