Greetings,
I'm having troubles with my furnace kicking on reliably when the temperature in the house drops below the thermostat setting. Sometimes it comes on by itself, other times I have to go downstairs and bang the top of the furnace to get it to kick on. Any ideas as to what part is beginning to fail? or where I should investigate first. Once i bang the top of the furnace the furnace kicks in and the heater works fine for a while.
thanks!
kw
Without knowing more about your furnace, little can be done. What make and model? Gas or oil? About how old is it?
It is a forced hot air furnace (gas). Ruud Silhouette II Gas Furnace
- I opened the top of the furnace and looked around... nothing looked unusual (i.e., no loose wires, nothing melting, etc...). Unit appears to be relatively new looking (although i've lived in the house for 14 years - so it's not really that new)
- Right below where I typically bang the unit to get it kick in is a pressure switch (round in shape) with a number on it: MPL 9300-v-0.35-deact-n/o-vs.
- When it won't kick on, i look into the site glass and see two green lights; 1 solid, the other blinks 4 times.
any ideas?
Thanks
If you have a pressure switch, that means you have a draft inducer fan.
Does that inducer fan start up when you turn up the stat?
Or does the furnace require banging first, for that to get going? If the later, then I'd suspect a bad connection, bad heating relay, sequencer, or something bad about that motor.
no, nothing happens until i bang the furnace.... then everything kicks in and works fine
Then it is one of those things I mentioned. Sequencer or heating relay acts like a 24 volt x 120 volt contactor to make the 24 volt thermostat current to allow 120 line voltage to the inducer, for the very first function of the furnace. Without that inducer running, the 24 volt safety circuit cannot function to allow current to get to a control board or module.
You can rule out ignitor, flame sensor, backed up condensate, bad pressure switch, blocked pipes, bad burners, bad gas valve, bad filter, and probably more.
It's got to be one of those things I mentioned in my post before.
Oops -thought of another possibilty that could be it! If you have a combination NO, NC 3-wire pressure switch that requires the pressure switch to close current when the inducer is NOT running yet (to thwart people who put jumper wires on one), that pressure switch may be sticky or bad contact inside it, and not closing down(or making good contact) after the furnace shuts off sometimes!! You could jumper the NC terminal and see if at least the inducer runs. Or even suck or blow on the pressure switch tube, into the pressure switch, to see if the pressure switch is sticky. You'd know if by doing so, the inducer were to start up. And by doing repeat tests with and without it, to rule out coincidence, then you'd know if the pressure switch was faulty or not.
thanks ecman - great help!.... any suggestions on the order of things to check?
- I'm pretty much a novice when it comes to these things and I am trying to fix it myself.
thanks!!
Let me know if you have a 2-wire or 3-wire pressure switch. IF it is only 2 wires, then my last idea does not apply. If 3-wire, then very possible culprit.
Reread my last post as I added more explanation.
2 wires. brown and purple (and 1 hose). I just went to the furnace now and opened it up (since it was getting a little chilly in the house). As soon as i jiggled the wires to the pressure switch, the clicks kicked in and the furnace started up.
btw - i read the update to your last post - not sure i understand it completely, but it seems to be ruled out since it is only a 2 wire (and not 3 wire) correct??
If the problem is at that inducer, maybe your furnace was designed with an extra component elsewhere, like on a board, that will not permit your inducer to start up if the switch starts out closed. And maybe after the furnace shuts off, it stays stuck closed, until you jiggle or bang.
Try to work softly and pull off the vacuum tube to the pressure switch, at the inducer end, and try BLOWING in the pressure switch to see if that starts it. If so, then they are utilizing some other electrical component many furnaces do not have, and hence do not need a 3-wire pressure switch, because their system does the same thing.
I have to get off this computer in a couple minutes. Please post back quick if you need help before tomorrow sometime! Unless someone else starts filling in, here.
While I sit here and wait, here is an explanation as to the why's again. The pressure switch is part of the safety sytem. One of the most key components. Well, what some people have wised up to is bypassing the pressure switch when they have some problem with the furnace drafting. It could be something as serious as a bad heat exchanger! Well, the company wised up to that trick, so sometimes they make the pressure switch have to work 2 ways. First, it has to be so that currrent is not supposed to flow through the pressure switch at first. If it senses that current can flow thru it, it knows it has been bypassed with a jumper. So what they do is make it so the pressure switch wiring must remain dead at first, until the inducer motor comes up to speed, and THEN allow current to continue the sequence process that leads up to ignition, then fire.
