BTU Heating Calculations, Manual J

BTU Heating Calculations, Manual J

PostBy: Yanche On: Sun Apr 13, 2008 11:11 pm

Heat loss questions have come up on the forum several times. It's usually in the context of stove, boiler or furnace size needed to heat a home. I'll try to outline the principles involved and the design procedures to provide an answer. There is a lot to cover.

Measurement Units

In countries like the USA which use the foot, pound, second units of measurement the quantity of heat is the BTU (British Thermal Unit). It's the amount of heat needed to raise the temperature of 1 pound of water 1 degree Fahrenheit. It's at standard conditions, sea level pressure, etc. ; conditions that only engineers worry about. Heat transfer is measured in BTUH (BTU per hour). In accordance with Thermodynamics Laws heat always moves from hotter to colder places.

Building Heat Calculations

A building can loose or gain heat based on the temperature difference between the inside and outside temperatures. The rate of heat transfer (BTUH) is largely dependent on the building materials between the inside rooms and the outside. Ignoring the effects of sun heating and wind, wait long enough and the inside and outside temperatures will be the same. So heating a home to a desired temperature requires the replacement of heat lost to the colder outside. Once heated you want to keep the heat inside, i.e. reduce the loss. That's what insulation does. The R-factor is an insulating material’s resistance to heat transfer. The greater the number the better. The R number you see on insulation materials is a value determined by ASTM (originally American Society for Testing and Materials) standard testing. It allows you to compare insulation material types. For a much more complete understanding on R-factor see:
http://www.alltechinsulation.com/Thermal_U_K_C_R_factors.asp
This link is broken, either the page no longer exists or there is some other issue like a typo.


The most recognized method for calculating building heat-loss/heat-gain is Manual J. This is a method established by the "Air Conditioning Contractors of America" trade association. See: http://www.acca.org/ In many states building contractors are required to use this method to calculated building and cooling requirements. It's written into the building code either directly or by reference. It's designed to replace the rule of thumb approach used for decades. It applies to both personal residences and large multi-story buildings. It will provide both heat loss and heat gain BTUH values. Used properly it can give accurate results. It's complicated and requires a personal computer running the Excel or equivalent spreadsheet. Like most construction trade association they sell membership and books. See: http://www.acca.org/store/category.php?cid=1 The Manual J books come with the spreadsheet software CD.

The Manual J procedure is so detailed that there are two versions of the design procedure. Manual J abridged edition and the full edition. The abridged edition applies only to certain kinds of single family type homes. It makes assumptions about the "diversity of construction", windows being on all outside walls, maximum number of skylights, no great differences in ceiling heights, etc. As you can expect the HVAC construction has struggled with education in getting its workers to use these new computerized procedures. In the past the Manual J procedure (version 7) was a check sheet type procedure and a simple totaling of numbers. Because of this new education need, the ACCA has on-line training. See:
http://www.acca.org/speedsheet/
This link is broken, either the page no longer exists or there is some other issue like a typo.
. You can download both the abridged software version and the training video. Saving the video for later playback will require special software on your computer. You should be able to play the video online. The video training is quite boring, at least to me, but it will give you an idea of how the calculations are made. There is a lot of special lingo that you have to wade through. What you can not get for free is the Table lookup values. These are the values for heat loss through building materials, windows and doors, etc. I know of no public sources for these values. You must buy the book. See: http://www.acca.org/store/product.php?pid=30 I have the book and paid the member price by ordering via my HVAC Community College instructor who was a ACCA member.

The downloaded Excel spread sheet is password protected. This prevents you from looking at the underling formulas used to calculate values. If you are a Excel expert you can easily bypass the password using techniques widely available on the Internet. Once you see the formulas you can make your own value Tables for the construction of your home. Hey, it's a lot of work to do, but it's possible. I just bought the book!

There's a lot more I could say about the Manual J calculations but I'll wait for responses and any real interest in this thread. Manual J will give an accurate number for heat loss (heating) or heat gain (A/C cooling). It will provide single room values (useful for zoning heating design) plus total building values. It's a must for A/C sizing, it's questionable if it's needed for coal heating appliance selection.
Yanche
 
Stoker Coal Boiler: Alternate Heating Systems S-130
Coal Size/Type: Anthracite Pea

Re: BTU Heating Calculations, Manual J

PostBy: gaw On: Mon Apr 14, 2008 3:34 am

Yanche wrote:There's a lot more I could say about the Manual J calculations but I'll wait for responses and any real interest in this thread. Manual J will give an accurate number for heat loss (heating) or heat gain (A/C cooling). It will provide single room values (useful for zoning heating design) plus total building values. It's a must for A/C sizing, it's questionable if it's needed for coal heating appliance selection.


Probably a bit much when deciding if you should buy the small, medium, or large stove, but for what I want to do, insulate and replace baseboard with radiators it will be useful to properly size the radiation to each room. Even heating with a coal boiler we are stuck with a few sizes must fit all.
gaw
 
Stoker Coal Boiler: Keystoker KA-6
Coal Size/Type: Rice from Schuylkill County

Re: BTU Heating Calculations, Manual J

PostBy: pret On: Mon Apr 14, 2008 1:02 pm

I'm sure the HVAC people used Manual J when calculating heat loss and gain on the house of a friend of mine. I am patterning my house after his - just about 800 square feet smaller. Using the same builder... same building specs... He was told his heat loss was 50K. It's a tight construction with inside/outside air exchange via the central cold air return. I am assuming the heat loss on my place will be comparable. I am interested in trying to determine the amount of coal I needed for a season of heating... which is where my question regarding the type of function heat loss favors. I've gained enough of an idea to predict the amount of coal within 10% margin.
pret
 


Re: BTU Heating Calculations, Manual J

PostBy: e.alleg On: Tue Apr 15, 2008 12:05 am

I went with the "EFM" calculation method. When the boiler couldn't produce enough heat I increased the feed rate a tooth :D Exact things give me a headache, I like vague generalizations best. What I'm trying to say is the manual J isn't for everyone, it's designed for HVAC Pros so the common man might find it hard to get through.
e.alleg
 
Stove/Furnace Make: EFM
Stove/Furnace Model: 520

Re: BTU Heating Calculations, Manual J

PostBy: CoalBin On: Tue Apr 15, 2008 6:58 pm

Yanche

I bet that book cost more than $ 50 ?

When I installed my AC system I ran my numbers in "Hvac-Calc" you can download a demo @ hvaccalc.com. Worked great/easy, cost $50 for a homeowner. Works for heat & cool :)
CoalBin
 
Stove/Furnace Make: Harman
Stove/Furnace Model: DVC-500

Re: BTU Heating Calculations, Manual J

PostBy: Yanche On: Tue Apr 15, 2008 9:42 pm

CoalBin wrote:Yanche

I bet that book cost more than $ 50 ?

When I installed my AC system I ran my numbers in "Hvac-Calc" you can download a demo @ hvaccalc.com. Worked great/easy, cost $50 for a homeowner. Works for heat & cool :)
You're right it's $64.95 plus shipping. I was not aware of "HVAC-Calc". Looks interesting. I wonder if it gives the same results as the ACCA software.
Yanche
 
Stoker Coal Boiler: Alternate Heating Systems S-130
Coal Size/Type: Anthracite Pea