Does parking a recently driven car in an attached garage warm the house?

[burritos]

Seems like it to me. So does it make sense to leave the garage door open in the summer and closed in the winter?

Also, do attic air fans provide negligible benefits when trying to rid the house of heat?

[icarus]

I think that the premise that warm cars can contribute to the heating load of a house is absurd. The garage SHOULD be insulated from the heated envelope of the house. There might be some (so small as to be a rounding error) reduction in heating load because the garage temp is marginally raised so that the Delta T between heated and un-heated spaces would be reduced but hardly worth noting. If on the other hand you could head a couple hundred gallons of water in the back of the car instead of sending the head out the Radiator, then plug that barrel of water into the boiler in your house you might have something. Of course you would have to calc the energy wasted carrying around several hundred pounds of water.

As for attic fans reducing summer cooling loads. The answer is very much yes. A well designed and installed attic fan can drop attic temps by as much as 50f. That same Delta T between the cooled living space and the heated attic is dramatically decreased, much more than the Delta of the garage example.

Even if you don't air condition you can keep your house much cooler buy cooling the attic. Some great ideas include drawing air out the attic and drawing it in from the cool basement. Not a perfect solution as the basement air can be quite humid.

Icarus

[qbee42]

There are good reasons to ventilate an attached garage. Air quality studies have shown that attached garages are major contributors to indoor air pollution. Hydrocarbons from the fuel and other fluids infiltrate, as well as exhaust gases. Attached garages are also fire hazards. Convenience and lot space usually trump safety, so most houses have an attached garage, if they have one at all.

Tom

[burritos]

Originally Posted by icarus

I think that the premise that warm cars can contribute to the heating load of a house is absurd. The garage SHOULD be insulated from the heated envelope of the house. There might be some (so small as to be a rounding error) reduction in heating load because the garage temp is marginally raised so that the Delta T between heated and un-heated spaces would be reduced but hardly worth noting. If on the other hand you could head a couple hundred gallons of water in the back of the car instead of sending the head out the Radiator, then plug that barrel of water into the boiler in your house you might have something. Of course you would have to calc the energy wasted carrying around several hundred pounds of water.

As for attic fans reducing summer cooling loads. The answer is very much yes. A well designed and installed attic fan can drop attic temps by as much as 50f. That same Delta T between the cooled living space and the heated attic is dramatically decreased, much more than the Delta of the garage example.

Even if you don't air condition you can keep your house much cooler buy cooling the attic. Some great ideas include drawing air out the attic and drawing it in from the cool basement. Not a perfect solution as the basement air can be quite humid.

Icarus

The temperature of our garage(after car use) is warmer than the outside temperature especially throughout the night. The 3 bedrooms over the 3 garage spaces(both sets east facing) also on average stay warmer than the west side of the house. Now the cars might have nothing to do with this occurrence, but the effort to open the garages to remove the delta T is negligible. In the same vein of the attic phenomena, if the sun can heat the roof and warm up the attic space, why can it also not do the same thing to the face of a garage? Also, could you imagine how hot that attic space would be if there was also a just used car trying to cool down?

[icarus]

Denis,

You would have to do some pretty complicated thermal testing to figure out if the garage was gaining more heat from the car or from the structure.

Remember, the heated space will lose heat from the walls into the garage, and if the garage is cooler than the bedrooms above, the bedrooms will lose heat through the floor into the garage. The net/net is that the garage will be warmer than the outside, if for no other reason that the thermal mass is warmer than the outside air.

My point is I wouldn't try to heat my house by parking the car in the garage.

Icarus

[hill]

I'd guess maybe 1 out of every 5 homes (besides ours) on our street are cracked about 6" open during warm nights like last night ... where it went from 90 degrees in the daytime, down to only 72 degrees at 5am today.


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[EZW1]

Im an electrical engineer, Now, I'm not premising what I'm about to say that my words are cast in stone, just wanted to reflect I'm offering a professional opinion.

