Please help!! CO leak in the system . Can't find it.

Long story short, I have a tankless water heater (Bosch 240) and Bryant gas Furnace (not sure of the model)

We have been having trouble with our gas fireplace and started to be concerned about CO in the home. We had CO detectors and none ever went off but when we upgraded to digitals, the numbers started scaring us. Consistenly showing up upper teens to around 10 on the meters with spikes in the 20's.

Okay, so we completely turn off the fireplace...gas valve and all and think we are done. NOPE, meters still are showing 10-17 with a an hour or so of zero readings then back up again.

Next step was to purchase an Extech CO10 Carbon Monoxide detector and sweep the system in the basement with the furnace on first.....nothing and then the vent piping on the water heater.....nothing!!

With the Extech meter, I have never picked up more than 4ppm upstairs...... thanks with the water heater cranking in the basement. Here is the strange part......most CO readings are coming from upstairs and NOT the actual room in the basement the units are in.

Couple direct questions as I know I have been ramblng a bit.

1) Can a good HVAC guy diagnose CO problems or is their someone else I should call about finding the source?
2) Can CO rise that fast like helium or something? Can't figure out what the heck I am picking up CO in the upstairs (above the units) and not in the actual room they are in.
3) Have you ever heard of a water heater/furnace vent pipe leaking inside walls heading for the roof? House is a newer cookie cutter. 2400sq foot 2 story with basement and double flu going up from basement.

Thanks for any input. I gotta wife and 2 daughters that are freeaking out on me and I gotta admit, I am a bit nervous living in a home with even small amounts of CO in it.

Need to call someone asap and just wanted to get input.

Thanks guys!

Not seeing the whole house, here are just some off the wall possibilities:

Backdraft of a roof vent into a kitchen or bathroom exhaust vent.
Leak of a flue pipe in an attic area.
Leak of a flue pipe in a wall area.


Call Dick Tracy, because you need some detective work!

Well update. Went to Lowes and wrapped every joint of the water heater flue with foil tape.

So far house has been reading 0 all day.

Let's hope it holds.

Wanted to apologize for being such a rookie on all this stuff. You guys must get a kick out of us DIY guys.

Doesn't make any sense compared to my explanation above (1st post) but for now it's working so keeping fingers crossed.

Flue pipe should not need to be taped at the joints, so I am leery that there is a backdraft issue.