Amana AMS90703BXA Gas Furnace

[Fermental]

Greetings,

We have a 2 year old Amana gas furnace that fires up, then the flame goes out when the recirc / blower fan starts. After this, the furnace blower starts up again, glow plug glows, the flame fires up and stays lit for about half a minute then goes out with a "floof" sound only to repeat the whole process over.

This only happens when it is well below freezing outside like 20 degrees F or below. During more moderate outside temps, all is normal (the flame starts and stays lit during the whole cycle). The filter is new. There is no troube code flashing (steady red L.E.D.) All in all, this thing fires up its burner around 5 or 6 times per cycle when the problem happens. The recirc / blower runs constantly. No problems there. The drain tubes seem clear as well.

There is a 1980's era manual "White-Rogers" thermostat in use. Could this be causing the wierdness? I've checked the intake / exhaust pipes. They seem clear.

Many thanks in advance.

[donn]

lp or nat gas?? if it's lp could be moisture in second stage regulator freezing and locking out ---you said, problem, it's NOT  firing at or about 20 degrees? makes an attempt then nothing--- sounds like lack of fuel---just an educated guess

[Fermental]

Thanks for the reply.

It's a natural gas unit.

All is well at above freezing temps. Below freezing, the unit fires then the flame goes out. Unit fires again during the cycle.....and again....and again. Usually it fires 5 or 6 times during a cycle before the house finally heats up enough for the thermostat to shut it down.

If it's warmer outside, the unit will stay fired up for the whole cycle. The control unit isn't flashing any trouble codes and it never goes into lockout mode. The diagnostic L.E.D. is steady red which means no faults according to the manual.(??)

Is this normal operation? The house always heats up, but it takes longer due to this wierd flame on / flame off thing resulting in luke warm air coming out of the ducts.

Do you think I have a problem with the outside natural gas regualtor during extreme cold?

[Fermental]

It finally set a code this morning. 2 flashes from the L.E.D. "Induced draft blower pressure switch contacts sticking" according to the manual.

Maybe the temperature of the outside combustion air being drawn into the unit  affects it?

I guess I need a new pressure switch?

[donn]

it does sound like a bad (proving) switch ---however ,it acts like it's starving for fuel-- cause of the foof you described- (after the first firing)-- is there anything else  on the  same gas supply line??

[Fermental]

Thanks, Donn.

I took the front cover off and looked everything over. I pulled the hoses off of the 2 pressure switches and blew through them. They seem clear.  I'm not really sure which switch is the "Induced draft blower pressure switch". Both switches have rubber tubes running to different parts of the blower housing. I have an Ohm meter and know how to use it. I'm just not sure when each switch is supposed to be open / closed during the cycle.

The problem happens most often when we turn down the thermostat at night, then fire the furnace up first thing in the morning. After several heating cycles, everything works normally; suggesting that something is sticking overnight and frees itself up after the unit heats up a bit.

I guess I'll just be a "parts changing monkey" and throw a new pressure switch in it.

[hvacdrd]

Start with all the things that the pressure switch is there for in the first place before making any part changes. Look at the intake and exhaust pipes for wasp nests, leaves, or anything your kids can stuff in there. Make sure all intake and vent piping is not sagging causing condensate to trap in the piping (this will make the pressure switch go nuts)

Then head to the condensate drain lines and trap. Make sure all are flowing freely and making their way out of the trap.

The only way to test the pressure switch is with a manometer (very low pressure readings - in inches of water column. 28 inches = 1 psi) most pressure switches are around the 1.0 in w.c. point.

The flame going out when the main blower starts usually isn't a good sign as far as the heat exchanger goes but I've never seen one fail with the Amana tubular design. If it is consistent timing wise then look for a change in the flame where it enters the heat exchanger when the fan starts and report back...

[Fermental]

Thanks, hvacdrd.

I agree that the pressure switch code is just a symptom of another problem. When the flame "fluffs and puffs" around in the combustion chamber, that's when it sets the code.

When I leave the manual thermostat set around 64 degrees, the unit works perfectly. At other settings, it acts up and starts that wierd cycling thing. The anticipator in the thermostat is set to ".7" per the furnace manual. The intake and exhaust pipes seem to flow well when the unit is in operation. There is one sag in the PVC inlet pipe, but I poked a (very small) hole in the bottom of the sag and no water came out. I pulled the 2 drain hoses and cleaned them, but they seemed clean to start with.

I pulled and cleaned the flame sensor. That seems to have helped. The problem is very intermittant now. The unit is only 2 years old and it's really clean inside. I suppose the heat exchanger could be a problem, but all of the metal looks brand new inside. The flame doesnt seem to change when the main blower starts.

Is it possible my thermostat is just wacky? It's a simple 2 wire t-stat.

I'll call a pro if I have to, but I just like to tinker with the thing.

[hvacdrd]

The easiest way to rule out the thermostat is to disconnect it and jumper the wires together. Then see what happens...if the unit runs ok then you are on to something...

[Fermental]

The T-stat checks out ok.

It's an updraft model. When I crack the upper cabinet combustion chamber screws loose a few turns and let some "house air" into the combustion chamber, the problem NEVER happens.

I'm going to take the screen off of the outside intake pipe and poke around in there with a coat hanger or something to check for obstructions. There are 5 elbows in the PVC intake pipe. This seems a bit restrictive to me. Are there any filters in the induced blower path?

 

Many thanks.  This site is great. Beer fund money on the way!!

[AccApp]

What is the BTU capacity of the furnace? What size and length are the intake and exhaust pipes?

[Fermental]

The pressure regulator on the gas valve was adjusted to lower the flame in the combustion chamber. Problem fixed.

There was too much flame for the amount of available air. The regulator was cranked all the way up by someone. We've only owned the house for a few months so I don't know a lot about the history.

Thanks to all for the help.