Heater BTU estimate

[Maurice Arney]

I've spent the last hour searching the internet for a calculation to estimate heater size for my new shop. Unfortunately, my confusion only increased with each search result. The formulas and calculators I found varied as much as 800% using the same data (I am suspecting that the web sites selling heaters might be inflating the BTU requirements for obvious reasons). I was wondering how other folks estimated how many watts or BTU they needed to heat their shop.

Thanks!

[Lee Schierer]

It gets pretty cold in NH, so you need to provide some more information. How big is your shop? What sort and how much insulation is in the walls and ceiling? How many and what type of windows and doors are there? Do you want to just work comfortably or are you going to do finishing in the winter was well? You want to size your heater so that on the coldest day of the year the heater will run 95% of the time but still keep the space at temperature. In the days when oil was cheap, bigger was better according to the furnace experts, particularly those companies that also sold the fuel. Now, you want better efficiency. I live in Northwestern PA and we heat our entire 1800 sqft house with a 50,000 btu heat geothermal pump, which works out to 27 btu's per sq ft. Normally you use 30 btu's/sq ft as a rough estimate.

You need to do a manual J calculation for your shop. Here's a link to a calculator that may help. Manual J

[Maurice Arney]

Thanks Lee. That may be a bit more complex than I really need. I should have mentioned that I am not installing a heating "system" but rather just looking for a space heater to warm the shop when I go out to work. I have been looking at so called "garage heaters" that hang from the ceiling. I just want to make a rough estimate based on the cubic feet of my shop and the anticipated winter temps. Most of the formulas I see will multiply the cubic feet by some factor and then adjust for poor, average, or excellent insulation. Trouble is, the results vary quite a bit from one method to the other so I don't know which ones are the best estimate.

[Chris Friesen]

The Manual J is the "proper" calculation, but I can throw out a data point at least. My garage unit heater is 45KBtu. I probably only needed a 30K but there weren't any in stock in town. I usually keep it just above freezing, then crank it up when I want to work, so extra heat output means I can get to work quicker.

I'm up in the Canadian prairies, we usually hit -40 at least once a winter. I have a double-car attached garage (one wall shared with house). It has 2x4 walls insulated to R11 with fiberglass batts, and blown in fiberglass in the ceiling that is supposed to be R30. Double garage door, nominal R16, but actually less than that due to windows. One insulated steel door to the outside, and one double pane sliding vinyl window on the back wall.

[Jim Neeley]

Maurice,

In your original post you're complaint was a 800% variance. The difference in the answer between plywood siding and a 2x6 insulated wall is 20-fold. The heat loss also varies linearly based upon the difference between inside and outside temperatures. With a 60* inside, the difference between 2*0 above (40* difference) and 20* below (80* difference) is another factor of 2.

If you sized it around 60* inside at 20* outside and the outside dropped to -20*, the inside would only warm to 20*.

It's be nice if there was a simple rule of thumb, but there isn't. Chris provides about as close as you get.

If you really, really don't want to run the numbers then you can either choose one you know is too big and have it run for shorter periods (more $$ upfront & higher energy) or go for a smaller one that *may* keep the place warm. That will save you $$, may or may not save you on energy, and sometimes your shop may be cold. If the one you buy is too small, you either live with it or pay even more to add a second heater or replace it with a bigger unit and resell the small one at a loss.

You makes your decisions and pays your money.. No decision is a decision in and of itself.

I wish I had a better answer but I'm not about to blow smoke and tell you to "trust me, I'm with the Government". I'm not.

[jay gill]

I can attest to the: "Dimplex DGWH4031 4000-Watt Garage/Workshop Heater". I had it previously in my the 20x20 basement of my townhouse that was 3/4 exposed and would hover around the 50 degree mark. It kept the shop toasty warm on a middle setting and the thermostat does shut the heater off.

I have since moved and have the heater installed in my 2 car garage ~ 25x30 with 11' ceilings. The walks are insulated and sheet rocked and it had no problem heating it to the mid 60s on the coldest winter days. The fan moves a good amount of air and the unit can be aimed down towards you for some quicker warmth.

As to my sizing methods I went with the (gasp) rules of thumb and experience. I reasoned if a 5,000 btu a/c window unit cooled my 14x14 bedroom growing up, 18,000 btus in this heater should handle 3x-4x the square footage. (Yes I realize lots of factors but not heating a house just semi-temporary heating for a low acquisition price). It does require a 220 circuit, so that should be taken into consideration.

