Disqus - Latest Comments for papaD

Re: Pressurization Control in Large Commercial Buildings

papaD — Tue, 16 Mar 2010 08:57:16 -0000

Mulltiple DP sensors only compounds the errors. How do you average them? Are their positions reflective of the true average building pressure flow? Where do you place outside tap? How do you reduce the volatility of the DP output for control without loosing the value of the data? How ofter do you have to zero and recalibrate the sensors? Is this done regularly in practice?

Stack pressure and wind pressure both influence NET building or space pressure flow. Some walls can show a positive flow while others could be negative, due to wind. Trying to overcome anything but a light wind's effects is usually not possible or may damage window seals, windows or doors. The objective should be to maintain a positive NET flow from the area of lower dew point temperature to higher dew point temperature. In most climates and seasons, this is inside to outside. In cold climates with higher inside humidity, you can't run the space negative and you don't want to push humid air into the wall cavity to freeze. NET neutral appears to be the best alternative.

If not DP direct measurement, then what? Volumetric tracking by zone or AHU is much more stable, controllable and can manipulate very small increments of DP using differential volume control.

Re: Multiparameter Demand-Controlled Ventilation

papaD — Thu, 03 Sep 2009 09:42:01 -0000

Although most major points made by the author are pretty accurate, some of the assumptions, indications and characterizations are not.

Let's be clear, ASHRAE 62.1-2007 and previous versions DO NOT regulate CO2, not interior space levels or differentials. It does set design and operating minimums for dilution ventilation rates required for comfort, essentially the minimum quantity sufficient to satisfy 80% of visitors to a space in terms of body odor perception. Period. The historic and current references to differential CO2 are contained in an informational appendix intended to show the basic derivation of the ventilation rates used in the tables, that where originally based on CO2 chamber studies in the 1930's.

Many misunderstand Demand Controlled Ventilation and automatically associate it with CO2. DCV is any method that adjusts the ventilation rate proportional to the number of occupants in intermittently and/or unpredictably variable spaces. CO2 control is not recommended for spaces with stable populations or low densities. This means it works best and is more justifiable in large conference rooms, theaters, lecture halls, gymnasiums, places of worship, etc. It is not suitable for total ventilation control in MOST typical buildings.

Therefore, any method or device that can estimate space population can be used to calculate the required minimum rate and reset the intake set point: mechanical or electronic counters, time-of-day schedules linked to occupancy assumptions, binary sensors (occupied-unoccupied), data from security systems, ticket sales, etc. Most of these are very economical and more reliable than CO2 alone.

Differential CO2 is useful to estimate ventilation rates "per person" only. They do not help to dilute building-generated pollutants (e.g. VOCs) or to directly satisfy the building component (per floor area) of the ventilation standard's requirements. Because since 2003, the ventilation tables have more than just "cfm / person" rates, the relationship between occupancy and differential CO2 is no longer linear and each space may require a unique algorithm for CO2 set point control.

The biggest problem with using CO2 alone, is that you are measuring CO2 alone. It is not a valid indicator of ventilation rates and as the author pointed out, can lead to some extremely large control errors, on either side of the minimum requirements.

For more information on the sources of error in differential CO2 ventilation control, see "CO2-based DCV - Do Risks Outweigh Potential Rewards?" in October 2004 ASHRAE Journal.

More reliability is needed than CO2 sensors alone can provide, if you intend to satisfy both IAQ and Energy imperatives without the risks involved with indirect control methods.

Re: Multiparameter Demand-Controlled Ventilation

papaD — Thu, 03 Sep 2009 08:51:41 -0000

I can't help with the cost, but know that the piping will not be cheap, and the single sensor is a good idea but it is more like scientific-grade instrumentation. Try David Bearg, AirExpert Systems, sagefarm@comcast.net.