two CO detectors, two different readings

midwest

I have two Testo 317-3 CO detectors and one Sensorcon Industrial Pro CO detector. Normally they read about the same. But last night, I noticed the Testo's reading 1 ppm, and the Sensorcon reading 7-8 ppm. Today the Testo's are reading 0 ppm, and the Sensorcon is reading 3-4 ppm. The Sensorcon is always "on", and typically rests on my nightstand.

I live in the Midwest and according to my weather radar app, we have a fair amount of wildfire smoke in the area again today. I'm wondering if the Sensorcon may be picking up on that. The Sensorcon picks up readings both outdoors, as well as inside the house. FYI - my only gas appliance is the propane furnace, everything else is electric. Plus it's cooling season so, the furnace has not been running.

I wonder, which CO detector to believe, the Testo's or the Sensorcon....??? Any thoughts would be greatly appreciated.

Side note - we are getting our 2016, 95% single stage Rudd replaced with a new two stage furnace and a/c as soon as our hvac guy can get to it. Back in early March, it began blowing out a nauseating burnt metallic smell (aldehydes) from the supply registers when the heat kicked on, which triggered 3-5 ppm readings on each of my CO detectors, and one day, caused my face to turn ghost white with blood red lips.

Knew immediately what it was, shut down the furnace and opened the windows. I'm sensitive to CO from prior exposures several years ago. Doesn't take a lot to trigger symptoms for me. And I have allergist tested / confirmed chemical sensitivities to aldehydes. Bummer but, it is what it is. Sadly, by April, the family cat became lethargic and had to be put down.

Our hvac tech inspected and couldn't find anything wrong with the furnace, aside from rust inside the heat exchanger tubes along the welds where the flames shoot in (I read an article by Jim that said, "....if you have rust, you have carbon monoxide..."), and a lot of rattling when the blower motor initially comes on with heat or a/c. The rattling isn't coming from the blower motor, it's somewhere inside the upper cabinet. The current tech doesn't have a combustion analyzer but, I did have another company come in a couple years ago and do a combustion analysis and told me "everything was good" back then. The current tech still doesn't think there's anything wrong with our furnace but.... he wasn't sleeping here last March / April when the burnt metallic smell was so strong. I won't go thru another winter feeling like crap and keeping windows cracked open for fresh air.

I think we got a lemon. Previous problems included the smell exhaust inside the upper cabinet since the day it was installed, even pegging the alarms on my consumer level gas sniffer and the LP company's super expensive gas sniffer. I even called NCI and got to visit with Jim Davis personally, looking for suggestions (Thanks for your time Jim).

It wasn't until March 2021, that it began leaking water from the exhaust pvc pipe that comes off the inducer fan inside the upper cabinet. Using a small mirror, I found a 3 inch crack on the back of that pipe. Hvac tech replaced that section, and upon inspecting the old part, commented, "....wow, this looks like it's been cracked since manufacture...." He gave the part to his distributor and six weeks later, told me that Rheem had put out a technical services bulletin on that part. Interesting, seems Rheem was a little concerned. At least the exhaust smell finally disappeared, my five year headache finally subsided, and I'd wake in the morning actually knowing what day of the week it was. That's how badly it effected me.

Anyway, thoughts on the Testo vs Sensorcon question would be greatly appreciated.

Thanks folks.
Susan

GGross

"The Sensorcon picks up readings both outdoors, as well as inside the house."

If your two CO detectors are monitoring different locations you can't compare the accuracy of the two based on those readings, as they are reading different spaces CO content

captainco

You can take your CO detectors and put them in a ziplock bag. Then take a couple of matches light them and then blow them out and put the smoke in the bag. Seal the bag and see how close they read to each other. Below 5 ppm it is hard to get CO detectors to read the same. Can't even get expensive combustion analyzers to read the same. They can be affected by temperature. The Sensorcom can be re-zeroed but you need to be in a place where there is no CO for sure.
Susan, I don't believe I said if you have rust you have CO problem, that would be soot. Rust is a venting problem, not necessarily a CO problem.

Please make sure whoever installs you new furnace does a combustion test in both stages and gives you those numbers. You can post them on here or call me.

mattmia2

I suspect your flapping and odor are some sort of debris that has fallen in to the furnace or lining from the furnace that has come loose and is burning on the heat exchanger. I doubt that would produce a detectable amount of co. Is there an ac coil above the hx?

midwest

GGross -- I apologize, allow me to clarify. I am reading the same space with the two detectors side by side. Example from tonight, inside the house, the Sensorcon show 2 ppm and the Testo shows 0. Same thing will happen when holding the two side by side, outdoors.

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Jim,--- Hello sir, hope you are well. I must apologize for misquoting you. I did a quick search and found the online pdf with the rust comment. You are correct. In the article I was thinking of, your rust comment was indeed referring to a venting problem. Under the paragraph, "One Last Thought"....
https://www.nationalcomfortinstitute.com/pro/files/articles/LesseningCOLeaksWithEquipmentSafety.pdf

And thank you for the info on it being hard to get CO detectors to read the same, and how to do a home test with the ziploc bags and smoke from a blown out match.

I'll ask our hvac tech if he's able to perform a combustion test on the new installation but I know he didn't have an analyzer the last time I asked him. The hvac tech that did do the combustion analysis was the owner from a different hvac company and I believe he has now retired. I've never been able to find another tech in my service area that does combustion analysis. Unfortunately, you said the same thing when you looked through your list of NCI trained techs when we spoke on the phone.