As to cost to operate I've seen $5-10 bumps in monthly electricity spending weekend Saturdays and Sundays running it for the day, so I'd estimate the running cost ~$2 per 8 hrs but then again it would shut off for periods of time. Your mileage may vary.

[Maurice Arney]

Maurice,

In your original post you're complaint was a 800% variance. The difference in the answer between plywood siding and a 2x6 insulated wall is 20-fold. The heat loss also varies linearly based upon the difference between inside and outside temperatures. With a 60* inside, the difference between 2*0 above (40* difference) and 20* below (80* difference) is another factor of 2.

If you sized it around 60* inside at 20* outside and the outside dropped to -20*, the inside would only warm to 20*.

It's be nice if there was a simple rule of thumb, but there isn't. Chris provides about as close as you get.

If you really, really don't want to run the numbers then you can either choose one you know is too big and have it run for shorter periods (more $$ upfront & higher energy) or go for a smaller one that *may* keep the place warm. That will save you $$, may or may not save you on energy, and sometimes your shop may be cold. If the one you buy is too small, you either live with it or pay even more to add a second heater or replace it with a bigger unit and resell the small one at a loss.

You makes your decisions and pays your money.. No decision is a decision in and of itself.

I wish I had a better answer but I'm not about to blow smoke and tell you to "trust me, I'm with the Government". I'm not.

Jim, I guess I should have clarified more... The 800% variance came from using the SAME input in several calculators. So something in their formulas varied significantly. I's not that I don't want to run the numbers but I can't seem to make sense out of that "Manual J" input screen. If you think about a single room in a detached building, then go look at the input fields in that link, it is hard to make any sense of it. I have a 16W X 24L X 9 1/2' ceiling room in an unheated barn with 2x4 walls and T-111 siding. I put R-13 fiberglass in the walls and R-19 in the ceiling. I've got one steel door (insulated), one 40 x 24 sliding thermo window, and 4" thick concrete floor.

It seems like there is no good answer so I'll just have to go by cumulative experiences from other folks with different sized heaters. Based on what I've read, I'm thinking a 5000 watt heater might be enough. I think I may go for the Dimplex heater and if needed, I can add an oil filled radiator to supplement.

[Maurice Arney]

I can attest to the: "Dimplex DGWH4031 4000-Watt Garage/Workshop Heater". I had it previously in my the 20x20 basement of my townhouse that was 3/4 exposed and would hover around the 50 degree mark. It kept the shop toasty warm on a middle setting and the thermostat does shut the heater off.

I have since moved and have the heater installed in my 2 car garage ~ 25x30 with 11' ceilings. The walks are insulated and sheet rocked and it had no problem heating it to the mid 60s on the coldest winter days. The fan moves a good amount of air and the unit can be aimed down towards you for some quicker warmth.

As to my sizing methods I went with the (gasp) rules of thumb and experience. I reasoned if a 5,000 btu a/c window unit cooled my 14x14 bedroom growing up, 18,000 btus in this heater should handle 3x-4x the square footage. (Yes I realize lots of factors but not heating a house just semi-temporary heating for a low acquisition price). It does require a 220 circuit, so that should be taken into consideration.

As to cost to operate I've seen $5-10 bumps in monthly electricity spending weekend Saturdays and Sundays running it for the day, so I'd estimate the running cost ~$2 per 8 hrs but then again it would shut off for periods of time. Your mileage may vary.

Thanks Jay... Saw this on Home Depot and think I'll give it a try. http://www.homedepot.com/p/t/2035689...2#.UbsswaPD_RY

[Wayne Crider]

Based on 35 years in the cooling and heating industry in Florida a 5k heater won't work for you on a cold day. 1st off they always give off less heat. Then remember that once a room goes cold the heater will be warming the structure and not you. It's as much about the temperature of the entire space including everything inside, including walls, floor, wood and machinery. Heat always flows to cold. The insulation helps keep the warm or cold load from increasing. Once the entire space is at temp, warm or cold, the load lightens and the less usage of energy. You will need to run a heater for awhile depending on the temperature of the space and outdoors before going out to work.

There are alot of extremely high efficient wood and coal stand alone furnaces available. You'll just have to run a flue pipe and keep them off the wall a little. With the efficiency they run at you'll lower your consumable bill and btw, they run pretty dam clean. You can even recycle some of your scrap.