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Carbon monoxide = CO (houses don't need CO2 detectors)

There are many common misconceptions and misunderstandings about furnaces, water heaters, and carbon monoxide that I hear repeated regularly, and I’d like to clear a few of them up. To start, here's a video discussion:

False: Carbon Monoxide is also called CO2. Carbon Monoxide is CO. Carbon Dioxide is CO2. Mono = 1, Di = 2. Just don't tell the folks at Walmart; they actually have a section on their website where they sell "carbon dioxide" alarms.

Walmart Screenshot

They're actually selling carbon monoxide alarms, but I'm guessing they have the wrong thing listed to help people find what they're looking for even if the wrong thing is typed in. Google is a master at this, so don't get me wrong, I'm not complaining. I'm just sayin'.

False: Carbon monoxide alarms are needed to sell a home in Minnesota. Carbon monoxide alarms are required within 10' of sleeping rooms, but this has nothing to do with a real estate transaction. You don't install carbon monoxide alarms because it's the law, you do it because it's a $15 life safety device. See R315 for the exact rules on CO alarms.

False: Backdrafting at a furnace or water heater means CO is filling up the home. Backdrafting means that exhaust gases are spilling back into the home, rather than leaving through the vent. A properly functioning water heater or furnace will not create high levels of CO. Even if a properly functioning furnace or water heater backdrafted into a home all day long, you might not ever see an elevated level of CO. Not that this is ok. This should still be considered a hazardous situation that requires immediate correction. Backdrafting has the potential to fill the home with CO. It will always contain CO2 (carbon dioxide), which can cause sickness and headaches in higher concentrations.

Hmm... maybe Walmart ought to start selling CO2 detectors for real ;-). They do exist, after all: CO2 Meter

False: Cracked heat exchangers create CO.  CO is caused by incomplete combustion, not a cracked heat exchanger. A heat exchanger is the part of a furnace that transfers heat from the flames to the household air. A functional heat exchanger keeps the household air and the combustion gases completely separate from each other.

A cracked heat exchanger might, under the right conditions, create elevated levels of CO, but this is not typical. If a furnace has a cracked heat exchanger, the combustion gases can mix with the household air. It’s usually just a little bit, but this is still unacceptable, and it means the furnace or heat exchanger should be replaced. More on that topic here: Heat Exchanger Cracks and Carbon Monoxide Myths.

False: High CO levels = cracked heat exchanger. See above. We usually test CO levels in the flue gas at furnaces, but not with the idea that this will tell us about a cracked heat exchanger. Heat exchangers fail when the metal rusts through or when it cracks. CO does not cause this.

False: High CO levels in the flue gas mean the furnace is leaking CO. If an appliance is venting properly, all of the exhaust gases leave the home. Even if a furnace is producing extremely high levels of carbon monoxide, this carbon monoxide will not mix with household air, provided everything else is functioning properly. If the appliance backdrafts or the exhaust gas leaks into the home through a cracked heat exchanger... well, that would be a different story.

Because of these possibilities, however, high levels of carbon monoxide are still a concern. If we find high levels of CO in the flue gas, we recommend repair. It doesn’t matter if the gases are mixing with the household air at the time of the inspection or not, because this condition could potentially change at any time. The Minnesota State Fuel Gas Code (Chapter 9, Subp.6) says that gas-fired equipment shall produce not greater than 0.04 percent of carbon monoxide on an air-free basis. This equates to 400 parts per million.

Wrong Term: Hot water heater. It's just a water heater. The heated water that comes out is hot. Yes, I know there are plenty of retailers who will sell you a hot water heater, but please re-read the beginning of this post about how you can buy a carbon dioxide alarm at Walmart.

To summarize, high levels of CO need to be fixed, cracked heat exchangers need replacement, and backdrafting is never ok. These three things are all independent, but a combination of these conditions is especially dangerous. When using these terms, make sure you have them correct. It makes a difference.

Author: Reuben SaltzmanStructure Tech Home Inspections

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Whole-House Humidifiers Harm Houses (still)

Whole-house humidifiers are at the root of many of the most serious moisture problems in Minnesota homes. If you decide to use a whole-house humidifier in your home, be warned: it's easy to cause major damage to your home. Last winter we had an unusually long cold-spell, and a lot of Minnesotans ended up with major water damage in the attics from frost accumulation. I personally investigated a lot of these, and I'll bet you can guess what I found.

