UBC First Nations House of Learning    

Mechanical Design Tips


Importance of Air & Water Balancing and Commissioning of Mechanical Systems

The main objective for thorough and accurate commissioning and balancing of the mechanical systems is to achieve the optimum performance. This is not only in the best interests of the Owner of the building but also for the architect and consulting engineer. If the specified design results are not achieved in the field, it reflects poorly on all those who are involved in the design and construction. There is a one-time opportunity to verify that the design objectives have been achieved and to measure and adjust the performance of all systems before the building is turned over to the Owner.

Based on the collective field experience we have produced the following set of practical guidelines that we would like to see incorporated to assist in achieving optimum results when we are balancing and commissioning these systems.


Air Handling Units "Balanceability" of Air Systems
Intake Louvre Location and Sizing Hot Water Boiler Piping Arrangement
Outdoor Air Ventilation Location of Cold Water Makeup
Air Quality Parallel Pumping
Ductwork Design Location of Expansion Tank
Fan Sizing Pump Connections
Use of Flexible Connections Pump Sizing
Supply Diffusers Ease of Fluid (Water) Balancing
Air Balance Verification Access to Heating & Cooling Coils
Variable Air Volume Systems Cooling Tower Selection
Building Pressurization  

Air Handling Units    
1. For units with mechanical cooling, select cooling coil for maximum 500 FPM face velocity.   Reason:
When the coil velocity exceeds 550 FPM there is likely to be undesirable carryover of moisture into the supply air.
       
2. Select total supply air flow for air outlet total plus leakage allowance (e.g. 10%) for constant volume systems. Keep in mind the plus or minus 10 % balancing allowance.   Reason:
There is always some leakage even in a well constructed ductwork system. Not all of the total heating, ventilating or cooling effect at the fan is available to the rooms served.
       
Intake Louvre Location and Sizing    
1. Determine the per cent free area for the actual louvre being specified (and supplied). Generally, most louvres have a maximum of 50 % free area.   Reason:
Some louvres have as low as 30 % free area. An undersized louvre can restrict air flow, resulting in low air flows, loss of cooling capacity and poor air quality.
       
2. If there is a choice, avoid facing intake or exhaust louvre directly into the wind.   Reason:
The wind can cause undesirable fluctuations in air volume. Strong winds may cause snow and rain to enter the air handling system.
       
3. If it is unavoidable to face louvre into the wind, increase size to reduce air velocity or provide intake cowl.   Reason:
Avoid carryover of snow and rain into system.
       
4. Provide adequate distance between any exhaust air outlet and any air intake.   Reason:
Unwanted heat or odours may be reintroduced into the building through the supply air system.
Outdoor Air Ventilation    
1. Always ensure that each and every occupied area will have at least the minimum ASHRAE rates of outdoor air ventilation (e.g. 20 CFM per person) under all operating conditions.   Reason:
During cold or hot weather extremes systems may operate with minimum outdoor air at the supply fan and with VAV terminal boxes to some areas at their minimum position for extended periods resulting in poor air quality because of inadequate outdoor ventilation air supplied to the occupants.
       
2. The balancing agency should be specified to verify the outdoor air flow to occupied areas as part of the air balance scope.   Reason:
If not specifically called for, such verification is not usually carried out or is done on an arbitrary basis (e.g. by setting the outdoor air damper to 20 % by eye).
Air Quality    
1. Be cautious about controlling outdoor air quantity to maintain a CO2 level. It is better to define a specific amount of O.A. as minimum and have the system adjusted to provide the minimum.   Reason:
Although CO2 level can be a very useful indicator it does not guarantee that air quality is good or bad. High CO2 levels (e.g. 3,000 ppm) may not be overly harmful and at low CO2 levels there may be other pollutants that cause the air quality to be bad.
       
2. Ensure that the design, controls balancing and commissioning result in providing adequate outdoor air delivery to all occupied areas under all operating conditions. For VAV systems, with the supply fan at minimum and VAV boxes at minimum air flow there should be 20 CFM of O.A. / person to each room.   Reason:
If there are rooms that have less than recommended O.A. for ventilation during peak winter conditions for example when the fan is at minimum and VAV boxes are at minimum position, then portions of the building may qualify to be a "sick building".
       
3. The amount of outdoor air delivered to occupied spaces during minimum outdoor air conditions and the minimum air volumes for all VAV terminals should be verified as part of the air balancing work.   Reason:
In many cases the all important (for air quality and energy reasons) minimum air volume settings may be done arbitrarily, e.g. by setting an outdoor air damper to be 20 % open by appearance. If the true amount is too low, the air quality suffers. If it is too great, then the energy consumption is excessive.
Ductwork Design    
1. Avoid using long runs of uninsulated supply air ductwork.   Reason:
There is a loss of heating or cooling capacity.
       
2. Avoid long runs of ductwork even if ducts are insulated (e.g. use more smaller systems instead of one very large system).   Reason:
There is a loss of heating or cooling capacity.
       
