Blocked Vent Switch problem

John W

Christmas Day 2009....

A Brooklyn family wakes up to a cold house.  The father dilligently trudges down to the basement to trouble-shoot the problem.  He concludes that the boiler's blocked vent switch is keeping the boiler from kicking in.  He jumps out the switch as a workaround and makes sure there is no buildup of fumes by pretending to be a canary in a coal mine and emerges from the basement as the hero....but the story does not end there.

System background: Single-pipe steam system connected to a gas-fired Burnham boiler that was installed 10 years ago.  Until now, the boiler has operated flawlessly.  The flue was inspected and cleaned about 18 months ago (one problem free heating season ago) by a professional chimney cleaner.  The flue goes from the basement to the roof of a three story (plus the basement) townhouse.  The boiler is the only piece of equipment connected to the flue.  The system has not been modified in any way since a hot water heater replacement 2 years ago.  The flue damper is working properly.  On startup of the system, I can tell the combustion gases are rising through the flue because the flue gets hot.  Until now, the system has worked properly for this heating season (since September/October of 2009).

Fix attempt: A few days ago, I got a replacement blocked vent switch and installed it.  The system worked problem-free for 3 days.  This morning there was no heat.  I jumped out the switch again to get the system running again.

Obviously, this is no way to responsibly operate a gas-fired piece of equipment.

Before I continue my attempts to fix the problem, I have a few questions:

1.  How does a blocked vent switch actually function?

The switch is located at the bottom of an open plenum that is between the combustion outlet of the boiler and the flue damper.  So I can see that there is no physical blockage at this point.  It can't operate on a temperature basis because there is no elevated temperature before combustion.

2. If there is blockage at the top of the flue or within the flue, why would the system operate for three days and then not?

I am about to climb up to inspect the top of the flue, but doubt that I will find a squirrel's nest or any other blockage.

3.  How would I test to see if the new (or old) vent switch is faulty (aside from putting it in service and not having the boiler work)?

I have a voltage/ohm meter, but I have no idea how to test the vent switch.

Humbly Yours,

John W

John W

Background continued

The Post author continues...

Before you tell me I am crazy to operate a system in this way, I have checked the top of the flue and combustion gases are leaving the flue.  They reach the top of the flue a few minutes after the boiler is started.  This seemed an unlikely cause of the problem, since sensing for a buildup of gases (due to a blocked flue) isn't possible prior to combustion and the vent switch is prohibiting firing of the boiler.

We also have a CO2 sensor in the basement that hasn't alarmed.  Not a fool proof way to be safe, but it's some solace.

I am trying to avoid calling a chimney sweep to find I don't have a blocked flue and then calling a plumber to replace a blocked vent switch to only have the problem occur again.

John W

[Deleted User]

assumption - vent damper

have you watched/verified the proper operation of the vent damper and/or the associated controls?

Mark Eatherton

Vent spill switch tripping indicates...

SPILLAGE, plain and simple.



Let's look at what can cause spillage.



A back draft condition generated by another mechanical means, i.e. GenAire grill, whole house exhaust fan, oversized bathroom fan and tightening up the home in general. This condition does not have to occur continuously to cause the back draft condition.



A back draft condition can occur due to the lack of outside combustion air. There should be 1 square in of combustion air per 5,000 btuH of burner capacity. 1/2 of this air should be near the ceiling and the other 1/2 should be near the floor.



Back draft condition can occur due to a higher outlet to the dwelling that is connected to the same structure (rare, but possible) whose negative pressure can cause the same back draft condition.



Another more common method relates to the use of programmable set back thermostats and a mechanical system/room with little or no heat. During times of set back, the chimney cools down. When the system comes out of set back and the boiler fires, it has a tough time establishing draft and the hood spills until the SPILL switch, trips. In any case, you've replaced the switch and it continues to trip, indicating that there IS a spillage problem, and NO spillage problem is a good problem.



Something has changed.



What is it?



What is the common denominator with the trip? Night time? Equipment use?



There are a HOST of other conditions that can cause back draft conditions, but these that I have just covered are the most common. In any case, you need to locate the reason and eliminate it before irrepairable damage is done.



ME

Mark Eatherton

Vent spill switch tripping indicates...

