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Bob Harper

Bob Harper

Joined on December 20, 2004

Last Post on August 16, 2014

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PTFE allowed?

@ August 16, 2014 3:39 PM in Gas Piping - teflon tape

Both the NFPA 54 NFGC and the IFGC allow PTFE tape for gas threaded connections. The attached line from the NYC code does not prohibit such tapes either. This is an interpretation issue. The white vs. yellow PTFE tapes has nothing to do with the gas itself but simply thickness. The yellow is thicker. You can use Blue Monster tape if you want to. I personally do not believe in using any sort of tape or wicking because I've seen and investigated too many valve failures caused by pieces of tape getting into the gas stream and partially clogging a valve seat. The code cannot specify one brand---that's a monopoly. They can specify certain properties or compliance with a national standard but otherwise your inspectors are full of BS. The biggest problem with PTFE tapes is the danger of over-tightening. They lubricate the joints so well you run the risk of cracking the female hub since NPT is tapered. Unless you have another more specific local ordinance, the one provided merely reflects the two major model gas codes. I personally use the blue goos here lately but most PTFE dopes, Rectorseal #5, etc. are all legal. I think the inspector is the dope here. If the dope is compatible with NG and LP, it's legal-done. More important fish to fry such as improper support or testing the joints.

gas codes

@ August 14, 2014 10:47 PM in Gas drip leg/reservoir

You'll need a listed shutoff then sediment trap immediately upstream of the MP regulator then another trap or pressure tap immediately downstream. The sediment trap not only protects the regulator but provides a means of attaching a pressure gauge on the 2-10 psi side or a manometer on the low pressure downstream side. Start with a capped tee off the appliance valve and work your way back for the minimum offset.

more wrong with this

@ July 20, 2014 7:39 PM in Exhaust Ell turned wrong way:

They cannot discharge the exhaust into a well like that. It will recirculate exhaust gases into the intake, which is probably why they flipped the ell even if they didn't have the grate covered with leaves. They should snorkel the exhaust up and out of that well pmi. That's a rain catcher as is. The acidic condensate will eat up that grate in short order as well as the galvanized steel well.

liner sizing

@ July 16, 2014 11:31 PM in Do I need a chimney liner?

I'm too tired to look into my sizing charts, which sit on my desk but let's look at the maximum liner size available:
a nominal "8x13" flue tile may range from ~6"x10" ID to around 7"x11" ID depending upon what part of the country, whether nominal or actual size, and whether or not the tiles are properly aligned, which they seldom are. Let's work with a 7x11 just for fun:
A 7" min. dimension can take a nominal 6" round allowing about 1/4-3/8" per liner wall thickness, which leaves almost no room for the liner to slide down but some. Draw the 7x11 rectangle on paper. Now draw 1/2 of each circle against each end. Since each half measures 3"+3"=6" subtracted from the width of 11" leaves 5". Subtract let's say one inch for liner wall thickness and some gap for the liner to slide down without dragging. That leaves you with a 6"x 5" box in between the two half circles. A 6" circle has an area of 28..27 inches square plus the 30" of the center box = 58.27" square of liner cross-sectional area. Now, before you look this up in your sizing charts we have to derate it 20% off the top, then another 20% if it has an offset from vertical. A 40ft x 6" liner without offset not factoring exterior chimney or climatic zone can handle two or more CAT I natural draft appliances not fan assisted for a combined ~222MBH.
If you want to use a rectangular liner the cold corners don't count, which is why I factored it on an oval liner. In the pic, the WH vent connector is too long, not properly pitched, not properly attached to the draft hood, does not have a min. 12" vent rise and is too far from the common vent. You should never start with an ell right off a draft hood. This WH will probably spill into the CAZ more than vent up the chimney.
Gotta hit the sack but hth,

correction if you don't mind

@ July 16, 2014 7:14 PM in Do I need a chimney liner?

