Energy Decision # 52 - High-Efficiency Commercial HVAC Systems Explained: Buy for Part Load

So here's the deal.

High efficiency commercial HVAC means equipment and controls chosen for part load because that's how your system almost always runs.

High performance equipment by itself can cut energy and cost by 10 to 40 percent.

The question is whether you're buying for the rare design day or the other 360 some days of the year.

If your electric bill is five, six or seven figures and heating and cooling drive a large share of it, this episode is for you.

Now, if you caught episode 12 on LED retrofits and lighting controls, HVAC is the next system on your list.

The decision is which high efficiency HVAC equipment and controls deserve your capital to lower operating costs and keep your climate control reliable.

HVAC is the largest single energy end use in a commercial building, roughly 39%, according to the whole building design guide.

That measures energy, not your bill, though.

HVAC's share of your bill depends on your electric and gas rates because...

A BTU of gas heat often costs less than the electricity running your chillers, fans, and pumps.

The whole building design guide estimates integrated whole building efficiency design can typically cut annual energy costs about 30% with a three to five year simple payback.

Payback on a single replacement depends on your load profile, utility rates, and whether you replace at end of life or early.

Part load is any time your equipment runs below its maximum output.

HVAC systems are sized for design conditions that historically occur only 1 to 2.5% of the time, then sized up again for pickup loads and a safety factor.

So most systems run at 50% or less capacity.

Most equipment only hits its rated peak efficiency at full load, and the full load winner can lose badly at part load.

That's why the whole building design guide treats part load performance as a critical sizing and selection criterion.

Weigh the part load rating as heavily as nameplate efficiency.

Greatly oversized equipment runs less efficiently and costs more than properly sized equipment, and an oversized cooling system may not dehumidify properly.

The expensive error is assuming every worst case at once, then stacking the highest safety factor on top.

Treat the safety factors and pickup allowances in ASHRAE 90.1 as your upper limit.

Before replacing anything, pull a cooling season and a heating season of trend data from your building automation system plus 15-minute interval data from the utility and have your engineer show actual peak load against installed capacity.

For new design, ask for an hour-by-hour load model.

in a current tool like Energy Plus or Trace.

So here's the deal.

High efficiency commercial HVAC means equipment and controls chosen for part load because that's how your system almost always runs.

High performance equipment by itself can cut energy and cost by 10 to 40 percent.

The question is whether you're buying for the rare design day or the other 360 some days of the year.

If your electric bill is five, six or seven figures and heating and cooling drive a large share of it, this episode is for you.

Now, if you caught episode 12 on LED retrofits and lighting controls, HVAC is the next system on your list.

The decision is which high efficiency HVAC equipment and controls deserve your capital to lower operating costs and keep your climate control reliable.

HVAC is the largest single energy end use in a commercial building, roughly 39%, according to the whole building design guide.

That measures energy, not your bill, though.

HVAC's share of your bill depends on your electric and gas rates because...

A BTU of gas heat often costs less than the electricity running your chillers, fans, and pumps.

The whole building design guide estimates integrated whole building efficiency design can typically cut annual energy costs about 30% with a three to five year simple payback.

Payback on a single replacement depends on your load profile, utility rates, and whether you replace at end of life or early.

Part load is any time your equipment runs below its maximum output.

HVAC systems are sized for design conditions that historically occur only 1 to 2.5% of the time, then sized up again for pickup loads and a safety factor.

So most systems run at 50% or less capacity.

Most equipment only hits its rated peak efficiency at full load, and the full load winner can lose badly at part load.

That's why the whole building design guide treats part load performance as a critical sizing and selection criterion.

Weigh the part load rating as heavily as nameplate efficiency.

Greatly oversized equipment runs less efficiently and costs more than properly sized equipment, and an oversized cooling system may not dehumidify properly.

The expensive error is assuming every worst case at once, then stacking the highest safety factor on top.

Treat the safety factors and pickup allowances in ASHRAE 90.1 as your upper limit.

Before replacing anything, pull a cooling season and a heating season of trend data from your building automation system plus 15-minute interval data from the utility and have your engineer show actual peak load against installed capacity.

For new design, ask for an hour-by-hour load model.

in a current tool like Energy Plus or Trace.