Building Heating is a Gas!

WoodHarbinger-479-EditBy Jeff Yirak, P.E., LEED AP BD+C, O+M

HVAC engineering may be all fun and games, but the main purpose these systems serve is no laughing matter. Whether at home or in a commercial building, maintaining a comfortable temperature for the occupants is a top priority. The type and complexity of HVAC system used to provide heating and cooling will depend on the size of the building, needs of the occupants, and climate of the location.

A hydronic heating system using natural gas is an effective and cost efficient choice for a variety of building types, from residential to large commercial. Natural gas is a fossil fuel frequently used in building heating applications, as it has a pricing favorability compared to electricity in many heating appliances, such as water heaters and hydronic boilers.

A basic hydronic heating system consists of a boiler, circulation pump, and terminal heating devices like radiators:

aerco boiler

Aerco Benchmark boiler. Image source: http://aerco.com/solutions/boilers/benchmark

circulation pump

A centrifugal circulation pump. Image by Kaze0010 via wikimedia.com, licensed under CC BY-SA 3.0.

radiator

A household radiator. Image by User:SimonP via wikimedia.com, licensed under CC BY-SA 3.0.

How big are these boilers? Small boilers are about the size of a refrigerator. The gas burner inside is similar to the burner on a gas stove, but while the stove burner puts out around 9,500 British Thermal Units, (BTUs), the boiler’s output is closer to 950,000 BTUs!

Why is the boiler output so much greater, and where does all the energy go?

Residential Heating

Unlike commercial buildings, most residential furnaces do not conduct outdoor air into the house. Houses usually have operable windows, manual exhaust fans, and maybe even trickle vents above the windows. Houses are generally qualified as naturally ventilated, since the furnace does not provide fresh air for ventilation. Since there’s a very small quantity (or no quantity) of cold outside air being introduced into the space, the furnace has only to make up the heat lost through the walls, floor, and roof.

Commercial Heating

Commercial buildings that use central air handling equipment are mechanically ventilated. The energy code requires a certain amount of fresh outdoor air to be brought into the building based on the square footage of the space and the number of people in the building. This amount is usually around 15 to 20 cubic feet of air every minute (cubic feet per minute, or CFM). That’s a volume of air about equal to the size of a dishwasher being moved into the building every minute for every person in it.

We may finally be getting on to some warmer weather in the Puget Sound area, but it was pretty chilly in past months. On a cold winter’s day, this outside air, being, say, about 20 degrees F, requires a substantial amount of heating before being introduced to the room. This heating energy, measured in BTUs, comes from the furnace or boiler. If a building had a population of 100, then we would be required to provide about 2,000 CFM (20 CFM per person, multiplied by the 100 people in the building). Assume we wanted the room to be 70 degrees F; we can calculate the amount of BTUs we need to bring 2,000 CFM of 20 degree F fresh air up to temperature with the sensible heat transfer equation. You multiply the CFM required, by the sensible heat constant 1.08 (this number has to do with the specific heat capacity of air), by the temperature difference between your outside air and the desired room temperature. In our example, we have 2,000 CFM, multiplied by the 1.08 constant, multiplied by 70 degrees minus 20 degree F (starting temperature of the air). Here’s how it looks in equation format:

2000 CFM * 1.08 * (70 degrees – 20 degrees) = 108,000 BTUs.

We’d need 108,000 BTUs to condition the outside air to 70 degrees F. However, this calculation doesn’t account for heat loss due to conduction through the walls, roof and slab, and due to leaks in the building. Besides conditioning the ventilation air, additional heating is required to counteract the heat loss of the building (which was the only heating load in the residential example). We would provide this additional heating by recirculating some of the indoor air back to the heating system, and we would provide a supply air temperature higher than 70 degrees F. Referring to our sensible heating equation, the airflow is about three times higher (CFM), the temperature difference is now more like 90 degree F supply air heated from about 55 degrees F mixed air (a mixture of the outside and return air) so you’d actually need more like 300,000 BTUs to keep the building warm.

Making the Most of Heating Energy

Commercial buildings use a lot of energy to keep the occupants comfortable. Much of this energy, such as used for conditioning fresh air, is necessary for the health and safety of the occupants, but smart building construction including a good thermal envelope and air barrier can reduce the energy required.

Having better insulation and an air-tight building is a great way to reduce the amount of heating energy needed to keep a building warm in winter, but also cool in summer, as it holds in conditioned chilled air. Good insulation pays dividends in summer and winter by reducing the energy needed to condition the building.

Controlling heat transfer is a vital part of our daily lives and is something building owners pay for throughout the life of the building. Understanding how to manage and improve it through accurate load information and efficient appliances will help keep utility dollars from (literally) going through the roof! Speaking of utility bills, check out my previous blog post on this topic for another good laugh. Trying to make sense of those is also a gas! (We engineers sure know how to have a good time, don’t you think?)

 

Follow Jeff on Twitter @JYirak_WH

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