Okay, let's run a heat loss calculation

Redsky (or others),

Any insights on how to determine the btuh requirements for a given space? Is there a site or calculator I can use to determine this?

thanks
Joel

A number of posters seem to be in this situation, not knowing how to properly size and select a heater. Rather than guess, let me show you how to do it correctly. It's not rocket science, and it will allow you to confidently, and safely, heat your garage or workshop as economically as possible.

No computer programs are required, and I can perform a hand calc using just pencil and paper in about 10 minutes for a simple space or building. A calculator will speed up the process, and might eliminate some arithmetic errors by those who have forgotten their math skills. It might take a beginner a bit longer, but anyone should be finished in less than an hour.

Let's get started.

First item is to collect the construction documents for the space. Don't have blueprints? No problem. We'll make some. Start with a couple sheets of 1/4" graph paper. This is the stuff that is marked vertically and horizontally with light blue lines, every 1/4". For our plans, each 1/4" segment on the graph paper will represent one foot of the actual dimension of the space. This is a 1/4" scale drawing: 1/4"= 1'-0".

First we need a floor plan. This is a bird's eye view of the space. Indicate the wall thickness as best you can estimate and show the doors and windows. On the inside of the space plan draw a dimension line from the top of the space to the inside of the bottom of the space and likewise for the left-right inside dimensions. On the outside of the space, provide a reference dimension for the doors and windows and label them as to what they are. Show a north arrow for orientation. That's it. The plan drawing is done.

We will also need exterior elevations. This is the view of the walls when looked at from the outside of the building, just as when you drive up to the building. On this drawing you need to show the windows to scale, as well as the doors. Provide a couple of dimensions to show the wall and window/door heights. Each wall requires a separate drawing. Keep it simple. Label the walls according to the direction that the wall faces.

These plans will serve multiple purposes so make a couple copies of each.

With the plans in hand we can start the calculation. Three steps are required: Takeoffs (or measurements of the building components (walls, windows, etc.), determining the insulating values of the components, and some easy calculations. As an example, I am going to size the heater for the GJ garage.

The GJ garage is an attached 2 car garage, 20'x24' with an 8' ceiling. Two walls are shared with the house so they can be ignored. There is one 2'x3' window, a 3'x7' passage door and a 7'x16" overhead door.

Takeoffs are as follows:

Walls: (44'x8') - [6 sq.ft.(window) + 21 sq.ft.(door) + 112 sq.ft.(OH Door)]=
213 sq.ft.
Door: 21 sq.ft.
Window; 6 sq.ft.
OH Door: 112 sq. ft.
Ceiling: 528 sq.ft.
Flr. slab: 528 sq.ft.

Now we have to determine the insulating value of each component. Try Googling "insulating value for construction materials" if you have any difficulties. I will show how the wall is determined as an example.

Wall Assembly

Component R-Value
Vinyl siding 0.61
Vapor Barrier wrap Negligible
1/2" sheathing 1.22
5-1/2" fiberglass 19.00
1/2" Gyp. Bd. 0.45
Total R-Value 21.28

Perform this same exercise for the ceiling, doors, etc.

Then we need to establish the design conditions, in this case 70 degrees indoor, and 10 degrees outdoor, or a 60 degree difference. For the floor slab we will take a shortcut and use a 40 degree difference.

Once all the R-Values have been established for each building component, convert the R-Value (the resistance to heat flow) to its U-Value, the actual transmission value of the insulation. Very easy. Simply divide 1 by the R-Value. For the wall shown above for example, divide 1 by 21.28 and you get 0.047. This is where a calculator can speed things up.

So the heat loss from the wall is as follows:
213 sq.ft. x 60 degrees x 0.047 = 601 btuh

The formula is area x temp. diff. x U-Value = heat loss. Very simple.

Add up the various components and you have the heat loss of the space. But wait a minute, we left out one item - infiltration, the cold outside air that sneaks into our garage despite our best efforts. This is a little harder to calculate but we can fudge a little bit here. If you have new, tight fitting gasketed doors and windows you can estimate infiltration as follows:

Windows: 20 cfm each
Passage doors: 30 cfm each
Overhead doors; 100 cfm

So for the window infiltration: 20 cfm x 60 degrees x 1.1 = 1320 btuh

That's it. Add up all the heat losses for the building components, plus infiltration and you will have the total heat loss for your space. Go forth and buy the correct heater! For those with fuel-fired heaters utilizing interior air for combustion, you will need to add that amount of outside air as well.

Now since you have a good set of plans, either existing or hand drawn, you can sketch in your installation and use the plans for your permit application. Note that for some jurisdictions there is a minimum size requirement, so that you may need to be bigger than 8-1/2x11. This will also be a good source for laying out your lighting plan and for general space planning so your hard work will be paid back many fold.

