Minisplit install resulted in gas boiler carbon monoxide problems. Ideas needed!

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North Shore, MA
Posted this in the boiler forum, but this is an HVAC issue as well. Hopefully ok to cross-post here...

We installed LG heat pumps/mini splits in our New England home. Home is about 2200sq feet, a mix of antique (built 1852) and newer addition. We have excellent windows and the home is well sealed and insulated. Probably tighter than most homes of that vintage.

New Equipment: We now have two 30k compressors (LMU300HHV, rated for very low temp heating) runnning: one 15K unit (large master BR/bath with high vaulted cieling), four 9k units (kitchen, family room, den, finished basement), and a 12k concealed duct/airhandler unit (LDN127HV4) in the attic with short runs to two bedrooms, and that has a return in the hallway between the rooms, top of the stairs.

Existing Equipement in the house: 35+ year old Utica gas boiler for baseboard hot water heating and heating our indirect gas hot water tank year round.

This setup has done a great job cooling our home, even in these recent heat waves. The PROBLEM: We are now experiencing carbon monoxide buildups in the basement. The boiler runs in these hot summer months to heat the water storage tank, and the addition of the minisplit system has definitely impacted the boiler venting. The boiler has its own doublelined flue that runs up from the basement, up through closeted areas on first and second floor, and pops out low on the roof line and extends another 10ft or so higher. Additionally, after installing the LG system, we were also seeing strong, continuous drafts being pulled down a chimney and out our wood stove. I was able to block that other flue effectively to stop that draft (and the chimney stench in the house).

Could we be experiencing that same issue with the boiler flue? That is, could negative pressure (?) created by the airhandler/return (?) be preventing the boiler from properly venting? Or some other theory? We NEVER had a CO problem before this system was installed, and the first time the basement CO alarm went off was within days of the install and initial usage.

Any thoughts, please! The HVAC installers don't do any traditional heating/boiler work, so they don't have much to offer. They did turn the fan down in the concealed duct unit in the attic from the factory setting of High to Medium-Low. We could still go down one more setting to Low. Local companies that service these traditional gas boilers don't do minisplit work, so I'm trapped at the intersection of very old systems and very new. What to do!?


The wife is still training me.
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Orlando, Florida
I think your assumptions on the negative pressure is correct, however, mini cassettes are per room and it just recirculates the air. To be sure set up the conditions that pulls air from chimney's. Then turn off all the mini split units and check for down drafts. Another way is open a basement window while the boiler is on.

With such as tight house, is there any fresh air exchanger? For now keep a basement window open, maybe just an inch and check CO levels.
Before the mini splits, was there any AC?


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Peace valley missouri
Minis can't effect the pressure inside the house the reason is they only move air from the same space. If they were connected to the the outside they would be more than likely to be connected to to outside on the return side and pressurized the house. Does the basement have any freash air opening and if so are they clean? With boiler running crack basement door and with a match see which way the flame moves. Like Worth wrote crack a basement window.
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