Introduction

Regardless of what energy conservation choices are made and sustainable strategies employed, it is still relatively rare for buildings to be Net Zero (requiring zero energy). Even if a building is able to achieve net zero, fluctuations in seasonal weather will typically mean a building needs to supplement its energy in some way. Typically this energy is supplemented through conventional fuel and/or storage.

Use and Movement

Every type of fuel needs to be converted into a useful form, such as heated air or chilled air, or heated water or cooled water. Once the fuel has been converted to a useful energy supply, the distribution of the energy usually requires an additional energy source to operate pumps and fans.

Selecting Type of Fuel

Fuel source selection depends largely on availability, but also the efficiencies and how much fuel will be needed should be taken into account. Another aspect is cost as different fuels have great fluctuations in price over geographical areas.

• For example, in urban areas, steam may be a byproduct of electricity generation, and may be readily available for purchase and hook-up. However, it is rare in suburban areas, and Oil or Natural gas may be the only fuels available which may require intermittent deliveries.

Natural Gas

The most efficient of all fossil fuels. It accounts for about 30% of the energy used in the United States.

• Cleanly burns & Fairly cheap

Propane Gas

Where natural gas is not available, propane can be used.

• It is delivered and stored in pressurized tanks.
• Odorless in its natural state, so an odorant is added to help detect leaks.
• LP (Liquified Petroleum) – Propane is a form of LP gas, but not all LP gas is propane

Oil

Being a petroleum based product, its cost and availability depends on the world market which is highly regulated. The oil is transported and then delivered to its location where it is stored in or near the building. Equipment required on site requires more maintenance than gas burning equipment.

• Comes in different grades: No 1, No 2, No 4, No 5 light, No 5 heavy, and No 6
  • No 2 – Most commonly used in residential and light commercial boilers
  • No 4 & 5 – Grades are used in larger commercial applications 

Electricity

Electricity has many advantages as an energy source. It is relatively inexpensive to install, simple to operate, easy to control, and flexible. It does not require storage – although it can be utilized if desired. It also does not require exhaust flues, or supply air.

Most companies will charge for peak use (use during high demand times). Therefore, heating with electricity during the cold can be extremely expensive. If utilizing electricity, it’s smart to offset this cost with passive and other active systems including storage.

• Electricity is ideal for radiant heating.
• If used to heat water, it is called a hydronic system.

Steam

Not considered a basic fuel like gas and oil, but in many urban locations it is a by-product of the generation of electricity. Steam is not generally used for heating although it can be found in radiators for commercial buildings.

Typically steam is piped into a building and used to heat water for water or air heating systems and to drive absorption-type water chillers for air conditioning.

Energy Chart

Energy Source	Type	Heating Value	% Efficiency
Natural Gas	Fossil Fuel	1050 Btu/ft3	70-80
Propane Gas	Fossil Fuel	2500 Btu/ft321,560 Btu/lbm	70-90
Oil	Fossil Fuel	No 2:137,00 Btu/gal to 141,000 Btu/galNo 5:146,800 Btu/gal to 152,000 Btu/gal	NO 2: 65-85
Coal	Fossil Fuel	25–35 MJ/kg	65-75
Electricity	Renewable	3413 Btu/kW	95-100

Calculating Amount of Heat Needed

Degree Days (Heating Degree Days)

Degree Days are a rough measure of how much heating is needed for human comfort in a particular location over the course of a year. This value is used to help estimate the size of a system that is needed to provide adequate heat.

To calculate

Found by taking the difference between a baseline indoor temperature of 65°F and the average 24 hour temperature for a specific location. 65°F is not considered high enough for winter comfort, but it is used as the baseline design temperature as there are more degrees expected to be contributed by humans, lighting, solar and other factors.

• If a specific location has a 24 hour average temperature of 65°F or more, then there are no heating degree days for that specific day (24 hour period).
• Degree Days are used to estimate yearly fuel consumption, size some passive solar energy systems, and to factor into other computations.

Example

The 24 hour average temperature for the City of Seattle is 25 on X Day. The degree days for the day are then 65 (design temp) minus 25 (avg temp) = 40 degree days. This is the number for this location for that specific day. To get the yearly degree days, you must repeat this process for every day in the year for that location.

Cooling Degree Days

Degree days are below 65, while another measurement is the Cooling Degree Days. This is for helping size air conditioning systems and for when the temperature is above 65.

Measurements

1 Ton of Refrigeration = 1 Ton of Cooling.

The cooling effect when 1 ton of 32°F ice melts to water in 24 hours. This is equivalent to 12,000 Btu/hr. 

The calculated capacity of a system, it can be determined by dividing the total heat gain by 12,000 Btu/hr

Heat Generation Equipment

Furnace

A furnace burns fuel inside a combustion chamber. Air is circulated around the chamber by a fan. As the cool air from return air ducts passes over the chamber, it is heated for distribution to the building. The hot exhaust gasses caused by burning the fuel pass through a flue that is vented to the outside.

• Typically has replaceable filters to clean and filter the air.

Forced Air Furnace Types

• Upflow: The return air is supplied at the bottom of the unit and the heated air is delivered above the furnace where it is then distributed.
• Downflow: Operates the exact opposite way. Typical when ductwork is located in a basement or crawl and the furnace is located on the first floor. May be required for when flood plains prevent mechanical from being below grade.
• Horizontal: When head space is limited, air is delivered from the side.

Boiler

Uses fuel to create hot water or steam. The fuel source can be gas, oil, electricity, or steam. In a typical boiler, tubes containing the water to be heated are situated within the combustion chamber where the heat exchange takes place. As with furnaces, the gasses and other products of combustion are carried away by a flue or chimney.

• When the fuel source is electricity or steam, there is no need for an exhaust flue.

Heat Pump

A heat pump can provide heat in the winter and cooling in the summer. Its name refers to the ability to move heat (pump it to a different location) and not ‘generate’ heat. The heat pump relies on the principles of refrigeration to take heat from a cooler location and transfer it to the hotter location.

• Principle: ‘Heat’ always moves from hot to cold.
• Heat pumps generally work best in moderate climates. For extreme climates they are typically needed to be supplemented with other forms of heat and cooling.

Summer

The heat pump is a standard air conditioner. A refrigerant circulates inside the heat pump through a cycle that includes a condenser and an evaporator. In the evaporator, the refrigerant is made to be cool, and therefore when circulated, it absorbs the heat from the inside air becoming warmer. The warm refrigerant is pumped to the condenser which is on the exterior of the building. The refrigerant is compressed to drastically increase the refrigerant temperature, which makes it hotter than the exterior air. This heat is then transferred/dumped to the exterior air.

Winter

The process during winter reverses itself essentially, and the heat pump acts as a standard heater. The refrigerant is made extremely cold when it is on the exterior of the building, and therefore it ‘sucks’ the heat out of the air (even when the air is cold itself), and then that refrigerant it sent into the building and compressed to make it hot, which then dumps the warm air into the inside space.

• As stated, this gets more difficult as the temperatures drop very cold, typically below 40°F, gas and oil become more efficient and more economical. This is because the refrigerant has its limitations to how cold it can get and how much heat is left in the air to take.
• To supplement heating, sometimes electrical resistance coils will be placed in supply ducts.

To increase efficiency, a dual heat pump and solar energy system can be used. The solar energy will provide heat between 47°F and 65°F. Below this range the heat pump works, until the heat pump needs additional support and the solar energy system kicks in also.