Posts Tagged ‘geothermal’

Control Systems: Intelligent Management of Indoor Climate

Thursday, November 12th, 2009

More Than a Thermostat

A combination of mechanical equipment, motor-operated zone dampers, zone temperature sensors, programmable controllers and an IP-enabled server comprise a Rockwall VAV system.

Single-Zone Constant-Volume

The simplest format control system has a basic programmable thermostat that sequences heating and cooling for multiple rooms served by a single fan/furnace unit: all spaces served by this unit heat and cool equally. While its simplicity is a plus, its inefficiency is a big minus. Unless every space served by the air handling unit has the same occupancy (people in every space over the same time period), unused spaces inherently waste energy.

Multi-Zone Variable-Volume

Although a multi-zone variable-volume (VAV) control system adds cost, it yields excellent long-term benefits to the owner.

  • Perfect air balance
  • Perfect comfort
  • Lower operating cost
  • Fully automatic – needs no human adjustments, automatic seasonal changeover
  • Intelligent – resolves issues and transmits appropriate messages to service personnel
  • Conserves energy by not conditioning unoccupied spaces
  • Effectively extends capacity over same-design constant-volume systems, depending on variability of loads

Rockwall VAV monitors the use of all zones, it is self-aware, it ‘learns’ and it ‘understands’ how each zone behaves. A variety of specialized sensors and techniques inform the control system when a space occupies and when it becomes vacant. Heuristic algorithms track human use of spaces over seven days to develop profiles to assure comfortable re-entry of each space. For optimum control, Rockwall Controls applies a broad array of zone sensors that best suit respective zones.

Unoccupied Mode

While unoccupied, Rockwall VAV modulates the zone damper and sequences the central air handling unit to allow temperature naturally rise and fall between a maximum unoccupied cooling set point (nominally 85OF) and a minimum unoccupied heating set point (nominally 65OF). Depending on materials (works of art, antique furniture, sensitive plants, etc.) within the conditioned zone, logical control may include a relative humidity sensor. This mode provides maximum energy conservation.

Occupied Mode

While spaces occupy, the control system modulates respective zone dampers and activates central air handling unit equipment to precisely maintain zone temperature. An algorithm weighs cooling and heating demand by aggregate zones of a given air handling unit and sequences refrigeration, heating and supply fan to satisfy all the zones.

Automatic Air Balance

The inherent beauty of VAV is its self-balancing characteristic. Due to the way each zone damper individually modulates to precisely control temperature while automatically compensating for changes in cooling load, assures precise delivery of conditioned air to each respective zone.

Add Space without Adding Cost

Depending on load variability, the differences in demand for either heating or cooling between spaces served by a given air handling unit, it is possible to extend a five-ton air conditioning to cover a seven-ton load. What’s the catch? There is no catch. All those features enumerated under “Prescription for Success” must align for Rockwall VAV to work satisfactorily.

Assuming not all zones occupy at the same time, capacity unused by unoccupied zones can easily be diverted to occupied zones through addition of a properly designed air distribution network.

Looking Under the Hood – A Technical Discussion

Conversion of a single-zone air-handling system to VAV challenges the central mechanical equipment. Operating as a constant-volume system, the fan moves constant airflow through the cooling coil. This is important, because the air must remain in the coil long enough for the coil to absorb heat from the air, but fast enough to expel condensed moisture from the fins of the coil. If too fast, space relative humidity is high; if too slow, the cooling coil can freeze over.

VAV introduces an important issue: tight coupling between the zone dampers and the supply fan. Our solution is to de-couple fan-coil airflow from zone airflow. Refer to the following diagram. 

Rockwall VAV with 8 Zones
Rockwall VAV with 8 Zones

 In order to optimize fan airflow to the heat exchanger coil, we must de-couple zone variable-air volume (VAV) airflow from the constant-volume (C AV) airflow required of the heat exchanger. A de-coupler air valve, responding to airflow, fan discharge pressure, fan motor current and the fan curve, modulates to maintain relatively constant airflow through the coil.

It is helpful to understand this discussion relates to geothermal air conditioning systems that use super energy-efficient ECM (electrically commutated motor) fan motors. These DC motors use approximately 75% less energy than conventional AC-powered fan motors.

ECM fan systems work in a low-pressure, low-velocity configuration for optimum

GE ECM 2.3 Motor Cutaway

GE ECM 2.3 Motor Cutaway

 efficiency. GE ECM motors are programmed by the air conditioning equipment manufacturer. Rockwall VAV communicates speed commands to the motor. The System analyzes motor current, input power, fan discharge pressure and leaving-air temperature to manage the fan for greatest operating efficiency. Click here to learn more about the GE ECM motor.

