sealed combustion furnace draws air from inside

csab_

100% relative humidity in 0 degree (celsius) is equivalent to about 30% humidity in 20 C. Outside air is almost never 100% humidity, and the temperature difference can easily be higher, so at the end you are drawing in 20% or lower humidity air, even if it's very humid outside. The water vapor that you create inside with cooking, bathing, even breathing, is sent out on the exhaust. So it makes sense to me that a system like mine would create dry air inside.

I found the manual of the furnace. It says that combustion air MUST be taken from outside. So much for the installer. I am gathering tools and material. The toughest thing will be to cut that 3.5 inch whole through the thick wood frame and the brick veneer, where there is little space available to work.

csab_

I've just finished the job today. It basically took all day (I'm slow and careful), but I'm so glad I did it. The hardest thing was to drill a 3.5 inch hole through wood and brick veneer. I had to buy a bunch of tools, plus the pipe of course, but it still only came out to about $100. (And now I have those tools.)

My CO monitor registered 25 ppm peak through the last few days. Although it is not hight enough to warrant an alarm, it is still disturbing. I reset the monitor and I hope that with the new system, my peak will never go beyond the detection threshold (10 ppm).

how

CO is produced by all gas heating systems. The venting systems attached are supposed to remove them. What do you think is the source of your 25ppm in your house. On another post you said you had no spillage from the HWT when your furnace was running and your detecter was "0".
This would hold my unabated attention in my house until I found out.

csab_

No idea. The only non-electric appliances are the furnace and the water heater. If it's one of the two, I'm leaning toward the old water heater that is vented into an old chimney.

That's right. It didn't seem like there was any spillage with the water heater and the furnace on. But that was only one experiment and I'm not that experienced to determine this. The detector was left in the furnace room for one night and it showed 0 in the morning. Then I moved the detector close to the bedrooms and it has been there ever since. I checked the "peak" today and it was 25.

One thing that was different that it was a lot colder in the last few days here. I'm talking about 20F as opposed to like 40F when I did the first measurement in the furnace room.

You bet I'm going to do the same!

how

I might of mentioned this before but the HWT spillage test should be done with the tank temp turned up to make it fire, the furnace also firing, all the windows and doors closed and the clothes dryer running.
Spillage isn't subtle from the draft opening at the top of the HWT. Your fingers will be cool at the vent opening is it's OK and uncomfortable hot if your getting spillage.

csab_

I don't have a clothes dryer.

When I did the test last time, both the furnace and the heater were on.

I repeated it today, and the matchlight was going straight up, maybe leaning just a tiny bit toward the flue. I can definitely keep my hands under the skirt of the flue indefinitely (when both the furnace and the heater are firing): all I can feel is the subtle warmth radiating from the metal flue.

But of course this is after the furnace intake is fixed. So maybe I did have (occasional) spillage and hopefully I won't any more. Let's see if I get any more positive CO readings.