Pressure switch problems

One tip is to inspect very carefully the actual barb connectors that the rubber hoses connect to on the pressure switches. They can become slightly restricted with oxidation or other particulate rendering them ineffective. HOWEVER, your comment that the flame "rolls out" has me concerned. Flame roll out is usually caused by a flu blockage. Many times, if the flue is not covered with a screen, small birds roost in them and hunker back farther and farther for warmth then die from carbon monoxide. If they die at or close to a fitting, they can fall down to the first elbow or 45 and create just enough blockage to effect the pressure switch, but not enough blockage for you to determine without cutting off the flu pipe and physically inspecting.

Another thing that causes flame roll out is a plugged p-trap in the condensing coil drain at the bottom of the blower assembly. It could cause flooding of the condensing coil which cause restriction of air flow through the system, resulting in tripping out on pressure switch.

You may have to cut the flu off in two places depending on how sophiticated you flue is. Usually its not too bad but you'll need pvc glue and primer and a couple pvc couplings to reinstall it. DO NOT operate the furnace with the flu disconnected.

If you find this as the cause, you may want to install a screen over the flu opening but use 1/4" mesh as too fine of screen (screen door) will create too much restriction and also effect the sensitive pressure switches.

The roll out switch is a safety switch. It is there for safety. The purpose of the switch is to cut the gas off if there is flame or excessive heat low in the heater as found when the heat cant exscape out the top. Many reasons for a roll out failure if you are tripping the roll out it is always a symptom of a larger problem.
Low gas pressure or lack of combustable air causes soot in the heater like that of a chimney. This will restrict the air flow through the heater. A spider web in the gas oriface or burner can cause a roll out flame to hit the roll out fuse. Indoor applications can easily be an issue with the exaust venting or fresh air supply. Outdoor applications it could be a wind down draft through the heater. Whatever the reason it is tripping the roll out fuse, by bypassing it you are allowing the gas and burn to continue in an unsafe condition which mean larger problems will arise possibly even the heater catching fire.

It sounds like an issue of either incorrect gas valve settings, or a case where the refractory kit has started chipping off on the flame bed.

Heres what you need to find out. Can your service tech tell you what the pressure is before and after the regulator?

On natural gas it should be before the gas valve at 7-14 inches of water colum.
After the gas valve should be 4 inches water colum.

For propane it should be between 12-14 inches water colum before the gas valve and 10.5 inches water colum after the gas valve.

If you tech cannot verify any of this, you need a new tech. This should have been the first thing to be looked at.

Note: The most common cause for flame rollout (which is what you describe) is the refractory kit chipping and cracking . Then peices fall onto the burner tray and cause the high flame condition that you mentioned. If the tech working the job has not checked before and after gas pressures, and cleaned and inspected the burner tray and the heat exchanger. Fire him. These are all the quick easy things that would get you down to the bottom of your current situation. I wish I could go more in depth with troubleshooting, but it is difficult on these forums. You can contact me with any other questions at [email protected]

With out brand and model # all i can do is to tell you to check heat exchanger for holes or flame roll out, Check limit switches if not then could be gas valve

If the water seems hotter than normal, then a faulty thermostat would be a good bet.
If water doesn't seem any hotter than normal, then check incoming water pressure.
usually 2 things cause that, too much pressure coming in, or thermostat faulty and not cutting out when normal temperature reached, thus higher temperature, and pressure. If both them check out, replace valve.

Are you sure that the flame sensor is installed correctly? If the flame comes on and the flame sensor is right in the flame, then the control module is not working right. It's not 'seeing' the flame sensor input. You can prove it by jumper across the contact wires on the flame sensor. If it stays lit when you jump out the sensor, it's not working. The flame sensor tip needs to be right in the pilot flame to sense heat.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/craig_3fa289bf857b1a3c

Definitly a gas valve that needs to be replaced and get another roll out switch after getting hot a few times they are demaged

Sequence of operation, Gas Furnace.

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the ignitor. The ignitor will glow and you will be able to see it if viewed thru the small inspection port. Once the ignitor gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the ignitor does not energize (bad control, bad ignitor), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not lite (dirty pilot), the burner does not lite (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

Most newer furnaces will have a diagnostic center or control built into the control module. You may be able to view it as it is running thru a small viewing port. Some models will require the removal of an access panel prior to finding it. On the reverse of the access panel, there will be a diagnostic chart that will aide you in understanding any error code recorded (usually a blinking light).

This is why we technicians earn the big bucks!

I the electrical gas switch valve set to on? If not...

Is there also a gas valve and is it on? If so...

Does the exhaust fan turn freely?
If not then then electrical gas valve will not open the gas valve. I have seen these bearing get loose and then the exhaust tips toward the inside inhibiting the fan from turning. Note: this is not the large blower but a smaller fan for the exhaust fumes.

Is the Vacuum hose from the exhaust to the sensor in good condition and attached at both ends? If not fix hose or connect both ends.

When repairing a unit, many tech will turn off the gas for saftey.