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August 20, 2009

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While working on a project about one month ago we realized (using this document) that since the heat loss caculations use the same engine as the heat gains (and there almost never heat gains from the ground) that Revit does not caculate heat losses through the perimeter of the foundations. Is this correct or have we overlooked something?

Basically, not being able to caculate heat losses through the perimeter foundation renders Revit useless for any serious calculations ( besides a quick estimate) in a northern climate (aka Canada with -23F design temperatures).

Have also sent this to my Mechanical Engineers for review.

In Canada we are referred to the CSA Norm entitled "F280-M90". This norm handles mainly residential construction situations where studs are accounted for and a lot of details around the walls and foundation constructions. If I am not wrong, this is implemented by the HOT2000 software provided by the government of Canada. The main advantages I found is that its tailored for typical house construction in Canada. I have hard time to correlate the same settings in MEP (and I'd like to do that)

I have only run through the tutorials as a student, but this module seemed pretty good. sure beats doing it the old fashioned way. I was impressed by some of the commands, especially trim for pipe. I am currently doing a short term contract for a consulting mechanical engineer correcting redlines - most enjoyable to apply what I learned in the tutorials to a real world situation.

From my Engineers:
• The input and calculation methods seem to be on track, pretty typical stuff, RTS is the right method, I think.
• I expect they will also use inch-pound measurements, although they only list metric at the moment (in places).
• Having ASHRAE typical glazing info is OK, but window manufacturers typically provide the U-values and shading coefficients, etc. with their product. REVIT should be retrieving this information from the glazing objects, wherever possible. I see where it indicates there may be something available in the future, but this is what makes the REVIT modeling special, isn’t it?
• The system is capable of VAV and CAV system calculations, good, this covers +95% of all HVAC systems but the tool should be capable of being expanded to other systems and variables as time goes by. This may be too big of an endeavor at this time, but as buildings get “greener“, more alternate HVAC systems will employed (chilled beams, radiant floors, etc.) and this product should be able change to account for these.
• There should be calculations for outdoor ventilation (per ASHRAE 62) and for air change rates (in places such as hospitals). Any calculated fresh air values should have the option of being output as a % of system air and as a separate value since the designer may wish to provide dedicated outdoor air systems, separate from the typical HVAC system.

To expand on the ventilation comment, though:

The information included in this brief only reference heating and cooling loads, so maybe there is another section that would address ventilation. However, I want to take the opportunity to comment on this because it is a limiting factor in many load calc engines.

Ventilation for each space should be based on either the ASHRAE 62.1-2007 or an air change requirement. The system should allow inputs that support either, then choose the greater value. For example, an space may require 20 cfm of outdoor air per 62.1. But, due to other design criteria, it may require 2 air changes per hour of outside air, which, let’s say, works out to 40 cfm. In this case, 40 cfm would be the requirement for outdoor air.

In addition, there are some design situations where total room air flow must meet a minimum value, expressed in air changes per hour. A hospital patient room, for example, needs 2 ACH of outdoor air, but also needs 6 ACH of total airflow. The air delivered to this space must meet both of these criteria. It should also be compared against ASHRAE 62.1 requirements.

When zones are created from multiple spaces, these requirements must be maintained. Sometimes the minimum is set based on total airflow for a specific space in the zone; sometimes by OA flow for a specific space. We have been using a spreadsheet to compare all of these requirements and determine the minimum airflow for a zone. In order for all of the calculations to be handled within Revit or through gbXML transfer to another engine, this data needs to be set as a property of the space, then carried through gbXML to the engine. It would be impractical to manage the data only in the engine or a separate spreadsheet while passing revisions back and forth through gbXML.

These air change requirements may seem foreign to a European mechanical software developer – I don’t know if they have a code that looks at ventilation that way. However, the Guidelines for Design and Construction of Hospitals and Healthcare Facilities (“the Guidelines”) are used in most of the US. That guideline has the effect of code and must be followed in design of hospital HVAC systems. If the calculation engine in Revit cannot manage these requirements, it will be dramatically less functional for design of healthcare facilities.

We would, of course, welcome any discussion on any of these issues.

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