As far as heating the house, unless you don't have any insulation between the garage and the house, I wouldn't worry about the heating effect. Thermodynamic equasions factor temperature differential, volume of mass absorbing the thermal energy, temperature coefficient of the mass, etc. Likely the hottest parts of your engine when you shut it down are the exhaust manifold, catalytic converter, and cylender head. They are likely sitting around 400F, 350F, and 200F, respectively. Their volume and mass is quite small compared to the volume of air in your garage - especially if you have a two car garage. Now, the difference in temperature between the average of the hottest components in your car and your garage is about 200 to 250 degrees. As your auto cools it will heat the air and my guess (with out conducting any calculations) is that when everything equalizes, your garage temp will rise about 40 degrees. Now, how much of that 40-degree rise will get into your house? Nearly nothing.

So the motto is: don't worry.

[jayman]

Originally Posted by qbee42

There are good reasons to ventilate an attached garage. Air quality studies have shown that attached garages are major contributors to indoor air pollution. Hydrocarbons from the fuel and other fluids infiltrate, as well as exhaust gases. Attached garages are also fire hazards.

Hence the need for a properly designed and installed HRV that keeps slight positive pressure on the house.

At one time, attached garages were required by code to have a cement (Usually cinder block) fire wall between the garage space and the house. This fire wall extended up to the roof. There was also a fire rated steel door to the house

Now, code only demands type "x" 5/8 drywall as the fire wall, and in most new construction it does NOT extend up to the roof. The fire rated door is steel clad, but not as heavy/tough as a commercial/institutional door

A lot of attached garages around here are uninsulated and bare studs on the inside. In my opinion, that is incredibly dangerous. As the soffits are fully vented, if the vehicle were to catch fire while parked inside, it would be like a blast furnace effect

I would have a cement wall between an attached garage and the rest of the house. The garage would be fully insulated with Roxul mineral wool insulation, and type "x" drywall over top

A properly insulated garage would have minimal effect on the house temp. Improperly insulated, I would think in wintertime the cold would be a lot more noticeable than heat gain

[hill]

Originally Posted by EZW1

Im an electrical engineer, Now, I'm not premising what I'm about to say that my words are cast in stone, just wanted to reflect I'm offering a professional opinion.

As far as heating the house, unless you don't have any insulation between the garage and the house, I wouldn't worry about the heating effect. Thermodynamic equasions factor temperature differential, volume of mass absorbing the thermal energy, temperature coefficient of the mass, etc. Likely the hottest parts of your engine when you shut it down are the exhaust manifold, catalytic converter, and cylender head. They are likely sitting around 400F, 350F, and 200F, respectively. Their volume and mass is quite small compared to the volume of air in your garage - especially if you have a two car garage. Now, the difference in temperature between the average of the hottest components in your car and your garage is about 200 to 250 degrees. As your auto cools it will heat the air and my guess (with out conducting any calculations) is that when everything equalizes, your garage temp will rise about 40 degrees. Now, how much of that 40-degree rise will get into your house? Nearly nothing.

So the motto is: don't worry.

Nearly nothing? I duno ...
If the garage & all the rest of outdoor temps are 90 degrees ... and the car raised the temps 40 ... that's 130 degrees. Let's say it's only 120 though ... just for grins. Many attached garages have direct access into the house. Seems to me that heat willll get sucked into the house each time the door gets opened ... like leaving the refrigerator open. Our home office and service porch open out to the garage. WHAM ... in comes that 120+ degree heat, maybe a dozen times a day, so that the office easily hits up to 80 degrees, real quick ... even though the rest of the down stairs is centrally cooled to 76 degrees. Ergo, we crack the garage door open 6" or so. Plus, we leave the side door open ... for cross ventalation. We only park one car in the garage ... and sometimes, to difuse that car's heat, I'll park it out on the street under a tree until it cools off.

Your results may vary.

[jayman]

When I still had my Prius, I never really noticed a huge rise in garage temp. Even on a +38 C day, driving in and powering off, the garage temp *maybe* climbed +10 C. The Prius motor doesn't run very hot

The FJ is different. On a +28 C day, the garage gets toasty after I park inside

I don't like leaving the garage door open. Skunks, stray cats, etc, are attracted to your garage