Calling all AC techs. Why would a furnace run fine without the access panel?

Question

When set for a call for heating, the Bryant furnacte, 376 CAV, will short cycle. 
The ignition sequence is okay: combustion blower energizes, pressure switch closes, HSI energizes, gas valve opens, gas ignites, blower comes on. It runs for two minutes, then all but the blower stops. I get 33 fault code.
The blower motor -  ok.
Capactior - ok
limit switches -  all ok
Air flow - Return and Supply air are ok.
Dampers - not any
wiring and conections - ok
blower motor - rotation ok
combustion blower - ok (would be a different fault code
PCB - ok, is getting power.

When I  tape down the door switch and leave the access panels off, the system will operate fine, until a call for heating stops.
So, the airflow is higher without the panels and it runs ok.

I can't think of any other reason, can you ?

Chuck · Accepted Answer

Sounds like your high limit is opening. This is similar to other limits, a metal disc, size of  dime, but this one is on a 3"-7" probe and sticks into your heat exchanger. If the heat exchanger gets to hot it opens and will reset itself after it cools down. It is on the back wall, close to where the inducer assembly is and you will see a backing with 2 wires to it.

You can either (1) check with a meter to see if it opens, or (2) pull the 2 wires off and jump them together which will keep the circuit closed**** If #2, don't leave it jumped for to long, you can damage the furnace if it gets to hot!!

UPDATE: Well Marko, if you jumped the high limit and it ran normally, then that's your answer; bad high limi or severly restricted air flow.
 If this is it, your high limit is either not reading the correct temp, or lack of airflow thru the heat exchanger is causing it to get to hot.

Greg D · Answer

33 LIMIT OR FLAME ROLL-OUT SWITCH IS OPEN - If open longer than three
minutes,code changes to #13.  Check for:
- Defective blower motor or start capacitor. - Dirty filter or restricted duct system.
- Loose blower wheel. - Defective switch or connections.
- Inadequate Combustion air supply Flame Roll-out Switch or fuse link.
- Open Flame Roll-out switch,or fuse link. Manual reset or replace

Can you follow these directions to get the CFM while your furnace is running with the door off. Then post your CFM and the temps you recorded. Then try it with the door on.
http://rjmurray.com/serviceBulletins/CFMverification.pdf

If this is a new problem. Check your return air grills, make sure they are not blocked or covered. make sure all supply air dampers and registers are 100% open. Check your filter, if you're not sure if its dirty, take it out and test it with out the filter for an hour. Also check the underside of the a/c indoor coil (if you have a/c). Try changing the blower lead to Med/High if its not on that already.

If this is a new system, your duct work may be undersized.

Whats the manifold gas pressure reading?
Whats the supply air Temp when it shuts down?
Also whats the high limit's setting?

EDIT:
It looks like your furnace is doing everything correct. The air flow is a little low which looks like you either have a duct restriction or the duct is undersized. 
Check out this residential air duct calculator and see if the duct exiting and entering your furnace is sized correctly.
http://efficientcomfort.net/asp/ResDuct_Web/ResDuct_Web.aspx
The high limit may be weak, they are really inexpensive. Try changing it, and if that still doesn't work you should add some supply/return vents, or resize your duct.

Calling all AC techs. Why would a furnace run fine without the access panel?

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