Design-build, from the load calculation to the balancing report

Almost every mechanical problem we are called to fix started as a sizing decision made without a load calculation. Rules of thumb, tonnage per square foot, and matching whatever was there before are all methods of guessing, and in a city where the air is 17 percent thinner than the assumptions behind those rules, guessing is expensive.

For residential we run ACCA Manual J, room by room, not just a whole-house block. For commercial we run a block load and then a zone-by-zone breakdown, using the ASHRAE Denver design conditions: 91 °F dry bulb and 59 °F mean coincident wet bulb at the 0.4 percent cooling condition, 1 °F at the 99.6 percent heating condition.

The report comes to you with the inputs visible: glazing area and U-value, orientation, infiltration assumption, insulation levels, internal gains and the occupancy schedule. If you disagree with an assumption, that is a productive conversation to have before the equipment is ordered.

ACCA Manual S is the step that gets skipped most often. It is the discipline of selecting equipment against the load you calculated, at the conditions the building will actually see, rather than against a nominal tonnage on a catalogue page.

At Denver altitude the corrections are real. Sensible capacity for a given airflow drops roughly 17 percent, so the required cfm per ton rises from about 400 to 450 or 480 actual cfm per ton. Fans generate less static pressure at reduced density and draw less brake horsepower for the same volume. Gas-fired equipment above 2,000 ft needs the manufacturer high-altitude listing, which typically means an orifice change and a derated input.

The selection sheet you receive shows the catalogue capacity, the altitude correction applied, and the resulting capacity at your design condition. Three numbers, so the correction can be checked.

The friction rate is not 0.10 in. w.c. per 100 ft because that is the number everybody uses. It is derived: take the equipment external static pressure rating, subtract the coil, filter, and accessory losses, and divide the remainder by the total effective length of the worst run.

On a residential air handler rated at 0.5 in. w.c. with a four-inch MERV 13 filter and a wet coil, the available static pressure for the duct is often closer to 0.25 in. w.c., which gives a very different friction rate and a very different set of duct sizes. Designing to 0.10 anyway is how a new system ends up measuring 1.1 in. w.c. and delivering 70 percent of rated airflow.

For hydronic we do the same thing with head, pump curves and the actual operating point rather than the nameplate one.

Commissioning and TAB appear as their own line on our proposals because they are their own work, and because a separate line is harder to quietly delete during value engineering.

Testing, adjusting and balancing means every terminal gets measured against its design airflow, dampers and valves get adjusted until they match, and the results get written down. Commissioning means the sequences of operation get verified against the written sequence, point by point, with someone signing that they did it.

You receive both reports. Where a terminal is out of tolerance, the report says so and says what was done. A system that was designed and installed correctly still does not perform correctly until it has been balanced.