Whats your ambiant CO2 in your grow room?

I just got my CAP PPM-3 tonight. This is my first dive into CO2. I went down and hooked it up in my room which is like 7x7, in my basement. I dont actually have any CO2 or a generator or anything, just the meter. I do have a NG hot water heater down there, but its 2 rooms away. The meter cranked up to 1430ppm and settled at around 1350ppm. It said in the instructions that it should be between 250 - 400 outside and "as high as 1500 indoors". Did I just happen to hit the CO2 jackpot? Or maybe I have a Carbon Dioxide/Monoxide leak. I should probably check the ppm or atleast install a Carbon monoxide detector down there. Is like a "natural" ppm of 1350 unusual? What is your C02 levels, especially if you're in a basement, when you're not pumping??

My grow in my basement with an oil burner and hot water heater running quite a bit cuz it's winter here in Philly, is about 600 on my COCO monitor/controller.
I know 300 is usual for outside air and just for fun I breathed into the monitor for a bout 30 seconds. It went off the charts. It registered like between 4 and 5000. No wonder people say you should talk to your plants. lol.
I think you need an accurate meter. Buy a mid range one . I went on ebay and poked around. they're not that expensive. You need one anyway.
Carbon Monoxide is ororless and absolutely toxic to people. Get a dectector and that is the most important investment you might ever make

You have to check it immediately once you enter the room, if I spend 15 minutes in my room my ppm jumps from 1300-1550 to 2200 or so just from breathing. Granted I use co2. Without the co2 on, it maintains 350-400ppm.

The highest baseline CO2 levels I've ever seen indoors was at my buddies he had 750 ppm in just about every room in his basement. No leaks of any kind he just had a old school gas burner for heat. 1500ppm isn't dangerous but you should still figure out if thats a false reading or if you have a leak somewhere. Concentrations of CO2 can build up really really quickly if a leak all of a sudden gets larger.

Did you calibrate the machine before going into the grow room with it?

Sounds false. Caps are known to be inaccurate so ive heard... My basement reads around 450ish ppm co2 naturally.