Garage Ventilation

CT seems like the right crowd to get the sort of input I'm looking for, but I couldn't find a more appropriate section than 'Wiring Closet' to ask this.

long story short: i have an attached 2 car garage that has an insulated rollup door, dual-pane window, and is fiberglass insulated (ceiling as well as walls). above the garage there is attic space (this attic space is not partitioned or compartmentalized in any way from the rest of the attic space above the livable areas of the house). in the summer, esp when there are two recently-driven cars parked inside, the garage is an oven. the cars continue to radiate heat for hours and there's nowhere for the heat to go.

a few months ago i had a contractor cut a couple of vent-sized (14"x5") openings through the exterior wall of the garage, about 6-8" above the floor, which has helped somewhat with air flow. of course this being winter it's hard to get a sense for how much this will help when it gets hot.

i've been intending to also install a ventilation fan that would pull in cool(er) outside air through those holes and exhaust the hot air to the outside, ideally through the ceiling so i don't have yet another hole in the wall at eye-level or above.

the challenge is that I don't want to exhaust the possibly-CO-containing air into the attic where it could get into the rest of the house. a brief conversation w/ my local city building inspector suggested this would be a violation of code anyway.

i came across this product from a company called Natural Light which seems almost ideal for my needs: a roof-mounted ventilation fan that comes with ducting to connect to a ceiling mounted intake vent with a heat-activated damper (close in case of fire) and thermostat-controlled switch. the all-in-one convenience is great as well as the fact that it's made for exactly my situation. the only problem is that it's a solar-powered fan.

now, i don't have anything against solar power per se (in fact i have a grid-tied solar PV system with a number of large panels on my roof) but for this application i think it's a bad idea: there's no battery, so the fan will only run when there's sun. the cars are likely to be put into the garage late in the day when there may not be much sun left (although there is more sun for longer in the summer, when it's likely to be hotter) and i expect the fan will need to run for more time to effectively eliminate the heat. so i'd prefer something designed to be hard-wired, which i could then wire up to be controlled via a relay or somesuch.

i looked around for other exhaust fans: besides the bath exhaust fans (which are too small and probably not suited for longer run times), i found this which is a ceiling-mounted fan that exhausts into the attic (with fire damper), or others which are basically small whole-house fans that also exhaust to the attic. it's not clear to me if one could easily / safely add ducting to one of these solutions and vent it through the roof and i couldn't find any mention of anyone doing such a thing online.

so i went back and called Natural Light, the manufacturer of the first solution. i found out they don't offer their attic fans without the solar panel but learned that the motor is basically a DC motor that seems to accept a range of .

So.. i got to thinking: can i 'convert' this solar-powered attic fan into a relay-controlled fan which is powered via mains power through a DC transformer?

obviously this should be possible, but i'd want it to be safe (code-compliant), and reliable.. esp. able to tolerate high temps (>120*F on a hot day) in my attic. have any of you done anything like this, or have any tips or guidance to offer? can i just buy an off-the-shelf DC transformer like one of these? is there anything else I need to bear in mind? any tips on the best way to make this a relay-controlled circuit?

and a follow-on question: the 850cfm fan requires slightly more sq ft of intake vent than i have cut through the side wall of the garage (1.13 sq ft is the stated requirement vs the 0.9 sq ft that i actually have). it turns out because of the spacing and placement of the studs in the (load-bearing) wall it will be hard to cut a third vent of this size.. i'm assuming i can just reduce the current to the motor to run it at a slightly slower speed to compensate?

thanks!

I don't see any reason why you can't do what you are looking to do except for paying for a solar panel that you won't use. As long as you follow code with the installation of the vent, all you will need is a power supply with a rating slightly above the amperage draw of the fan. I wouldn't worry too much about the intake venting. I would just run the fan at it's rated speed with an appropriately sized power supply. Depending on how good the seals around the doors are, some air may be made up there. I would also make sure that the garage is air sealed with regards to the house, it should be anyway but it could save you a few pennies if you make sure you aren't pulling any conditioned air from the house.