Henry stated if the chimney has a clay tile liner it does not need a liner. Actually, the correct approach is that ALL chimneys must be lined and when a clay tile liner is damaged, eroded or otherwise cannot meet its intended purpose then the chimney must be relined or rebuilt. When changing fuels, upon sale or transfer of property or upon a malfunction, a full level II inspection is required by NFPA 54 because '54 refers you to 211 for chimneys as does '31 for oil. I can guarantee you that a properly performed inspection will reveal defects that warrant a liner in almost all cases, not including the other code considerations for cold exterior chimneys.
If it matters any, I'm a Certified Fireplace and Chimney Inspector and sit on both the NFPA 211 cmte. and UL Standards Technical Panel 103, which writes the UL listing for chimney liners, vents and factory chimney.
My main point is,just because some terra cotta flue tile is visible at the top or base does not automatically mean the chimney can be considered "lined". More to the point, most tile flues were improperly built with improper mortar and have suffered deterioration of both the mortar and the hard fired face of the tile. Sizing alone is often grounds for a liner.

blowing air into gas lines?

@ July 15, 2014 9:43 PM in Bouncing Gas Pressure

First of all, if you plan on blowing any gas under pressure above 1/2 psi, you'll have to disconnect every appliance. Next, you'll have to replace the gas cocks if the pressure exceeds 5 psi. If you blow compressed air, you will probably just add a lot of water into the piping where it will find a low point and puddle and, it will not blow out standing water. If you had a short section and wanted to blow it out, rather than going to all the trouble of rigging up dry N2, just replace the offending section. I know of no method that has been proven reliable at clearing gas piping once contaminated. Moreover, should there be an Ooops! down the road, they would do everything they could to pin it on you and your blow down procedure. Deliberately causing rust inside a gas pipe is insane, grossly negligent and probably criminally negligent.

root cause analysis

@ July 14, 2014 10:09 PM in CO deaths because someone cut off the exhaust:

Crew untrained or improperly trained
Crew unsupervised or ineffectively supervised
Job not properly laid out before hand
Job not post-view inspected by qualified inspector both in house and municipal
combustion analysis not performed on ambient air
no unlisted low level CO monitors installed.
No training for homeowner on need to integrate low level CO as a condition for remotely firing combustion appliances or transmitting alarm signals such as via a 4-20 mA loop signal to an alarm, strobe and integrated telephone notification.

Question: were there UL listed CO alarms installed and any word if they sounded? If alarm company sold and installed integrated home remote control system without proper CO protection, they are negligent. Ditto for dual sensor smoke alarms.

bonding CSST and gas lines

@ July 14, 2014 10:03 PM in Bonding CSST:

All metallic gas piping must be bonded. All CSST must be bonded per the listed instructions. You just can't bond underground metal piping and you can't use gas piping as a conductor. That means using a UL467 listed clamp with a #6 solid copper wire back to the buss bar on the main distribution panel in most cases. The clamp goes on the brass nut on the end proximal to the meter. Some AHJs will allow you to bond it to another well bonded steel pipe. The bonding ties it to an electrical grounding conductor or ground rod. I can drive one in in about two minutes with my electro-pneumatic hammer and a ground rod attachment. The bonding puts the CSST at equipotential with the EGC so lightning would probably go to the EGC instead of blowing out the tubing causing a gas leak. I call out every unbonded CSST I encounter and usually get the job to bond it properly. I usually have to install a ground rod a second stage regulators and meters since it is rarely done before hand. When bonding galvanized or painted steel, you must rough up the surface to make solid contact. HTH

liners

@ July 14, 2014 9:52 PM in Do I need a chimney liner?

Your Realtor should have recommended an NFPA 211 Level II inspection since you bought the house. That would have undoubtedly revealed the need to reline the chimney. All chimneys since 1927 have required intact properly sized liners.

Most mfrs. now require a listed liner to connect to their appliances.

If that water heater vents under positive vent pressure, shut it down at once and have it replaced tomorrow with a CAT I chimney vented model or vent that one per the listed instructions out the side of the house. You can NEVER common vent positive vent pressure with negative vent pressure chimneys or vents.

When relining, have a pro do it and make him show you his calculations in the code for sizing not only the liner but the vent connector manifold as well. Try to get as much vent rise straight up before any offsets. If no spill switches, have a qualified pro install them on bullhead tee with double acting barometric damper. Regardless, when all done, require combustion analysis on each appliance individually and collectively and the ambient room air. Then install unlisted low level CO monitors on each floor and within 15 feet of any sleeping rooms. The liner must use the listed top support plate/ system and rain cap. Have them bring it in like a hockey stick without a tee then once its in the room, transition to galvanized connector. Can you provide pics and the appliance BTU input ratings?

voice of experience

@ July 13, 2014 7:49 PM in Bouncing Gas Pressure

Yes, you must do your calculations and as Timmie says, the proof is in the pudding regarding performance at the appliance under full load.

