A MECHANICAL ELECTRICAL CURRICULUM FOR CONSTRUCTORS

INTRODUCTION

Mechanical Electrical (M/E) subsystems account for 20 to 25% of building construction cost and a large part of building operating cost. It follows that a substantial number of construction graduates may pursue careers constructing, operating, and maintaining building M/E systems. Construction curricula typically dedicate 2 to 5% of their available hours to the M/E subject area; this paper begins with a brief overview of course goals and then focuses on drawings and operating cost estimates as valuable components of an effective M/C curriculum.

COURSE GOAL AND OBJECTIVES

The M/E curriculum goal is to provide a foundation of information and experience that enables graduates to make informed value , judgements. Curriculum objectives are outlined briefly below; these objectives are not proposed as mandatory standards for a M/E curriculum. Rather, they are presented as a starting point for discussion of two topics addressed in this paper.

Objectives

Students successfully completing M/E course work should be able to:

The following discussion emphasizes objectives 7 and 8; and proposed that these objectives can be realized most effectively using student prepared drawings, and student prepared operating cost projections.

DRAWINGS

Materials, structure, M/E subsystems, and construction are interrelated elements of the building process. Informed judgments and decisions before and during construction should be based on a working knowledge of each individual element and its relationship to all others. Evaluation of an alternative curtain wall panel for example, should include U values, weight, sealing and wind bracing comparisons as well as the usual initial cost and installation time considerations.

Over more years than I care to remember, I have concluded that students can best begin to understand the interrelationships of building subsystems by sizing, selecting, and detailing an element and its relationship with other building elements. Preparation of a set of M/E drawings for a commercial building is a curriculum approach that can help students understand the M/E topic area and its impact on the building process.

Drawing preparation is particularly effective because it requires selection and detailing of a M/E installation based on previously completed student calculations of heat loss and gain, electrical, lighting, and water requirements. The drawings reinforce lecture discussions and confirm the relationships between orientation, material selection, and M/E subsystems. In addition preparing drawings, provides familiarity with available M/E contract

drawings that are an essential part of all construction projects. Finally, drawings are the practical route to understanding the space requirements necessaryy to accommodate and integrate M/E subsystems with building structure, materials, and construction.

I believe construction students who have developed a set of M/E drawings are better able to:

Unfortunately, correcting and evaluating M/E drawings is not the easiest way for an instructor to cover course material, but the potential student benefit from the drawing effort can produce stronger graduates.

ESTIMATING OPERATING COST

A second valuable component of an effective M/E curriculum is operating cost analysis; it helps students understand the impact of design and material decisions and is the basis for informed value judgments concerning construction alternatives.

An operating cost projection is the logical culmination of student efforts in M/E design and equipment selection; it can focus student interest and develop a sound basis for evaluation of M/E subsystems. Selecting the "best" M/E system for a given building naturally involves cost as well as comfort considerations; and the operating costs tie M/E subsystems into nearly all other building subsystems. Materials, insulation, orientation, even structural detailing, affect the operating costs incurred to maintain comfortable indoor conditions.

Operating costs for building lighting or water supply are easily estimated based on past experience records from similar buildings. However, HVAC costs are a bit more challenging because of three areas of uncertainty. Actual infiltration-ventilation conditions, actual equipment efficiency, and actual "full load" operating hours, are all factors that vary considerably with building quality, building operation, and climate.

I use equivalent full load hour maps as a starting point for estimating annual HVAC operating costs; they are approximations based on limited experience, weather maps, manufacturer's information, and a good deal of (hopefully) inspired judgement. The following 7 pages from Efficient Buildings heatinq and cooling summarize the process I use to estimate operating costs and evaluate construction alternatives.

You will notice the maps are very small and very general. While I am confident about their application in specific areas where I have historical utility cost records, I have not confirmed their applicability throughout the U.S. Should you decide to apply them, confirm or adjust heating and cooling hours based on local experience. The best source of "full load" operating hour data for HVAC installations is your local electrical utility and heating fuel supplier. They maintain extensive historical data that correlate electrical and fuel consumption with weather conditions and building type.

After improving on my general maps and adjusting my approximations concerning heat contributed by people and lights, confirm your operating cost estimates by checking several existing buildings where historical utility data is available. Then pass your results on to students for improvement and application. Students respond to an economic basis for evaluating construction alternatives and their impact on operating costs with interest and enthusiasm.

The following 7 pages from Efficient Buildings are reprinted by ASC with permission.

5.0 Estimating Annual Energy Requirements

Calculated values for heat loss and heat gain may be used to estimate annual energy requirements and costs. The heating and cooling hours maps at right give estimated full-load operating hours for heating and cooling equipment. Maps are rough estimates; better values can be obtained from local utilities. Do not apply map values to buildings that are heated and cooled occasionally, like churches and dance halls, or to buildings operated 24 hours a day. To estimate annual energy requirements in BTU/yr (BTU per year) multiply the peak heat loss and gain values for a building by the annual full-load heating and cooling hours shown on the maps.