Good luck with your projects. Hope this is useful.

As always, offered only as opinion

Disclaimer: Surface air films, heat transmission directions, etc., etc., etc., omitted for this exercise. Don't PM me. I know!

That helps a great deal.

How would I do the heat loss calculation of a 4" thick slab with 3" of R12 insulation on the under side and perimeter and rear being 8' - 10' below grade with the front being grade ? Would you use a mean number for the below grade of say 4' ?

Another question. My garage has a heated suite above which is kept at a slightly warmer temp. What should I do for the heat loss of the ceiling ? Is there loss ?

Or, you can just forget all the silly numbers, put in a decent heater and try it. Your winter will be warmer with a decent heater than with months of calculations.

This information is excellent. I could calculate that a single solid wood garage door would lose more heat than all 3 of the insulated doors that we are looking to purchase combined ( not including infiltration ) . Money well spent on the better insulated units !!

By these numbers, I will be losing more heat to infiltration at a rate of over two times in the garage. I am sure that these numbers have a bit of a fat safety margin built in however it goes to show that we need to spend much more time sealing it up and making sure the doors fit just right.

I will go with the silly numbers

Weedwaka said:

Another question. My garage has a heated suite above which is kept at a slightly warmer temp. What should I do for the heat loss of the ceiling ? Is there loss ?

Click to expand...

no, probably a small gain, but just ignore it...

Weedwaka said:

That helps a great deal.

How would I do the heat loss calculation of a 4" thick slab with 3" of R12 insulation on the under side and perimeter and rear being 8' - 10' below grade with the front being grade ? Would you use a mean number for the below grade of say 4' ?

Click to expand...

Keep the calculation simple. Use a weighted or average number.

Concrete has an average R-value of about 0.1 per inch for standard mix.

As always, offered only as opinion

Weedwaka said:

Another question. My garage has a heated suite above which is kept at a slightly warmer temp. What should I do for the heat loss of the ceiling ? Is there loss ?

Click to expand...

No heat loss. Maybe even some transfer (minimal) to the garage.

As always, offered only as opinion

Thanks

I've pondered this issue for my building. Its a little different than the common residential garage however, being a standard "red iron" type steel building with the vinyl backed insulation installed and then the sheeting over it, roof and all. No ceilings, etc. 60x60x16 with a 12/2 roof, thats 66,600 cubic feet of interior. I suppose it would not be difficult to calculate if I had a "standard" R value for the roof and walls. I have a 12x14 garage door, a 10x10 roll up sheet door, and a 56x14 door opening, all that essentially have no R value as the garage door is glass, the roll up door is sheetmetal, and the 56x14 is translucent fiberglass.

Any guess as to the R value of the thinly insulated metal walls and roof?

Charles

Redsky49 -- Thanks for posting this. I did some of these calcs in college about 15 years ago. This was a helpful reminder.

I did about six pages of rough calcs for my garage.

I was surprised at how large the infiltration number added up to....

4 garage doors, 2 man doors, 5 opening windows, 53 degree delta T results in about 33,800 BTUs.

I calc's the rest of the heat loss for the garage envelope to be about 15,500 btus.

This would make a total BTUs of about 50,000 for a 1600 sq ft garage.

Just curious what other people have calc'd out....

I'm ultimately trying to size a boiler and the gas line.....

Ya know there isn't but so many sizes of furnaces out there.
Anybody with any experiance doing installs/replacements that's remotely worth a rats *** should be able to size up a furnace by spending 10 mins walking through a house.

Good tutorial/refresher Redsky49! Miss your posts. Always good sage advise, even if offered only as an opinion��

Thank you very much. Im in the process of trying to get permits to install a hydronic floor heat system in my garage. I have been getting the run around from the county. First told I don't need calcs then told I do (Im just installing the tubes at this point and probably won't hook them up for a year or 3). Ive probably looked at 15 different sites with "Heat loss calculators" but none that Ive liked or seem to work well. With this, Ill just be doing my own calcs for the Nazis at the county.

zmaxmotorsports said:

Ya know there isn't but so many sizes of furnaces out there.
Anybody with any experiance doing installs/replacements that's remotely worth a rats *** should be able to size up a furnace by spending 10 mins walking through a house.

Click to expand...

And typically if your buying a boiler you add additional for future zones or the abnormal below winter temps. Not to mention how many times your wife or kids open the garage door or even forget to shut that dang thing.

We use wrightsoft. No guesswork!

Or just look at the chart which is amazing close to the calculations

Got a new tight building go to,the smaller size.........got leaky 100 years barn but it insulated go up one or two sizes...........it's not all that complex.

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It's a garage and like others said there are not that many choices

Perfect, thanks Showkey. Ill have to print that off and show the county building inspector. Im sure they'll take that no questions asked.....