I will try what you suggested with the tube and blowing into it. Right now (since i jiggled the wires) the furnace is working fine so I may have to wait until the morning (it usually stops working after a few hours and I'll be in bed by then).
- Let's say it does trigger it on when i blow in the hose - are you saying that the problem would be that the pressure switch is getting stuck closed? (if so, can that be fixed easily? or would i need to buy a new pressure switch?? )
btw - Here is the manual (pdf file) that i found for it via Google:
http://216.122.22.11/FetchDocument.a...c-b70faeb12ba4
It probably would have to be replaced. But, if say you can see the pressure switch's plunger, you may try spraying it with silicone spray or WD-40, with the furnace switch turned off! (highly combustible vapors when first sprayed!) and wipe off any over spray.
Good nite.
ok... i will investigate further with the wd-40 tomorrow and let you know how i make out. i realize you have to get off the computer now - so thanks for all your help!!
ecman,
i tried your suggestion of blowing through the tube at the inducer side - but nothing happened (i couldnt even blow air through???). So i reattached the hose.
Anyway, as soon as i moved the wire to the pressure switch the switch clicked and everything started up at that point. When i get some time later today I will try cleaning the switch and putting some WD-40 on it. I'll let ya know how I make out....
If you have a votlmeter, test for 24 volts at the pressure switch. It should be dead going though it (open), before the inducer fan starts. You can test this anytime, with the furnace off. This switch should only allow current through it (closed) when the inducer runs.
ecman,
no, i dont have a volt meter. I wasn't able to see the pressure switch's plunger either (so i couldnt lubricate it with WD-40). The whole pressure switch is encased in a plastic casing. I was only able to re-seat the brown and purple wires - and i added a little electrical grease to the contacts. So far, it's been running ok (just did it about an hour ago). We'll see what happens over night - that is usually when it fails to kick back on.
If it still fails to kick on, what do you suggest next? A Pressure Switch replacement? I see that they are about $75 bucks online - not too bad since i can install it myself.
This is the pressure switch I have: MPL-9300-v-0.35-deact-n/o-vs
2-Wires 1-Hose New Upgraded Furnace Pressure Switch(Rheem Ruud): American HVAC Parts
Hello, I'm having a similar (possibly same) problem with my furnace. I have the same model vent pressure switch in my Rheem furnace. The error light indicates a closed pressure switch. Blowing does nothing because it's already closed. I have to suck then blow a few times on the hose connected to the pressure switch to make it operate. It used to work for about a week before I would have to mess with it again. Now it stops working after 1 day or less. When it's not working the furnace will just blink the error code for closed pressure switch it does not attempt to start, even if I turn the power switch off then back on. Is there a way I can test for sure if my problem is the pressure switch?
all i did was re-seat the wires to the pressure switch as well as putting a little electrical grease on it. So far, so good (knock wood) no issues for 2 days now. Ecman was very helpful Rob, maybe he will jump in and help you at as well (i'm a novice)
Originally Posted by rob-650k7
Is there a way I can test for sure if my problem is the pressure switch?
If you own a volt-ohm meter(multimeter), you could take off the wires and set meter to ohms and test across the terminals to see if there is continuity. If there, is, it is closed. It should be open at first, and only close after the inducer motor closes the contacts in it. If it indeed is closed when the furnace is off, then it is getting stuck after the last run cycle. And either you have to try to free it up with a lubricant (a dry graphite used in locks, perhaps) or buy a new pressure switch.
Ecman, I think you're right about it getting stuck closed. We have the thermostat set low at night and in the morning when it first calls for heat the furnace doesn't normally come on. The furnace is working at the moment, so I'll wait for the fault code then test the pressure switch with my multimeter set to ohm. Since my last post I've taken apart the exhaust duct work and found what appeared to be white sand collecting in the bottom of the draft inducer so I vacuumed that out. There was also some in the inlet where the pressure switch hose connects to the draft inducer so I cleaned it out with a small nail.
I wanted to update my post. I tested the pressure switch with a multi meter and found it was getting stuck closed. After cleaning the exhaust piping and replacing the intake filter on the furnace the pressure switch was still getting stuck. I even tried squeezing some powdered graphite in the pressure switch in hopes that it would begin operating smoothly. It didn't. A few days ago I bought and installed a new pressure switch. It appears to have fixed the problem. Thanks for your assistance on this forum.
Rob
Tags: furnace, heater, kicking, reliably, forced, pressure switch, getting stuck, pressure switch, with furnace, stuck closed, 3-wire pressure, 3-wire pressure switch, allow current
