Need a 90+ furnace and pressure switch 101 please

[Iceman_pdx]

Gentleman,

Happy New Year and may our industry prosper in 2013

I need a 101 on pressure switches. I'm used to pressure switches with just a vacuum hose that closes the switch. I'm on a Lennox system with a + and a - hose connections. Both connect to the collector box. Is one sensing the negative pressure on the inducer motor and the other sensing the positive or discharge of the inducer motor? I read some or this senses the combustion and discharge? Wouldn't they hook up at combustion chamber and other at the collector box? Simple answer I'm sure so thanks guys

Here's problem I was on the call and I'm the second company there, and this is second time there pressure switch has gone bad. I just installed my normal pressure switch with sinle hose connection and up and running for now. Looks like the installer Joe Blow ran the minimum pipe size on both pipes, and burner looked like wasn't burning right and I removed the combustion pipe to add more air and gas furnace seems to be burning stronger more flame, so system is lacking air, and I believe they ran that from garage all the way thru roof, so system needs to be increased to 3". They also seem to be having to remove water from the drain box as well. Question is water getting into pressure switch, and will undersized venting cause this water issue and are all these things toeing into one common problem? So 2 questions this and how thie above pressure switch works? Thanks for the 101 guys

[Cosmicmuffin]

Believe I had one of these calls...,the collector box needs to be cleaned (and all condensate hoses). Condensate water backs up...,trips P.Switch. Takes you hours to get there..,system drains off before you arrive! Took me 3 trips to figure out! Must admit Im a little confused on those dual hose P.Sw. also.

[2141]

Usually one hose goes to the collector box and one goes to the burner box. it is possible that the last guy put the hoses on wrong. I have seen this before. I have also seen water in the pressure switch. you can tap it and usually get some out. if it does have water in it the switch would be replaced even if it works. the micro switches inside will not last long and you will be back soon.

[littlebigdad]

I use a magnahelic to check vent pressures. You need to connect the neg and pos to get a true reading. As for the water check the traps and drain lines also they need to have a vent after the trap. The a/c drain line could also cause problems if its connected to the furnace trap. The manual tells you how they want it done

[chaard]

Red tubing goes to - port. Black tubing goes to + port. There has been an issue with these collector boxes. The ports need to be fluted. If not change the collector box. The drain traps on side are prone to cracking. We had an issue also with press switches failing.

[rundawg]

I'm on a Lennox system with a + and a - hose connections. Both connect to the collector box. Is one sensing the negative pressure on the inducer motor and the other sensing the positive or discharge of the inducer motor? I read some or this senses the combustion and discharge? Wouldn't they hook up at combustion chamber and other at the collector box?

On the Lennox systems, the Combustion Air Inducer (CAI) is installed on the cold end header box. The box has an internal channel where the combustion air inducer creates negative pressure on unit start up.

The channel contains an orifice used to regulate flow created by the CAI. The box has pressure taps for the CAI pressure switch hoses. The switch measures the pressures across the CAI orifice, or difference in the channel and the box.

On start-up, the pressure switch senses that the combustion air inducer is operating. The switch closes a circuit to the ignition control when the difference in pressure across the CAI orifice exceeds a non−adjustable factory setting.

If the flue or air inlet become obstructed during operation, the switch senses a loss of pressure differential, and opens the circuit to the ignition control.

If the condensate line is blocked, water will back up into the header box and will also reduce the pressure differential across the switch. The pressure switch opens if the differential drops below the set point.

[Milk man]

What is the Model number and is it on propane?

You need to use the proper switch. I reads the differential pressure across the Cold End Header Box. (collector box).

Since we are in an open forum you will have to use your experience. Keep it basic. Everything has to be clean, from pressure switch ports to drain lines.

If the trap is cracked or broken the furnace will not drain properly. You will see a rush of water out of the trap when the draft motor stops if your trap is not sealed.

[Milk man]

Nice explanation Rundawg.

[Milk man]

Red tubing goes to - port. Black tubing goes to + port. There has been an issue with these collector boxes. The ports need to be fluted. If not change the collector box. The drain traps on side are prone to cracking. We had an issue also with press switches failing.

What have you done to solve the trap problem? I have a certain furnace that is nothing but trouble with restricted ports with a new CEHB and broken traps.

[Andr00]

These collectors have an annoying issue where they plug up with a white substance - looks like by-products of combustion to me, but I don't know. We just replace the collectors because they'll just clog and clog again otherwise. No callbacks once they're replaced.

[pacnw]

Red tubing goes to - port. Black tubing goes to + port.

I thought, and expressed this to our Rep who did not get it as he is obviously a desk guy, it was stupid to do the colors this way.

as anyone who does any mechanical work can attest to with black and red involved, black is - and red is +. just seems to make a possible issue when common sense is not used.

just my $.02

[pacnw]

initial version of some unit pressure switches are defective and there is a free replacement.

collector box cracks were USUALLY due to installation with other than hand tool or overzealous installer not wanting it to run away.

[Milk man]

These collectors have an annoying issue where they plug up with a white substance - looks like by-products of combustion to me, but I don't know. We just replace the collectors because they'll just clog and clog again otherwise. No callbacks once they're replaced.

Not always. A G51MP-090 on LP has an ongoing problem even with the new CEHB. This is also the furnace with a broken trap each year.

[chaard]

What have you done to solve the trap problem? I have a certain furnace that is nothing but trouble with restricted ports with a new CEHB and broken traps.

Just replace the trap. And make sure the trap is not stressed. Having issues with condensate collars cracking as well. Lennox hasn't done anything to solve the problem so we are at their mercy.

[pcp]

The pressure switch is marked positive and negative and so is the collector who cares what color the hoses are?

[chaard]

These collectors have an annoying issue where they plug up with a white substance - looks like by-products of combustion to me, but I don't know. We just replace the collectors because they'll just clog and clog again otherwise. No callbacks once they're replaced.

White substance is crystallization. Co workers say its due to too much heat in the collector box. The water is boiling.
I'm still skeptical of this.

[chaard]

The pressure switch is marked positive and negative and so is the collector who cares what color the hoses are?

No its not.

[pcp]

Maybe not all of them but the ones I've worked seem to always be marked

[rundawg]

Nice explanation Rundawg.

Thank you.

[Iceman_pdx]

Thanks "DandyMe" I got Lennox instructions now I understand about pressure differential on the switch. Thanks