Intermittent Ventilation Run Cycles

Best Practice Recommendation

To avoid the need for a high ventilation rate to provide equivalent ventilation effectiveness, keep the duty cycle under 3 hours. With a short duty cycle, the increased ventilation rate for intermittent operation is roughly the inverse of the fractional on-time. For example, operating 1/3 of the time requires 3 times the airflow. Operating 1/2 the time requires 2 times the airflow.

How Do You Calculate Intermittent Ventilation Run Cycles for Whole-Building Ventilation?

To introduce the same volume of outdoor air for whole-building ventilation when the system is operated intermittently, the ventilation rate must be increased to compensate for shorter on-times.

Both ASHRAE Standard 62.2-2007 and ASHRAE Standard 62.2-2010 use the formula below to calculate the airflow required for equivalent ventilation. However, the two versions of the standard have different tables for the ε factor in the equation (the ventilation effectiveness factor).

intermittent fan flow rate  = 

continuous fan flow rate


(effectiveness factor ε) x (fractional on-time f)

where:
continuous
fan flow rate
 =  Required whole-building ventilation rate for the house or apartment if the system was operated continuously (Step 3).
f  =  Fractional on-time for whole-building ventilation, or the percentage of time the whole-building ventilation equipment will operate in one duty cycle (the length of one complete on/off cycle).
Example 1
If 1 hour on/3 hours off, fractional on-time is 1 hour out of 4 hours or 25%. Duty cycle is 4 hours.
Example 2
If 20 minutes on/40 minutes off, fractional on-time is 20 minutes out of 60 minutes, or 33%. Duty cycle is 1 hour.
Example 3
If 1 hour on/1 hour off, fractional on-time is 1 hour out of 2 hours or 50%. Duty cycle is 2 hours.
ε  =  value for Ventilation Effectiveness for Intermittent Fans from Table 4.2 in ASHRAE Standard 62.2. Note: Table 4.2 is different in ASHRAE 62.2-2007 and ASHRAE 62.2-2010.

ASHRAE Standard 62.2-2007 Table 4.2

Title 24 Requirement

ASHRAE 62.2-2007

The 2007 ventilation effectiveness table is much simpler than the 2010 version, but a severe penalty for intermittent operation kicks in when the ventilation duty cycle exceeds 3 hours. If the system runs at least once every three hours, 1.0 can be used as the ventilation effectiveness, ε. The maximum duty cycle allowed is 12 hours.

ASHRAE Standard 62.2-2007 Table 4.2
Ventilation Effectiveness for Intermittent Fans
Daily Fractional On-Time, f Ventilation Effectiveness, ε
f ≤ 35% 0.33
35% ≤ f < 60% 0.50
60% ≤ f < 80% 0.75
80% ≤ f 1.0
N = 12.8 x Qfan x Tcyc / Afloor
where:
Qfan  =  ventilation air requirement (Step 3) in cfm
Tcyc  =  fan cycle time: total time for one on/off cycle
Afloor  =  floor area in square feet

ASHRAE Standard 62.2-2010 Table 4.2

Title 24 Requirement

ASHRAE 62.2-2010

The 2010 ventilation effectiveness table is much more complicated but also much more versatile. It applies only a very minor penalty for a duty cycle of up to four hours but will accommodate duty cycles up to a maximum of 24 hours. However, Table 4.2-2010 requires an additional calculation of the required turnover, N.

ASHRAE Standard 62.2-2010 Table 4.2
Ventilation Effectiveness for Intermittent Fans
Fractional On-Time, f Turnover, N
0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 6.0 8.0 12 20 40 100+
For values not listed in Table 4-2, use the next higher value for N or the next lower value for f.
Linear interpolation is allowed.
0.00 1.00 0.95 0.88 0.78 0.60 0.00                  
0.05 1.00 0.96 0.90 0.81 0.67 0.41 0.00                
0.10 1.00 0.96 0.91 0.83 0.72 0.55 0.21 0.00              
0.15 1.00 0.96 0.92 0.85 0.76 0.63 0.44 0.18 0.00            
0.20 1.00 0.97 0.93 0.87 0.79 0.69 0.56 0.40 0.03 0.00          
0.25 1.00 0.97 0.94 0.89 0.82 0.74 0.64 0.53 0.26 0.02 0.00        
0.30 1.00 0.98 0.95 0.90 0.85 0.78 0.71 0.62 0.42 0.24 0.00        
0.35 1.00 0.98 0.95 0.92 0.87 0.82 0.76 0.69 0.54 0.39 0.14 0.00      
0.40 1.00 0.98 0.96 0.93 0.89 0.85 0.80 0.75 0.63 0.52 0.32 0.02 0.00    
0.45 1.00 0.99 0.97 0.94 0.91 0.88 0.84 0.79 0.70 0.61 0.45 0.21 0.00    
0.50 1.00 0.99 0.97 0.95 0.93 0.90 0.87 0.83 0.76 0.69 0.57 0.37 0.13 0.00 0.00
0.60 1.00 0.99 0.98 0.97 0.96 0.94 0.92 0.90 0.86 0.81 0.74 0.61 0.45 0.27 0.14
0.70 1.00 1.00 0.99 0.98 0.98 0.97 0.96 0.94 0.92 0.90 0.85 0.78 0.68 0.55 0.46
0.80 1.00 1.00 1.00 0.99 0.99 0.99 0.98 0.98 0.97 0.96 0.94 0.90 0.85 0.77 0.70
0.90 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.99 0.99 0.99 0.98 0.97 0.96 0.93 0.88
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00