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Discussion Starter · #1 ·
Hi,
I have a very old (not sure, but could be 30-year old with the house) Bryant furnace (model 518A030). It used to rarely skip a start, but two days ago it just stopped solid with error code 31. Based on Internet information, it seemed that the Pressure Switch (PS) was the most likely root cause, but mine worked just fine (open-close) and the problem turned out to be the Draft Safeguard Switch (DSS), which is serially connected with the PS and was stuck open. I shorted the DSS and the furnace is now working, but could not find much information on its role and replacement.

In one document, it is described as
"Also known as a vent limit switch. They are NC switches usually mountd on the draft hood if the furnace has one. They monitor the draft hood temperature and open when there is any spillage of flue gas."
So I am concerned about potential safety if gas leaks, but the DSS was stuck open even when the furnace was not operational for 2 days, therefore it could not be detecting real gas to stay open. My CO detector is not showing any concern either.

Hope I can get a more definitive response of DSS function and replacement. Thanks!

P.S. Attaching a couple of pictures with the DSS and the shorted (working) configuration.
 

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There is something wrong with the venting system. It is probably UNSAFE to operate if the switch tripped. If you have a UL approved Co detector. It won't alarm quick enough, as UL approved detectors are designed not to alarm quickly/easily, so as to prevent nuisance fire department call outs.

That switch has a manual reset button on it. So that the furnace and its venting system is checked at the first sign of a problem. If it was an auto reset switch. A lot of people would die before anyone knew there was a problem.
 
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Have you tried to reset the switch? Don't "short" or jump the switch out, or any safety switch for that matter. If you have electrical issues use a meter for troubleshooting.
The vent switch is temperature based, it's not a CO or combustible gas detector. I'd make sure all the exhaust and chimney were clear.
 

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Funny thing about safety and limit devices on heating systems . They are there to protect your property an life. A malfunction safety device ob a furnace indicates something that needs to be fixed now before the furnace is used again. It it's a 30 years old furnace it should probably be good idea to have it inspected by a competent pro.
 

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Discussion Starter · #6 ·
Thanks a lot for the responses!
May be naively, but I expected a somewhat different type of DIY advice, more along the lines of how to determine root cause and finally fix the problem. I am generally reluctant to call someone else until I understand enough about the issue.

Anyway, I was somewhat comfortable bypassing the DSS temporarily because it was clear that it had tripped on an event in the past, which could be intermittent and no longer present. Count also the cranky wife impact... I did check the vent pipe, which goes straight to roof and is unobstructed (aside from a rain cap at the top). The narrow white plastic pipe (I guess for condense prevention) also seems clean, though the system looks very different from those I saw on YouTube and the white pipe is not detachable.

Today I got back to the Draft Safeguard Switch (DSS) and pressed on the switch more firmly, so it tripped back to closed position (did not expect to require such force to push back). I hooked the wires back to the DSS and will test tonight if it trips again.

>> roughneck: The vent switch is temperature based, it's not a CO or combustible gas detector. I'd make sure all the exhaust and chimney were clear.

This is very useful information, I was not sure what exactly the DSS senses and could not find detailed clarification online. However, did suspect vents and check those.

>> beenthere: If you have a UL approved Co detector. It won't alarm quick enough, as UL approved detectors are designed not to alarm quickly/easily, so as to prevent nuisance fire department call outs.

Hm, interesting, I was quite cautious (still am) about a CO leak, but would definitely expect the home CO detector to show something before people in the house die. Otherwise, what's the point of having it...
 

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Remove the vent pipe from on top of the furnace and look inside for dead birds or roasted bird bits as they sometimes fall down and in.

If you have a negative pressure in the house from drawing air down the chimney because of using a fireplace, clothes dryer, central vac, kitchen exhaust, bath exhaust at the same or a few of them it can trip.
 

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Thanks a lot for the responses!
May be naively, but I expected a somewhat different type of DIY advice, more along the lines of how to determine root cause and finally fix the problem. I am generally reluctant to call someone else until I understand enough about the issue.

Anyway, I was somewhat comfortable bypassing the DSS temporarily because it was clear that it had tripped on an event in the past, which could be intermittent and no longer present.

It didn't trip sometime in the past. It tripped the day you noticed no heat. The condition is most likely still there.

Count also the cranky wife impact... I did check the vent pipe, which goes straight to roof and is unobstructed (aside from a rain cap at the top). The narrow white plastic pipe (I guess for condense prevention) also seems clean, though the system looks very different from those I saw on YouTube and the white pipe is not detachable.

Today I got back to the Draft Safeguard Switch (DSS) and pressed on the switch more firmly, so it tripped back to closed position (did not expect to require such force to push back). I hooked the wires back to the DSS and will test tonight if it trips again.

>> roughneck: The vent switch is temperature based, it's not a CO or combustible gas detector. I'd make sure all the exhaust and chimney were clear.

This is very useful information, I was not sure what exactly the DSS senses and could not find detailed clarification online. However, did suspect vents and check those.

>> beenthere: If you have a UL approved Co detector. It won't alarm quick enough, as UL approved detectors are designed not to alarm quickly/easily, so as to prevent nuisance fire department call outs.