Now, I wanted to share a case where I had a similar problem. My first thought was a regulator upstream but it turned out to be a low point in the piping forming a bit of a trap. I could feel and hear the vibration from the turbulence. Re-piped it and got some water/ emulsion out and it worked fine. No evidence in the sediment traps. In the old days of wet gas, "drips" were installed right inside the building and the pipes were pitched for drainage.

I investigated a case in the mountains with LPG delayed ignition. Turned out it was gravel (rocks) in the copper tubing. The LP supplier carried copper tubing uncapped in the rear of the truck they use to deliver and set up service. Gravel from driveways must have gotten into the tubing. Before changing a valve, I cracked the gas cock and gravel bounced off my hand! Got it on camera.
Just a thought.

passive makeup air

@ June 26, 2014 10:03 PM in boiler venting

Passive MUA does not work reliably and under certain conditions may hurt

why a ventilated cleanout door?

@ June 12, 2014 9:26 PM in Donley Bros. Co. Vented ash clean out

Would one of you please explain why you would want a ventilated cleanout door? Specifically, what is your application? Indoor or outdoor fireplace? What do you think it does for you? 

codes can get you killed

@ June 2, 2014 11:02 PM in gas range,no vent?

The ANSI standards allow for 800 ppm of CO from a gas oven. The rationale for this is that ovens usually don't fire at more than about 3hrs. At that rate, in a typical building with a minimum ventilation rate of 0.35 Air Changes per Hour (ASHRAE 62.2), theoretically, the ambient CO would not reach levels that would cause a UL listed CO alarm to alert. That's a lot of ASSumptions but it gets worse. They base this off the UL 2034 listing of those crummy CO alarms. The way their sensors are set up they will not alert until your blood has reached a carboxyhemoglobin level of 10%. In other words, the UL standard is designed to protect against CO death only--not CO poisoning. Therefore, your gas oven can legally poison you daily as long as the dose is not immediately fatal and keep on doing it year after year. Now, do gas ovens burn cleanly or produce more CO than the standard allows? Try this test: zero your combustion analyzer and place the probe at the back of the range top where the combustion fumes exhaust and leave it sampling. Light the broiler with the door closed and let it warm up. Note the fluctuations in CO levels. Now, open the door so the cooler room air rushes in on that overhead spreader plate and watch the CO levels jump! What about a gas range? Set up your analyzer running then place a pot of cold water over a burner and watch it jump. Now, breathe deeply and tell yourself this is all approved by code. The tests are with just the bare burners and no with pots of cold water on them or with an oven door ajar as commonly used. It is not run with tin foil lining the floor of the oven either.

Note that microwave ovens listed for use over ranges typically must be 66" above the floor, which provides only about 13-16" of clearance from the burners. Combustible cabinets, however, must be at least 30" above range top burners. These numbers are based upon a maximum input of 68,000 BTU/hr. Large "professional" cook tops may require higher clearances. "Commercial" cook tops are not allowed in residential applications hence the cheating on naming them to get by the code. Some of these flamethrowers get waay too hot for residential applications and typically have stated clearances to combustibles with requirements for noncombustible materials in proximity.

Gas ranges do not require exhaust fans in residential applications.

UL listed CO alarms protect fire departments from responding to so many alerts. They provide a false sense of security. Get unlisted CO monitors instead.

gas dryer sizing

@ May 29, 2014 8:40 PM in LP Dryer

Sizing a gas dryer is not any different than any other gas appliance. You determine the input BTU/hr. rating of the appliance along with all the other gas appliances in the home then run a Longest Run method calculation as shown in the gas codes and GAMA tables. Each appliance comes with its own regulator to modulate the pressure to the burners. You will need an approved terminal within 6 feet of the appliance with an approved shutoff. From there, you can use an approved flexible appliance connector properly sized, up to 6 feet long for dryers. Do not reuse flex connectors or flare fittings. You are not required to provide a sediment trap at the shutoff (often erroneously referred to as a "drip leg" or "dirt leg") but they are still a great idea.