You can use any DC power supply rated for the amperage load and voltage. You could also choose to control the fan with an appropriately rated relay tied to a thermostat or other device like a home automation controller...I see you have an HAI controller.

I don't know rules on the HAI units but you could have something like this:

Whenever the garage door opens
turn on relay x for 60 minutes

or something close to that.

If you have any gas appliances in your garage (hot water heater or funace) be VERY careful as a fan could change the air pressure and possibly cause a problem. I have the same hot garage problem and I an using a solar powered fan with some luck. Just venting to outside the house. Like you mention, the sun goes down when you need it most so I was thinking of creating a battery storage so the fan would run later in the day, but I haven't done it yet.

Things to consider

1) Good point on the gas appliances. Exhaust fans would cause a downdraft in the chimney. We can't run our kitchen exhaust fan while a fire is in the fireplace for that very reason.
2) When you say your garage is an oven, what do you mean? Actual temperature?
3) What is the outside temp during the hours you would be venting? If it is 100 inside the garage and 92 outside, not much good is comming of exhausting.
4) Are you venting the garage to help with the house temp or because you actually want the garage cooler? If the garage only shares one wall with the house, having it 10 degrees cooler probably will have minimal impact on your AC (provided the wall is insulated). If the concern is the heat from the garage and the attic temp rising as a result, why not just vent the attic? There are lots of good ways to do this, many of which are passive (ridge vents with soffit vent).

thanks folks for all the replies!

gatchel: thanks. i wasn't sure whether plug in "wall-wart" style dc transformers of the kind i linked to above were rated to be run at attic temps (i.e. > 100*F). the ones i've been able to find datasheets for talk about the conditions under which they were tested which were generally less than 40*C/100*F. it's a good point about making sure the door between my garage and house is sealed enough so i'm not pulling conditioned air into the garage (or worse, risking air going the other way when my hvac is running).

ano: yes this is a good point. does the concern you raise pose an issue for all gas appliances, or are some designs immune to this issue? i have a gas-powered hot water heater but i don't have a good feel for whether i should worry about backdraft: i was figuring as long as the air vents allow enough air into the garage to replace the air being removed through the ceiling, there wouldn't be negative pressure.. but there certainly will be some air flow. the water heater is probably abt 7-8 years old if i had to guess, and has an electronic ignition but i don't know much else about it.

Lou: answers to your questions follow:
1) interesting. i will have to look more closely at our hot water heater. i've actually been thinking of replacing it so we have more hot water (either a larger water heater, or a hybrid or instant water heater) but i don't know if that would make any potential backdraft situation better or worse
2,3) actual temps: i'm not sure because i haven't installed the HAI temp sensor in my garage yet, but i would guess the temp differential between the inside the garage vs outside is 10-20*F (at the high end of the range on a hot day esp with hot cars in the garage). so even 80*F outside and easily 90-100*F in the garage
4) i'm interested in venting the garage because i want the garage cooler: like a lot of ppl we use part of our garage to store things and the high temps are not good for them. it also makes going through the garage to run trash or recycling out really unpleasant. the garage shares two walls with the conditioned parts of the house but i agree, we don't expect cooling the garage to have an impact on the rest of the house (the walls are insulated). and, yes, ensuring we have good ventilation in the attic is a whole 'nother issue we're working on (we have ridge and soffit vents, i've supplemented with some attic fans, and installation of some baffles to help air flow through the soffit vents are planned as well as installation of radiant barriers underneath the roof to keep some more of the radiated heat out of the attic).

I wouldn't mount the power supply in the attic per say. If you plan on going that route I would put it in a serviceable location and run a low voltage wire to the attic, maybe near the HAI panel, etc. Make sure you wire is capable of handling the current and rated for in wall use if you plan to run it that way.