Back to Residential …

Adequate Capacity – Air Conditioning Systems

Monday, October 26th, 2009

¬Return to Residential Main

Simple ‘Rule of Thumb’ Sizing

Obviously, conditioned floor space, expressed in square feet, is a chief factor when calculating air conditioning capacity (tons). Depending on local experience, contractors may allow a certain amount of airflow per square foot. In this example of a 10,000 square-foot residence, let’s set a ‘rule of thumb’ at one cubic foot per minute (cfm) for.

10,000 ft2 X 1 cfm/ft2 = 10,000 cfm

Next, we set a second ‘rule of thumb’ of 400 cfm per ton refrigeration.

10,000 cfm/400 cfm = 25 tons

A much more conservative value of 300 cfm/ton can be used to evaluate the margin.

10,000 cfm/300 cfm = 33 tons

A real-life example: a 26,000 ft2 Texas residence features a 100-ton ‘chiller’ that circulates refrigerated water to 24 fan-coil units. (Read more about fan-coil units)

At 400 cfm/ton, an ambitious low-bid contractor might estimate 65 tons. A more conservative contractor, using 300 cfm/ton might come up with 86 tons. In this real-life case, the ratio is 260 ft2/ton.

What’s the correct tonnage? Would you believe even the most conservative calculation proves to be inadequate?

Why?

Simple calculations may work for the small system situation where it is actually hard to go wrong. Structures, residences over 3,000 ft2 need to be scientifically evaluated. Various software products exist for this purpose and these software products are intended to be used by professional air conditioning contractors and mechanical engineers. You will not find a “How To” book at Home Depot to solve this problem – “you can’t do it and they can’t help”.

What to Do – Leave it to the Professional

Professionals measure dimensions, create a floor plan and evaluate several important features of the structure.

  • Windows – type, size, relative compass direction they face, solar screens, reflective film, etc.
  • Insulation – type (glass, cellulose, foam), thickness, condition
  • Roof – type shingle, radiant barrier, attic ventilation, construction (gable, flat, cathedral, etc.)
  • Ceiling height
  • Geographical location (latitude, altitude, ASHRAE design temperatures)
  • Use of spaces – game rooms, gymnasiums, indoor spas and pools, trophy rooms, bedrooms, media centers, etc.
  • Computer rooms, home office, etc.
  • Garage spaces
  • Vestibules and passage ways

Symptoms of Poor Design

Some problem signs are patently obvious – one register in an 800 square-foot space is a good one. Loss of control on hot summer days is another. If it’s 100 degrees outside, I expect my system to hold the inside around 73 degrees, and it does, because my system was professionally designed and professionally installed.

My Choice for Air Conditioning

Geothermal easily comes in first over air-cooled conventional residential equipment and it definitely surpasses commercial ‘chillers’ for cost of ownership and overall reliability. (Read more about geothermal…)

Residential Air Conditioning Systems

Monday, October 26th, 2009

Home Sweet Home

We live approximately two-thirds our lives in and around our homes. A man’s home is his wife’s castle – a simple truth I learned long ago. Our homes are safe havens from high-stress business and work environments, a center of entertainment – valuable asset. Some of us transform our ‘castles’ from casual residences to art galleries and even into mini-museums. Irrespective to fun times away on vacation or travel abroad, “there is no place like home”.

Expectations

We want a healthy, comfortable indoor environment at the lowest possible cost. This is true whether your ‘castle’ is a modest structure or a mansion on a country estate. Stable temperature, just-right relative humidity, fresh odor and dust-free – these are the characteristics of the perfect indoor climate.

Prescription for Success

Key to a satisfactory air conditioning system is an alignment of numerous features:

Good News/Bad News

Any home can have near-perfect to perfect air conditioning when essential factors line up. Knowledgeable design and installation by skilled craftsmen are requisites to have this finished product. This is the good news.

Unfortunately, too many poorly designed, poorly installed residential air conditioning systems get into homes. It’s almost impossible to go wrong with the “rule of thumb” method for small cottages, but all systems over 3 ½ tons should be carefully designed. (Read more…)

Air-Cooled vs. Geothermal

Geothermal systems win “hands down”, when there is adequate terrain for geothermal wells or, better, an adequate pond or lake nearby. My geothermal system has nothing visible outside – no condenser coils to clean, no noisy condensing unit, simply invisible underground piping. (Read more…)