3. If long runs are unavoidable, make allowance for supply air temperature changes for more remote areas (provide additional air)   Reason:
If the air temperature rises several degrees by the time it reaches the farthest rooms, then more air is required to achieve design cooling effect.
       
4. On all branch supply ducts use air supply outlets that have a similar pressure drop (e.g. avoid mixing diffusers and registers on the same branch).   Reason:
It is difficult to balance mixed outlet systems without resulting in air noises at the registers.
       
5. If it is necessary to mix grilles and diffusers on the same branch duct, locate the balancing dampers for the grilles as far back from the outlet as possible.   Reason:
If the balancing damper is near the grille and has to be throttled in there will probably be undesirable air noise.
       
6. Where space permits provide some straight run of duct immediately downstream from the supply air unit or fan. If this is not practical right at the fan or unit, provide some straight runs for each main branch supply duct.   Reason:
For air balancing, the most accurate means of measuring total air flow is to use a Pitot tube traverse on main duct or individual traverses on all main branch ducts.
7. Locate air outlets to always be accessible (e.g. not inside heating cabinets or above architectural ceilings.   Reason:
If air outlets are not reasonably accessible it is difficult or perhaps impossible to measure the air flows to individual outlets.
       
8. Avoid unorthodox duct configurations, e.g. abrupt expansions or contractions.   Reason:
Drastic changes in ductwork size or shape result in excessive static pressure losses that may lead to reduced air flow available.
       
9. When using long runs of acoustically lined ductwork, base static pressure calculations on the friction loss of the (rough) insulation surface.   Reason:
The static pressure loss through lined duct is excessive compared to smooth metal ductwork.
       
10. Never locate a supply diffuser directly on the bottom of a supply duct. Always provide a branch duct and provide a balancing damper at the takeoff.   Reason:
It is usually impossible to balance the air flow to a duct mounted diffuser without having excessive noise. The air velocities are always uneven on the four sides of the diffuser so the air velocity on the high side (in the direction of air flow) is very high.
Fan Sizing    
1. When calculating S.P. for components allow safety factor to allow for field conditions.   Reason:
Published data is based on ideal laboratory conditions, e.g. even air flow across reheat coil or air filters.
       
2. When sizing air flow for fans, be aware of duct leakage. A well constructed supply duct for example may have 10 to 15 % air leakage. Refer to SMACNA Manual to estimate leakage allowance.   Reason:
If no allowance is made for duct leakage there may be a shortage of air available at the outlets.
Use of Flexible Connections    
1. Avoid using long or twisted runs of flexible connections to VAV terminal boxes.   Reason:
The static pressure loss through flexible ducts is much greater than through smooth metal ductwork, especially when there are severe bends.
Supply Diffusers    
1. Avoid where possible locating diffusers adjacent to walls or near ceiling drops.   Reason:
When the supply hits the wall it may cause drafts on work stations located nearby.
       
2. Provide a "cushion head" (extension of supply duct past diffuser) for about 10 to 12 inches to provide more even velocity on all sides of each diffuser.   Reason:
If the air velocities are uneven, there is more noise for any air volume. The air noise depends on the maximum air velocity.
       
3. When selecting diffusers, assume that the actual air flow will be 10% more than specified. Don’t select a diffuser at its maximum rating.   Reason:
The air balancer normally has an allowance of plus or minus 10 % when setting the balancing dampers.
       
4. When selecting a diffuser or approving a substitute, avoid air velocities more than 1200 FPM.    
       
5. Avoid using adjustable diffusers for air systems used for both cooling and heating. If an air system is used for both it is less problematic to use fixed horizontal flow pattern.   Reason:
Building owners and operators are unlikely to change the diffuser patterns from horizontal to vertical and back each season. If left in vertical pattern for cooling there will be drafts.
Air Balance Verification    
1. If there is reason to suspect that the design air flows have not been achieved, then arrange for the balancing agency to carry out spot checks as soon as possible after report is submitted using same instruments and personnel where possible.   Reason:
A reputable balancing agency is willing to assist in identifying and resolving performance problems. Verification of air balance should be done soon after balancing is done before possible changes are made to system by building operators.
Variable Air Volume Systems    
1. Identify clearly on drawings the minimum air volumes as well as the maximum air volumes for all VAV terminal boxes. Avoid using minimums much below about 35 to 40 % of the maximum air volumes.   Reason:
If the minimum air volumes are too low, then there is increased risk of inadequate outdoor air ventilation during winter conditions.
       
Building Pressurization    
1. The balancing specifications should call for the balancing agency to check the building pressure under all extreme conditions (e.g. full outdoor air or at minimum outdoor air).   Reason:
Excessive positive or negative pressures in a building can cause undesirable noise, security or building material problems. At the completion of the air balancing operation is the best time to resolve any pressurization issues.
"Balanceability" of Air Systems    
1. Avoid the use of splitter dampers. Instead use balancing dampers in the branch or main supply ducts.   Reason:
When a splitter damper is adjusted it has an unpredictable affect on the air flow in both the main and the branch ducts and makes it more difficult to balance an air system.
       
