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Ross in Hillsboro, OR's Journal Entries

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Posted by Ross in Hillsboro, OR on 9/6/2009 9:34:37 PM

These heat load calculations were based on the book, Modern Hydronic Heating For Residential and Light Commercial Buildings that I mentioned before.

I just found this excel file and was reviewing it.  There is a page for the "ASHRAE 97.5% Design Dry Bulb Temperature" for my location... you need to look up this value.  ASHRAE is like the Free Masons or the Moonies.  It's a club with a secret handshake, and a secret manual... (just joking).  But getting the ASHRAE data out on the Web is difficult... they want to SELL you the ASHRAE handbook with the data in it.

I can't say that any of the information in this spreadsheet is accurate, but you get an idea of the work a good HVAC contractor will be doing if they are going to plan your heating system.  A good heating contractor will have some type of software for calculating all this stuff.  I did it manually (set up the spreadsheet).  As I said, I don't know if any of this is accurate... but the idea is to share the information I extracted from the book to give you an idea of the "planning" on hydronic heat that can be done.

There are various tabs of the spreadsheet:

Room by room tab: The "Room by Room" is based on a table from the book.  

Rough square footage tab:
 Based on the plan views for our particular project.

Parameters tab
: is to figure out things specific to your area. What your average outdoor temperature is and your designed "temperature difference."

Wall R-value calculations:
I also have a sheet for calculating the R-value of your wall... this is a chapter in the book on how to do this.

I used the cross section of a typical wall type specified by our architect.  You have to consider the difference for the region where the studs are in contact with the drywall vs. the cavity on a stick frame.  I used R-21 batt insulation.  We may end up using spray foam to get a higher R-value... and/or I may add 1/4" foam to get a bit more R-value.

Foundation slab heat losses:
We have a slab-on-grade first floor, but this would be applicable to a garage/shop area as well.

Infiltration heat losses:
Again based on formulas in the book.

I expect that I am way off on some of these calculations... it's like being a freshman engineering student in college...

For my house, I have come up with a heat load calculation of 44,947 BTU.  This seems to be about one half of what two of my HVAC contractors came up with, so either I'm way off -- missing something, or the contractors are oversizing the boiler. I still have much to learn.  I'm fine getting a 92,000 BTU (27kW) boiler, as the cost differential isn't much, but every dime I save is a dollar I don't have to earn... so perhaps I should hit the books some more, and talk to some more HVAC contractors.

I think I would like to talk to the number one contractor on my list (whom I can't afford), and ask them what they would charge me to reevaluate my numbers... it's probably worth a few bucks to me to have someone knowledgeable check my figures.

I'm just happy I found the spreadsheet (on the computer)... I spent hours (possibly days) researching this and learning to build the data for the house.


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