You would not need a Medium Pressure regulator upstream of the appliance unless the house is provided with a 2psi gas system. In that case, you would need an approved shutoff and sediment trap immediately upstream of the regulator then another trap or other approved access point immediately downstream of the regulator. The reg. would need to be supported on both sides and have a ground union to allow removal. The MP regulator must have an approved vent limiter if located in a ventilated space or an approved vent to the outdoors if located in an unventilated space. You must have sufficient space to pull the appliance out and still get to the gas piping and rear of the appliance for service.

Follow your codes on venting. In general, plan on 26 ga. galv. single walled duct with male ends downstream joined with UL181 a/b-fx tape-no screws. May use flexible transition duct from appliance to rigid only, max 8 ft., cannot penetrate walls or floors and must be listed to UL 2158A which few are.

Passive MUA

@ May 28, 2014 7:21 PM in Crazy idea

ASHRAE conducted a study and prove passive MakeUp Air (MUA) is very unreliable. However, there are a number of problems with simply ducting outdoor air directly into a return plenum:
-if the air is too cool, you can actually damage a heat exchanger. The MUA would have to be brought in far enough back so it gets tempered with stale indoor air. This requires a metering system.
-Outdoor air can be laden with moisture, odors, noxious fumes and chemicals, bugs, etc.
-If you bring in too much MUA close to the unit then that forces a reduction in return air from the far reaches of the building. The result is air stagnation and a drop in IAQ.
-if you bring in too much MUA, you can over-pressurize a room causing air to be forced into interstitial wall cavities. This can lead to mold in walls and "soot" stains on carpets.
-if the MUA is too cold, it can throw off combustion analysis which can lead to CO production, sooting, and a drop in efficiency
-If you bring in too much MUA, your TESP will fool you into thinking your lousy ducts are great and you'll adjust your fan curve improperly
-The duct you use to introduce MUA would need to be insulated to discourage condensation, pitched for drainage of said condensation, preferrably have an in-line duct heater to temper the air, have a fan to force the MUA in so that slight pressure gradient shifts won't adversely affect flow, slave the fan to the AHU's fan speed (multi-speed fan), need a high limit switch to provide a safegard on the inline heater, provide an thermocouple/ snap disc/ RTD to monitor incoming air and turn on the inline heater only when needed, provide an activated removable charcoal filter, provide a MERV 8-12 particulate filter again with access, have all joints sealed with UL 181 closure systems and provide dehumidification for summer months.
-the intake must be well away from grade, exhaust penetrations, lot lines, noxious fumes, stinky things, etc. yet be accessible for inspection and service
-the intake would need a backdraft damper to close it off when not in use
-the intake would need to be very strong with a fine mesh to keep out critters and bugs yet still have sufficient net free area to provide adequate air flow
-be large enough to supply the requisite air flow.

Other than that, there's nothing to them!

making my point

@ May 28, 2014 7:01 AM in Carbon Monoxide/Smoke Alarm Interlock

Sorry guys but you advocates seem to be making my point. You are putting them in the wrong places. You expect a human to enter a toxic environment to hush/ investigate the alarm. You claim no negative effects: how many times have your sensors failed to alert when they should have? How do you know when one of your alarms alerts? Do you have any sort of central station monitoring with data logging? Sorry but this issue is far more complex than LWCOs or spill switches. My overriding concern is the false sense of security you are breeding with these installations. No, I don't see this getting "perfected" in this generation by a long shot. 

history of mfd gas

@ May 12, 2014 7:43 PM in Galvanized pipng for gas

http://en.wikipedia.org/wiki/History_of_manufactured_gas
This explains what constitutes "manufactured gas" and some of its main derivatives along with the history. You will see the bulk of it up until the mid-20th cent. was derived from coal. There was a brief period where various hydrocarbon species were also employed The use of "naptha", which is a generic term covering dozens of various hydrocarbons, was not to burn it to form another combustible gas. If you burn naphthalene (moth balls) you get: C10H8 + 12O2 = 10 CO2 + 4 H2O + 5154 kJ  The point is, combustion of it once cannot be piped off and burned again. Yes, there was some blending of all sorts of aliphatic and aromatic hydrocarbons. It was Katie bar the door until the petro-chemical industry figured out how to make money off the previously thought of waste. A prime example of this was gasoline. It was initially a waste product from the production of kerosene to light America's homes until Edison came along. Then Ford found another use for it and the rest is history.
Yes, we are discussing using galvanized steel pipe when directly exposed outdoors but not buried. The inside and the outside of the pipe gets coated with zinc but threading removes this protection so you must apply paint, oil, dope or some other protectant to exposed threads. You can use galvanized steel indoors if it is otherwise approved for use in that jurisdiction (see my first post). It's just overkill in most cases. If you pipes are rusting indoors, fix the humidity problem.