Double post sticky key... Sorry

John W

Controls verification

I have verified that the vent damper (took apart the 6" round ductwork so I could see it) is operating properly.  I can also tell when it moves becuase I can hear the metal "feather edge" tinkle against the inside of the flue when it rotates.

The Low Water Cut Off is functioning properly and NOT preventing the boiler from operating.  There is an auto feed that is working as well, but that isn't set up to prohibit the boiler from firing (only the LWCO will do that).



NEWSFLASH: My neighbor just called me with a no heat question (I happen to be the neighborhood guy who knows enough about his boiler to advise others.  I always warn them I know enough to get myself/them in trouble.).  Turns out he has the a pretty similar system.  I checked if gas was flowing (it was), pilot light was lit (it was), LWCO was in operation mode (it was), auto feed was working (it was).  The damper wouldn't open when the themostat was calling for heat, so I jumped out the blocked vent switch.  Gues what?  The damper opened and the boiler fired up.  The blocked vent switch is the EXACT same model.  My guess is that we have outsourced all of our blocked vent switch manufacturing to China and/or India and are reaping the rewards of letting the market deliver us our goods.  I know I am dangerously close to politics here, so I will veer away.



Unless anyone has any better advice, I am going to try to replace the switch again.

John W

Holding out on you

I seemed to have left an important detail out of the background information of my post.



I recently insulated the front wall of the basement and installed a properly sealed door.  This has effectively cut down (or off) on the fresh air introduction into the basement.  It would seem I have starved the boiler of it's original fresh air source by tightening up the envelope of the house.  Time to find a duct route from the outside to the boiler room.



So once a blocked vent switch trips, it needs to be replaced?



Thanks Mark for educating me on the spillage issue.

Mark Eatherton

Well,there you go!!

Had you not had a functioning spill switch in place, the appliance would have kept chewing through the room air, eliminating it, causing the air to be drawn down the same vent the products of combustion are trying to exit in.



You can't sustain flame in the presence of CO2. consequently, the ignition system keeps trying, and eventually catches a flame, and the production of carbon monoxide is severely compounded, and eventually people get sick and die.



Your vent switch worked as designed.



Consider yourself and your family members extremely lucky people.



As fuel costs rise, and people start dong "tightening up " of their homes, such as you did,more people will be overcome with CO poisoning. The Fed's are behind the conservation effort push (read cash for caulkers), and as usual, done wrong, people will get hurt.



Whooda thunk...



Proceed with EXTREME caution.



ME

John W

Remediation

The blocked vent switch function now makes more sense to me.  Once it gets too hot (due to backup up combustion gases), there must be some sort of fusable link inside it that melts.  It then prevents the boiler from firing.  So when the first instance of blockage/spillage occurs the switch doesn't prevent it the boiler from firing.  It halts the boiler operation and prevents it from occurring again.  Basically, the whole system needs a reset (with corrective measures).



So now that that is settled...I will have to find a route to the exterior (back of the house to avoid unsightliness), chop a hole in my wall (with a lintel), install a louver, install an insulated intake duct (don't want to have condensate on a cold duct) to the boiler room, and get a damper interlocked with the boiler operation.  I might have been better with the drafty door.  Not really.  I do consider myself lucky and now a little wiser.



Thanks Again.



John W

Mark Eatherton

Old equipment...

When the ANSI standards first started required electric spill switches on appliances, they use to use a fusible link type of switch, which it sounds like you have. As time went on, they realized that nuisance spillage (cold starts) would cause these switches to blow, so they went to a manually resetable spill switch, which is the norm now. Even at that, a tripped switch is an indication of a problem in need if identification and elimination.



Leave the door ajar and see if your problem abates itself.



You are welcome. We're here to help.



ME

[Deleted User]

yes, usually resetable

and that's what's indicated in the burnham i/o manual that i have.  and now that you've limited the available combustion air, the trip may also be letting you know that now there isn't enough combustion air due to the boiler flues being dirty.  when were they last brushed?  yearly is the recommended interval    

