A little heater mathematics

[Boddah2]

I have a workshop I keep above freezing until I’m am working in there and crank the heat. My question is it better to keep it set at 3000w or 5000w ? I’m guessing to keep at temp above freezing the 3000 w will be running longer but use less power or at 5000w be running less using more power? Idk


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[Flounder]

The theory is that your heating costs will be related to the insulation of your structure and the differential temperature between the inside and outside. If the 3000 meets your needs and gets it warm enough I assume it is a lower initial cost and that would be a better option. If you want to heat up faster then the 5000 is the way to go.

after rereading your post I think you already have the heater and it is just a setting you adjust. Overall cost should not vary too much for just keeping it at a set temperature

[boarder2k7]

Cost does not change based on the size of your heater. Q = dT/R so the only things driving your cost are the temperature differential and insulation value of your building.

The heating capabilities should be sized for maintaining the desired temperature at the most adverse temperature differential (say a -10° design point or something) plus adequate overhead for recovery (a plant that will only maintain at -10° will never be able to warm you up from a 40° idle temp to a 70° comfort temp for example.

Within reason (some complex efficiency calcs here that only really apply to fuel based heat sources, not electric like you're using) the size of the heat source does not matter. A thermostat will manage it so you don't overheat the space and will use the minimum energy possible since it isn't running when you don't need it.

If you don't have a thermostatic control on the heat source, I'd buy one that can handle the 5000W load, leave it set to 5k, and let the thermostat take care of it for you.

[Siress]

boarder2k7 has the right simplification.

If reducing operational expense is your goal, you need to minimize heat loss first and foremost. This starts with the roof and making sure it's built properly. Then add a ceiling to close off as much of the unneeded overhead space as possible and insulate it thoroughly - I caution against spray foam insulation, but there are some really sophisticated options out there that include things like radiant heat barriers. Then, and only then, work on the walls - seal up any gaps, insulate what you can, double pane glass (or add an insulating layer), that sort of thing. There are rules about R value, so look into those... Then work on the flooring. If you can raise and insulate the flooring from the concrete foundation it will help keep your feet warm, but I don't imagine that's preferable in a shop space.

From there, do what you can to use cheap fuel; you've already chosen electricity so we'll move on.

Then it's the uncertain battle of avoiding excessive heat gradients within the insulated space you've created. This is done through careful placement of the heater(s) and thermostat, or with sufficient convection (i.e blowers). The easiest way to do this is to have a network of heaters - as many as you can - placed in the coldest parts of the room. The thermostat should always be as near to the middle of the space as possible and at shoulder height. If you have small, enclosed rooms you wish to heat as well, that gets more complicated unless you can add a system just for each room.

It really depends on your space, but I'd guess your best bet is 5000W and ceiling fans blowing downward that kick on when the heater does.

[Boddah2]

Ok ok I knew this would get over complicated haha. Let me start again. Workshop has insulation but probably not what it should. Single pane glass drafty doors yes. Thermostat is built in to heater. I know there are a ton of variables and I’m not rebuilding this thing😀. Just hypothetically basic answer with no real variables... must drive some of you nuts to even think that way. Yes! I will be getting a ceiling fan as it’s nice hot up there! So 5000w is the way to go? Also is it better to leave the temp a little higher all day so that when I crank it up when I come in to work it has less temp to climb?
I hope I made this simple. Thanks!


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[Siress]

Sounds like your heat loss is really high. Best leave it as low as possible when not in use.

[Mr. Hick]

Ok ok I knew this would get over complicated haha. Let me start again. Workshop has insulation but probably not what it should. Single pane glass drafty doors yes. Thermostat is built in to heater. I know there are a ton of variables and I’m not rebuilding this thing😀. Just hypothetically basic answer with no real variables... must drive some of you nuts to even think that way. Yes! I will be getting a ceiling fan as it’s nice hot up there! So 5000w is the way to go? Also is it better to leave the temp a little higher all day so that when I crank it up when I come in to work it has less temp to climb?
I hope I made this simple. Thanks!


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Are we talking about a plug in style space heater? If we are I would caution you against leaving it running on its highest setting, unattended. If you are talking about a hardwired unit with thermostat that's one thing, but any plug in style spaceheater can smoke an outlet and burn your shop to the ground when it's sucking down 15amps almost continuously. Be leery.

[ghilliesuit]

How often do you work in the garage?

