Heat loss/heat gain considerations
February 1, 2015 by Robert Bean
When I joined ASHRAE in 1990, I did not imagine that 25 years later HVAC equipment would be selected and installed in the absence of a heat loss and gain calculation. It is like bad reruns on late night television; the Btu guessing continues even as the new CSA F280-12 is launched across Canada.
To put the guessing into perspective, imagine the confidence level of airline passengers if ground crews and pilots guessed what fuel weights were needed to get from departure to arrival points; or how about the confidence level of train commuters if bridge crossings were designed by hunches rather than structural engineering calculations? Simply stated, calculating heat loss and gains is a fundamental exercise required in HVAC before other design sequences can begin.
This is why. Load calculations drive three not-to-be ignored elements of plant capacity: fluid and air flow rates, and operating temperatures.
No one who is paying the utility bill should tolerate the selection of a heating/cooling plant based on sizing clichés.The installed power drives energy input and its conversion drives health, safety and IEQ factors, and affects environmental emissions plus capital and operating costs.
Fluid/air flow rates and pressures which influences pipe/duct sizes and mechanical characteristics are established from load calculations. These are tied back into health and safety considerations, as well as capital and operating cost. The selection of pipes and ducts based on flow, drives velocity and establishes the friction load and thus circulator and blower motor horsepower requirements. To run those motors is going to take electricity and money all the while contributing to green house gas emissions. There is the potential for parasitic heat gains, noise, and vibration and erosion corrosion while running those motors to move Btus around.
Since loads influence operating temperatures they are intimately connected to IEQ, health and safety factors and energy efficiency, which is attached to economic and environmental issues; and yes (again) capital and operating cost.
The following is not an exhaustive study of heat loss and gain but rather a commentary on items that designers ought to consider when performing calculations.
Whether you are using an ASHRAE methodology or CSA Standard to do a load calculation, the least understood but most influential element in load calculations is the transfer of energy via short and long wave radiation. Notice the term “transfer of energy” instead of “heat transfer.” Unlike conduction and convection (including infiltration and exfiltration), there is no heat in radiation only electromagnetic waves of energy which are converted to heat energy when absorbed (resulting in a rise in temperature at the surface); or converted from heat energy to electromagnetic waves when emitted (resulting in a drop in temperature at the surface).
Changes in surfaces temperatures due to radiation affect the conductive flow rate of heat through the materials bound by the surfaces. The direction of that travel inwards or outwards from the enclosure will determine whether the radiant energy is accounted as a heat loss or heat gain (and why the radiation process is often referenced as thermal radiation). Some key points designers should know about radiant transfer is that short wave solar–high intensity electromagnetic waves (Figure 1, item 1) can pass through glazing systems (Figure 1, item 2); an amount dependent on the glass and film characteristics but long wave radiation cannot (Figure 1, item 4 and 6). In this regard windows are like check valves in that they will let short wave radiation in but the resulting long wave length radiation will not be transmitted back out through windows. This energy is absorbed and conducted (Figure 1, item 5). Solar loads (short wave) on opaque exterior surfaces (solids), are absorbed (sans discussion on colours and reflectance) and converted to heat, which is conducted through and released as interior long wave radiation. This long wave radiation is then exchanged with other interior surfaces depending on their thermal optical characteristics.
There are of course radiant losses from building enclosures to the outside. You can see this with infrared cameras but those losses originated in part as the absorption of long wave radiation on interior surfaces (and heat transfer through convection next to interior surfaces). It is then ultimately conducted as heat through the enclosure (Figure 1, item 8) and at the exterior surface it is converted and emitted as long wave electromagnetic energy (Figure 1, item 4) ; Together with losses due to convection (Figure 1, item 11) and exfiltration (Figure 1, item 10), these are accounted for as heat losses.
There is also the case where shortwave radiation
(Figure 1, item 3) is absorbed by slabs on grade (a heat gain) and through conduction to ground (Figure 1, item 7) also partially accounted for as a conductive heat loss. This can also occur where short wave radiation lands on the interior side of an exterior wall where the solar gain is partially converted to a heat loss via conduction, and long wave radiation and convection at the exterior surface.
Conduction losses and gains occur through all sides of the enclosure (Figure 1, items 5, 7, 8 and 9) based on the temperature differentials across the wall, windows, floor/slabs and roof/ceiling; and the combined weighted U-value of the panel. The operative phrase is “weighted U-value” meaning a wall constructed with R-20 insulation is in fact not an R-20 wall after considerations are made for the area, and the R-values of framing members and window and door areas.
For typical framing factors of 15 per cent to 20 per cent an R-20 wall is actually an R-12 to R 14 wall. Also the type and thermal performance of insulation materials is affected by temperature changes, which is why climate driven R-values should be used for greater accuracy in performing load calculations. Insulation is rated at 75F (23.9C) but deviations up and down from this value result in decreasing R-values as much as one to two R-values at typical design conditions.
This means a designer may be using the rated R-value of the insulation of say R-20 when in fact adjusted for framing and temperature could potentially drop the wall to an actual R-10. It is this author’s experience that many software programs used by HVAC designers do not account for the consideration of U-value weighting and climate on actual enclosure performance.
Many software programs simplify the transmission calculation from and through the enclosures by using a combined heat transfer coefficient which includes the convective air flow across the surface (Figure 1, items 11-15). Designers should note that programs often use a still air coefficient, which does not accurately represent the air flow typically created by all air systems or higher velocity dedicated ventilation systems. The still air coefficient would likely be an accurate representation for spaces conditioned with systems such as radiant. As noted in Figure 1, the convective coefficient is different for vertical surfaces, and is dependent on horizontal surfaces facing up or facing down and whether that plane is an interior or exterior surface.
Many seasoned designers agree that the estimation of infiltration/exfiltration has
been more of an art than a science in the past. But understanding of air leakage through buildings has advanced over the past decade. Some load calculation programs account for measured input values from blower door tests. Where these test are not required or not performed, it is still possible to more accurately estimate air leakage using such references as ASHRAE Standard 62.2 or procedures that consider construction materials and assembly techniques, building geometry including building heights, approximation to exterior wind/pressure barriers (other buildings, trees and so on) and orientation to prominent wind direction.
Today, infiltration and exfiltration are not generally seen as acceptable means to ventilate a building. We now understand that this process is a source of contamination in the form of particulates, gases, odours, moisture and sound.
Also, the air leakage process introduces an essentially uncontrollable latent load to the interior space. Along with other interior latent loads from people, cooking, cleaning and mechanical ventilation these additional loads must be accounted for in the calculation procedure.
Our ability to accurately predict loads is increasing with our understanding of building systems and how people operate their spaces and appliances. However, software does not always accurately reflect reality. Designers and their clients ought to understand clearly that performing load calculations is a first step but by no means the final step in establishing thermal comfort and energy efficiency.
In the case of thermal comfort, this separate calculation is the natural follow up. Unfortunately, the exercise to determine comfort from load calculations is rarely done, much to the annoyance of yours truly. <>