2. Opposed blade balancing dampers are preferable to single blade dampers on rectangular branch ducts.   Reason:
Single blade dampers in some cases may cause problems downstream from the damper.
       
3. Provide balancing dampers well back from the air outlets and inlets.   Reason:
In cases where the damper has to be throttled more than usual there may be excessive noise generated.
Hot Water Boiler Piping Arrangement    
1. The boiler pumping and piping should provide full flow of water through each and every boiler. This can be achieved by using boiler loop pumps or primary / secondary piping with three-way control valves.   Reason:
As the flow reduces through a boiler the temperature rise increases and the ability of the water to carry away the heat from the boiler decreases. This can result in excessive wear on the boiler and on nuisance tripouts.
       
2. It is preferable to locate the heating circulating pump on the supply (not the return) side of the boiler.   Each boiler has a maximum pressure rating. If the pump is pumping towards the boiler, the boiler must have a higher pressure rating and there is increased likelihood of exceeding the relief valve pressure.
Location of Cold Water Makeup    
1. The best place to locate the cold water makeup connection is on the line to the expansion tank.   Reason:
The water pressure at or near the expansion tank remains constant regardless of system conditions.
Parallel Pumping    
1. When 2 or more pumps are operating in parallel, plot out the system curves and pump curves to ensure that the two pumps will have adequate flow.   Reason:
There may be little or almost no increase in flow for two pumps versus one pump, depending on the pump characteristics and the system curve.
       
2. Whenever there are two pumps in parallel, always provide a check valve on the discharge of each pump.   Reason:
The check valve prevents reverse flow through the idle pump (resulting in loss of capacity for the system).
Location of Expansion Tank    
1. Locate the expansion tank piping connection on the suction side of the heating pumps (i.e. pump away from the expansion tank).   Reason:
The expansion tank size has to be increased if it has a greater pressure. There is always less pressure on the suction side of the pump compared to the discharge side.
Pump Connections    
1. Provide straight pipe entry connections to pumps. Alternatively provide a suction diffuser.   Reason:
Poor entry conditions may result in reduced pump performance.
       
2. Avoid series pumping in all cases where the flow rates through any piping circuit is likely to change under different operating conditions.   Reason:
Excessive flows may result in noise problems under some operating conditions. Low flow may result in inadequate cooling or heating capacity.
       
3. Mechanical drawings should always provide flow diagrams to show design intent for connecting pumps and associated equipment.   Reason:
Essential design elements may be omitted if not clearly called for.
       
4. Provide pressure and temperature plugs for measuring conditions at heat exchangers, coils and other major equipment.   Reason:
Provision for measuring conditions facilitates water balancing and trouble shooting.
       
5. Use primary / secondary piping arrangements arranged so that the flow of each secondary circuit is independent of the flow through the primary circuit or other secondary circuits.   Reason:
This avoids undesirable situations e.g. where "the strongest pump wins" and maintains stability under all operating conditions.
Pump Sizing    
1. Base pump selection on the maximum flow and head when all coils, heating elements etc are calling for full heat. Provide a reasonable allowance for field conditions.   Reason:
Field conditions (e.g. substitutions for heating equipment, dirty strainers, alterations to piping runs etc) may result in increased pump heads compared to theoretical values.
       
2. When glycol (antifreeze) solution is used instead of water, calculate flows and heads based on actual properties (specific heat etc) of fluid used.   Reason:
Glycol solutions typically have a lower specific heat compared to using water.
 
Ease of Fluid (Water) Balancing    
1. Provide pressure gauges (on suction and discharge) and flow measuring devices for all circulating pumps for major systems.   Reason:
A meaningful water balance requires accurate measurement of total flow.
       
2. Provide flow measuring devices at all major branch lines and devices such as cooling or heating coils.   Reason:
It is awkward and less accurate to determine flow based on temperature drops or rises.
Access to Heating & Cooling Coils in the Ductwork    
1. Always provide an access door in the ductwork upstream and downstream from each coil.   Reason:
Cleaning the coils requires access for compressed air on downstream side and vacuum on the upstream side.
       
Cooling Tower Selection, Location
and Piping
1. Avoid locating a cooling tower near a wall, enclosure or baffle.   Reason:
The cooling capacity of the cooling tower may be reduced if it is possible for the air to recirculate.
       
2. Ideally, provide one cooling tower for each water chiller with matching capacity.   Reason:
Improved economy and flexibility of operation.
       
3. Provide an equalizing line if it is necessary to have two cooling towers connected to a common condenser water line.   Reason:
Avoid drain pan overflow for cooling tower with greatest flow of condenser water.