If there is a gas water heater in the garage, this would be a "no deal" for me. I wouldn't take the chance pulling CO back into the garage. Ignore my earlier statement about having enough venting. Unless of course there would be a way to guarantee the water heater had a dedicated fresh, combustion air supply. Then and only then would I consider it. You could take a pic of the water heater with the vent pipe and I/we could give you a totally non-professional opinion. ( I am not a plumber but I stayed at a holiday in once)

Side story: We recently did some house tightening, new windows, sealed the ceiling penetrations...We noticed that about a week ago we were running the dryer (in the same room as the heater) and the range hood. The heater is an oil burner with the typical barometric draft regulator on it. I just happened to go into the heater/dryer room one day and it smelled horrible. The dryer and range vent were pulling air back down the chimney. Now we open the kitchen window when the range hood is on high. This is until I can get in a makeup air supply for the heater room. The downside to trying to save money on heating and cooling costs, I guess..

OK, so you want a cooler garage becuase you want a cooler garage.

But if you have a gas fired appliance in there you very much can have trouble sucking the exhaust into the garage instead of it going up the chimney. An on-demand heater may be better because the venting on them is powered (has a fan), but it still wouldn't be safe. To be safe, you would need to put your gas appliance in its own sealed area with a fresh air intake from outside.

Alternatively, consider blowing outside air into the garage and let the hot air exit passively. I am thinking you would not want to blow into the garage from a roof vent, however, since it would be pushing the hot air convecting off your sun seered roof into the garage. It would need to be blowing air in from a sidewall penetration. But that is what you were trying to avoid. So, I don't know. The roof venting just doesn't seem like it is a very workable solution.

I have to agree with the others - the attic vent will create negative pressure on your garage; I would imagine a positive-pressure situation would be much better for all the reasons mentioned above. To accomplish this, you'd still want a roof vent, but a passive-one - and in one or both of your existing vents, you'd want to have a fan drawing in air forcing it into the garage.

Around here there is a requirement for a fire-rated barrier between the house and garage. There are actually two layers of drywall on the walls shared by the house and garage plus the entire garage ceiling. I'd check with your local building inspector to learn the requirements in your area before I cut any openings through the ceiling. Additionally, you need to know the fire rating of the duct. A unqualified statement that it has a fire damper without a rating probably won't impress the inspector. If you proceed with a fan after all the discussion about the water heater issues, I suggest you install the fan in an outside wall rather than the ceiling.

Photon said:

Around here there is a requirement for a fire-rated barrier between the house and garage. There are actually two layers of drywall on the walls shared by the house and garage plus the entire garage ceiling. I'd check with your local building inspector to learn the requirements in your area before I cut any openings through the ceiling. Additionally, you need to know the fire rating of the duct. A unqualified statement that it has a fire damper without a rating probably won't impress the inspector. If you proceed with a fan after all the discussion about the water heater issues, I suggest you install the fan in an outside wall rather than the ceiling.

Click to expand...

No double drywall requirement here. But drywall has to be fully intact (no penetrations) except when "plugged" in an approved way.

js19707 said:

1) interesting. i will have to look more closely at our hot water heater. i've actually been thinking of replacing it so we have more hot water (either a larger water heater, or a hybrid or instant water heater) but i don't know if that would make any potential backdraft situation better or worse

Click to expand...

Speaking from experience, if you go with an instant unit, you do not need to worry about back drafing when the unit is on. At least with the one I had, because of the sheer VOLUME of combustion it did in the small amount of time, it had to be installed with sealed stainless steel exhaust and ... I can't remember the intake...I think was PVC...
but it had it's OWN connection from intake and exhaust, was also powered blower to suck/blow the intake/exhaust.

The style you need to worry about, only has an exhaust, which are typically the smaller units. I would recomend against getting one at a big box store, as compared to one as a plumbing supply store...they are crap. The ratings do not match very well either, very undersized.

I do miss my unit. BTW for your edification, "typical" NG water heaters are ~30k-35k BTU units. the "better" whole house instant water heaters (at least the level I bought at) was 150Kbtu. Where I put it, ~35' from the meter. I ended up needing to run 1" lines to the unit. By math a 7/8" would have been adequate. BUT, 1" was cheaper since Lowes goofed up (I paid $2.99 each for 10' pieces). Had one cut down to something like 5-7'. One 90degree, which the 90 counts as 2'...so something like 35-40 foot.

