Hi gents,

 

I have a 3 car garage which is part of the house (a bedroom above and the wall borders the dining room).  It is unheated and that’s not been a problem, but now with the boat sitting in the garage I’d like to heat it some in the winter to take the chill off so I can be out there working on things.  We aren’t the frigid north where it is 0 degrees outside.  For the most part it is in the 20-35 range outside which pulls the garage to 30-40 inside.  I am going to insulate the garage doors to mitigate temps better and that will give me a couple degrees.  But I want to add a heater to try to hit 55-60 degrees or so (70 would be even better).  It would probably even  lessen my main furnace heating a little to keep the garage a little warmer.  

 

I have a 220 dryer plug that is a 14-30.  That limits me to about 5000 watts (18k BTU give or take) on an electric heater.  I might be able to get that to 7500 W if there is a 40 amp breaker already there, but I don’t think so.  Anyone else here run a 5000W heater that can input on how effectively it can heat up a garage?  Online guidance seems to say that 30k BTU is about right but since this is part of the house I suspect it can do with a little less.

 

For less money I can get more BTUs in a propane heater, but I don’t want that for an enclosed space with no ventilation if I can get away with an electric option that I just turn on the switch.  

 

Any thoughts?

 

thanks,

rick

 

For homes a general rule of thumb is 10 watts per sf based on 8’ ceilings. I don’t know the size of the room

 

You can probably hang an electric unit heater off of the 30 amp dryer circuit. 
 

EDIT:

I said 30 amp circuit, I know you have 40. Your connected load shout never be 100% of the circuit breaker rating. Design/connected load should be 75% of the circuit rating hence the 30 amps

Ceiling fan will also help. I have a fully insulated 3 car with my boat in there (no heat) and the circulation greatly helps in the winter time. I also run it in the non winter when I’ve been fishing in the rain and need to dry everything out.

 

I have a natural gas pipe in the corner of my garage and have considered adding a heater too. From my research, a 30,000 BTU is what I would need.

@Jigfishn10- I double checked and it is a 30 amp breaker as I suspected.  I didn’t check wire gauge, but I would wager the builders didn’t put in a heavier wire than necessary, so I doubt I can get to a 40 A circuit.  I was looking at 5000 W units because nominally 5kW = 21 A draw which is just right on a 30A circuit.  7500W is 32 A and needs a 40A breaker that I’m not sure I am comfortable doing.

 

The garage is nominally 35x20 with 14’ ceilings. I haven’t measured height, but my 12’ ladder leans against the wall almost upright with some more space to go.  At 10W/sqft that would be 7000 W which is about what I was seeing elsewhere online.  That’s the calculation that is making me think about putting in a 40A breaker (only if the wiring can handle it).  That said, my question is if that 7000 W is for a standalone building and bringing it to full temp (72ish), then will 5kW be okay in my situation and I don’t have to swap breakers.

 

@gim- adding a ceiling fan is a non starter.  That would mean running wire or at least cutting in a box and pulling power from a garage door opener.  That’s more work than I’m willing to go to.  I’d just get a propane torpedo at that rate.  I wouldn’t mind it for the drying air circulation.  Right now I keep a dehumidifier in the garage that I put into the footwell of the boat when I get wet (plus a separate fan blowing under the seats.  It works, but a ceiling fan would be a nice add.  That said, as much as I have the ability to scab one in, I think running the same fan and dehumidifier with a propane heater (or wall heater) in there would be good enough.  

 

Is your garage connected to the house or separate (by walls)?

7 hours ago, casts_by_fly said:

 That said, my question is if that 7000 W is for a standalone building

Standalone building? Does it have an electric panel in it?

 

EDIT: Disregard . I just read your opening post again

When it is colder than I want to be in I put an electric space heater in my garage and point it toward where I am working.  It isn’t great but it helps.  
 

My garage borders our sun room.  I have considered cutting a hole in the wall and adding vents on both sides but haven’t seriously looked into it.
 

 

I had a boat garage connected to the main house by roof only via a covered patio. It was 28x26 with a 10 foot double overhead garage door. Ceiling height was around 14 foot. It was super insulated and had a ceiling fan for air movement. I had a gas heater installed but it was ventless which became a problem because of moisture build up when I stored the boat wet. I ended up with a mildew problem which required me to repaint the walls after mitigating the mildew. So my only caution to you is make sure the electric heater does not cause a similar problem after insulating. 

Based on the information above, you are probably looking at:

2 - 20amp 2 pole 240v circuits

7 - 48" 240v baseboard heat

1 - T-stat or you can do local control

 *  Above is based on 8' ceilings and a rough guess. In no way should this be used as a design ... more info is needed to come up with a design

 

Add 2 - 48" 240v baseboard per additional 2' of ceiling hgt

 

Seems cheaper to drain the fluids and fog the engine on the boat on yearly basis

 

Good luck

 

9 hours ago, casts_by_fly said:

Is your garage connected to the house or separate (by walls)?

 

Its connected.  Shared walls on the back and part of the side.

 

I wish I had a drain in there.  When my vehicles come in with snow, ice, and debris from the roads here in the winter, it melts and I get puddles on the floor.  I squeegee it out every day but a drain would help that issue.

On our previous house I simply used a 1500 watt infrared heater as temporary heat.

If I had plans for a busy day in the garage I'd cut it on the night before set at 60*...... cheap and effective but not as a daily routine.

 

On our current retirement home knowing that the garage would be my hangout, went with radiant heat and a drain in the floor so I could wash vehicles and start the boat.

