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HVAC Control Settings That Save You Money

By: Maria Ramos, PE

These strategies apply to variable air volume (VAV) systems with reheat, which is the

most common system design for larger buildings.

1) Allow VAV box minimums to be set at 15% or lower. Frequently, designers will use

a box minimum of 35-50% to provide ventilation at all times, which assumes that the

Owner will operate a boiler during the summer to reheat the excess air. So even if thespace isnt occupied, the air handling system is furnishing a substantial amount of air,

and reheating it if necessary to maintain temperature. Setting box minimums lower

minimizes reheat and overcooling. If your building has occupancy sensors, use these

to set the box minimums to 0% when the space is unoccupied.

A decision needs to be made whether your building will have summer reheat, i.e.

will you operate the boilers in the summer. Even though it is recognized that

simultaneous heating and cooling is wasteful and often doesnt achieve theobjective of properly ventilating each space, it remains by far the easiest and most

widely used strategy for delivering ventilation without overcooling oroverheating. Air is delivered to the space via a VAV box, which has a minimum

setting of 35-50%. If this amount of air overcools the space, then the space goes

into heating mode, which leaves the box at its minimum airflow with 55 degree

supply air. The supply air is then heated to 85 degrees or so to return the space tosetpoint. This is particularly wasteful in a school building where, on a typical

summer day with minimal use of the space, most of the spaces will be in reheat.

If reheat is undesirable, then the VAV boxes could have a occupied/unoccupied

minimum position, so at least when the spaces are unoccupied then the buildingoperator could allow the VAV boxes to go to zero minimum, or 10%, ifcontinuous airflow is desired. The VAV boxes will still respond to temperature

increases, so the space will be ventilated periodically. The cooling setpoint could

remain 75 degrees or so, which would prevent stuffiness and potential moldgrowth.

2) The best strategy to reduce the amount of summer reheat, if you are in the planning

stage of building a new building is to consider a dedicated outdoor air system(DOAS). In a conventional VAV system, there is no possible means of ensuring that

each individual space receives the amount of outdoor air which the occupancy

suggests that it would need, since the outdoor air is introduced at a system level.Heating or cooling air (depending on the season) is used to deliver outdoor air to the

space, and if a CO2 sensor determines that more air is needed, then ventilation air is

increased at the system level. However, this does not insure that the ventilation air isreaching the space which has an increased occupancy. The ventilation air will

increase in the space with increased occupancy only if the space also has an increased

heating or cooling demand, since the ventilation air is always just a percentage of the

supply air.

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With a DOAS system, ventilation air is delivered to the individual spaces, so if a space

has an increased demand (which the system would know because of a CO2 sensor), thenthe ventilation would directly increase to that space. Then, the heating and cooling

system could be operated strictly based on demand, and if there were no heating or

cooling demand, the VAV box could operate down to zero percent.

This approach is most feasible when planning a new building, however, it is not

impossible to retrofit. Since it is such a superior system operationally, it should alwaysbe considered.

3) Make sure that your system has supply air temperature reset. Supply air temperature

reset will set the temperature of the supply air based on demand in the space. Forexample, if the air handler is in cooling mode and all boxes are at their minimum

settings or calling for reheat, the supply air temperature should be reset upwards. The

supply air temperature should continue to be reset upwards, to a maximum of 62-63

degrees, until the position of the worst case VAV box is 90%. Some systems areoperated at a constant 55 degree supply air temperature, which increases the chances

that more air will need to be reheated.

4) Make sure CO2 sensors are working properly. For some reason, most jobs are

completed and the contractor leaves the site, without the CO2 sensor ever reading

properly. Look at this output on the screen. It should read, depending on thelocation, 200-1200 PPM. If it reads 50 or 5000, or some other value out of range, it

hasnt been properly tuned. This sensor also seems to need periodic checking to

make sure that it continues to read properly. Check it every six months or so. Mostmodern systems reset the amount of outdoor air based on this reading. If the reading

is artificially low, your building could be getting an inadequate amount of outdoor air.

If it is artificially high, the outdoor air damper could be fully open at most times,which increases heating and cooling costs.

5) Set duct pressure settings on VAV systems at their lowest possible value. All VAVsystems have a duct static pressure sensor, usually located two-thirds of the way

down the ductwork, which tells the supply fan what speed it should be maintaining (if

the fan has a variable speed drive). Depending on the system and the location of the

sensor, the fan may need to provide less static than the setpoint in order for the VAVbox furthest out in the system to get full flow. There is no reason to maintain the duct

static at 1.25 or 1.5 inches if the VAV box furthest from the air handler can get full

flow at 0.75 inches of static pressure. Setting the static reading too high wastes fanenergy. Some designers are changing the static pressure based on the position of the

worst case VAV box. This has some merit, but is too difficult to achieve in practice.

Too many changing variables make a control system impossible to tune.

6) Make sure that any installed energy recovery systems are operating properly.

Enthalpy wheels, the most common type of energy recovery equipment in schools,

should be operated in heating mode and in cooling mode, but not in economizer

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mode. If the wheel is operated in economizer mode, the incoming ventilation air will

be preheated by the wheel, and then, will need to be subsequently cooled back down.

Some wheels utilize a VFD for frost control and for economizer operation, whichslows the wheel to reduce effectiveness. The best strategy for economizer operation

is to bypass the wheel, which may be accomplished in the air handling unit itself,

with bypass dampers around the wheel, or in the ductwork. Have your controlcontractor demonstrate that this is working properly.

7) Determine how your return fans are being operated. Some return fans track supplyfans, and other times, are controlled based off a pressure sensor either building or

plenum pressure. If you have air flow monitoring on the return fan, then this is the

best, most reliable means of controlling the return fan. If air flow tracking on the

return fan is not present, the only method that seems to work reliably is tracking thesupply fan. Pressure tends to fluctuate too much, which causes the speed of the return

fan to also fluctuate.

8) In general, do a thorough sanity check on all the values on the DDC screen. Look forthe following:

a) Boxes should have a flow sensor and usually, a temp sensor. Is any box reading

way out of range (200 degree supply air or 5000 CFM to a relatively small room)?

b) Do VFD speeds seem reasonable for what the air handler or pumps are doing?

c) Are bypass valves in the correct position? Is the bypass valve at 100% and theVD controlling the pump at close to full speed? This might indicate a problem.

9) If it is possible to work from a short version of the sequence of operation, and havethe control contractor force values for air temperatures and so on to simulate modes

of operation, this is the best way to verify that your system is working properly.