Possible carbon monoxide leak?

[Michael A.]

I recently purchased a duplex that needs repairs. I’m in Los Angeles and the property is in Ohio. I visited the unit recently for 3 days. It’s my first rental property and I’m hoping someone can help me out.

One side of the duplex is occupied and the tenant tells me that the previous landlord did not help with anything. After talking with this tenant she revealed to me that she would get headaches throughout the year and couldn’t figure out why. She’s been living there for a couple of years. I had an HVAC and plumber go down into the basement. HVAC looked at the older furnace and determined that it is up to code. Plumber looked at the old water heater and there was charring on it. The chimney pipe also was very corroded with holes in it. The water heater pipe went very far into the basement and the rest couldn’t be seen.

We concluded the water heater must be causing exhaust to enter the unit. I bought a brand new water heater that vents directly outside the unit. It uses its own chimney and the old chimney pipe is no longer in use. That night the tenants took a shower with no issue. The next day at around 5pm they took a shower and an old carbon monoxide detector alarm went off. 

Tenant said it’s an old battery operated detector. I’m wondering why it would go off. I had thought I solved the problem but now I’m worried that gas is being leaked from something else. The HVAC said the old furnace was still good enough to be up to code, so I don’t think gas is coming out of the heating vents in the unit. Some things I observed. I noticed tenant has a portable heater. Bathroom with shower is on second floor. 

I want to install new detectors. Does anyone have advice on where they should be installed?  The unit has 3 beds, 1 bath, living room, kitchen, basement. Water heater and furnace are located in basement of this unit. Who would I call to install this?