Re: furnace code light flashes 3 times /pressure switch...
Check the intake and exhaust plumbing for any obstructions. I had same issue, turned out to be ice in the air intake plumbing on outside of house. This may be the problem. The pressure switch can be tested by pulling hose from purge motor that goes to switch and applying a small amount of suction. The switch should make a clicking sound.
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If its a gas furnace it could be the heat sensor on the pilot flame unit. When the sensor is failing it stops telling the control that the pilot is lit properly and the furnace goes thru another start up sequence. You can try to scrape any flash and dirt off of the heat sensor rod to clean it or remove it and clean or replace the sensor or the pilot assembly. There are plenty or furnace parts on Amazon. The next most likely thing is the relay board is ready to fail that part is ~$80 but you need to get the part number right and be experienced with wiring. WARNING-DANGER B4 you do anything to your furnace Turn Off The POWER first! If there is no plug or switch near by, shut of the circuit breaker! If you don't know what any of this means then you are likely not a good choice to mess with this device Be Safe :)
something is stuck in it, the humming is letting you know that its stuck. assuming you dont have a disposal turning wrench try this. unplug the unit under the sink. using an old wooden broom handle,, place it inside the disposal , and try and turn the unit. once you free it up, use a flashlight to look inside and remove whatever is jamming it up. once this is done, plug it back in and try it again. if the humming has stopped and it doesn't turn on. at the bottom of the unit is a reset switch. push it in, and try it again ( turn off the power as you rest it)
This sounds like a problem with the flow switch(es) that control the operation of the pump motor.
When the pressure decreases I assume that the pump has switched itself off - check this before going any further, but I will assume that this is the case. (If the pump is still running when the pressure decreases then you have a really strange problem and I would need to think again).
The pump motor is controlled by flow switches on the outlet side of the pump - when the shower valve is opened the flow of water activates a switch that supplies power to the pump motor. There will be one flow switch on the hot outlet from the pump and another on the cold outlet.
I assume that one or both of the flow switches is not operating correctly. This can be either a fault with the switch(es) or insufficient water pressure on the inlet side. Has anything changed that might have reduced the pressure on the inlet side of the pump? Have you moved the water tank that supplies the pump? (cold water storage tank)
If there is adequate pressure on the inlet then one or both of the flow switches is not working correctly. To further diagnose the problem try turning the shower valve to cold water only and test the operation of the pump, then turn it to hottest and test again.
If the pump operates correctly on cold only or hot only then the associated flow switch is OK, and the other one is faulty.
Replacing a flow switch is not difficult if you are reasonably handy, and I am happy to provide further help if you need it.
If your furnace has a filter or fan compartment door, be sure it closed tightly and that the filter is installed correctly - there could be an interlock switch that is preventing the thermostat from operating. Also, if you haven't already turned off the power to the furnace, do that, wait a minute and then turn it back on.
Obviously, make sure the thermostat is on, is set to heat and calling.
You make a good point about overheating on heat exchanger temp.
Since you don't have to reset anything for it to recycle, then I assume you have one of the newer forced draft furnaces.
There should be leds on the control board inside the furnace compartment that will blink a code to you, telling what the shutdown reason was. Blinks and code should be on a label inside the blower door panel, or inside the control box cover.
Let me know what they are, and I can give you more direction. I am assuming this furnace worked at some point, and no changes have been made (including closing of duct vents).
The problem you describe sounds pretty much like your gas valve is going bad. There is a brand by Honeywell called a Smart Valve which is anything but smart, when the electronics in the valve start going bad the furnace will run great for a day or two and then the next day may not run at all and then an hour later my run and then an hour later may not run again. Very sporadic operating furnace is usually the Honeywell Smart gas valve going bad.
you should have 24 volts between yellow and white wires at the stat when the stat is set to heat and set as hot as it will go. burners should light first then 3 to 5 minutes later the fan motor should fire up. if burner doesn't light check the wires going into the top of the gas valve see if you have 24 volts arriving there from the stat. if you do then the gasvalve is probably bad. you can bypass the action of the stat by jumping red to white on 90 percent of the stats made and in a few seconds the gas valve should click open gas flow to main burner where pilot ignites it. another easy check you can do is flip stat fan switch to on. fan should come on at once. if not then you have either no high voltage to unit or no low voltage ( as in a bad transformer). if the fan comes on you know you have high volts and low volts...by cover i assume you mean the decorative cover but there second cover with the mercury bulbs and all is still there. if you have only the bare subbase then new stat time. sounds like an old furnace if it has a standing pilot, it should be checked for cracked heat exchanger by a pro with a n electronic carbon monoxide detector. NOT a diy item due to the real danger of co poisoning.