UPDATE: Gas boiler Carbon Monoxide problems after minisplit installation. Opinions needed!

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JBNE

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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 LG 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!?

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wwhitney

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Absolutely you are experiencing backdrafting on both the chimney and the boiler flue. [And by blocking the backdrafting at the chimney, the effect at the boiler flue is probably increased.] Some fan or other phenomenon is causing the air pressure at the boiler to be lower than the outdoor air pressure, and the draft mechanism of the boiler isn't strong enough to overcome it. I'm not so familiar with boilers, does your 35 year old Utica unit depend on natural draft?

As to identifying what is causing the backdrafting so that you can fix it: Did you have any air sealing or other building envelope changes made when you had the mini-splits installed? If you did have new air sealing done, then the most likely explanation is that the previous leaky condition was providing requisite make up air for the boiler, and now it is no longer getting enough make up air. This is a phenomenon that any company doing air sealing needs to be aware of and needs to check for, before and after the work.

But to diagnose what is causing the backdrafting, I would suggest starting by turning off all air moving fans anywhere in the house, and then seeing if the boiler properly drafts. If it still backdrafts, it could be caused by a very strong stack effect: the top of your building envelope (the top floor ceiling if your attic is ventilated) is very leaky, hot air is rising out of the building envelope quickly enough that the boiler flue in reverse is supplying air into the basement to replace it. In this case the solution is to improve the air sealing at the top of the building envelope (best option) and/or to add intentional outdoor air inlets into the basement to provide the replacement air for the stack effect (and combustion air for the boiler).

But if the boiler drafts OK with all the fans off, then you can try turning on the fans one at a time to figure out which fan is depressurizing the house excessively. For example a large range hood that exhausts to the outside can do that if there isn't any explicit makeup air provided; the solution would be a range hood make up air system that brings air into the house whenever the range hood is running. The new mini split air handler installation could possibly contribute to depressurizing the house, if the attic is ventilated and the supply duct in the attic has a large leak, so that the unit is pulling more air from the house than it is pushing back into the house. However, that wouldn't be my first guess,

Another possibility is replacing the boiler with a sealed combustion unit that draws combustion air from outside, so that it is immune to air pressure at the boiler. Of course, given your recent mini-splits, it would pay to reconsider the demands on the boiler in choosing a replacement (likely smaller or a different type of product).

Backdrafting and CO can be deadly, so it would be most prudent to shut off the boiler until you know what is causing it (e.g. range hood) and can avoid causing the backdrafting. Or perhaps leaving a basement window open would be enough to mitigate the backdrafting in the short run.

Cheers, Wayne
 
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JBNE

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Thanks very much for the reply, Wayne. Yes, the old boiler is natural draft. We had a lot of air sealing and added insulation to the attic, but that was almost a year in advance of the minisplit install, and in all that time we never had a boiler CO problem (and no issues running bathroom or kitchen fans, dryer, etc). So I can't help but think this is somehow tied to the additon of the minisplits. But if that is a closed systems with no exterior venting, I just can't quite understnad the cause/relationaship. The ducted handler in the attic is pulling air from the house through the return (but only back into the handler and back out the BR vents). But maybe enough to create that negative pressure and downdraft in the boiler and stove flues?
 

wwhitney

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Can you be more specific on the dates of (a) air sealing completion (b) mini-split completion and (c) when you started noticing the backdrafting? Also, is the attic ventilated, and did the air sealing include paying attention to air sealing the top floor ceiling / attic floor?

If the CO problem started recently and around the same time as the mini-split work finished, but this is the first summer since the air sealing, it's possible the backdrafting is a summer-only phenomenon since the air sealing. Otherwise, it certainly is appealing to come up with a mechanism for the mini-split installation alone to cause backdrafting. The obvious one is a leak to the attic on the supply side that I mentioned, but I would think that a small air handler on a mini-split wouldn't be moving much more air than a kitchen range hood, so I'm skeptical of that. I wonder if by making the top story cooler, the larger temperature delta between that floor and the attic increases the stack effect through leakage through top floor ceiling.

Cheers, Wayne
 

JBNE

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Sure. Foam [crack] sealing work was completed last July (2021) and was especially focused on the attic floor. They also sealed throughout the house in various closets corners, where various pipes emerged, under-sink plumbing, and extensive foam sealing of rim joist areas in the basement. Basement is much tighter. If I remember correctly, house blower test before and after brought the CFMS from over 5K to closer to 3K. We then added a much deeper layer of loose cellulose insulation to the attic in December. Last summer (our first in the house), we had no AC whatsoever. Not even window units.

The first time (ever) that the CO alarm in the basement went off was winthin a day or two of the minisplit install/first usage, in late April. It has only happenened a handful of times--though twice this week--since the install. But it sure feels like it somehow coincides with the minisplit.

I was also wondering about the new (post air conditioning) effect of now having a stack of cold, conditioned air in the boiler flue that runs up through two stories of the house before exiting through the roof. Could that have any impact on the natural flow of air/boilder venting?

And The attic has a ridge vent, a small static vent, and a humidity (or heat?) controlled mushroom vent with a fan. This was all exisitng, pre-HVAC.
 