Hm, interesting, I was quite cautious (still am) about a CO leak, but would definitely expect the home CO detector to show something before people in the house die. Otherwise, what's the point of having it...
The CO level can be high enough to be harmful(70 PPM) for 4 hours before it will alarm and still be UL listed. It has to go off in X time for different levels.
 

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Not too long ago locally we could have 100ppm for up to 8 hour shifts, with 16 hours between shifts. I don't recall very well, but i think the 4hr limit was 150-200ppm, or somewhere like that. You get symptoms at that point, but it used to be considered OK. Fast forward and now limits have changed. Home alarms follow those older limits, but prolonged exposure even at 50ppm can cause lasting effects.

Cheers!
 

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supers05 said:
Not too long ago locally we could have 100ppm for up to 8 hour shifts, with 16 hours between shifts. I don't recall very well, but i think the 4hr limit was 150-200ppm, or somewhere like that. You get symptoms at that point, but it used to be considered OK. Fast forward and now limits have changed. Home alarms follow those older limits, but prolonged exposure even at 50ppm can cause lasting effects. Cheers!
The occupational exposure limit is now an 8-hour time weighted average (TWA) of 25 ppm and a 15-minute short-term exposure limit (STEL) of 100 ppm. The STEL must not be exceeded more than four times in an 8-hour work shift, with at least 1 hour between any two successive 15-minute periods. As far as I know, it's been this way here for at least ten years. Of course, this is in industry, NOT in homes, and home CO detectors aren't used for measuring CO in industrial settings. The acceptable limit for CO in homes from gas appliances is zero ppm.
 

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The occupational exposure limit is now an 8-hour time weighted average (TWA) of 25 ppm and a 15-minute short-term exposure limit (STEL) of 100 ppm. The STEL must not be exceeded more than four times in an 8-hour work shift, with at least 1 hour between any two successive 15-minute periods. As far as I know, it's been this way here for at least ten years. Of course, this is in industry, NOT in homes, and home CO detectors aren't used for measuring CO in industrial settings. The acceptable limit for CO in homes from gas appliances is zero ppm.
Like i said, times have changed, and different jurisdictions would have changed at different times. The point was that people were regularly exposed to 100ppm and expected to work the next day. Wasn't particularly healthy, but neither was breathing asbestos fibers, or decomposing CFCs, or MIC... Some poor souls are still subjected to these.

Cheers!
 

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Discussion Starter · #12 ·
OK, with the DSS manually reset, the furnace worked just fine tonight, will see if it trips again, but the vent is clean. Is there anything else (intermittent perhaps), which could cause DSS to trip? Good to know that it is temperature sensor, not a gas one (thanks roughneck).

Regarding safety, I am still trying to understand the risk (if any) of operating the furnace with DSS shorted and a CO detector nearby. I've got a Kidde KN-COEG-3 (CO and explosive gas detector) from Home Depot, which I think is mandated in California. The user manual states:
Carbon Monoxide PPM Levels
Dangerous Levels:
When someone is experiencing symptoms of CO poisoning and CO readings are generally above 100 PPM. Anytime someone is experiencing the symptoms of CO poisoning this should be treated as an emergency.
High Levels:
Generally above 100 PPM, with no one experiencing symptoms. This should be treated as an urgent situation.
Mid Levels:
Generally between 50 PPM to 100 PPM. This should be cause for concern and should not be ignored or dismissed.
Low Levels:
Generally below 50 PPM. Kidde recommends you take action to eliminate the source of CO.
>> beenthere: The CO level can be high enough to be harmful(70 PPM) for 4 hours before it will alarm and still be UL listed. It has to go off in X time for different levels.

Having the initial concern when turning on the furnace with DSS shorted, I was actually watching the detector reading at 0PPM, but it seems from the manual that the manufacturer recommends action even below 50PPM, so supposedly it would raise an audible alarm if present.

BTW, a friend informed me that San Diego Gas & Electric has a useful free customer service program to come and check each gas appliance for any leak, including CO. Seems like a good sanity check, which I plan to do...
SDGE Gas Appliance Check
 

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Co from the flue is lighter then air. So is your Co detector plugged into a ceiling recep, or low near the floor.
 

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Never ever run a furnace with a safety jumped out. It's there for a reason.
Bypassing it could result in overheating damage to the furnace, venting, cause a CO condition inside the structure or cause a fire.
It's there to shut the furnace down in the event that it's not working properly. If the switch doesn't work, it must be properly replaced before the unit is allowed to operate again.
Plus, if something did happen, and the fire inspector found the safety(s) jumped out, you'd have quite a bit of explaining to do.
 

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Flames may roll out the front of the furnace and burn the wiring etc. Also will spill CO from poor combustion. If there is a downdraft or not enough draft then the heat and flames come back out the front of the furnace.
 

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The density of Carbon Monoxide at 20 °C ( room temperature) is 0.96716 compared to air (1.00)

Meaning it is the slightest bit less dense then air. (will eventually rise in static air) However, the flue gases will be very hot and will rise carrying all products of combustion with it. It will travel the the floor above fairly easily. Your detector has to be in that path. CO is not the only problem product of combustion though.

CO2 levels will be high enough to cause diminished brain compacity, along with other ill effects on the body. It's not sensed by your alarm. In high enough levels for long enough, you won't be able to recognize the CO alarm when it does go off. High levels of CO2 mean lower levels of O2, which causes more CO. It's only a matter of time.

Cheers!
 
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