Funny thing, most meters are rated at 250kbtu, so total the load in your house, if you want to use an instant unit at 150kbtu, ensure the rest of the house is less (you mentioned a fireplace 50k, I assume gas oven 75k if all burners on the cook top and oven is running, furnace 55K?). From my guess, that's 180kbtu, then addon a 150kbtu for the water heater...you would overload the meter. Unlike electricity, NG needs to be totaled such that EVERYTHING can on at the same time. They don't want a loss of pressure, since there are that devices count on that pressure to not go out (pilot flames?).

My last house, I ended up getting a 30kbtu fireplace, and my stove was 55kbtu, so I was right on the edge. If you are not sure in ANY WAY, get a professional!!

After I ran everything in my house, I actually called the local utility, since I was so close to being at my meter's limit. They came down with a meter and we measureed the water column level, while everything was running full blast. I was good. I'd suggest the same if you want to DIY.

If you need more, they will put in a different meter (your cost), but typically (from what they told me) it will be a second meter...such as one that would get installed if you had a kiln or something like that in the house. i.e. instead of them dropping the pressure of the NG to less than 1PSI to feed your house, they will run a few PSI to the other meter, which means 500-750kbtu.

Sorry for the rant.

--Dan

I have had ventilating the garage on my list for a few years. I blocked out in the framing for a vent fan through the side wall at the top of the wall. It's on the north side since that is the "back" of the house. I hadn't completely figured out how best to let air in. I have windows on the south side only and could open them slightly (and block them so they couldn't be opened further). Better might be low vents in the wall on the north side but then I would have to run the fan duct for output across the ceiling to pick up hot air across the room.

I have radiant heat in the slab which works great.

Ceiling is fire barrier with 1 layer of drywall. And they allowed a wooden pull down ladder in the middle which pretty much defeats any fire resistance. I actually asked the inspector about that - he said it did compromise the fire barrier but as long as it wasn't completely open it was their standard practice to allow such things.

No gas appliances there so no problem with back drafting.

Don't have any great insights to add but am hoping to get a few ideas for my own setup.

drozwood90 said:

Speaking from experience, if you go with an instant unit, you do not need to worry about back drafing when the unit is on. At least with the one I had, because of the sheer VOLUME of combustion it did in the small amount of time, it had to be installed with sealed stainless steel exhaust and ... I can't remember the intake...I think was PVC...
but it had it's OWN connection from intake and exhaust, was also powered blower to suck/blow the intake/exhaust.

The style you need to worry about, only has an exhaust, which are typically the smaller units. I would recomend against getting one at a big box store, as compared to one as a plumbing supply store...they are crap. The ratings do not match very well either, very undersized.

I do miss my unit. BTW for your edification, "typical" NG water heaters are ~30k-35k BTU units. the "better" whole house instant water heaters (at least the level I bought at) was 150Kbtu. Where I put it, ~35' from the meter. I ended up needing to run 1" lines to the unit. By math a 7/8" would have been adequate. BUT, 1" was cheaper since Lowes goofed up (I paid $2.99 each for 10' pieces). Had one cut down to something like 5-7'. One 90degree, which the 90 counts as 2'...so something like 35-40 foot.

Funny thing, most meters are rated at 250kbtu, so total the load in your house, if you want to use an instant unit at 150kbtu, ensure the rest of the house is less (you mentioned a fireplace 50k, I assume gas oven 75k if all burners on the cook top and oven is running, furnace 55K?). From my guess, that's 180kbtu, then addon a 150kbtu for the water heater...you would overload the meter. Unlike electricity, NG needs to be totaled such that EVERYTHING can on at the same time. They don't want a loss of pressure, since there are that devices count on that pressure to not go out (pilot flames?).

My last house, I ended up getting a 30kbtu fireplace, and my stove was 55kbtu, so I was right on the edge. If you are not sure in ANY WAY, get a professional!!

