Shop Heating Question that is baffling me beyond belief!

[David Cramer]

Hello

I am very new to the Creek and have a shop heating question that I can't get a straight answer to.

A lot of guys keep their shop at let's say, 50 degrees, and then turn it up to, let's say 65 degrees when they are going to actually be out there to work.

I asked a friend, what's the difference between just keeping it at 65 and have it go on and off by itself when needed to maintain 65, versus turning it down to 50 degrees and then have it work hard for a long time to get to 65. Aren't they equal and if so, why not?

If it cycles naturally to maintain 65, doesn't that amount of natural gas equal to........... the gas it takes to maintain at 50 and work hard straight for the amount of time it takes to increase 15 more degrees?

I know I am wrong, but I just don't know why. They seem like they'd be even. Any heating experts who would care to explain the reasoning, I'd be more than grateful!!! My neighbors don't know either and everyone I've spoken to is split on it, but I've yet to ask a heating professional what the true answer is.

Thanks a lot! Great site ya have here people! Not much bickering to speak of. Unheard of on most woodforums I've checked out. Someone is doing something right. Keep it up!!!

Dave

p.s. How can Missouri get more snow, WAY more snow, than Ohio/Michigan? Elni-who-no ?

[David G Baker]

Dave,
I have a 30x40 well insulated pole barn that is heated with a natural gas fired 75,000 BTU Hot Dawg heater. I do not work in the barn too often but store many things that I do not want to freeze so I keep the thermostat set at around 50. When I work in the building I turn the heat up to around 65. The heater comes on and stays on for around 15 minutes. I dress warmly because I am in and out of the building frequently. The temperature in Mid Michigan has been in the teens to twenties at night for the past week or so. During the day when I am in the building it has been in the mid to high twenties and the heater does not come on very often. I would say that it comes on about once every 30 minutes.
The way I have been using my heater has been very comfortable. My average monthly gas cost for the barn is around $30. I will probably never keep the heat at 65 degrees 24/7 so I can't tell you the cost difference.
David B

[Kyle Kraft]

I do the same thing. 50 degrees when I'm away and 60-65 when I work. Same as Dave about $30 more to heat the shop...where else can you get so much bang for the buck? Plus my machinery loves it!! Especially when we get a 60 degree day and 150% humidity in March.

[Todd Tolhurst]

Does it take more energy to keep your oven at 375 degrees constantly, or to only turn it on when you want to cook a frozen pizza?


Whenever you heat your space, heat will escape. The greater the difference between your heated space and its surroundings, the greater the heat loss. If you keep the heat low when you're not using the shop, you minimize the heat loss. The amount of heat lost from a 65 degree shop for many hours or days of idle time far exceeds the amount of heat needed to warm the shop from the 50 degree idle temperature to 65 degrees.

[Dick Strauss]

David C.,
The amount of heat lost to the outside is based on the temperature differential (the differnce between the inside and outside temps). Time also factors into the equation but is less significant. If you have a higher differential, you get greater losses. So a 50* garage will lose less heat to the atmosphere (at say 30*) than a 65* garage.

By turning it down to 50* you'll lose heat based on a temp diff of 20* for 90% of the time and 35* for 10% of the time instead of 35* for 100% of the time

This is the reason energy companies ask you to turn down your thermostat during Winter (closer to the outside temp) and up for Summer (closer to outside temp).

[John Schreiber]

I've heard this argument before, once even from an HVAC repair man. I'm not a heating expert, but I'll write down how I think it works and please tell me if it makes sense.

If your outside temperature is 30 and you are setting your shop thermostat to 30, you wouldn't use any energy to heat your shop.

If the outside temp stays at 30 and you set the thermostat to 35, you'll have to use a little energy to keep your shop temperature 5 degrees above the outside temperature.

If again the outside temp stays at 30 and you set the thermostat to 70, you will have to use a lot more energy to keep the 40 degree temperature differential between inside and outside.

So far this is obvious. If you aren't ever going to change the temperature in your shop, the lower you keep the thermostat, the less energy you are going to use. This is because the energy required for heating a given space (everything else being equal) is dependent on the difference in temperature between the inside and the outside. When there is a large difference between inside and outside, you lose more heat and you have to use more energy to keep the building warm..

Now, if you go out to the shop once a week and change the thermostat from 35 to 70, you will have saved a lot of energy not keeping the shop as warm all week. If you go out every day and raise the temp to 70, but set it down to 35 when you aren't there, then there is still a long period when little energy is needed to maintain the difference in temperature between the inside and the outside. If you were to go out to the shop every hour and turn the thermostat up to 70, then when it reached 70, you were turn the thermostat back down to 35; then come back to turn it back to 70 at the next hour, you would still be using less total energy than if the shop stayed at 70 the whole time.

So no matter how you slice it, I think you come out well ahead, from an energy use point of view at least, by turning down the temperature as much as is safe when you aren't using the space.

Make sense, or am I confused?

[David Rose]

OH! And don't forget that most "yellow" glues need their required temps until they FULLY setup. Otherwise they will chalk, which isn't good.