The garage keeps my sanity in the cold winter months.

 

I do like the idea of cutting vents in the wall which would allow the use of a fan but you'd need a 2nd vent to act as a return.

They make large vents with louvers so you could close them if desired.

you can get a 30-36K BTU mini split for like 2k or less. Should be able to get by with a 30amp breaker. If you are limited to 18K BTU or lower the cost is only about 1-1.5k depending on the unit.

 

Youll get AC and heat with this too. 

Mini split would be the way to go.  My garage is just for cars (boat won’t fit) and if I need to do anything in the cold, I have an upright kero heater that will sweat you out.  I use it for emergency heat in the house when needed.  

Hi gents-

 

A mini split would be great, but I’m not spending $1k on this for all I need.  A plug is portable turbo heater is a $150 spend.  A wall mounted unit is in the same general ballpark and it would be off the ground and out of the way.  A propane turbo heater is half that.  So this is the spend range I’m looking at.  

Could always find an old pellet stove on Craigslist and plumb chimney through exterior wall horizontally. There are lots of them on my local Craigslist for $300 to $500.

 

We use one in our 40x70 steel building that our batting cages are set up in.  Works really well to heat that space.  I have one in my personal living room as well.  Heats up the whole house.

I’m considering this one. With the additional fan. It’s vent free.

 

https://www.menards.com/main/heating-cooling/heaters/gas-wall-heaters/dyna-glo-trade-30-000-btu-dual-fuel-vent-free-convection-wall-heater/bf30dtdg-4/p-1559111496867-c-6867.htm?srsltid=AfmBOorqxNrDNWVlLRO2tTb4IUEJPeZZijd4DfRqmSc_g4gjlcP5DF4N

Wood stove ftw. 

 

This is corded but you may have to change the dryer outlet device. Wire can remain as long as it has a neutral leg. I would also consider a fan as @gim mentioned just to move air around. I'm also thinking that you want to avoid anything with a flame inside a garage. But that's just me

On 1/4/2026 at 6:47 PM, casts_by_fly said:

For less money I can get more BTUs in a propane heater, but I don’t want that for an enclosed space with no ventilation

The Mr Heater 'Buddy' line is rated for indoor use...no ventilation needed. Available on Amazon or from Home Depot, Menards and Fleet Farm

I decided to go with the propane turbo heater (60k BTU). As much as I’d like to have it hung up and out of the way with a thermostat, I would be practically limited to 5000 watts which is about 17k BTU. Based on all the calculations, that would be pushing the limits to heat a 3 car garage with 14’ ceilings. Instead, for the same spend I can get 4x the BTU and give up some of the convenience. I’m confident that 60k BTU will heat it up pretty quickly and being propane there should be no smell and minimal risk of noxious gas if using it with the doors closed. I already have 2 propane tanks and propane is easier to manage than diesel/kerosene (neither of which I have cans for already). I’ll deal with the floor space it takes up (in the winter) and in the summer I’ll put it in the basement.

I’ll have it next week so I’ll report back on how it does.

I meant to suggest something like that, I’ve seen them be fairly effective at work (if it’s what I’m thinking ). They are loud but nothing a little Bluetooth can’t conquer

I've got one that I run in my personal shop at home. Thing sounds like a jet engine but it will heat that shop up in no time.

Yeah, that was my thinking. 60k BTU, turn it on for 15 minutes before I go out there to let it warm it all up, and then shut it off. Put on some tunes and go to work.

The heater arrived today. No assemby required other than connecting the hose to the tank and to the heater so off to the garage I went. I threw it on high for 15 minutes and when I came back out the air temp was easily 10-15 degrees warmer. It was enough that I would have been able to work out there in normal clothes and not wearing a jacket. The air temp before heating was 39 per the car which had been sitting in there all night. Of course the boat, the car, and the concrete slab weren't heated up that much so when I shut it off the air was back down to 41 about 30 minutes later when I went back out.

Considering I already had a propane tank that was full enough, this seems like a just fine solution for $100. I've got some work to do on the boat so we'll see how it does for an extended period over the next two weeks.

@casts_by_fly It should do perfectly fine. I used to use one of those in a garage to fix dirtbikes and cut rifle barrels in half to full day stretches up in middle NY during the winter. As you said, sometimes what you are working on takes a while to thaw out but you can get it very comfortable with one of those quickly.

Every so often I did crack a door though to get some fresh air in.

I can confirm that it works perfectly well. We got a foot of snow this weekend over a couple stretches. I also was planning to fish Sunday morning so had some boat organizing/rigging to do (ended up not going). It takes about 15 minutes to get the temp from 35 to 'liveable' which I would guess is 55-60. Another 15 minutes and the boat hull is warm to the touch and the air is upper 60's. It isn't heating the concrete slab, but I didn't intend for it to. When I was snow blowing, I turned it on when I left the garage so that in 20-30 minutes when I came back in the garage would be warm-ish for the blower to melt the remaining snow off and for my outer garments to dry out.

If the plan was to have a garage workshop in this scenario where I was spending more than an hour our there at a time or if I was working in heavy metal equipment that holds the cold, then a wall mounted heater with a thermostat would be a better option. Once you get the walls and floor warm it could kick on and off with air temp changes and the metal equipment stays fairly warm. The propane heater doesn't have the wattage to heat up a UTV for instance if you needed to work on it. But pulling in the front of the truck to change a headlight? Yeah I'd just put this heater on in the area and take the chill off.