The How-To Guide for CALCULATING HEAT LOADS AND JOB …

CALCULATINGHEAT LOADS AND JOB SIZING

The How-To Guide for

TABLE OF

CONTENTSSECTION 1: INTRODUCTION

SECTION 2: JOB SIZING FACTORS

SECTION 3: HEAT LOAD CALCULATIONS

Air Density

Building Heat Loss

Equipment Needed

SECTION 4: PARTNER WITH AN EXPERT

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01HOW-TO GUIDE FOR CALCULATING HEAT LOADS AND JOB SIZING SUNBELT RENTALS

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In The Construction Heating Handbook: Selecting the Right Heaters and Fuel for Your Project, the third task is to calculate heat load and job sizing. With that information, you’ll be able to determine how much equipment and fuel you’ll need on-site. That’s critical for protecting the structure, allowing tasks like concrete pouring, and keeping your workers comfortable – and ultimately for staying on budget and on schedule.

You may be tempted to design a temporary heating system the same way you would a permanent system. But the circumstances differ considerably. In the early stages of construction, you may have a tight roof but walls of sheet plastic or canvas. Or maybe you have walls that are almost complete but door and window openings covered with plastic. Either way, cold air can seep into every part of your building, and you can lose an enormous amount of heat through the ceiling or roof. In fact, heat loss may be 5-6X as great as it will be once the building envelope is complete.

Although some contractors think all they need to know is the volume of the structure that requires heating, many other factors come into play. Below are only a few of the considerations you should take into account to determine your heat load and the equipment and fuel you need.

How do I calculate how much

HEAT I’LL NEEDFOR MY JOB SIZE?

SECTION 1

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DUCT-RUN LENGTHSFor long lengths of duct, you’ll need high-static blowers, which will limit your choice of heater type to indirect-fired heaters. But keep in mind that if your equipment is outside, you’ll lose about 7.5% of your heat in the duct run.

DESIGN TEMPERATURES The amount of heat required to heat a space depends on the temperature difference between the inside and outside. For the outside, you need to determine the lowest temperature over the heating season and use that for design. Otherwise, you’ll risk not being able to heat adequately. A psychrometric chart for your area can give you the temperature information you need for each month.

DESIRED MOISTURE LEVELSIn a heated space, moisture is not typically an issue. When you heat air, it expands, and the percentage of moisture lowers. However, propane and natural gas both contain water. In direct-fired heaters, that moisture goes into the air as vapor. If you can’t exceed a specified level of relative humidity in the space you’re constructing, an indirect-fired heater will be a better choice.

ALTITUDE At higher altitudes—from about 2500 feet on up—an equal number of BTUs will heat up more air. The lower air pressure at higher altitude, along with the lower air density, means it carries less heat but is easier to heat up. But the lower air pressure can also make your heater slightly less efficient.

BUILDING ENVELOPE/ENCLOSUREYou’ll need to consider where you’ll be in the building process when the temperatures drop enough that you’ll need heat. You may have a structure that’s using plastic sheeting to close it in or one with finished exterior walls and insulation.

HEATER EFFICIENCYHeater efficiency ranges from 100% for direct-fired heaters sitting inside to 87% for indirect-fired heaters sitting outside. Note that for most of the calculations in this guide, we haven’t taken heater efficiency into account—but you definitely should.

JOB SIZINGFACTORS

SECTION 2



HEAT LOADCALCULATIONS

SECTION 3

• Air density

• Building heat loss

• Equipment performance

Let’s take a look at how each one impacts your calculations.

To calculate the heat load for a building, you need three main numbers:

AIR DENSITY

Air density varies based on both temperature and altitude. Colder air will be more dense that hot air, and air at sea level will be more dense than air at altitude. Let’s take a closer look. At sea level, the air density is .0750 lb/cf @ 67ºF. Taking the inverse of the density gives us the amount of space a pound of air occupies. So:

1/.075 lb/cf = 13.33 cf/lb

Let’s consider a 150 kW-heater with a fan delivery volume of 5000 cf/min (cfm). Multiply by 60 minutes to determine how much air the heater delivers in an hour:

5000 cf/min x 60 min/h = 300,000 cf/h.