Back to the mfd. gas; back then if a heater gunked up with soot or backpuffed into the home, it was not something you sued anyone over. It was your misfortune. A lot of homeowners owned brushes and swept their boilers and chimneys regularly themselves because there weren't any chimney sweeps. The houses leaked like a sieve so any fumes flushed out quickly and every lived with various forms of soot, dust, dirt, etc. We didn't have forced air ventilation systems for the most part until into the 1950s. So, as people became aware of the waste from coal gasification plants polluting their neighborhoods, the fumes poisoning their families, the homes become nicer with white wall to wall carpet, the homes being built like mason jars thanks to the Yom Kippur War, and litigation becoming the fastest growing livelihood in the US coupled with the delivery of much cleaner and cheaper NG ran mfd gas out of business. However, those old appliances set up to burn that junk are not suitable or compatible with burning NG. Moreover, a heater that old is grossly inefficient by today's standards and is highly suspect at having multiple failures in the heat exchanger, burner compartment or producing high levels of CO. Those units also were typically attached to undersized chimney flues which would work back in the day but not anymore. These aren't "Ozzie and Harriet" houses anymore and neither should their heaters be vintage.

other gases

@ May 10, 2014 10:22 PM in Galvanized pipng for gas

Before WWII, much of the gas was "manufactured gas" or "city gas". You'll see old energy company names such as "XYZ Gas Works" or "XYZ Electric and Gas". Well, the *gas* back then was probably made by the gasification of coal--not natural gas or propane. There are gobs of different types of mfd. gas and they became known by where they were produced. The Gas Engineers Handbook is full of tables on these various gases, their chemical analysis and properties. You had all sorts of names for them such as "water gas" where steam was indeed injected to names like "blue gas". One of the common constituent gases was carbon monoxide. There were all sorts of other nasties mixed into these gases.
 If you watch old pre-WWII melodrama's on TV, you might see a scene where someone was asphyxiated by turning on the kitchen range. Well, NG and LPG are not toxic directly. They do displace O2 but otherwise are not poisonous. CO certainly is and that is how a lot of murders and accidental poisonings happened before NG. What is interesting is how many appliances are still out there with rating plates stamped into them indicating the appliance was set up for "MFD Gas" and never modified. These boilers and furnaces are major safety concerns. Combustion analysis will undoubtedly reveal very lousy combustion and often very hazardous. I always red tag and lock out these appliances when I find them. They are almost always sooted up. Even where someone did some sort of conversion, there are questions.
Yes, those drips were rather large and often full of *stuff*.

chemistry 101?

@ May 10, 2014 9:33 AM in Galvanized pipng for gas

I am not a chemist and don't play one on TV but here is my rudimentary understanding: For hydrogen sulfide to react with the zinc in galvanized steel, several things have to happen. The most important, as I understand it is moisture: the H2S needs to get into a solution form of sulfuric acid in direct contact with the zinc: >zinc sulfate and hydrogen. Next, anything that scratches or degrades the surface of the zinc  to allow an entry point of the acid to really set up shop. The third is time. Lastly, is the condition of the zinc--a coating of zinc oxide is totally different from bare zinc and react differently. That forms zinc sulfate and water. If you don't have moisture in the pipes, then it becomes hard to get corrosion for any reason. This is why they had "drip legs" in buildings. A drip is NOT what everyone else erroneously calls it at the appliance. That is a "sediment trap". A drip should be located as a gravity sump right where the gas enters the building. It is much larger than a sed. trap at an appliance and, like a sed. trap, must be "accessible" to unscrew the cap, drain the condensate and inspect it. Some systems actually pitch the entire system back to a drip, just to keep the moisture and condensate out and away from the appliance.
As for copper, this is probably your best resource as it is written easy to understand:  http://www.copper.org/applications/fuelgas/pdf/Official_Copper.pdf 
I will dispute their claim about the low incidence rate of gas valve problems. I work in product liability and litigation support. I know of a number of cases where copper sulfide was found to be the culprit. Thus, for a long time, special internally tinned copper tubing was available for use with NG or LPG that was suspected of containing too much H2S.
Galvanized steel pipe should be fresh so there's not a lot of zinc oxide coating on the inside to flush downstream. If you keep old fittings, as I do (yes, I do gas piping), I run a baby bottle-type brush through all galv. nipples and fittings. Use clean, fresh gas cocks and never reuse regulators or flexible appliance connectors.
HTH

