New load calculations are coming your way

July 6, 2000
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MINNEAPOLIS, MN — Commer-cial and residential contractors may see changes to the ways they calculate cooling and heating loads in the not-so-distant future.

The 2001 ASHRAE Handbook will have a load calculation chapter that’s “radically different,” according to forum speakers at the recent American Society of Heating, Refrigerating, and Air-Conditioning Engin-eers (ASHRAE) Summer Meeting, held here. These “new” load calculation methods also will be incorporated into Manual J and N updates by the Air Conditioning Contractors of America (ACCA), according to president Roger Jask.

Last year, an ASHRAE technical committee ran a comparison of two new load calculation procedures, heat balance (HB) and radiant time series (RTS), with old methods, CLTD/CLF, TFM, and TETD/TA. While HB and RTS are being referred to as new, they are, in fact, older and relatively simpler than those they will replace.

The new methods

At one ASHRAE Summer Meeting forum, HB was referred to as “the flag bearer of ASHRAE.” And while both HB and RTS are considered simpler than current methods, contractors and engineers still will need computers to crunch the numbers.

According to a committee member, the HB method indicates the balance of all heat transfer in a room to determine the cooling load.

In the RTS method, assumptions of a steady periodic condition are repeated every 24 hrs. Radiant loads within the space have to be absorbed within the space and disposed. Convective and radiant components are split and expressed as a curve.

The method also takes into account the time lag of heat transfer through opaque materials, like walls. Its companion is the conduction time series, in which heat gain through walls and roofs over time is expressed as a curve. The assumption is that there’s a steady flow through the wall.

Load comparisons

The method compares convective and radiant curves, and allows users to see each component of the cooling load. Simple algebraic equations illustrate when each peak load occurs. With it, the user can analyze the load component by component.

“Many times you catch problems by looking at components of load and say, ‘We screwed up on input here,’” said a forum participant.

The RTS method was described as “backed-out mathematics. It gives an estimate at the very best.”

However, most load calculation methods can be considered relatively inexact.

“We are dealing with anything but an exact science here,” responded an engineer. “It’s not a load calculation; it’s a load estimate at the very best.”

Load vs. energy use

One attendee asked, “Are both load and energy calculations coming together? I think that’s the eventual outcome.”

The short answer is that yes, calculation procedures are coming together. “But there are still two goals, one for the design guy [customer comfort] and another for the energy guy.”

Engineers warned against the blind faith younger engineers seem to have in the computer. “The computer does not do anything that we cannot do. It just does it faster and with fewer errors.” The saying of garbage in, garbage out also came to mind.

A participant quipped, “Measure it with a micrometer, mark it with a pencil, and cut it with an axe.”

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