Can anyone point me towards a protocol for establishing correct values to be used in the MRT (mean radiant temperature) sheet of the ASHRAE Thermal Comfort Tool 2.0? It asks for the temp. and emissivity of the “Glass/Panel Data”. There are so many factors to consider in order to establish that temperature. Do I need to do a Therm model on the window and frame and use the value the coldest point as in the PH standard? Or do I try using a weighted average of frame and glass? Or can I get by with just the glass temp. Thanks in advance.
Note: The help menu for the thermal calculator tool does not work with my computers operating system so I apologize if this is covered there.
Lindsay Austrom
Mechanical EngineerStantec
21 thumbs up
August 20, 2012 - 5:13 pm
I haven't been able to access the ASHRAE Thermal Comfort Tool v2 software help menu either. Look for a "Final Report 1332-RP" PDF that is saved in the same folder as the software -- it appears to include text from the help menu.
I typically use the assembly (glazing and frame) U-value and design indoor/outdoor conditions to calculate an inside surface temperature. I use the same equation for all exterior surfaces, each with their respective U-value. For lack of other info, I leave the emissivity at the default values (0.90 for opaque surfaces, 0.84 for glass). If you have separate temperatures for the glass and frame, I think an area-weighted average should be fine.
ASHRAE is working on a User's Manual for Standard 55, which (if I read the RFP correctly) should be available in early 2013. It will include explanations and example calculations on how to apply the Standard. You could also send an interpretation request to ASHRAE.
Eric Morrow
ArchitectDore & Whittier Architects, Inc.
August 20, 2012 - 10:55 pm
Lindsay: Thank you for your reply. Unfortunately, I did not get the "Final Report 1332-RP" PDF file on my recently purchased copy of the Thermal Comfort Tool. Not sure why not.
My next hurdle is to identify the appropriate exterior boundary condition for my Therm models. I have searched the internet and have found no definitive source for that temperature value. Any ideas on that? I could see using 28.6F which is the average monthly temperature for the coldest month as calculated by NOAA.
Lindsay Austrom
Mechanical EngineerStantec
21 thumbs up
August 21, 2012 - 2:14 pm
Look in the folder on your hard drive where you installed the software, not on the CD. The PDF should be included with the installation when you ran the EXE file.
For the exterior boundary condition, what outdoor design temperatures were used to size the HVAC systems? These should be documented as per ASHRAE Standard 55 Section 6.1.1.1 and often reference published ASHRAE climatic data (99.6% heating design temperature, 1% cooling design temperature and humidity) or local code requirements. It looks like NCDC/NOAA also publishes an Engineering Weather Data CD-ROM. Use your own judgement for spring and fall design conditions -- the main concern then is high humidity and whether the HVAC systems have enough dehumidification at part-load.
As an aside, do you have radiant heating or cooling in your project? If not, and since you mentioned Passivhaus, your project may have a good enough building envelope that you can assume the operative temperature is equal to the indoor air temperature, and avoid calculating the MRT altogether (see Standard 55 Appendix C for details).