My zoned Train unit for the house will not let us set low temps below 65 or highs in summer above 50. I asked the installer why and he said what some of the others here have said. Somewhere around 7 to 10 degrees is the point where diminishing returns take place in a well insulated building. In other words, it takes more BTUs to move more than 7 degrees than it does to keep the temp all day and night. I still would like to go below 65 on winter nights. But maybe in a couple of years they will change their minds, and I can reprogram for lower lows. ;-)

David

[Brian Hale]

........... I still would like to go below 65 on winter nights. But maybe in a couple of years they will change their minds, and I can reprogram for lower lows. ;-)

David

Keep a candle burning under your thermostat

Brian

[Perry Holbrook]

As an ex-energy engineer I often found it hard to convince building owners why they should set back their heat t-stat and night. For some reason they thought it made common sense if you left it set at a constant temp it cost less than turning it down and back up.

Unfortunately, thermodynamic laws are not based on common sense. John explained it pretty well.

One thing that makes this a llittle more complex is thermal mass. Most shops have concrete floors and a lot of heavy equipment. Both are heat sinks and will cool down and heat up slower than the air. If the temp is allowed to drop too low, the thermal masses fall to ambient temp and will continue to cool the air for some period of time even after the air has heated up.

The energy calculations get much more complicated when this thermal mass is considered. My guesstimate is for my shop a night time temp of 50 is about right day time 68. I have an automatic t-stat that does it for me but remember I'm in the shop 12 hrs/7days.

Perry

[Dick Aubochon]

As an ex-energy engineer I often found it hard to convince building owners why they should set back their heat t-stat and night. For some reason they thought it made common sense if you left it set at a constant temp it cost less than turning it down and back up.

Unfortunately, thermodynamic laws are not based on common sense. John explained it pretty well.

One thing that makes this a llittle more complex is thermal mass. Most shops have concrete floors and a lot of heavy equipment. Both are heat sinks and will cool down and heat up slower than the air. If the temp is allowed to drop too low, the thermal masses fall to ambient temp and will continue to cool the air for some period of time even after the air has heated up.

The energy calculations get much more complicated when this thermal mass is considered. My guesstimate is for my shop a night time temp of 50 is about right day time 68. I have an automatic t-stat that does it for me but remember I'm in the shop 12 hrs/7days.

Perry

Perry,
That was my thinking as well. What about the energy expenditure to reach thermal equillibrium? That is a factor with all the cast iron, steel, etc. in the shop is it not? I still think the idea of a set back is a good one. My shop is in an outbuilding garage that was built about 60 years ago, so you can imagine how loose the building is. I leave my heat at 50 deg F almost all the time, except when I have a glue up, or a finish drying.

[Mike Cutler]

Heck.. I'd be happy just to get the shop to 50 degrees. It's about 16 deg. in there right now.

It all comes down to how many calories, or BTU's it takes to get the room back up to temperature, versus how many does it take to keep it at a given temperature.
You're better off setting the temp back when not in use, just don't have a huge delta, or you expend more energy than you save bringing it back up to temp.
While on this subject. It may be worthwhile to check out the newer generations of Smart Temp controllers. It's not just on/off anymore. The controllers are actually self tuning PID ( Proprotional+ Integral+Derivative) controllers. Very nice little units.

[Al Willits]

Honeywell has done extensive work on set back thermostats, maybe visit their web site and take a look.
But there does seem to be a point where if the differential is to great, the cost to bring a house/garage/shop back to temp negates the savings.
I don't remember exact numbers and it might vary with climate conditions, (web site talks about this I believe) but seems to me the 10-15 degree number comes up a lot.
I only need to deal with Minn so I don't remember the varied setbacks, but I think the website has a chart for different parts of the country.

fwiw some of the new multi stage furnaces don't recommend setting back at all, others run a 2 stage thermostat which runs on the low fire mode till heating requirements are greater than the lower input can handle, then switch over to a higher input.

Al...who's kinda a HVAC guy...imho

[Ken Garlock]

The whole problem is also modulated by the amount of insulation surrounding the volume you are heating/cooling. Insulation is cheap, use a lot of it.

With good insulation, the room will cooler slower and heat faster in the winter, and the reverse in the summer.

[Kurt Strandberg]

I have a 30X40 pole building with 9' ceiling and I have a 75,000 btu hanging unit, I keep it at 45º when I am not in there and when I am going to work in there, like today, I turn the heat up to about 70º untill it shuts off then I turn it down to about 55º for the rest of the time I am out there.

I also have the thermostat at about 7 feet of the floor.

[Robert Mickley]

its a moot point in my shop, it gets s warm as the fire will make it.

So to sum it up there are several factors
How goos your insulation
How much iron is in the shop.

Personally If I ever get around to doing a new shop, I'm going with a heated floor, lots of insulation, good windows and doors and I'm leaving the temp set at the same place all the time. I build a fire in the shop every morning. Even on days like today when I've only been out there maybe an hour so far today. When I walk in I want it warm