Testing Enclosures : Sub-Floor Only Testing Using Whole Room Values

Typically, sub-floor only tests are performed by mounting a fan on the walked on floor to measure the total leakage of the below floor space. That is the holes below the floor and the holes through the walked on floor. The walked on floor is usually so leaky that this test fails.

The next step is to then measure the below floor leaks that the agent would actually escape from. Essentially, we want to measure the size of the “drain hole” below the floor since this hole size determines the loss rate. This is done by neutralizing the flow across the walked on floor with a fan mounted in the door to the room or covering the floor with plastic.

In cases where the walked on floor has too many penetrations to allow for neutralization, a whole room test may be performed. This test will almost always err on the side of being too conservative. For example:

If the Total room leaks are: 2.91 sq ft, in a room with an 18 inch under-floor, the retention time at 75% of the under-floor height would be 7.4 minutes. This assumes that half the leaks measured are below the walked on floor. This would seldom be the case. Typically a maximum of 25% would be below the walked on floor and may go as low as 1%! In the case of 25%, the retention time would be 11.8 minutes. This estimate can be based on a leak location test using the Leak Audit feature on the Retrotec CA2001 software or an estimate using other means.

Choosing Minimum Protected Heights for Sub-floors

See step 14 from Retrotec’s Level 3 manual repeated below. Note that for subfloors, protection for the entire height is not an option because this can only be done if there is mixing and mixing is not practical in most subfloors because it would blow the agent out into the room above.

Measurement Procedure

Operating the floor blowers only, measure the ELA at the Total Leak tab or put in an overestimate of the floor leaks.
Turn these blowers off and measure the "Static Pressure at Time of Fan Test". Stop test if over + 2 Pa and eliminate before proceeding. Any pressure should be eliminated or at least reduced to below 1 Pa if possible.
Establish a 10 Pa room pressure with the blower(s) mounted in the door. This is a balancing pressure so flow pressure does not matter.
Pick a likely range for the Floor Fan(s) and turn it (them) on. Adjust the fan(s) to only 25 Pa of flow pressure. If more than one Floor Fan is being used, always have all of them adjusted to the same flow pressure on the same range.
Remove the low-flow plate from the Room Fan(s). Turn on the Room Fan(s) until the bottom of the allowable room pressure range is achieved. (e.g. usually 10 Pa). The Room Fan(s) will not be adjusted again during the test.
Check for neutral pressure. If air is still coming up, the Floor Fan(s) has to be turned up to its top speed, e.g. 200 Pa of flow pressure, and the floor checked again. If air is still coming up, a less restrictive range must be selected. If the air was going down, the Floor Fan(s) is sucking too much, and a more restrictive range must be chosen. The correct range for the Floor Fan(s) is determined when at the bottom of the range (flow pressure of 25 Pa) air moves up into the room, while at the top end of the range (flow pressure of approximately 200 Pa) air moves down into the sub-floor.
Commence a process of bracketing to narrow the "window" of Floor Fan flow pressures which defines when the pressure across the raised floor shifts from positive (air moves up) to negative (air moves down). The leakier a raised floor is (i.e. unsealed cutouts etc.) the wider this window will be.
For example, in a 5,000 ft² room, using two Floor Fans, when both were on range 3, with 100 Pa of flow pressure, smoke clearly moved up from the floor, while at 125 Pa of flow pressure, smoke clearly moved down. At some point between 100 Pa and 125 Pa of flow pressure therefore, a neutral pressure was established across the raised floor.
If it is not possible to clearly define the exact flow pressure at which a neutral floor pressure is obtained, note the flow pressure on the Floor Fan(s) when smoke clearly moves down.
Reconfirm the room pressure. It may have changed slightly during the test. If it has moved off of the bottom of the allowable target range (e.g. 10 Pa) adjust the Room Fan until it is back on. Double check the neutral floor pressure if necessary.
Enter the Room Pressure gauge reading into the computer (usually 10 Pa)
Enter the range used for the Floor Fan(s)
Enter the Flow Pressure(s). DO NOT add them together in any way.
Choose a Default Minimum Protected Height of 75% of sub-floor height unless the protected cables are below that height where a lower protected height may be considered. There should be NO static pressure during the retention period and mixing is not an option so protection for the entire height of the sub-floor space is not possible.
The retention time will be displayed

Choosing Minimum Agent Retention Time for Sub-floors

As with all retention times, sub-floor times must be sufficient to allow qualified personnel to arrive. Typically 10 minutes is chosen without much thought and may be too long or too short depending on the circumstances. This time must be longer where:

the cables cannot be de-energized and may continue to create sufficient heat to reignite the fire.
the cables are massive and will stores large amounts of heat.

This time can be shorter, if:

qualified personnel are available 24 hours per day