Have to totally agree! Very Very dangerous. Get CO/CO2 detectors ASAP! Blowing air into the room could very well force CO/CO2 into the house. I DK what the best solution is, but I like MonkeyZeus's suggestion about forced of an active blower.

House is not totally sealed. When the pressure is reversed like when it's cold out, I have no problem drawing air for the water heater, propane furnace, and 2 wood burning stoves while keeping all windows and doors closed. What seems to be the problem is when it's hotter outside mid-day in the summer the cooler air inside creates pressure not allowing fresh air to come in.

So you think just drilling the hole into the garage and maybe cracking the garage window would do it (not needing a fan to blow inside it)?

@FranklinGray Sorry, I'm not trying to split hairs. Totally sealed or not, summer or not, whatever; that room is not getting enough fresh air, period.

@FranklinGray Is this a new house build? A new water heater install? I'm confused how it passed inspection.

No, it's 41 year old house with an old heater. I bought it in Feb and had it inspected in Jan. It's not a problem in winter....or even at night. Just during the hot days. I totally agree though that it's not getting fresh air. Maybe if I can seal off the room from the rest of the house and drill those holes it will solve the problem. The issue is sealing off where the water pipes and furnace air ducts go.

@FranklinGray Maybe your chimney is undersized for two heaters. Maybe your chimney has a dead animal blockage. Maybe in the summer time there are strong cross-winds which push air into your chimney. There are far too many maybes but IF you plan on tackling this yourself then at least properly test your attempts by performing a water heater chimney draft test such as youtube.com/watch?v=AZCWEonZvUs

Like right now. I don't smell the exhaust like I normally do in mid-day, but it's 77 degrees inside and 66 degrees outside so the pressure is stronger outside. Wind now is like 3 knots...almost non-existent. I took a closer look at the water heater...it is 10 years old. An OA Smith. What I found odd is it has electrical wires going inside from the pilot light control box, but I couldn't see any external wires going to the heater and I could see anything talking about a battery. I would expect if there was a battery there would be something saying open here for battery.

You're detecting this exhaust by smelling it? The thing that makes carbon monoxide so dangerous is it has no smell. Neither does natural gas, until they put a chemical into it to make it detectable. Have you done draft tests? Do you have proper ventilation into this room (a vent, fan grille, etc)?

The weather is getting much cooler this week so I doubt I will have this problem anymore until next summer. I guess I will have to wait till then to hire a professional to come out as it's hard to fix a problem that isn't a problem at time of inspection. He would only be able to guess, but yeah, I like the idea of drilling a 6 inch hole with screen to keep the field mice out and sealing the room from the house. At the very worse, just the garage will get the fumes and that is easy to clear by just opening the door.

There is no vent into the room just the 3/4 inch gap at bottom of the door. Yes, I can smell it. It is propane and I believe they put extra stuff in so you can smell leaks but it doesn't burn clean. I know it's the exhaust smell because when I was up on the roof installing an antenna and the heater was running it was very strong blowing right into my face. I got a propane leak tester which also tests for CO2 and it didn't register anything when the house was full of the smell.

But the tester manual did say it would only detect large quantity of CO2.

So I guess we all agree that I should install a vent to the garage and see if that fixes the problem, right?

@FranklinGray I assume CO2 is a typo; carbon monoxide is just CO and odorless. Anyways, the answer is to supply adequate fresh air to that water heater year-round or else the next visitor to your house could be a coroner.

I think I might have found the previous owner's solution. There is a 2.5 inch hole drilled into the wall that leads to the cabinet under the sink in the bathroom. The bathroom has a vent with fan on it. I guess they just opened the cabinet door and closed the bathroom door with fan off. It will get up to 78 degrees this weekend so that might be a good day to test that theory. Thanks all. At least I know now that it's a venting issue. Do most houses have a vent coming down into that water heater/ furnace room from the roof as well as the chimney?

sorry to be spamming this site, but now I'm getting a little confused. When you guys say 'vent', are you talking about the chimney where the exhaust goes out? I thought you were talking about air intake, not exhaust. When I watched a video talking about how to vent a water heater, it referred to the chimney as the vent.

@FranklinGray It doesn't help that the definition of vent is ambiguous. Vent: an opening that allows air, gas, or liquid to pass out of or into a confined space. The chimney is a vent pushing bad air to the outside. You are in need of a vent that supplies that room with fresh air.

@FranklinGray "Do most houses have a vent coming down into that water heater/ furnace room from the roof as well as the chimney?" Generally no. That is a cumbersome configuration which puts more holes in your roof than necessary and has the ability to intake the bad air which is being exhausted. Fresh air is usually taken in through the side of the house via wall or rim/band joist.

I think my problem is a combination of a bunch of things: 1) chimney is a shared chimney with furnace with lots of turns. 2) water heater chimney pipe is about 1/2 inch smaller than the hole on top of heater 3) draft dome isn't all that wide and maybe too far off the heater 4) utility room not vented for incoming air well 5) house is kind of sealed. I do have to open the door when first lighting my stove downstairs that has a chimney that goes outside and up the wall. Upstairs where it has a straight chimney I usually don't have to open the window but sometimes I do.

so lots of resistance (turns and lots of pipe going from basement to roof) to rising exhaust, easy escape of exhaust at dome and hard to draw fresh air to replace exhaust. These are the same issues I have when starting a fire downstairs. Before learning the tricks I used to smoke the house up half the time trying to get the smoke to go up the chimney.