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DIYorBust

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Since it's a ducted unit, it could be creating negative pressure at the boiler, although it could be coincidental. Are you certain the issue only occurs when the air handler is running, and have you had the boiler inspected by a pro to make sure it is working properly? If you can confirm that it is a negative pressure problem, two solutions to explore would be bringing makeup air to the boiler room, such as by installing vents through an exterior wall, or ducting to the exterior. Alternatively, perhaps a power vent could be could be considered to increase the draft. As wayne points out, the boiler could be replaced with a direct vented unit, but this might not be economical unless the boiler was due for replacement anyway. Is the minisplit system capable of providing primary heat for the house? If so that may be a option vs replacing the boiler.
 

Fitter30

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Easy way to see if the house is in a negative. With everything closed up crack a window with a lite match see which way the flame points. 3k cfm is still not a tight house a lot better than 5k tight house is around 1100. Has anyone inspected the chimney and chimney cap. Take some pics of boiler venting and outside roof / chimney.
 
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John Gayewski

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I think i would open a window in the basement to add a fresh air makeup. See if that fixes the issue. If opening the window fixes it then your likley not within the minim fresh air supply requiments for an atmospheric boiler.

If you tighten up an old house who's heating needs were designed from a loose standard your going to lose your ability to feed the fire at the designed rate. I have seen people build a room around the boiler and duct in fresh air.
 

jadnashua

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There are good and bad things about a tight house. Any combustion device needs a draft. Many modern, high-efficiency boilers now use outside air, so does not need any internal air, or leakage to burn. To preclude those problems, a HRV may be required.

I agree that it sounds like you have negative pressure, pulling the exhaust gasses back into the house. Note, even low levels of CO over time can lead to health problems...the CO detector needs a higher level or longer time before it goes off, but you could be experiencing issues prior to the alarm.

If the ducts are not sized properly, the fans on the mini-split could be creating the issues.
 

JBNE

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Update.
Problem still persists sporadically. I installed a CO monitor with a digital display a couple weeks ago. It also has a memory feature that records peak levels. No CO whatsoever detected for a couple weeks. Then suddenly within the last few days, I've had two CO spikes in the basement, including one this afteroon with level recorded at 700. Scary stuff.

The minisplit installers were back and confirmed there are definitely no leaks in the short runs of the ducted unit in the attic. It's well sealed and really cleanly installed.

So I was trying to pay particularly close attention to other factors preceding these last two CO detections. In the first, my wife was filling up the high volume whirlpool tub in master bedroom (so the boiler was cranking to refill our indirect water tank). Bathroom fan was also running. And just today, there was no hotwater usage leading up to the CO spike, but the dryer was running. And (not sure if this could be relevant), we had just experienced an extreme downpour outside resulting in EXTREME humidity in the 90 degree weather. Very heavy air.

But keep in mind that we run the dryer ever day (at least once). While we rarely use that high volume bathtub, our family of four showers daily and we use various bathroom fans and kitchen fan, yet don't experience these CO spikes regularly. So those factors are not exactly the obvious sources.

Again, all this seems to coincide with the minisplit installs, and less immediately, also our weather sealing and extra insulation efforts.

Turning to my boiler, I want to ask about the venting/damper setup. Is that something to look into as a possible factor? I'm not familiar with this style.

Any other thoughts?

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JBNE

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And to be clear, I realize that an internet forum is not my ultimate solution to this potentially very serious situation.
Just looking for ideas to present to local people who have had no answers for me. I have had multiple people come by, none of whom have had answers/solutions. That last guy that came by (from one of our regions largest and *most reputable* heating and HVAC company) called his boss while at my house and candidly told me that the boss said "Run away. We don't want to touch that situation." Another traditional boiler guy, thinking it must have to do with the AC, said to bring the minisplit guys back because he had no ideas.

I will keep looking for a local company that will hopefully have a path forward, but in the meantime, just seeing if any experienced folks here have any ideas that I can bring to the table. That's the sole reason I'm reaching out.
 

Fitter30

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That damper doesn't use a actuator uses bimetal to open it. Unless im not seeing it. So boiler fires but it takes a few seconds for it to open when boiler flame is burning producing hi Co. Boiler side there are black marks on the flue pipe right off the boiler showing a leak. The bottom connection of the elbow looks like its short. The crimp is only half used. Pull the damper off replace with new flue pipe and adaptor to ell. Does the flue pipe get bigger going into the wall? A motorized damper on call for heat opens before boiler fires. Bi metal dampers usually have a place for a limit switch if the damper doesn't open trips limit burner shut off.
 
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JBNE

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Thanks for the reply. Yes, flue pipe does get larger, as pictured here. So are you saying that a new (motorized?) damper might be something to mention when I get another local tech over here?

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Fitter30

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Flue design shouldn't be lets guess. Know you heard this size matters. I want know btu input ratings and pics of other equipment flue pipe sizes and pics of equipment. Pics of roof where flue exits where flue to roof peak and where is the the flue over the peak best guess from the ground.
 

DIYorBust

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I would be very worried about this situation. Since you already called several pros, here are some actions that I might contemplate in your shoes:

1) Call your fire dept to investigate.
2) Hire an HVAC engineer to evaluate the situation.
3) Install a direct vent boiler, or retire the system and use/upgrade your minisplit system for heat.
4) Install a co detector with interlock to the boiler that will shut it down if a CO alarm occurs.
 

Fitter30

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Just noticed the metal tape covering the old water heater connection back of boiler tee. That should be capped or changed out to a ell and get rid of the tee. Need the btu rating of boiler and brand and.model. That main flue going up and out what is the o.d.
 

John Gayewski

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I'm about 90 percent sure there's no makeup air to this unit and it needs a fresh air supply. It would be a shame to hire an engineer to say you need makeup air which is the simplest thing.
 
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