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thickness of housekeeping pad

thickness of housekeeping pad

How do you determine how thick to make a concrete housekeeping pad, or do you just assume that a 4" or 6" thick pad will be sufficient for most cases?

RE: thickness of housekeeping pad

If the pad is just for housekeeping, 4" is adequate. For airhandling units verify that there will be enough height under the unit to trap the cooling coil condensate drain (trap height depends on static pressure in the coil casing & trap configuration depends on if coil is draw thru or blow thru). For pumps,large fans, rotating machinery, provide concrete inertia base per 2003 ASHRAE Handbook, HVAC Applications, Chapter 47 Sound & Vibration Control, Table 42 Selection Guide for Vibration Isolation.

RE: thickness of housekeeping pad

thank you

RE: thickness of housekeeping pad

In the project I am working on, the static pressure of the air-handling unit is 40". Do we have to build a housekeeping pad 41" high to trap the cooling coil condensate drain? Is there any other solutions with 4" high pad? The air-handling unit is blow thru.

Can anyone give some detail of the trap configuration for blow thru unit?

Thanks in advance for advise.

RE: thickness of housekeeping pad


I don't know where you're located but the securing of plant to a "houskeeping" pad may be subject to seismic requirements also. Particularly with regard to the closeness of the hold-down anchors to the edge of the pad.

Comment re sfxf:

40 inches static! That's a staggering pressure for an air handling unit. Is it an industrial application?

RE: thickness of housekeeping pad

 Tell me when your going to turn that air handler on,
we can check with the National Geologic Event Agency
and see if they record any seismic activity on their equipment.
 Is that for a wind tunnel ? I hope your ductwork
don't colapse and get sucked into the fan.
 Sorry about the jokes, but wow, that's a lot of pressure.
 Improper trapping leads to several problems. If the trap outlet is too short

 the negative pressure created at system start-up will pull water from the trap into the air handler. The "seal" is destroyed, producing the same effect as an untrapped system. If the trap outlet is too tall

 negative pressure will prevent drainage, causing the condensate to back up into the system resulting in property and equipment damage.

RE: thickness of housekeeping pad

It is industrial application.
The AHU is on during the day and
off at night.


Thank you for the reply. The two fitures
are draw-through AHU. In my application,
it is blow-through. It should be positive
pressure at the outlet of the drainpan.
Do we still need the pad to be 41 inches
high? I don't quite understand why.

Thanks again for help!

RE: thickness of housekeeping pad

Are you sure that is 40" static pressure?

Trane has a detail on their website for blow thru coil trapping.

I wouldn't mount it on a 40" high concrete base, i'd look into a structural steel frame.

RE: thickness of housekeeping pad

Minimum condensate drain size:
   0-20 tons.... 1"
  21-40 tons.... 1-1/4"
  41-60 tons.... 1-1/2"
  61-100 tons... 2"
 101-250 tons... 3"
 251 & larger... 4"  

  AC condensate flow...
  Range: 0.02-0.08 GPM/ton
  Average: 0.04 GPM/ton
  Unitary Packaged AC equipment: 0.006 Gpm/ton
  Air Handling Units (100% outside air): 0.100/gpm/1,000 cfm
  Air Handling Units (50% outside air): 0.065/gpm/1,000 cfm
  Air Handling Units (25% outside air): 0.048/gpm/1,000 cfm
  Air Handling Units (15% outside air): 0.041/gpm/1,000 cfm

  Pipe size shall not be smaller than drain pan outlet.
  Minimum size below grade and below ground floor shall
  be 2-1/2". Drain shall have a minimum slope of 1/8"/ft

  Verify pipe sizing and discharge requirements with
  local authorities and codes.

  Depth of trap must exceed by one pipe diameter the
  total static pressure of fan.
Try this web site:

Here are some interesting trap details:

You need the 41 inch high trap because you don't want 40 inwg of air pressure to be blowing into your mechanical room.

RE: thickness of housekeeping pad


Theoretically if you have a fan generating 6” of static you need a trap which is 6” deep plus a safety factor to take care of any surges. If it’s a blow-through configuration the interior of the air handler casing will “see” positive static pressure. If it’s a draw-through configuration the pressure will be negative. Either way, I’d contact the manufacturer and get them to calculate exactly what the pressure will be because when the air comes off the fan there’s a velocity component i.e. it’s not all static. Within the casing, the static pressure, at the point where you need to put the drain may only be ¾ of fan static. Your pad doesn’t need to be as high as the water seal is deep. I’ve seen these water seals in the ceiling of the floor below – is that possible? Rather than build a pad over 40” high I’d dig a bloody hole (i.e. form a sump) in the plant room floor if it’s on grade. Just thinking out loud here – what about a steam trap type of device? They’re designed to let liquids pass and close off when the gas tries to pass through.

RE: thickness of housekeeping pad


It is a good idea to use steam trap type of device. THe liquid drain traps by Spirax Sarco will work.

Thank you very much!


RE: thickness of housekeeping pad

I agree, a float type trap may be a better option in your case, or why can't you just use an NRV in the drain line?

Regards, : Solving your problems before you get them.

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