With out specifics of the units (efficiency ratings, etc..) - we can only assume the units are equal. This will result in a proportional relationship between the 2 units. 3kW unit will draw and impart 3/5 the energy/heat when compared to the 5kW unit. Conversely the 5kW unit will impart 5/3 the energy/heat when compared to the 3kW unit.

Assuming 3kW is enough to overcome the thermal heat sink that are your garage walls, all the objects in your garage, etc.... and overcoming any loss of heated air ...Then (I think haha) we can comfortably say that there will be no significant difference. The 5kW unit will achieve set point temperature 66% faster. but will on/off cycle more frequently due to reaching set point and cooling down VS. the 3kW which will need to run longer to achieve set point (Longer on periods and fewer off cycles). The monetary impact will be more a ratio of set point temperature to outdoor temperature and the rate of thermal loss (insulation value). The colder it gets, the more you will spend regardless of the unit (assuming equal efficiency) - the most impactful thing you could do would be to make improvements to insulation value.

For reference, the average rate in my state is $00.135 / kWh. The unit consumes 5kW per hour so my /hour usage cost is $00.675/hour. The name of the game would be to reduce runtime. It has a thermostat which is great for when it is ON to regulate. Something which controls the on time is a must. That could be an outlet timer, a smart switch/socket, etc...Reduce set point temp. Increase thermal efficiency. Reduce time in garage (run time for heater). Or install a more cost efficient heat source.

I would use the 5k purely because it will more rapidly achieve set point (allowing you to get to work quicker). Some quick and simple things like sealing up man doors, garage doors, and windows will help. Makes sure garage doors have a seal at the bottom. If your garage has an A frame roof line - make sure the A-Frame structure is separated from the working area by some kind of ceiling. Even taking painters plastic and stapling up a ceiling and taping the seams will help .

I suspect you will be using the garage for less than 16 hours a week (8 hours on Sat and Sun). 16 hours for multiplied by 4 weekends would cost (in my state, on average) ~$45.00. That is for 64 hours of continuous operation (unlikely with a thermostat). You could put a kWh gauge on the outlet to determine what uses less energy over a period of time. One week using it only when you in there. One week leaving it on all day with it at setpoint. Obviously, it wont be a controlled experiment, but it should reveal if there is a significant difference.

Install brand new smoke detectors, if you don't have a known good one in the garage. Get a fire extinguisher. keep the space heater away from anything that could easily catch on fire (AKA dont put it on top of your Gasoline cans, on your cardboard recycling container).

Either way, good luck!

[flyweightnate]

I'm in a similar boat - garage shop, just recently insulated, with a 3kW electric space heater. If you have 5kW I assume gas?

Regardless, I'd leave it at 5kW. The thermostat will keep the room the same temp, but the higher setting means it can get hot faster when you're ready to work. The only downside to having it at 5kW while maintaining 40F is that it's more strain on the heater, so it might wear faster.

Maybe keep it at 3k overnight, but switch to 5 when you get ready to work?

YMMV but I absolutely found the ceiling to be my biggest heat load in my garage (a total mess of cracked drywall until this year), and it made conditioning impossible. Patching it up made a huge difference. I plan to insulate eventually, but that's gotta be paid for. A sweep on the bottom of the door made a big difference, too.

[punkncat]

.02

There are a lot of good points above, so there is that.

IME, even the lowest setting on the heater is likely to keep things above freezing. Sort of impossible to know without square footage and other information, but as reference consider that many people in REALLY cold ass places use nothing more than an incandescent bulb in the well house to keep the water flowing. I have seen similar used for under home as well, in that situation.

Hang a thermometer.

Use the lowest setting and see if the temps stay in a desirable range to reduce the freezing or damage to whatever you are protecting. Adjust as needed.
Crank it when you actually come in the shop.

[Tarsun2]

your heat loss will be higher, the hotter you keep it. (duh, right? lol)

keep it just above freezing until you're in it, then crank it up if you need to.

[Mr. Hick]

Well? Can we have an update? I hate it when things are left unfinished.

[Boddah2]

Well... it’s been kinda mild so far this winter. But I have kept it at 3000w set for just above freezing. Then cranked it up full 5000w and it gets up to comfortable temp in 30-45 min. Also just installed a small ceiling fan. Have yet to check the electrical bill but it’s better than the stinky propane I was using before. Hope this puts you to ease! Haha


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