HVAC Questions? Ask an HVAC Expert for Answers ASAP
Hello.Sorry you are having problems. My name is XXXXX XXXXX I will do my best to help.Please wait to leave rating until we are finished.
This might be a tough one to troubleshoot via inter net, especially since onsite techs have not been able to find the issue.I will offer a few suggestions to check, If no avail or not helpful,do not leave rating and I will open to other experts who might have some other ideas,
As for the pressure sensing switch, several things could be at fault. Make sure condensate is not collecting in the rubber hoses instead of draining properly,Make sure the actual furnace condensate drain flows freely with no blockages or standing water.Make sure intake and/or exhaust is free of obstruction.As small a thing as a spider web or dirt dobber can effect the switch enough to cause issues.Check all rubber hoses for cracks or debris.You can remove the rubber hoses and blow through them and clean fittings with a straightened paper clip. If all has been checked, you might want to check with an authorized Carrier dealer to see if a high altitude pressure switch is available for you furnace (if standard switch is currently being used.)
Now for the pilot lockout, this is usually caused by a gas pressure problem on this particular furnace.You may want to have gas pressure to and from valve checked. It needs to be 5 to 7" WC into valve and 3.5 to 5 coming out of valve if you are on natural gas. If on LP, you should have around 13" Wc in and 10"wc out of valve. Also if your gas piping does not have a drip leg installed to collect condensate, moisture can accumulate in the gas line causing erratic pressures.
These are a few ideas that might help. Thanks for allowing me to work with you.
Thanks for the reply. I, as well as local techs have checked all the hoses, intake and exhaust ducts, and drain lines. No water is in evidence at this time. I have not had a pressure related fault since replacing the pressure switches, but offered that history in that the current problems started at that same time. A local repairman replaced the igniter unit this fall, but the problem still exists. I suspect that some parameter of the start sequence is out of limits, but hasn't been located. I might add, when the start sequence is successful, I can hear the gas valve cycle. during incomplete start sequence, there is no indication of gas valve movement (no click),but the igniters are firing. Are there other sensors that could cause the problem? Thanks.
This unit uses flame rectification, this is a DC signal of micro amps that returns to the ignition module via grnd/sense terminal. If it has no flame sensor, it uses the burner /pilot assembly's bonding which transmits the weak DC signal to the module to prove a flame exists and is safe to continue operating. I have a routine I use for these , I clean the pilot assembly's mounting brackets surface areas and the burners surface areas touching the pilot assembly mounts, basically we are making a nice conductive path to home , being the grnd/sense terminal, some say burner ground. In adition I use 105C degree rated wire and connectors and run a ground wire in addition to the units grounding back to the terminal that is used for sense or grnd, usually they are coon in the boards. Remember it is only transmitting the faint signal that passes through the flame, a flame is a poor conductor and only transmits microamps DC and that is easily stopped by a patina or moisture or corrosion etc, it does not take much and it is easily disrupted and restored. In fact many times new components seemingly rectify an issue, often the restoration of a ground path for the flame signal is really what fixed everything and if you try the old equipment it is operative about 40% of the time according to manufacturers. Johnson controls makes a nifty flame simulator that simulates a nice DC signal that will work on any flame rectification system! it plugs in with the existing flame proving circuit and clamps to ground with a clip, If it wont run with this, no question the boards/module is defective. For the pressure switches I use tattle tales to tell me if any control opens,even if a brief second, that is hard to spot unless gazing at it while it pops opens for 1 second , pressure switches on the threshold will waver and not be evident by watching , tattle tales light up if opened.
If this continues after typical protocols a in depth analysis must be made noting the water column measurements of the intake vs exhaust which the pressure switches monitor for correct differentials. The flame sensing issue is much less complex.The 2 pressure switches monitors components that must operate within parameters deciphered by a differential between the 2 being maintained.
So the flues routing, sizing, length and termination must be analyzed, as will the combustion air makeup to the mechanical room/home/area unit is located and while under worst case scenario condition, meaning all bath vents, kitchen vents and dryer and Jenn aire etc etc must be on and doors closed, and see if a negative pressure condition ever exists and why.
There is a systematic analysis protocol to be done still, the techs there just have never been shown how these things actually run.
Thanks for the reply.I understand the flame sensing mechanics. However the issue is there is no flame. The start sequence fires the igniter but will not open the gas valve.
The information is helpful and gives me a place to start. Thanks. I have submitted a rating.