Example: House

Find the total annual heating and cooling BTU for a house with a peak heat loss of 77,273 BTUH and a peak heat gain of 33,357 BTUH (p. 17 and 27) located at point "H" on the maps.

Example: Office

Find the total annual heating and cooling BTU for an office building if the calculated peak heat loss is 406,850 BTUH and peak heat gain is 630,162 BTUH (p. 19 and 29) located at point "0" on the maps.

5.1 Estimating Annual Energy Costs

BTU/yr can be converted to $/yr (dollars per year) if you know the cost of energy and the efficiency of the heating/cooling equipment. The following calculations use the preceding house and office examples with a variety of equipment efficiencies and energy costs. Indoor fan energy is not usually included in equipment efficiency ratings, so allow 10% extra for residential air handlers and 20% extra for commercial air handlers with longer duct runs.

The equations are given below:

House: Estimate #1

Heat with oil at $1.00 per gallon and a furnace that is 70% efficient. Cool using electric equipment with a SEER of 8; electricity costs $0.10 per KWH. (1 gal. oil = 140,000 BTU; SEER 8 = 8 BTU/watt)

House: Estimate #2

Same example, but use a heat pump with a COP of 2.5 and a SEER of 7. (COP 1 = 3,400 BTU per KW)

Office: Estimate #1

Heat the office building with natural gas that is 70% efficient at $0.50 per thermal; cool with electricity at $0.10 per KWH and SEER 9. (1 therm = 100,000 BTU)

NOTE:

The preceding calculations can be applied with some confidence to residential and smaller commercial occupancies. However, they do not apply to 24 hour a day occupancies like hospitals or airline terminals. Large buildings that operate chillers, cooling tower fans, cooling tower pumps, chill water pumps, high velocity air supply fans, and return air fans; may have overall operating SEER's as low as 2 or 3.

5.2 Evaluating Energy Conserving Alternatives

Preceding pages proposed a method for estimating annual heating and cooling costs. The same method may be used to evaluate energy conserving alternatives. Calculations are based on the office building example used earlier in the text for heat gain and loss estimates (p. 18 and 28). Alternates #1 thru #4 assume 1,600 full-load hours for annual heating and 1,400 full-load hours for cooling; alternate #5 uses 1,500-full load hours for heating and cooling.

Alternate #1: Slab Edge Insulation

An insulation contractor proposes to reduce building heat loss by installing slab edge insulation for $3,000. Evaluate the proposal by finding the first­year on investment.

Given;

Slab Edge length is 494 linear ft. (allow 1 sqft. per linear ft.)

U value without insulation is 0.8, with insulation U is 0.1. Winter TD is 60°F.

Heat is electric resistance at $0.08/KWH (each KWH yields 3,400 BTUH).

Alternate #2: More Roof Insulation

The roofing contractor offers to increase the thickness of urethane roof insulation from 3" to 4" for $2000. Evaluate this proposal by finding the first­year return on investment.

Given:

Roof Area is 10,800 sqft.,

U value for 3" roof is 0.05, U value for 4" roof is 0.04; winter TD is 60°F, summer ETD is 36°F. Heat with a 80% efficient gas furnace with gas at $0.75 per therm.

Cool with an SEER-7 unit; electricity costs $0.09 per KWH.

Alternate #3: Reduced Fresh Air

The HAC consultant proposes to weatherstrip the building and reduce the ventilation rate to 10 CFM per occupant (a total of 1,6000FM). Added costs for controls and weatherstripping total $2,000. Estimate the first-year return on investment.

Given;

Old design: 2,970 CFM winter, 2,400 CFM summer. New design: 1,600 CFM for both winter and summer.

Winter TD is60°F; summer TD is 20°F, GD (Grain Difference) is 40.

Heat with a 75% efficient oil fired boiler using $1 per gallon oil.

Cool with a SEER-8 electric chiller, electricity costs $0. 07 per KWH

Alternate #4: Reflective Windows

The glazing contractor proposes to provide reflective windows instead of clear windows for $2,470 extra. Evaluate this proposal by estimating the first year return on investment.

Given:

Shading Coefficient (SC) for non-reflective glass is 85%; SC for proposed reflective glass is 45%. Solar Factor (SF) in summer months is 30 (for north and shaded south glass). Glass Area is 2,470 sqft.

Cooling SEER is 7, and electricity costs $0.10 per KWH.

Alternate #5: Improved Heat Pumps

The HAC subcontractor proposes to furnish more efficient heat pumps for $10,000 extra. Evaluate this proposal by estimating the first-year return on investment.

Given:

The standard heat pump has a COP of 2.1 and an SEER of 7.3; the high efficiency heat pump has a COP of 2.8 and an SEER of 11.1. Building heat loss is 323,600,000 BTU per year. Building h eat gain is 945,000,000BTU per year (Note: 1,500 full-load heating and cooling hours are used to calculate BTU per year for this example). Electricity costs $0.08 per KWH.

* Estimates of annual cooling savings based on reduced peak heat gain may be exaggerated because of solar load variation. More accurate projections may be developed using actual solar data for the building site.

2.6 Office Example, Heat Loss Calculation

Design Conditions

Winter temperature is 10°F (for location "0" on map p. 14). An indoor temperature of 70°F is assumed; temperature difference (TD) is 60.

Project Conditions

Fresh Air: estimate infiltration at 0.75 air changes per hour in winter (0.5 air changes per hour in summer). Estimate ventilation rate at 15 CFM per person. Fresh air CFM due to infiltration is the largest value in winter. (p. 10) Glass: double glass; U = 0.6 (p. 9)

Lighting o not take heat gain credit because lights will be off on cold nights.

People: estimate 160 occupants, but do not take heat gain credit because people are absent on cold nights.

Ceiling-Roof: detail is below; U = 1/R total = 0.05 [1/(0.2+18+0.7) = 1/18.9 = 0.05] (p. 9)

Wall: see detail below; U = 1/R total = 0.07 [1/(0.2+0.4+1+0.5+11+0.5+0.7) = 1/14.3 = 0.07.]

(p. 9)

Floor: slab on grade; factor is 2 BTUH per sqft. (p. 13)

Slab Edqe: (not shown) no insulation; no heating ducts; U = 0.8 (p. 13)

Doors Single glass; U = 1.1 (p. 9) Equipment: Do not take a heat gain credit for equipment because it does not operate on cold nights.

Quantities

See Plan and Detail at left for dimensions. Fresh Air: building volume is (21,600)(11) _ 237,600 CF. At 0.75 air changes per hour the expected infiltration is (237,600+60)(0.75)= 2,970 CFM. Ventilation rate for 160 people at 15 CFM each is 2,400 CFM. Use the larger infiltration value for winter heat loss.

Glass: windows are 4' high; total window area is 2,400 sqft.

Ceiling-Roof: area is half floor area (2 floors) or 10,800 sqft..

Walls: net wall is 10,880 sgft. Gross wall area is 10,950 sqft.; add for soffits and sills 2,400 sqft.; deduct for windows and doors 2,470 sqft. Floor: slab on grade area = 10,800 sqft. Slab Edge: 494 linear feet.

Doors: 2 @ 5'x7'= 70 sqft.

Winter Heat Loss = BTUH

Insert appropriate U values, constants, factors and temperature difference; then complete calculations and total BTUH. Add 10% if ducts are outside the conditioned space.

2.12 Office Example, Heat Gain Calculation

Design Conditions

Maps show a summer dry bulb temperature of 95°F and a wet bulb temperature of 76°F (p. 14, location "0"). Desired indoor conditions are 75°F and 50% relative humidity. Time of peak heat gain is estimated as 4 P.M. Temperature difference, TD = 20; and grain difference, GD = 40 (p. 23).

Project Conditions

Fresh Air: select largest of infiltration at 0.5 air changes per hour or ventilation at 15 CFM per person (p. 10).

Glass: double glass U = 0.6. The (SC) shading coefficient for clear double glass is 0.85 (p. 22). Lighting: estimate 3 watts per sqft. of floor area for this office occupancy, and revise as necessary after completing lighting design. Office building lighting installations can range from 1 to 6 watts per sqft. depending on luminous intensity, lamp type, and fixture design.

People: 160 occupants seated doing light work. Ceilina-Roof: U = 0.05; weight is 7 pounds per sqft.; color light. ETD = 36 (p. 22). Walls: U = 0.07; weight is 45 pounds per sqft.; color dark. The west wall is hot at 4 P.M.; its ETD is 40; the weighted average ETD for all walls at 4 P.M. is 23 (p. 22).

Doors: U = 1.1 (tempered single glass). Solar gain for doors is included in Glass. Equipment: Allow 1 watt per sqft. for electrical equipment operating at the time of peak heat gain. Also allow 10 hp. for air handler fans.

Quantities

Fresh Air: use largest value. Infiltration at 0.5 air changes per hour = 1,980 CFM, (21,600)(11)(0.5)+60 = 1,980.

Ventilation at 15 CFM per person = 2,400 CFM, (160)(15) = 2,400.

Glass 1,030 sqft. faces north (includes 70 sqft. of glass door area); 1,440 sqft. faces south. Lie h in : totals 64,800 watts at 3 watts per sqft. People: each occupant produces 250 BTUH

sensible plus 150 BTUH latent. Ceiling-Roof : area is 10,800 sqft. Wall: net wall area is 10,880 sqft.

Doors: area= 70 sqft.

Equipment: allow 21,600 watts at 1 watt per sqft.; plus 10 hp. for air handler fans.

Summer Heat Gain = BTUH

Insert appropriate factors and temperature difference; then complete calculations and total BTUH. Add 10% if ducts are located outside the conditioned zone.

Refer back to heat loss example on page 18 for more detail.

Heat Loss - Heat Gain, Summary