I first blogged about this topic nearly ten years ago, and nothing has really changed. Whole-house humidifiers continue to cause major damage to Minnesota homes, and HVAC salespeople continue to sell them. This is certainly a divisive topic; I was recently involved in a discussion with another home inspector who insisted that humidifiers are fine.

Here's a video discussion of the rest of this post:

The good

First, let's take a look at the claimed benefits of whole-house humidifiers, which I'm taking right from Aprilaire's website.

Health: Adequate humidity supposedly reduces respiratory infections and symptoms related to allergies and asthma. That all sounds good to me. I'm not qualified to comment on any of this stuff, but I believe it. When my house is very dry, my sinuses definitely aren't right.

Comfort: There's no debating that. Dry, cracked skin is no fun.

Preserve wood: Hardwood floors are sensitive to humidity levels, and so are other things such as wood furniture and cabinets. If your home is too dry, you could end up with cracks and warping. There's no debating that.

In short, humidifiers can definitely do some good things for both your home and health... provided you use the humidifier properly.

The bad:

Damage caused by whole-house humidifiers frequently outweighs the benefits. I've seen too many frost-covered attics, mold-covered attics, ruined ceilings, and ruined windows to think anything else. In every one of these cases, there's a whole-house humidifier at work. Take a look at some of the images below for examples of what I'm talking about:

Moisture at window

Water damaged window

Frosty attic

Frost in Attic 3

Manual Humidifiers

I'm not a total doom-sayer when it comes to whole-house humidifiers. There are actually two types of humidifiers available; manual and automatic. Manual humidifiers are dumb, and probably ought to be banned. The problem is that the colder it gets outside, the less humidity your Minnesota home can tolerate. Here's what the folks at Aprilaire recommend, which all sounds good to me:

Humidity Guide

Humidity levels around 30% and up are comfortable enough, but things start to feel really dry below that. The average Minnesota homeowner has no chance of following the instructions that come from the manufacturer. To maintain the proper level of humidity, you need to know what the temperature is going to be, and then adjust the settings on the humidifier accordingly to match that chart. I kid you not. Here’s an excerpt from the owner’s manual:

“it is important to anticipate a drop in outdoor temperature and reduce the setting accordingly to avoid excessive condensation. For example, with an outdoor temperature of 20°F the correct setting will be 35% RH. If the temperature is expected to fall to 0°F, then merely reduce the setting to 25% several hours prior to the temperature change.”

I know they're not trying to be funny, but that's funny. Nobody does this, and nobody should be expected to do this. The consequences of not doing this are the conditions that I showed in the photos above. This is my beef with whole-house humidifiers.

My recommendation is usually to not use manual whole-house humidifiers unless you’re an extremely ‘type A’ homeowner, which means you’ll check the weather forecast and adjust your humidifier all the time.

Automatic Humidifiers

Automatic humidifiers are designed to do exactly what you think; they stop adding humidity to the indoor air when the outdoor temperature drops. This is done through an outdoor temperature sensor and a smart controller. All good stuff.

Humidifier installation diagram

I have no problem with these units, provided they're installed properly and operated in automatic mode.

Unfortunately, many of these units are installed without the outdoor temperature sensor, and the installer simply switches the operation mode from automatic to manual. There's a little switch that's hidden behind the control panel cover for this.

Manual humidifier control switch

There are some clear labor savings involved with this installation. I'm not going to speculate as to why or how this happens, but I know it happens.


If you have a manual humidifier, either turn it off or be sure to obtain, read, and carefully follow the manufacturer's instructions. If you simply set your humidifier to a "judicious" middle setting and keep your fingers crossed, you're going to have moisture problems.

If you have an automatic humidifier, make sure it's installed to operate in automatic mode. There needs to be a temperature sensor installed outside the house, and the unit needs to be set to automatic mode.

If you're going to have a whole-house humidifier installed, go automatic and make sure it's fully/properly installed.

Author: Reuben SaltzmanStructure Tech Home Inspections

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