[Bldg-sim] Equipment Sizing

Mon May 24 09:53:15 PDT 2010

Varkie,

I fully agree with you, and most of the following diatribe is not meant for you, but for the larger design community.  None the less, I invite you to correct me publicly where you believe I may have missed the boat.  I'm still learning!

When you can specify more than one cooling unit to meet the same load, you can afford to oversize a lot before energy efficiency is affected.  It's mainly the residential and small commercial sectors that concern me regarding excessive over sizing vs. efficiency and humidity control.

But in every case, more capacity costs the client (owner) more money, so proper sizing is important, especially during difficult economic times like these.  If you are like me, you tend to take ownership of the HVAC design, and costs may not be as important to you as they are to the client.  An excellent design may be rejected by the design team based on cost, and the easiest way to increase cost, along with the probability of rejection, is to oversize the systems.

With regard to sizing heating systems, I believe there is more leeway in terms of equipment (but not floor space or subsystems) costs, and the practical need for greater capacity is more common (morning warm-up after setback, for example).  A good design will consider the real capacity needs of the building without exceeding them more than necessary.  It's more difficult to define over sizing in the large C&I sector because proper sizing depends on more variables, so it comes back to the design engineer's integrity more often.  A highly skilled designer will tend to have more confidence in his ability (unless he has been "burned" a few times due to design errors) to calculate the real loads more accurately, considering all the variables.

One less confident (possibly including a highly skilled professional with burn scars) may tend to assume "conservatively" on all variables simultaneously, thus over-calculating the required loads, and then beef those up more than necessary.  Who, except another highly skilled (and brave, and independently wealthy) professional, is capable and willing to challenge this approach?  And even then it is one's opinion against another's.

If redundancy is needed (hospitals, for example), then there is still a practical limit to the need.  It just becomes more indeterminate, or more difficult to define.  This is where interaction with the design team is more critical; this time the HVAC designer needs to ask appropriate questions, listen carefully and offer verbal guidance to the team before he can properly size the systems.  We should not fail to consider that installation costs are always greater with larger systems, regardless of building sector.  All else being the same, this usually applies to O&M costs as well.  The integrity question is really this; "Do I mitigate my personal risk more through willful overdesign, or do I assign more value to the owner's financial objectives and/or limitations?"

To achieve a proper balance, the designer must first determine the true capacity requirements with accuracy and confidence, and then add only a modest oversize factor to that.  It isn't easy, but our choices here eventually establish our levels of self-respect and our professional reputations within the design community.

Glenn

________________________________

From: Varkie C Thomas [mailto:thomasv at iit.edu] 
Sent: Saturday, May 22, 2010 11:35 AM
To: Haynes, Glenn
Cc: Jason Humbert; bldg-sim at lists.onebuilding.org
Subject: Equipment Sizing

I changed the topic name to �Equipment Sizing�.  I thought I would add my 2 cents worth also.  So now Bldg-Sim has 8 cents worth.

The first building project that I was associated was at the tail end of the M-E design at JB&B of the Federal Reserve Bank Building in Minneapolis.

http://www.lera.com/projects/ofc/federalreservemn.htm <http://www.lera.com/projects/ofc/federalreservemn.htm> 

The building opened in January (I think) in the early 70s.  ASHRAE winter design at 1% is -16 F.  When the building opened it was about -30 F (it can go down to -40 F there) with strong winds.  Occupants in the US are not dressed (with full arm and leg heavy woolen underwear) to tolerate low indoor temps even for one day.  

Summer design in Minneapolis at 1% is 92 DB 75 WB but the DB can exceed 100 F.  Occupants can tolerate a little discomfort on the few days that extreme summer conditions occur.  It�s still better than no AC.  Few, if any, buildings were air-conditioned, even in the US, before 1940.  The moral of this story is design for extreme winter conditions in very cold climates.  Judgment, experience, and common sense have to be applied.  It depends on the location.  In a place like Singapore the temp varies from a low of 75 F to 95 F all day and all year.   There are no extremes.  All buildings in tropical countries do not need heating systems.

Inefficient energy use occurs when there is only one unit of the equipment and it is oversized.  When there are two or more units, one unit starts until it reaches maximum, then second unit comes on and the two shares load.  The units are rarely operated at minimum load.  This is the default in DOE2 but LOAD-MANAGEMENT allows you to sequence the use of primary equipment in any way you want that is appropriate for the project.

The lighting and equipment design criteria was 5 watts /sf and 3 watts /sf for buildings designed before the energy crises in 1974.  No one cared about energy before then.  Actual lighting density was nowhere close, and there was very little equipment in offices.  This means for a million sqft building you end up with three chillers.  One is standby which comes into operation when a chiller fails or one chiller has to be shut down for maintenance.  Specifying standby primary equipment affects first costs and does not affect equipment efficiency.

Energy programs are for comparing alternative energy conservation measures.  There is no need to size equipment for occasional extreme weather conditions.  But I think auto-sizing is based on weather data (not design data or median weather data) so equipment is sized for the worst condition of that year.  This means at every other hour of the year the equipment is operating inefficiently at part load conditions.  Equipment sizes estimated by energy programs are used in the design process.

Equipment schedules in design documents are based on actual equipment selected from manufacturers catalogs.  The name of the manufacturer and the model number are specified and then �or equal� added.  Equipment schedules are not based on design calculations or generic data because it is possible that real equipment cannot meet the performance data.  You need the dimensions of the equipment (with clearances for maintenance) to design them into the space.  TRACE energy program is based on actual real equipment performance data specified my model numbers.  HAP is based on real equipment, but they don�t mention model numbers.

I think the Code of Federal Regulations 10CFR434 (ENERGY CODE FOR NEW FEDERAL COMMERCIAL AND MULTI-FAMILY HIGH RISE RESIDENTIAL BUILDINGS) used to allow you to size two identical units of primary equipment each at 66% of the maximum design.  This would be commercial buildings with large heating and cooling loads where you would have two units.  Perhaps it was a special case where failure to perform at all times was not an option.  I couldn�t find it the latest register.

http://www.wbdg.org/pdfs/10cfr434.pdf <http://www.wbdg.org/pdfs/10cfr434.pdf> 

10CFR434 - 403.2.2 Equipment and System Sizing.

Heating and cooling equipment and systems shall be sized to provide no more than the loads calculated in accordance with subsection 403.2.1. A single piece of equipment providing both heating and cooling must satisfy this provision for one function with the other function sized as small as possible to meet the load, within available equipment options. Exceptions are as follows:

(a) When the equipment selected is the smallest size needed to meet the load within available options of the desired equipment line. 

(b) Standby equipment provided with controls and devices that allow such equipment to operate automatically only when the primary equipment is not operating.

(c) Multiple units of the same equipment type with combined capacities exceeding the design load and provided with controls that sequence or otherwise optimally control the operation of each unit based on load.

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