Pyrolysis, Thermolysis, and Other Terms You Don’t Care About

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Last time I wrote about the vent pipes for gas furnaces and water heaters needing clearance to combustible materials, including drywall and wood framing. This is not true for some newer furnaces that have plastic vent pipes, but almost all metal exhaust vent pipes require at least one inch clearance to combustibles. I explained the need for this by copying something from the Glossary in my reports:

Waste pyrolysis refers to a chemical reaction that occurs in wood. Normally wood will begin to burn at 400 to 600 degrees F. However, when wood is continually heated to a temperature of 150 to 250 degrees F, the wood ignition point can drop to 200 degrees F. This is why it’s important to maintain proper clearance around gas appliance vent pipes and wood burning appliance chimneys. NOTE that installing sheet metal between a vent pipe and combustible material is not adequate- metal is a good conductor and the heat will easily transfer to the combustible material.”I received an email from Lon Henderson, a home inspector and home inspector trainer in Denver. He sent me definitions of “Pyrolisis” that state this reaction only occurs when there is a lack of oxygen. And this would not happen in a home or attic.

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As usual with the internet, I could find some evidence to back up my definition. But I think he is correct and I will be taking that term out of my report.

Lon asked for evidence that the ignition point for wood can drop if the wood is continually heated above 150 degrees. He had found some evidence/studies to dispute this. Again I found some evidence to support it. I love facts and figures.

Lon has admitted that the flash (ignition) point for wood can drop, but only under certain circumstances. Lon convinced me that the chemical reaction I refer to is not pyrolysis. He suggests it is more likely due to thermolysis, which is a “dissociation of chemical bonds or decomposition of compounds by heat.”Either way, we both report on a furnace or water heater vent pipe touching a drywall ceiling or lumber in the attic. And I still have to chuckle when someone spends 10 minutes arguing whether it’s necessary. All manufacturers recommend it, and it’s a two-minute fix for a handyman with a drywall saw or cordless jigsaw, so why wouldn’t you make this improvement?

Speaking of facts and figures, here’s how they can be used. Say if a gas appliance vent pipe has proper clearance, there is a one in a thousand chance of it staring a fire. But if the vent pipe does not have proper clearance, there is a four in a thousand chance. I could use this information and say that even if your vent pipe does not have proper clearance, there is only a .04 percent chance of a fire. That sounds pretty safe. I could use the same information and say if your vent pipe does not have proper clearance it is 4 times more likely to cause a fire. That sounds more onerous. (I made the numbers up for demonstration purposes only.)And if you didn’t care about pyrolysis, you may not care about this either. I wrote recently about the ‘new’ rule regarding gas water heaters. I had some emails to let me know I was wrong. In a way I was, because the ‘new’ rule I was referring went into effect in 2003. Any water heater manufactured after 2003 has to be an FVIR type, or Flammable Vapor Ignition Resistant. This is a safety upgrade to prevent fires.

But another rule went into effect in April 2015, so I will admit the FVIR rule is no longer the “new” rule. The new rules are by the Department of Energy (DOE), and state any water heater manufactured after April 16, 2015, has to meet new energy use requirements. These requirements are very strict on water heaters over 55 gallons. Water heaters, especially over 55 gallons, will be changing a lot. There will be gas condensing and heat pump water heaters, solar and tankless water heaters, electronic ignition (no pilot light) water heaters, etc. All new stuff that will make water heaters, in my humble opinion, less reliable and more expensive to buy, maintain and repair.

This may be another case of Uncle Sam making requirements before an industry is ready, and without thinking of how it can affect the average citizen. Because another way the water heaters will meet the new energy requirements is to increase the insulation. Which means newer water heaters will be larger. If your water heater is in a garage or basement, this likely won’t be a concern. But if your water heater is in a closet, it could cost more to relocate the new water heater than the cost of the water heater. I have heard, but not verified, that at least one home warranty company includes up to $1,000 over the cost of the water heater to relocate it if necessary because of these new DOE standards.

Some predict this will especially be a concern in manufactured homes, where water heaters are almost always a tight fit in an interior or exterior closet. And many people that live in manufactured homes are low income or retired/fixed income. Now it may cost them $500 for a water heater and $1,000 to modify the home or relocate the water heater. I’m not sure how much the new water heaters will save you on your gas or electric bill, but it will likely take a long time to recoup that $1,000.

Hopefully manufacturers will overcome this by using smaller tanks or more efficient insulation so the overall size of a water heater is similar.

Randy West owns Professional Building Consultants in Prescott. He is state-certified and has performed more than 7,000 home inspections in the Prescott area. West serves on the Home Inspector Rules and Standards Committee for the Arizona Board of Technical Registration.

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