baseboard heat calcs, BTU per Sq foot?

[copper123]

Hi,
I have been asked to heat a small garage with electric baseboard heat. Can somebody give me a good link or a some advice on how you figure how much wattage would be required? I suppose there are many factors that need to be included. R-value of the wall, ambient air temp the area needs to stay at, and I suppose the heating degree days of the area I am in.
Thanks in advance.

 

[gndrod]

Re: baseboard heat calcs, BTU per Sq foot?

copper 123, System basic room heat = va/ sf per Climate zone area location. (i.e WA state has has two weather zone areas. Zone 1 is based on 5 to 8 watts/ sq ft. Rule or thumb for a baseboard heater is 250W @ 240vac per lineal ft. A 4 ft. base board is 1 kw output and will balance out well in a 8' high x 120 sf room per zone 1 in WA)

rbj, Seattle factor [1 kw = 3,412 BTU]

[ April 08, 2005, 04:48 AM: Message edited by: gndrod ]

 

[copper123]

Re: baseboard heat calcs, BTU per Sq foot?

thanks Gndrod!
I will try and find out what zone I am in. I appreciate the info.

 

[hardworkingstiff]

Re: baseboard heat calcs, BTU per Sq foot?

In Tidewater Va. in the 70's we used to use 10 watts per sq. ft. as a guideline.

 

[kiloamp7]

Re: baseboard heat calcs, BTU per Sq foot?

I have also seen the 10VA per sq. ft. R-O-T
referenced.

 

[copper123]

Re: baseboard heat calcs, BTU per Sq foot?

Thanks guys.
I also came up with some other general figures that were on a website.

1KW = 3412 BTU's
Very General Guidelines BTU, per Sq foot
25-30 BTU's needed warm climates
35-40 BTU's needed moderate
45 BTU's needed cold.

My garage
12x26 = 312 Sq foot
Cold climate, 45 BTU's need
312x45BTU's = 14040 BTU's
14040 BTU's / 3412 = 4.11 KW

Using 10 watts per sq foot as you fellas suggest.
312 x 10 = 3120. 3.1 KW

Looks pretty darn close to the moderate BTU number from above.
I'm up north, so I think I am pretty close.
Thanks for the help!

 

[charlie]

Re: baseboard heat calcs, BTU per Sq foot?

Besides the temperature outside, you have to consider the insulation or the building. With enough insulation, you can heat it with a candle.

 

[copper123]

Re: baseboard heat calcs, BTU per Sq foot?

Hi Charlie,

Do you know any formulas that can help find that?(Factoring in R-value or ambient air temp)
The folks I am working for say they only want to keep the garage at 30 above. Also, I was thinking about the entire insulation thing as you described. My thought was that the BTU's are calculated using insulation recommendation for the particular climate zone that you are in. So, my thought was the BTU's or watts they recommend for a particular climate zone would create a ambient air temp inside of maybe 72F? So,, would it be wrong to think if you dropped the ambient by half, you could drop the BTU's or Watts by half. I am sure this elementary rational is wrong for some good reason, but I thought I would throw it out there.

 

[BruceH]

Re: baseboard heat calcs, BTU per Sq foot?

There are computer programs that you can download to calculate heat loss or gain. One I know of cost 50 bucks for 2 months of usage. There is also a free one you could get to calculate available from Slant-fin, I think. Being it's just a garage, this might be overkill and you could get away with rule-of-thumbs if you want. I have the slant-fin and if you want to give me details of insulation and construction, I'll run a calc. for you.

 

[suemarkp]

Re: baseboard heat calcs, BTU per Sq foot?

You're looking for an ACCA Manual-J, but this will be overkill for a simple garage. You need to know the winter design temperature for your area (its typically your winter "lowest high temp", but I have a book that lists it for various cities in the US). You also need to know the temperature you want to keep that garage at. This difference between the two temperatures is the delta T (dT).

You need to know the R values of the walls, floors, ceilings, doors, and windows (R value is 1/U if you know the window U value. Assume R=0.8 for single pane window, 1.8 for double pane, 3 for double pane low-E argon filled). You also need to know the area of each of these too. Wood (such as doors) has an R value of about 1 per inch.

Finally, you need to guess how much air infiltration you get. This is measured in Air Changes per Hour (ACH) and depends on how leaky the doors, windows, and walls are. Guess an ACH of 1.0 if you have no idea and ACH=2 if you can see daylight through parts of the walls...

Calculate your conductive heat losses with this equation for all walls, windows, ceilings, doors, and floors and add them together (note -- wall area is the length*height - window and door area):

BTU/Hr = Area * dT / R

If the floor is concrete, use this equation to find the floor conductive loss:

BTU/Hr = Perimeter * 0.8 * dT, where perimeter is the floor perimeter in feet.

Heat loss to infiltration is:
BTU/Hr = ACH * Volume / 60 * 1.08 * dT where volume is the volume of the building's conditioned space in cubic feet.

Add all of these heat losses and this is your total BTU/Hr requirement. If using electric resistance heat, divide this by 3.41 to get KW of heat required to produce the temperature you want. If this building is allowed to get real cold and you want to heat it up only when its used, it could take a long time with this BTU value. You need to increase this substantially if you want a quick heat up of a cold space.

[ April 11, 2005, 04:48 PM: Message edited by: suemarkp ]

 

[copper123]

Re: baseboard heat calcs, BTU per Sq foot?

holy smokes, suemark
thanks for all the info. I will start to digest it now.
I sure appreciate the general knowledge that is on this site. Really blows me away sometimes.