From a consumer perspective that link is invaluable thank you. I could call 100 hvac people on the phone and the people answering the phone wouldn't be able to spell combustion. By having a website like that it makes it possible for me to find someone. As much as I have been troubleshooting here with the techs, it isn't what I want to do. It isn't really my engineering interest. You nailed it when you talked about guessing. I want to pay someone who understands fundamentals and isn't only guessing. I just haven't been able to find them. I haven't been satisfied with the guesses from the onsite folks hence this thread. Measuring combustion is the key to all of it as pointed out by folks in this thread.
Temperature rise and blower motor speed
One point I left out and just researched. First two techs measured rise and noted it was high per my last posts. I asked them about adjusting blower speed, not knowing anything other than this sounded like common sense. Both told me No, it is is a one speed blower and can't be adjusted. Third tech/sales guy said the blower speed should be increased from medium low to medium high. The unit ran well after that and within rise spec but I did shut down after about 40 minutes because I wanted to get inducer motor installed. At third tech also advised replacing inducer motor.
I now read the 58yav manual and they are very specific that blower motor needs to be adjusted to house specifications and correct rise. For all I know this unit has operated outside of rise spec for years which according to Carrier reduces hx life. What I cannot understand is why these first two techs couldn't have addressed that properly by adjusting blower speed. I now have two thermometers to measure rise myself but again alongside of measuring combustion something simple like blower speed and rise seems to me to be a key thing to check, isn't it?
Inducer motor has been replaced and unit has run for two hours with no problems. Previously it reset within about 15 minutes predictably. If I hit any issues in the future, I will call on a technician who can do combusion analysis, none of my technicians came prepared to do that. Here is a summary for my scenario:
Scenario: Carrier code 31 on gas furnance that had run for about 10 years without significant issue. DSS switch is tripped, requires physical reset. DSS trips consistently after about 15 minutes of heating house. Chimney adapter in-use blowing into clay-lined chimney. All vents OK (ingress, egress.) Filter of course clean. Unit very clean.
Measured symptoms and steps taken:
0. Fully inspect chimney and runs to chimney for safety and to assure it's clear. Verify run in-specification for Carrier chimney adapter. All of this was done and chimney was OK. It was determined that 5 feet of furnace vent should have been double walled. Note this vent is away from people and other equipment. Steel lining of chimney flues is the most straight-forwarded lowest risk approach. Tech stated that in our area of country, clay chimneys used extensively, chimney adapters are common.
1. Heat exchanger stated as OK by two techs. Rise between return air and heated air was too high, outside of specification. Specification was 45-75. Two technicians failed to adjust blower motor speed (increase in this case) which is what is prescribed by Carrier to bring rise into specification. Third tech did this. This change brought the temperature rise back into specification.
2. DSS switch and pressure switch were replaced. Unknown if these parts ever had issues.
3. Inducer motor was replaced. Unsure if this part was bad either, this was tech #2 guess, unit seemed to behave differently when motor was pulled and put back-in.
After the above steps were taken, unit is now running normally (measured rise at 53 degrees) although it has not been combustion tested. In retrospect, I would have first found an HVAC technician who could run a combustion test, none of the technicians I called-out had the equipment or could do one. To find a technician in an area who can do a combustion test, as described previously in this thread by another poster, visit http://www.stopcarbonmonoxide.com/index.cfm?p=find . CO protectors are installed around the house and in the furnace room. This thread noted that consumer units are not as good as they good be. I also purchased a CO meter just for my own fun and enjoyment.
I would look into a chimney liner as we have rasied the eff. of furnaces the flue gas is cooler and may reach its due-point (aprox.140')
before it exits the clay tile and this due being from the product of combustion is carbonic acid wich will eat away at the chimney and much more thus causing much more $$$$ spent than the liner will cost
I know clay tile is used all over though this is what has been learned over the past 30 or so years due to the effic. rise
did it ever occur that when the tech blocked off the "draft diverter" that the temp went down.
this tells me that it improved the draft, which sucked more flue gasses OUT of the house, and lowered the flue temperature. I would find a company certified in CO testings and have them properly tune and repair your venting problem
If the appliances were properly tuned you wouldn't need a flue liner because the flue gases would never condense and eat away the chimney.
a chimney liner is a nice shiny expensive drain pipe for an appliance.
Originally Posted by gravity
Clay lined chimneys have been illegal to use without a liner for 20 years here because they're unsafe and inefficient.
Gravity is correct. Making a chimney smaller reduces its ability to vent. If smaller chimneys vent better, how does a campfire draft so well?
Originally Posted by syndicated
Just because something meets code does not mean it actually works. Codes are developed through consensus by individuals with no real knowledge or experience with combustion equipment.
Worry is a really gross misuse of one's imagination. -- PHM
you have a 6" round line and a 8" round line both hooked to a common return at the same static pressure..
which one sucks more air
To the OP. I understand your furnace has ran this way for 13 years. It does not mean that it has properly vented though. Once certain conditions are meant the flue gasses may not all be going UP which is why the switch is doing its job.
There is a certain way to hook up a appliance to maintain proper draft year around and if draft can not be met it will turn off the appliance.
I highly suggest you call someone who has been certified in the CO field to properly tune your furnace and fix your venting problem.
I would hate to see another victim of CO
Originally Posted by chuckcrj
I really hope that's not how you've been going about doing your job.
Chimneys work due to convection. Hot air rises remember. The hotter the air is the faster it rises.
Have you ever seen a 35,000 btu water heater vent into an 8" chimney work properly. By the time the gas gets up to the cap it's cool and barely moving!!
Campfires draft well because the gasses are coming off the fuel at like 400-600* F and the surrounding air is cold and dense by comparison.
It's the same principle that allows hot air balloons to fly.
In the back of my code book there is probably 100 pages of chimney sizing tables based on height, type of appliances, type of vent, type of vent connector, tightness of house and input of appliances.
These were designed by engineers and technicians over many years using math and physics.
Btw, an 8" duct moves more air than a 6" because there's a fan in front of it and the air going through both pipes is the same density.
The back of your code book is based on CONSENSUS and has never been field tested. They are computer generated numbers. I hope you don't believe everything you read.
For someone who has been certified in Carbon Monoxide, Combustion Analysis and have performed many field tests and verified function of chimneys. I know what I'm talking about.
I attached to pictures. If you go by the code, the chimney adapter installed on the OP furnace is illegal because it retards the flow of exhaust gases. If you had a combustion analyzer you could verify this in 5 minutes.
Also, another document that states the code book is consensus and has never been field tested...
Hmmm.........i think i would believe field tested data with my analyzer then some theory in a book.