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mattmia2 -- Interesting theory. The rattling sound is more metallic in nature, like a loose piece of metal banging around. It can be heard even upstairs in our ranch home, when the furnace or a/c kicks on. I've run a bore camera in through the hole where the high limit switch sits and looked around to see if I could visualize any of the heat exchanger moving when the blower fan turns on (NOT the heat, obviously) but, it's impossible to get a good view in such a compact space. I did however notice a couple of small 6-8 inch sections of heat exchanger tubes that were charred black, compared to the white-ish silver color of the rest of the tubes. No flame roll out though, so that's good.

There should be nothing in the plenum since a few years ago we had the old fiberglass duct lined plenum replaced with clean sheet metal plenum because the duct liner was falling apart and getting into the air stream causing severe skin itching and turning the filters on upstairs air purifiers black in two weeks. Yes, the a/c coil sits above the heat exchanger.

The odor, as best I can describe, is like a burnt metallic smell that burns the eyes and triggers headaches and dizziness in myself and my elderly parents that live with me. Its an interesting odor, and not one that I've experienced in any other place that I've ever lived.

We had two different hvac techs and one electrician inspect the furnace back in March. Interesting that it was the electrician who touched the plenum above the a/c coil and said, "That's so hot that I can't keep my hand on it.... my furnace doesn't run that hot." Same electrician temp checked some supply registers and found them to be 130-135 degrees, smelled the same burnt metallic odor, and thought something was definitely wrong but didn't know what since he's "just" an electrician. He checked the wiring inside the furnace cabinet, said everything checked out okay, but added that he could smell whiffs of gas or exhaust, which is something our hvac tech claims to be unable to smell due to working in the industry for several years. I still smell it once in a while too but can't pinpoint it. It's not the smell of mercaptin... its more a gas / exhaust odor in the upper cabinet. I decided to shut the gas valve off on the side of the furnace for the summer.

Eventually one gets tired of chasing after problems. So we've decided to scrap the whole thing and replace it a new system as soon as our hvac tech has time to install a new furnace and a/c. In 50 years, I've never experienced so many issues with an hvac system as I have with this one.

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Thanks for your time gentlemen.
Greatly appreciated.

mattmia2

It could be the blower isn't running as fast as it should be so the hx is getting hotter than it is designed to(or is very clogged). There should be a rating for max discharge temp and rise between return and supply on the rating plate.

captainco

midwest - It is a shame consumers have to put up with such a lack of skills. Having a similar problem with a consumer in Long Island. Even the ones with combustion analyzers don't have a clue what to do!

ChrisJ

captainco said:

midwest - It is a shame consumers have to put up with such a lack of skills. Having a similar problem with a consumer in Long Island. Even the ones with combustion analyzers don't have a clue what to do!

Is this a recent problem?

captainco

ChrisJ - Yes

midwest

mattmia2 said:

It could be the blower isn't running as fast as it should be so the hx is getting hotter than it is designed to(or is very clogged). There should be a rating for max discharge temp and rise between return and supply on the rating plate.

The second hvac tech that inspected the furnace back in March, actually pulled the blower and crawled inside with a flashlight to look up thru the secondary heat exchanger, main heat exchanger, all the way up to the coil, and said it was totally clean. That was a testament to my OCD about changing the MERV 7 filters every three months on schedule. Such an easy thing to remember to do and note on a post-it note stuck to the return plenum next to the furnace cabinet. I've never understood how people can forget to change an hvac filter for years at a time. lol

A couple years ago, our hvac tech set the blower speed down from high to medium because the a/c wasn't running long enough to wring the humidity out of the indoor air. Once it was set to medium, we had much less of a humidity issue in the house. I can see and appreciate the "fine line" that you guys have to walk to find "balance" in an hvac system. Much respect to all of you.

The temp rise on the data plate shows a recommended 35-65 degrees. I used a regular meat thermometer at the filter rack and then about 4 ft above the a/c coil and found 70-75 degrees. The hvac tech said the supply air temp needed to be measured around the first bend of the supply plenum and not in line-of-sight of the heat exchanger, which he found to be 65 degrees. I have to trust his judgement as a licensed hvac tech but, it didn't solve the burnt metallic smell.

midwest

captainco said:

midwest - It is a shame consumers have to put up with such a lack of skills. Having a similar problem with a consumer in Long Island. Even the ones with combustion analyzers don't have a clue what to do!

Jim, totally agreed. We live in a very rural area. The nearest "city" is 65 miles away, which is well out of the normal 40-50 mile service radius of most companies. We have two hvac companies within a 30 mile range... one was the original installer, who took two weeks to come out when we were smelling raw propane in the house. His excuse was, "....they all smell like propane." When he finally got out here and opened the cabinet, he said, "Wow, that really does smell like propane !" (No kidding !). And this is a guy with a masters level license in plumbing / heating / hydronics. I was super irritated, but let him install a new gas valve, and have never called him back again. The other local guy does service only, no installs, and is the one we got the second opinion from. And our current hvac guy, that we switched to after lack of urgency with the original installer, is kind enough to drive out here from 40 miles away. The choices here are few and far between.

Also want to take a moment to tell you that, I still smile when I think about our phone conversation from a couple years ago. I called NCI, expecting only to visit with a secretary that might know of an NCI trained hvac tech. When she said she didn't "...but would transfer my call to someone who would know..." and YOU picked up the phone. I was shocked. I had read a lot of your articles in my research but, I never expected to visit with you directly. The fact that you are willing to give 15-20 minutes of your time to someone who isn't even an hvac tech, but "just" a consumer trying to solve a problem.... is just amazing. Your willingness to share information and genuine desire to help people even down at the consumer level, is something that is sorely lacking in most industries.

Thank you Jim. I really appreciate you being willing to visit with me that day.

captainco

Thank you Jim. I really appreciate you being willing to visit with me that day.

There are great contractors in the HVAC industry, but they can be really hard to find. On top of that we have many rule makers that force us to do some of the dummest things and don't allow us to do our job which makes it even hard for great contractors.