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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5648
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a 373LAV furnace with a 33 code. I changed my filter

Customer Question

I have a Bryant 373LAV furnace with a 33 code. I changed my filter at the beginning of October, checked it today and it's still clean. My blower stays on and it does not ignite. In checking my 33 code possibilities, my flame rollout switches both have .4 ohm resistance when checking across the terminals. I do not have the red-button type. Ideas?
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.

Hello,

There are two (2) blowers on your furnace... the small inducer fan (blower) and the main blower that circulates warm air through the house... I need to know very specifically which blower you mean.

LISTENTO FURNACE START UP
Stand next to the furnace after the thermostat has been turned off for 5 minutes. Listen intently as someone else turns the thermostat to heat and sets it for 85F.

You should hear the small inducer fan start right away.. then a faint click after 1 to 3 seconds, that is the pressure switch closing.

....then in 15 to 45 seconds you should hear 2 more faint clicks, that is the ignition relay and gas valve relay closing.

....then within 5 seconds you should hear the main burner fire..

----------

If the main burner goes off after 2 to 5 seconds or within a minute or so tell me about it and the number of seconds involved.

Repeat that a few times until you get familiar with the sounds, then tell me in detail how far the sequence goes before it stops.

It is also a good idea to replug all of the connections in the unit, that burnishes any oxidized terminal connections and will sometimes solve the problem,especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course.

We can go from there and get to the bottom of this quickly by these means... there will be subsequent tests on the printed circuit board. Using a volt ohm meter is very useful in very many cases, but not all cases, in some cases those reads can be *entirely misleading due to feed back in AC electronic circuits (not seen by the way in automotive DC circuits).

Customer: replied 1 year ago.
As soon as I put the cover panel back on, the big blower that fills the house with air comes on. It does not matter what my thermostat is set at - in fact, it is set to OFF. The 33 code gives me a short list of things to check. I am at the flame rollout switch stage.
Customer: replied 1 year ago.
I have to go to work now. I will be back at this issue around 4 pm PST.
Expert:  Phil replied 1 year ago.

Thanks,

I will be in to about 10 pm tonight, then back all day tomorrow.

There are several things that will cause the blower to stay on at all times.

- a bad printed printed circuit card, fan relay contacts burned closed. (70% chance at this point)

- The furnace high limit switch is tripped, the card programming runs the fall all the time in that

case to insure the furnace does not over heat, (20% chance)

- Flame roll out switch open, 0 to 5% chance.

Please attach a well focused, close up photo of the wiring diagram glued inside the service panel,you can use the paper clip icon at the right end of the toolbar at the top of this dialog box,Include a picture of the thermostat wire connections to the printed circuit board in the same area.

If there is no paper clip icon, look for the '+attach files' link at left of your send button.

I can mark up the wiring diagram with trouble shooting instructions.

You can also perform a heat stress test on the printed circuit card..it is about 70% reliable.

Use a hair drier to slowly and gently warm the card to 120F(warm to the touch but not hot and not over140F)... if you get any change in behavior that means the card is bad...

A micro crack has opened in the printed circuit at a solder joint, or inside its CPU in that case .... or a short circuit has developed as the card expanded when you heated it.

Constant expansion and contraction as these electronics heat and during their duty cycles over the years is what wears these cards out.

It is also a good idea to replug all of the connections in the unit,that burnishes any oxidized terminal connections and will sometimes solve the problem, especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course.

We can go from there.