Now divide by the inverse of the density to determine the pounds of air the heater delivers in an hour:

300,000 cf/h / 13.33 cf/lb = 22,500 lb/h.

Now, note that 1 kW = 3414 BTU/h. So, here’s what the 150-kW heater will deliver in BTUs:

150 kW x 3414 BTU/h/kW = 512,100 BTU/h.

We can now calculate how much heat we created with the 150-kW heater, given that it delivers 22,500 lbs of air. To do that, we need to know the specific heat – the BTUs required to heat 1 pound of air 1ºF. This number is .24 BTU, so:

512,000 BTU/h / .24 BTU/ºF = 2,133,333 ºF/h.

Now, we can divide by the weight of air we’ve put through the heater to calculate the temperature rise:

2,133,333 ºF/h / 22,500 lb/h = 95ºF.

CONTINUED



We add the incoming air temperature of 95ºF to the outside air temperature to determine the temperature the heater will deliver. If it’s 20ºF outside, for example, then the heater will deliver 115ºF.

Let’s introduce a constant – really a unit conversion factor – that you’ve probably seen in heat load equations. Often you’re trying to input the fan cf/min and output BTU/h. Here’s how it works out:1 BTU/h = 1 cf/min X 60 min/hour X.0750 lb/cf x .240 BTU/ºF X 1ºF

If ΔT is the temperature differential in ºF, then:1 BTU/h = 1.08 X cf/min x ΔTºF.

We know our fan delivers 5,000 cf/min. So here’s the calculation for BTU/h:5,000 cf/min x 1.08 = 5400 BTU/h to increase temperature 1ºF.

We know the heater delivers 512,000 BTU/h. So here’s a simpler calculation for the temperature increase:512,000 BTU/h / 5400 BTU/h X 1ºF = 95ºF.

But above 2500 feet, the density of air makes a greater difference. If we’re in Denver, at a mile high, the density of air is about .0615 lb/cf @ 67ºF. So the conversion constant becomes .89.

So 5000 cf/min x .89 BTU = 4450 BTU to increase temperature 1ºF.

So here’s the calculation for the temperature increase once again:512,000 BTU/h / 4450 BTU / 1ºF = 115ºF.

AIR DENSITY CONTINUED



BUILDING HEAT LOSS

You need to calculate the heat load for your structure to determine how much heat you lose and need to replace every hour. You may find it tempting to simply use a basic calculation, based on the building cubic footage (the square footage multiplied by the ceiling height), the temperature differential, and a building factor. That factor may vary from .135 for a sealed building to .160 for a tent. Here’s a simple formula:

BTU required = Building cf X ΔTºF X Building factor

This formula doesn’t take into consideration insulation or, more importantly, openings in the building envelope or air infiltrations that could increase heat loss through surface areas. For greater accuracy, you should closely examine each factor that impacts building heat load. These factors are related largely to the building quality, the specifications it needs to meet, and the codes in place. The heat loss depends on the following:

• “R” values of the roof

• “R” values of the walls

• “R” values of the windows

• Surface areas of the roof, walls, and windows

• Internal heat load

– The number of people inside the building and their level of activity

– Machinery operating in the building

– Traffic volume in and out of the building

– The number of exhaust fans and how often they operate

– The number of doors and windows and how often they’re opened

• Temperature outside

• Required temperature inside

Overestimating the “R” value for roof, walls, and windows is a common error. A good rule of thumb is to use the building code value when buildings are under construction. The following calculations will give you the BTU/h lost per element:

• Measure the surface areas of the roof, walls, and windows

• Use the inverse of the R values.

• Multiply by the difference between the outside and required inside temperatures.

• Sum these up to get the building heat load.

CONTINUED



BUILDING HEAT LOSS CONTINUED

Now comes the difficult part – factoring in all the intangibles that depend on everything from the kind of windows used in the building to the number of stairwells. To make a useful calculation, you need to determine the impact of each factor individually and then add them all together. Then add the building heat load for a final Heat Load value.

Once you have the Heat Load value, you can calculate cf/min needed to keep the building at the required temperature. For now, let’s assume the heat loss is 900,000 BTU/h. The required temperature is 55ºF and the heater can supply air at 150ºF:

So 900,000 BTU/h / (150ºF - 55ºF) = 9473 cf/min.

