Trane YCD150 Sensor Locations

[MickeyLeesServ]

I am working on a rooftop unit that is a Trane YCD150 Gas Furnace. I am trying to find where the pressure and flame sensor are located on this unit and after disassembling the unit, I was unable to find either sensor. The induced draft motor is working and the ignitor is burning however the gas valve is open for 5 seconds and then closes again 5 seconds later. I have already changed all of the parts including the High Limit. The control board blinked twice indicating that the problem is a system lockout. Does anyone know where these sensors are found or if they are perhaps integrated inside another part? Thank you and any help would be appreciated.

[jackintheboxtec]

The flame sensor should be located in front of the flame! So it can sense the flame.

[monkeyman#1]

I have already changed all of the parts including the High Limit.

:eek2::rules:

[dandyme]

I am working on a rooftop unit that is a Trane YCD150 Gas Furnace. I have already changed all of the parts including the High Limit.

that's just not right

[jayguy]

:eek2::rules:

never been said better.

[VTP99]

:eek2:A guys gota learn somewhere.:gah::whistle:

anyway the last one worked that way :eek2:

[929rc51]

The hot surface igniter doubles as the flame sensor on this trane. There is no pressure switch, let me explain, first lets start with the process
1. Call for heat.
2. Indoor fan starts.
3. Forced draft motor starts
4. HSI warms up
5. Gas valve receives 24vac
6. ignition.
6A. If flame is detected good to go
6B. If no flame is detected gas valve is no longer powered and steps 4-6 will be repeated for a total of 3 times before lock out occurs.

The reason for there not being a pressure switch is because Trane runs there gas valves at a negative pressure(-0.2) to be exact. This is also why the indoor fan starts first instead of your usual fan limit switch bringing your fan on when the heat exchanger gets up to temp. Follow me here, if there is a cracked heat exchanger or something blocking the outlet of the heat exchanger the heat exchanger will now be in a positive pressure not allowing the gas valve to open(by theory).

Since you said you tore the unit down looking for the sensors I assume you have all ready inspected the heat exchanger for cracks finding none. First thing is first since you've tested your HSI knowing its getting 120v and you've had your ammeter on it seeing a range of about 2-3 amps @ warm up and you know your gas valve is getting 24v(you have done all this right?), get yourself a manometer that does negative pressures and test your gas pressure on the outlet of the valve. If it isn't @ -0.2 set it there.(you may need Trane's special attachment to get the cap off of the adjustment screw, I have seen split ring pliers used) If the unit doesn't act right at the proper pressure something else is wrong. Check the screen in front of the draft motor. If everything seems correct but it just isn't lighting replace the HSI. Even though it doesn't test bad I have seen them get weak and not light. You may want to try switching it with another unit if there is one, they pretty much all use the same exact igniter but be careful they are very fragile.

One other thing I will point out if there is a building automation system(BAS) on this unit you need to remove the BAS board and plug back in the factory plug to run in test mode properly,but don't forget to plug it back in when your done.

Hope this helps and in the future don't just change parts because your costing the customer money and not learning how to diagnose in the process.

[VTP99]

The hot surface igniter doubles as the flame sensor on this trane. There is no pressure switch, let me explain, first lets start with the process
1. Call for heat.
2. Indoor fan starts.
3. Forced draft motor starts
4. HSI warms up
5. Gas valve receives 24vac
6. ignition.
6A. If flame is detected good to go
6B. If no flame is detected gas valve is no longer powered and steps 4-6 will be repeated for a total of 3 times before lock out occurs.

The reason for there not being a pressure switch is because Trane runs there gas valves at a negative pressure(-0.2) to be exact. This is also why the indoor fan starts first instead of your usual fan limit switch bringing your fan on when the heat exchanger gets up to temp. Follow me here, if there is a cracked heat exchanger or something blocking the outlet of the heat exchanger the heat exchanger will now be in a positive pressure not allowing the gas valve to open(by theory).

Since you said you tore the unit down looking for the sensors I assume you have all ready inspected the heat exchanger for cracks finding none. First thing is first since you've tested your HSI knowing its getting 120v and you've had your ammeter on it seeing a range of about 2-3 amps @ warm up and you know your gas valve is getting 24v(you have done all this right?), get yourself a manometer that does negative pressures and test your gas pressure on the outlet of the valve. If it isn't @ -0.2 set it there.(you may need Trane's special attachment to get the cap off of the adjustment screw, I have seen split ring pliers used) If the unit doesn't act right at the proper pressure something else is wrong. Check the screen in front of the draft motor. If everything seems correct but it just isn't lighting replace the HSI. Even though it doesn't test bad I have seen them get weak and not light. You may want to try switching it with another unit if there is one, they pretty much all use the same exact igniter but be careful they are very fragile.

One other thing I will point out if there is a building automation system(BAS) on this unit you need to remove the BAS board and plug back in the factory plug to run in test mode properly,but don't forget to plug it back in when your done.

Hope this helps and in the future don't just change parts because your costing the customer money and not learning how to diagnose in the process.

I'm thinking almost right. Just a few things that come to mine. Not a forced draft motor but a combustion blower motor. As far as the pressure sensor how many wires are going to the motor. Now negitive pressure in the heat exchanger. I think no because of where the motor is located. At the front of the the heat exchanger. Blowing and not sucking as with a draft inducer at the end of a heat exchanger. Sucks the gas out of the gas valve.
To check the ignitor read resistance accross it. Now by no means am i a Trane expert but there are Trane experts here on this site and maybe one of them will correct me. :anyone:

[929rc51]

I'm thinking almost right. Just a few things that come to mine. Not a forced draft motor but a combustion blower motor. As far as the pressure sensor how many wires are going to the motor. Now negitive pressure in the heat exchanger. I think no because of where the motor is located. At the front of the the heat exchanger. Blowing and not sucking as with a draft inducer at the end of a heat exchanger. Sucks the gas out of the gas valve.
To check the ignitor read resistance accross it. Now by no means am i a Trane expert but there are Trane experts here on this site and maybe one of them will correct me. :anyone:

I do stand corrected, combustion motor is right. And the heat exchanger will be positively pressured its the gas valve in the negative. If the HE is cracked the indoor fan will off set the primary a/f mixture as well as cause the flame to blow around the HSI. Thanks VT.

[timebuilder]

No one has asked the typical initial questions, so I will.

Are you a professional HVAC tech, and this is a client-owned unit?

Is this your first Trane RTU?

What was the initial service call, and who placed it? Does your company handle the HVAC PM contract for the unit?

[manny238]

Wow. This is to funny

[VTP99]

I do stand corrected, combustion motor is right. And the heat exchanger will be positively pressured its the gas valve in the negative. If the HE is cracked the indoor fan will off set the primary a/f mixture as well as cause the flame to blow around the HSI. Thanks VT.

I often wondered why the indoor blower came on first. Just never reserched that. Thanks

[chillerout1]

is the orifice clear ive had insects crawl in and plug them up