Joe_S

Same Exact Scenario

i have a similar situation as the original poster, or maybe i worked up to it.   found an intermittent issue where the furnace would not fire on our 5 year old Peerless MI series. the installer came in checked the water switch then reset the spill switch after which it came on.  problem recurred next morning and i tried to reset it and noticed that the vent damper was shut.  i fiddled it and it creaked open so i replaced it thinking the problem would be solved.  it worked for a day or so then  i had to do resets again every morning.  i then replaced the spill switch (L275 for my system) suspecting that.   it worked for three days then stopped working again (temps dropped 10 degrees overnight).  i reset it and the furnace fired again this morning.   In the thread, someone pointed out a scenario with setback thermostats, which I have used forever going back to our old oil fired burner.   they have worked flawlessly until this season.  nothing has changed that might change the flue or draft characteristics.  the installer said the flue "felt ok" and that he felt a draft (i was told that wasn't enough) so i am having a chimney sweep come in to check / clean it out tomorrow. i will post further changes and the final outcome.   in the meantime i would welcome any other pointers or things to check.



TIA

Joe_S

quick update

i am also suspecting the switch from peerless went bad. i was taking surface temps with a laser thermometer and it was around 235 degrees near the box but around the switch it was less than 200. nothing abnormally hot tjem it just popped again while i wan't looking - i reset it and turned the whole thing on using the thermostats. the damper opened and the furnace lit (because i had previously reset it), so it seems to be popping prematurely. i had read somewhere that the peerless spill switches were cheap and made in china and easily broke. comments?

Mark Eatherton

All bimetal switches are sloppy....

The spill switch is a bimetal switch, which are inherently sloppy. Their typical specifications are set point plus or minus 10% of total range... So, if your set point is 275, then it could make or break as much as 28 degrees F on either side of its set point. That aside, if you are experiencing spillage, the flue gas temperatures could be as high as 350 degrees F, or more depending, and that is well above the activation point of the sensor.



As for the use of a non surface contact thermometer, it generates a cone shaped beam. The further away from the target you are, the higher the average surface area it is seeing, the lower the temperature shown. Hold it right on top of the spot you are checking for most accurate reading.



In all of my years of working on these systems, Ive not seen one failure of a bimetal switch. If its tripping, its telling you something is not right.



On dampers, there is usually a damper bypass switch. Try setting it to the constant open position and see if your problem goes away. Sometimes, just keeping a little heat in the chimney keeps the draft going, and avoids cold start up spillage.



If you don't have a CO detector, GET ONE TODAY!



ME

Joe_S

yes, i have CO detectores

Thank you and yes, I had the damper open in the service position and the limit switch tripped when I walked away for a few minutes. When it tripped, the damper was open at that time and the area under the flue vent had already dropped below 200 (it's in the 30's outside) when I got to it. I reset the damper service switch so that it's working again, and I reset the spill switch. It hasn't tripped since then. I also have a chimney sweep coming in to check the flue.



I checked the temps right on the switch mount point, i had it pointed straight on. So if trips at ~250, that's normal? When I measured before I walked away it was around 220. I don't think it could have touched 240 even. I could stick an oven thermometer on there for good measure. Should I get a 300 degree switch maybe?



And yes, I have CO detectors.

Joe_S

replacement part note

the notes that came with the part indicated that the original system spec was for 250 or 275 and the new part was 275 only. so it seems they adjusted upward for a reason.

Joe_S

chimney / backdraft?

i have pretty much determined that it's not the spill switch because i've replaced it twice and it still trips intermittently.  i have watched it go through an entire cycle without tripping and the temps hover around 220 - 240 at peak (so it could be tripping due to this).  then the system shuts down via thermostats.  in the morning when the temps are raised by the thermostats, i find the switch sometimes needs to be reset.   it seems definite that the system is overheating somehow - probably from inadequate venting?  when the switch is not tripped,  it starts up normally but i will have to check early in the morning if the cold chimney is causing a spillage.  my thinking is that it had already tripped at that point.



in a separate post, i noted the result  of my chimney inspection - basically that i got scammed when i had the furnace installed 4 years ago to replace an ancient oil system.  there is only a 2 ft section of liner at the bottom and 6 ft from the top.  the chimney needs about 35ft(?) of liner.  the top section was crumpled to fit into a 5" cap.    the chimney sweep says it's aluminum (improbable?) and "may be melting".  this has been working flawlessly the past 4 years including last year's nasty winter here in NJ.  there was no blockage noted.  i will have to sort thru getting that addressed - nasty business.



in the meantime,  can the high temps/resets be ascribed to the chimney?   if i open a basement vent window - is that supposed to help identify the cause?  i don't know if i had mentioned that there has been no structural changes to the house since last year.