Bryant Plus 90 Will not Stay On!


  #1  
Old 03-02-14, 07:44 PM
S
Member
Thread Starter
Join Date: Mar 2014
Location: United States
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Bryant Plus 90 Will not Stay On!

I have a Bryant Model 350MAV060120F..and also a 350MVA06060F...the issue is with the larger unit..we came home from a trip away...turned on furnace...fired up and ran as usual...then stopped, even though the thermostat was still "calling for heat"...Checked it out today...when you turn up the 'stat...induction fan starts...ignitor fires..burners light and after warm up..blower runs...all good...but then about 10-15 min into the heat cycle...psssssssssst off go the burners! Induction fan continues to run.
Blink code 31...pressure switch open...so I cycle it again and sure enough..measuring voltage across the pressure switch..I get .oXX volts for a good while..then it jumps around to about 3-4 volts....then the burners go out and at the time the voltage across the switch jumps into the 20's!...So it's open...
Lucky I have a second furnace with the same control components...so I switch the pressure switches around...the 120K BTU unit still acts up..but with the PS from the larger unit..the small basement 60K BTU furnace runs along just fine till it hits the thermostat setting.
I checked the pressure hoses and their connections...all good, these furnaces were first fired off in the winter of 2003..so the rubber is still supple and no clogs...
I'm at a loss..all the other things..inadequate inducer flow, plugged stack etc would result in low pressure and trip the switch from the get go..none of those safety issues just suddenly appear...so I'm scratching my head here!
Any help appreciated big time! rick
 
  #2  
Old 03-02-14, 09:23 PM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 6,519
Received 94 Upvotes on 87 Posts
Check to make sure that all PVC pipes leading into the combustion-air intake and vent/
exhaust are free from any cracks and sags. Also check the combustion-air intake and vent/
exhaust pipes on the outside of your home for blockage
I would still check the vent piping for a restriction.
I have also seen a condensation drain restriction cause a pressure switch lockout with 90% furnaces.
 
  #3  
Old 03-02-14, 09:50 PM
S
Member
Thread Starter
Join Date: Mar 2014
Location: United States
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
OK...I'll check those..but if there was a restriction...I'd think it would open the pressure switch during induction fan run up...B4 burner ignition if the PS is supposed to be a safety device preventing the furnace from coming on with the system in an unsafe conditon..would it not? It ran for with burners showing perfect blue flames for a good 10-15 min after burner light off....then they just shut down and the "pressure switch open" code started flashing...thanks for the ideas! rick
 
  #4  
Old 03-04-14, 07:25 AM
S
Member
Thread Starter
Join Date: Mar 2014
Location: United States
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
I checked out the drains...clear and clean...as for the vent pipes...it's all schedule 40 PVC...2 " from the furnace out the utility room wall and up to the basement ceiling..then switches over to 3" for the rise to the roof...so no danger of crushing or collapse with that..which would explain sudden restriction. The basement is fully insulated, I've never seen it drop below 40+ degrees, even with teens outside...plus the weather in the last two weeks has been mild here...well above freezing, so ice isn't a possibility in the basement vent piping. I guess I could drop a fiber optic inspection camera down it to look for a bird that might have died from CO while sitting on top of the vent end...but how likely is that?
The furnace "healed" itself yesterday when I turned it on for about the 3rd or 4th time to take one more set of "time to failure" measurements...ran just fine till it hit the stat set point...About the only thing I can think of is that some excessive rain came down the flue and settled in the inducer area..slowing it down a little?...All my 10-15 min start up and runs must have dried it out?...That would explain the self cure...but why wouldn't such contamination slow down the inducer from the get go and cause the pressure switch to sense an unsafe vent draft to prevent burner ignition, instead of letting the burners run for 10-15 min?...The logical sequence to me is...inducer fan runs for a period long enough to establish adequate draft..the pressure switch senses the presence of "in spec" draft and allows the burner ignition process to occur. What's wrong with my logic?...thanks for the help! rick
 
  #5  
Old 03-17-14, 06:41 PM
D
Member
Join Date: Mar 2014
Location: USA
Posts: 1
Upvotes: 0
Received 0 Upvotes on 0 Posts
Donnerhund

I've had this furnace for 15 years and will relate the one thing that I do know about. Years ago it happened once and this very cold winter several times. Furnace didn't light up and I discovered some of the moisture-rich exhaust was getting taken into the air intake pipe where the warm moist exhaust condensed and froze in the air intake opening. I tried diverting the exhaust, then lengthened the vertical PVC pipe on the intake about a foot which solved the problem. CHEAP FIX!
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: