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

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

 

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

 

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

 

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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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.

 

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

 

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.

 

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

 

Even without a car in it, during warm sunny days, the temperature in the garage will rise, in fact, if the garage doors are any color other than white and have full sun on them for the better part of the day, the garage will get pretty darn hot. My garage is uninsulated open rafter next to my house with a brick wall separating them. It gets hot in there. OTOH since it is well ventilated, parking any of my cars, or all three after driving them the rise is negligible, it just stays warmer longer.

 

My garage door is R-10 insulated, with "arctic" weatherstriping. The door faces south, and even on the hottest summer day, the interior of the garage does NOT heat up. Door is standard white

 

My garage is huge, about 30' deep at one end, 26 at the other end, and about 40'+ wide. It is uninsulated, no ceiling, just open rafters, but the doors are insulated, and weatherstriped, but they are a light brown in color and absorb the suns heat like crazy, plus I have a dark brown roof. It gets mighty warm in there. Some day I do plan to insulate and drywall the garage, but I have so many other projects to do, that it will be years until I get to that point. I am still working on getting my roof done after 3 years. I dont trust anyone else to do it the correct way, the last people who worked on it did such a bad job I wound up replacing part of the plywood due to rot. Last two weekends were spent rebuilding one section of roof so that water would flow off instead of swirling around and backing up where a valley met the gutters. Not only that, but they didnt weave the shingles but laid over from one side to the other and cut down the valley, and in every valley, leaves and debris got caught up in the grooves and held water, damaging the shingles. When I am done, that will never happen again. I have replace all the utilities with energy efficient ones, both furnaces are 98+, both LP water heaters are tankless, the only tank one I have is a 50 gallon electric under the kitchen, and double wrapped with insulation.

 

I am curious about the 50 gallon water heater under the kitchen. What is it used for, and why won't the demand water heaters do the job? If it is only for kitchen use it sounds kinda big, not to mention how big is your house that needs a 50 gallon kitchen water heater PLUS 2 demand Lp heaters?

Icarus

 

That does sound big just for the kitchen. Maybe he meant 5 gal? That's a common size to be found under a utility sink

 

Unfortunately the average builder doesn't work like that. If they can save $50 on insulation they will. Result is that there is no insulation in the space between my garage and the bedroom above it (seems to be common here). So it is not absurd to consider the effect. Between carpeting/padding, floor deck, drywall, and air space it is probably R3-R5. The floor is not hot or cold in winter, but that doesn't mean losses are not occuring.

As you say though the impact is reduced by the fact that the temp of the garage is not as extreme as outside etc. However, I've had good results opening the garage fully during summer whenever the weather cools sufficiently.

Heat in the garage can go one of two ways: external surfaces, or internal surfaces (into the home.) When major internal surfaces are uninsulated then an increased fraction will go in that direction.

Parking several thousand pounds of heated material (at perhaps 100 F average temp. on a bright summer day) is going to have a measurable impact.

In winter much of the shell of the vehicle will be cold by comparison. So the engine heat is unlikely to make up the difference of that cold mass. In winter I keep the garage doors closed as much as possible.

 

I've seen that in 1920's construction. Unfortunately, builders p*** and moan about cost until intelligent things are eliminated from code. If we had homes the way builders wanted to build them they would be completely uninsulated.

As it is insulation and HVAC installs are typically half-assed affairs. I located the vendor who did the ductwork here and, without telling them this, I asked a few probing questions in order to find out who the best supply house was in the region. When I told the person I had been tightening the ductwork, she couldn't understand why that would be helpful. Pretty much a lost cause trying to educate such folks...afterall the family has been in the business "since 1943." Which begs the question, to they have 66 years of accumulated experience, or 1 year of experience 66 times over?

If/when I tear out the drywall ceiling in the garage and insulate it, I also need to insulate the ductwork there. The long runs are uninsulated of course and one (possible two) appears to be leaking condensate onto the garage ceiling below. I've already completely replaced another run that was resulting in lots of condensate in a closet below--replaced the sheet metal with insulated flex duct.

 

Enough Science has been applied to this question, I'd like to chime in, with fictional nonsense.

Those of you old enough will remember the episode of Happy Days where Fonzi almost died living above the Cuninghams Garage when Mr. Cuningham left the car running by mistake. Alright, you won't remember because it never was an episode, but I think it would of made an interesting one. My point being, living in a garage was good enough for Fonzi, you never heard him complaining of "heat" issues. He evidently was able to support an active social life. I don't remember a space heater being present, and this was suppose to be in Millwaukie.

Conversely, The Waltons at one point burned their entire house down. (Real Episode) and they didn't even have a garage, attached or otherwise.

So if we look at vintage episodic telivision of the 70's I think we can conclude that garages are magical realms best left alone or inhabbited only by Iconic leather jacketed hoods.

As far as any "heat transfer" benefits? All I can say is Good Night John Boy and Aaaay!

 

My house is a very long ranch, the kitchen is at one end, too far from the bathrooms where the tankless is. There is a room addition, a so called mother-in-law suite at the other end, and it is close to the kitchen, but the design of the basement prevents me from running a hot water line to the kitchen, and it would take minutes to get the hot water to the kitchen in either case. The reason I have the 50, I think it is fifty, it could be 35, it's squat, no more than 40" high, but it is over 2' in diameter, is because I had one of those tankless electric ones that could not heat the water up high enough to be of any use and cost a boatload in electric. I replaced it with a high efficeincy tank. The one I got used less energy than all the smaller units available. It was either this one, or a 2.5 gallon one which I know would be drained in no time by the mope who washes the dishes. I wanted a squat one so that I can build a cabinet/bar around it in the future and have hot water available downstairs too. I am using about 2/3rds less energy with this heater, than I used with the tankless one.

 

Got it,

There are circ. systems out there with sensors that work with demand water heaters. They sense when you come in the room, fire the water heater and the pump so that by the time you open the tap, the hot is right there. (Probably not cost effective in your case).

Icarus