If the heater can supply air at only 120ºF, then:

900,000 BTU/h / (120ºF - 55ºF) = 13,846 cf/min.



EQUIPMENT NEEDED

Now you can calculate how many heaters you’ll need. Let’s assume the building is 100,000 cf and losing 900,000 BTU/h. The specification requires an inside temperature of 55ºF. You’ll use a 150-kW heater with a delivery temperature to the room of 115ºF. The 150-kW heater delivers 5,000 cf/min.

REQUIRED AIR DELIVERY CALCULATIONLet’s first calculate how much air we need to deliver to the room to hold 55ºF:

900,000 BTU/h / (115ºF - 55ºF) (1.08) = 13,889 cf/min

Each 150-kW heater delivers 5,000 cf/min, so the calculation for determining the number of heaters needed is straightforward:

13,889 cf/min / 5,000 cf/min = 2.78 ~ 3 150-kW heaters.

FUEL CONSUMPTION AND COSTTo calculate fuel consumption, you need the same information we’ve already used. So let’s assume you’re using a direct-fired heater. The heater uses outside air (make-up air) at a temperature of 20ºF and the fan produces 5000 cf/min. The air leaves the heater at 170ºF. We are burning propane for heat and a gallon produces 91,600 BTU. The heaters are sitting inside, so the efficiency is 100%. Here’s the calculation:

G/h = 1.08 X 5000 cf/m X (170ºF - 20ºF) / 91,600 BTU/G = 8.8 G/h of propane.

But what if the heater is using inside air (recirculating) at a temperature of 55ºF. Then:

G/h = 1.08 x 5000 cf/m x (170ºF - 55ºF) / 91,600 BTU/G = 6.8 G/h of propane.

The fuel efficiency clearly improves when recirculating air, but you also need to consider the combustion byproducts and safety when using a direct-fired heater indoors.

CONTINUED



EQUIPMENT NEEDED CONTINUED

Now let’s look at fuel cost. Each fuel type produces a different amount of heat and costs a different amount per gallon. That impacts the cost to use each type of fuel. Let’s look at an example for electricity and then for diesel.

ELECTRICThe calculations for an electric heater are straightforward. A 100-kW heater will provide 341,400 BTU/h. If the cost per kWh is $0.14, then here’s the calculation for cost per day:

100 kW x $.14/kWh x 24 h/day = $336.00/day

DIESELMost heaters will shut off once reaching a set temperature. The heater blower capacity determines the temperature delivered into the building.

Assume the maximum temperature delivered is 180ºF and minimum 160ºF, for an average of 170ºF. For this scenario, we’ll use a MAC 750 heater that delivers 450,000 BTU/h. The heater needs to be set to deliver 1,700 cf/min to reach the 170ºF average temperature. Also assume the temperature of air entering the heater is 20ºF. We are burning diesel and a gallon produces 134,000 BTU.

Now, let’s calculate the fuel usage:

1700 cf/min x 1.08 x (170ºF - 20ºF) / 134,000 BTU/G = 2.06 G/h

At $3.00/G, we can calculate the daily cost per heater as follows:

2.06 G/h X $3.00/G x 24 h/day = $148.32/day for each heater.

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We hope that seeing these calculations gives you a better understanding of the factors that impact job sizing for construction.Although the calculations for air density, heat loss, heaters needed, and fuel consumption and costs are straightforward, the number of variables and intangibles can make it as much science as art. That’s where years of expertise and experience come into play, as well as a computer model that can help you answer what-if questions.

Two problems you definitely don’t want to face are undersizing or oversizing your solution. Undersize and your structure and workers will be cold, making it impossible to complete tasks on schedule and on budget. Oversize and you’ll be paying more than you should to protect your structure, complete your tasks, and keep your workers comfortable.

We’d be happy to provide a free job sizing estimate for your next construction project to ensure you’ve right-sized the job and your needs. Our computer models take into account everything from local weather to altitude to ensure as accurate an estimate as possible. For more information, contact us today at 866-830-6143 or visit us online at sunbeltrentals.com.

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SECTION 4


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The How-To Guide for CALCULATING HEAT LOADS AND JOB …