After I ran everything in my house, I actually called the local utility, since I was so close to being at my meter's limit. They came down with a meter and we measureed the water column level, while everything was running full blast. I was good. I'd suggest the same if you want to DIY.

If you need more, they will put in a different meter (your cost), but typically (from what they told me) it will be a second meter...such as one that would get installed if you had a kiln or something like that in the house. i.e. instead of them dropping the pressure of the NG to less than 1PSI to feed your house, they will run a few PSI to the other meter, which means 500-750kbtu.

Sorry for the rant.

--Dan

Click to expand...

Good info, but I would disagree with equating btu number with quality. You really shouldn't think btu, you should think GPM at a temp rise. You need to consider your local water temp during the coldest time of year. Then subtract that number from 120. This is your required degrees of rise of the water. Then figure out how many gpm's you would want available at max draw. Add up the various things that you would want to be able to run simultaneously. Then check the spec charts for the units and find one that will heat the water fast enough for you.

There are plenty of high quality units at all different btu levels.

It is also common with these units to have more than one scattered around the house keeping them close to the point of use.

OK thanks folks for a very interesting thread, this is all quite useful! FWIW, I believe my current hot water heater is an "atmospheric draft" type (see here for details) and so backdraft is a real concern if i add an exhaust fan. so i'll have to think about this some more, a positive pressure setup is maybe something for me to think about..

on a separate note, i read through another site talking about the importance of maintenance on hot water heaters, and all the various potential risks (lots to worry about). they also have some guidance on choosing a new water heater; they're pretty down on tankless water heaters, fwiw. i can't vouch for the site--i don't know if what they say is true, or if they're actually objective--but it was an interesting read.

i think i'm going to accelerate my plans to replace my hot water heater (i want more hot water than we get now, if two showers are running at the same time it is only a few mins before the water moves from nice and hot to tepid). another tank water heater seems to be the more economical choice, and may perform better based upon what i've read.. i wasn't expecting that last bit.

I'm not sure where they are getting the info. Stuff like saying tankless hot water heater co's come and go and then leave their customers with no warranty. Well their are a lot more mom and pop tank heater co's than tankless. The big players in tankless hot water heaters are not going bankrupt anytime soon. Co's like Rinnai and Bosch are pretty safe bets.

Tankless hot waters are probably not for you if you live in Juno Alaska. If your cold water in the winter is 35 degrees, then tankless is a tough option. If your cold water is 60 degrees, you'll love them.

Tank hot water heaters always leak. Unless you just replace them every 7 or 8 years "whether it needs it or not". When they leak, they can cause huge expense depending on the location. In my 42 years of life, I have had 3 tank hot water heaters blow out. Currently I have 2 tankless in my home, one in my office and one at my inlaws house. Two of them are now 10 years old. They have never seen a single day of service or a single problem.

We never ever run out of hot water. Never.

One feature that is very very nice is the fact that they hang on the wall like a thick picture. Essentially they take zero space. And if you mount them between the studs, then they really do take zero space. And becuase they take so little space, you can put them very close to point of use.

Tankless need no maintenance. There is no anode rod to check/replace and their is no sludge to flush. Tankless continuously self flush. There is also no cocern about legionella.

In new construction, tankless tend to cost the same or less. Tank hot water heaters in TX are usually in the garage and they require their own special closet. Tankless require no such closet. When you consider what your house costs per SF and then you add the cost of that little closet (constructing it as well as loss of sf), the tank style heater is costing you thousands.

A retrofit job is going to be more costly becuase you probably need a new gas line. You'll have to weigh that against the added SF of space you get. If you locate it in the same spot, you won't be moving it closer to point of use so you won't get the bonus of quick hot water. Mountainview CA should be a great climate. I assume it is like SF so you probably don't get very cold.

Don't get a tankless if you think it is going to save you all kinds of money in efficiency. They really don't. They do provide endless hot water, they take up virtually zero space, they don't leak (just don't let them freeze), they can be located in many more locations, and they are zero maintenance. Those are the reasons IMO to consider a tankless. In new construction in a milder climate, I would never do it any other way. In a retrofit, you need to look at your particulars.