then vs now

@ May 9, 2014 10:10 PM in Galvanized pipng for gas

Back in the day, when the gas was not as well refined as it is now (wet gas), the impurities, especially hydrogen sulphide, caused some corrosion associated problems. With the advent of the EPA and other regulatory agencies, the gas has been scrubbed rather well. Still, private wells abound so you can encounter wet or sour gas. It's always best if you have a laboratory analysis to confirm it but not too many techs carry a gas chromatograph on their vans.
I would be more concerned about lousy gas with copper tubing because the potential to form copper sulphide flakes. This stuff is often referred to as "black dust" or "black flakes" and yes, it can and has clogged gas valves. These clogs have lead to explosions, delayed ignition, sooting,  carbon monoxide production, etc.
Most of the things old timers were taught were based upon actual conditions, rumors, urban legend and just plain falsehoods from generations before. Yet once something gets into the codes, its there to stay. For instance, draft hoods made sense to the great minds of the world 80 years ago when houses leaked like a sieve. Today, we don't know how many people die or get sick annually thanks to these contraptions but they should have no place in modern weatherized buildings. Unfortunately, they tend to work as designed.
When a local gas utility REQUIRES galvanized steel pipe, that ought to tell you something.

gas codes

@ May 6, 2014 8:55 PM in Galvanized pipng for gas

Both the IFGC and NFGC allow galvanized steel pipe where the gas contains less than 0.3 grains of hydrogen sulphide per 100 SCF. Around Philadelphia PECO requires galvanized steel pipe outdoors and penetrating the wall. FYI, this requirement also applies to copper.

false sense of security

@ May 5, 2014 7:21 AM in Carbon Monoxide/Smoke Alarm Interlock

Hp, not to bust on you but I'm curious of your take by line item on my initial post. Do any of my points ring with you? For instance, what if this UL listed alarm alerts in a boiler room? Who is going to hear it and recognize it as a problem that requires action? When a UL listed CO alarm alerts, what does that mean to you in regards to actual exposure to CO?
Let's say one of your alarms is sounding in a boiler room under an apartment building. A tenant is motivated to open the door and enter to see what is causing the noise. Is that a problem to you?
On a side note, the alarm Hp specified is a cheap ionization sensor smoke alarm that senses free burning flames and not smoldering ones that take a photoelectric sensor. FYI,to be of any real benefit, ALL smoke alarms should be dual sensor.
In the case of an apartment bldg. for ex., let's say your alarm alerted and shut down the boiler(s). How do you know the boiler(s) was/were the source? If you have an underground parking garage, it must have a hard wired CO alarm system that goes back to a control panel to alert "someone" and do "something". Does this bldg. have such a system, is it working and how do you know? Do they hire a factory authorized pro to perform the initial calibration and test them do it again annually replacing sensors as needed? Does anyone perform any sort of test on these systems when they are first installed? In other words, are you certified to test these CO alarms and do you carry a calibration kit including test gas and flush gas with you? If not, how do you know what you installed is doing its job? Again, not to single you out but to illustrate my point. This law merely provides relief to a few people's conscience but does not come close to addressing the real issues. I am a mfrs rep. for MSA and sell professional hard-wired systems daily. It kills me to see UL listed systems but that is what is written in the codes so my clients are forced to comply. You have to reach such levels of sophistication before you get any sort of break from insurance carriers as one point. BTW, if a listed CO alarm is alerting in a boiler room, you don't want anyone walking in there without SCBA and backup. If a boiler is producing sufficient levels of CO to cause a UL listed alarm to alert then those room levels are already very high. If you kill the boiler, you remove the primary exhaust system for the room. The chimney cools and all that CO spills back into the CAZ and the ambient levels skyrocket. That's why post purge should be mandatory but that's another fight. So, someone enters by themselves with no one else in the world aware they are there or why. They silence the alarm, become overcome and pass out then die. Your body is found by itself only hours later when someone goes to investigate why the building is getting cold.
 Sorry but the more I study this, the worse its looking.
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