Gas and Oil Home Heating Furnaces - Weil-McLain HE 5 starting problem

The blower comes on, and the ignitor glows, but the main burners won't light all the time unless I tap the pressure switch with a mallet. After a couple of taps, the relay clicks and the main burners ignite. If I don't tap the pressure switch, the blower will continue to run and the ignitor glows, but nothing else happens. Is this a faulty pressure switch or a control board problem? I don't want to just start replacing parts. Thanks in advance for the help.

Usually the igniter wont heat up unless the pressure switch is closed. I'd start by using an AC voltmeter to identify whether the pressure switch is energized on both sides (closed) or only one side (open) when you are having this problem.

Just to make sure I understand correctly, there are three terminals on the switch, and if it is opperating correctly, I should read voltage on all three terminals with respect to ground when the ignitor is glowing. Correct?

Yes. The terminals are probably labeled C (common), NC (normally closed) and NO (normally open). These are the terminals of the SPDT switch that is controlled by the pressure.

The C terminal should have 24 VAC to it whenever the thermostat is calling for heat. The NC terminal should also have 24VAC to it when the thermostat calls for heat, and should switch to zero volts when the inducer motor comes up to speed and the pressure switch closes.

The NO terminal should have 24 VAC to it when the inducer motor comes up to speed and the pressure switch closes. When this terminal is energized, the next step in the sequence of operation is usually that the hot surface igniter starts to heat up.

Ok, Checked out the switch, has 26 VAC at common, and 26 VAC at the normally closed until the blower starts, then normally closed goes to 0 and normally open goes to 26 VAC. The igniter glows, and the furnace goes more than 45 seconds (from the manual) with no main burner ignition. There is no voltage to the gas valve. After several minutes of the blower running, and the igniter glowing, if I tap on the pressure switch, there is a loud click, the main burners light, and the igniter goes out. The furnace then runs fine until the next time it has to start up. Then the above repeats. From time to time it works fine without having to tap on anything. Could this be the controller? The troubleshooting chart in the book says if the igniter glows and the blower runs but the gas valve doesn't open to replace the wiring harness. This doesn't make sense to me.

Any suggestions?

Just to make sure I understand correctly, there are three terminals on the switch, and if it is opperating correctly, I should read voltage on all three terminals with respect to ground when the ignitor is glowing. Correct?

The idea of a 3-wire switch is to prove that the vacuum diaphram plunger of the vacuum switch can move.... proving it's worthiness to be able to shut down the furnace if a malfunction. To prove this, the plunger must retract, after the furnace run cycle is over, back to it's original position, which is the NC (or normally closed) position. That forms a circuit with the C(common)terminal). This then allows the ventor(inducer) fan to run. When that comes up to speed, the vacuum switch plunger moves the other direction(gets sucked in) and now completes the circuit between NO (normally open) and C(common). So, when ventor is running now and the ignitor wants to try to go, you will have 24 volts AC (VAC) juice flowing thru NO - C......and no longer NC - C.

To answer your question then, you should see 24 volts from NO to ground and from C to ground, when the inducer is running. And from NC to ground and C to ground when the inducer is not running.

Ok, reading through the HSI manual (Fenwal hot surface ignition system) it seems like somewhere between step 3 (45 second ignitor heat up) and step 4 (7 second trial for ignition) a) valve opens - low fire position b) power to ignitor off, somthing is going awry. How does the trial for ignition occur? Is there a sensor or probe that may be dirty? And if so, where is it located? How can I check the low fire position?

I never saw posts 4 and 5, when I made post 6 ???? It is like my screen flew past those. I only saw post 1 -3. ??? That is why I went into that explanation. And I see SP made a similar explanation.

Are you testing for voltage by testing from NO to metal ground? Or are you testing in series between NO and C? The reason I ask is that when you test in series(between NO and C is in series), the voltage patterns reverse. 24 volts on the meter actually means you have 0 volts traveling. And 0 volts on the meter means that you really do have 24 volts traveling.

What seems odd - obviously - is that IF your tests confirm the pressure switch stays closed for the whole 45 seconds, then why does tapping on a good pressure switch make it go? If that is what is happening, then your rapping is probably jarring something else. It could be even a marginally loose jack connector at the control module even. I've had trouble with those pin-type terminal connectors already, if say you have some of those instead of just all spade connectors. I had a couple of those pins that plugged into the ignition control module causing one time to get igntion, while other times would not, until I discovered this, and opened up that split in the pin some, to widen it some, to make a tighter connection.

Yes, I was checking voltage between the terminals and ground. Thanks for the input on the connectors, I'll check the pins there next as well as trying to wiggle the spade connectors for loose connections. I agree it doesn't make much sense that tapping a pressure switch that's testing good makes the burner light, but right now it seems to be the only thing that is working to get the mains lit. So, until I find the real problem...tap, tap, tap.

So, until I find the real problem...tap, tap, tap.


Actually, you want to identify the relationship between the tapping and the gas turning on.


Ecman is fierce on checking edge connections, and that sounds promising in this case. Also any of the wires or connectors.

Measure the voltage on the connection for the pressure switch at the ignition control and see if it fails to get voltage intermittently.

Try wiggling and manipulating that spade connector going to the NO connection on the pressure switch to see if that might be failing.

Use can use the tapping to track down which part the tapping actually affects. From your testing, it shouldn't be the pressure switch, since that seems to check out OK based on the test results you report.

But tapping there could be telegraphing itself to some other weak part. That's what it sounds like, anyway.

Well, one more evening of trouble shooting later... checked all electrical connections, everything is tight and clean. Spread the pins on the connector to the controller. Have good voltage to the controller from the pressure switch, good voltage through the rollout TFE. The click I'm hearing when tapping is the gas valve... figured this out by jumping 24 VAC to the valve after the ignitor had been on for 2 minutes. Got gas flow and the mains lit, as soon as I removed the voltage, mains went out. Seems to be a problem in the controller. Possibly something loose inside? Can this thing be opened up?

Thanks for all your troubleshooting tips guys, I appreciate it.

What does voltage to MV to common or metal ground say, when the gas valve is not jumpered but just allowed to run normal - when you have had to tap to try to get the gas valve to go?

Reads 0 volts with respect to ground at the gas valve terminals. No voltage comming from the controller to the valve. Have good voltage going to the controller from the NO terminal of the pressure switch.

It does sound like the controller, that the gas valve wires plug into, is bad. If the flame sensor or even an HSI flame sense was bad, at least the gas valve should get power and open for a few seconds before shutting off. But you are not getting that. You are at that stage now where either you take a chance and replace it, or call a tech who may charge you 3/4 of what the control module costs, just to show up at your door. Your call.

But just make sure first that with your pressure switch tests, that you leave your meter probes on there 45 seconds, or after teh HSI goes out, to be absolutely certain that when that valve does not get juice, that the pressure switch is not shutting off. The reason I mention this again is from a post of your where you said that when you had 26 volts from NO -C, that you did not get any gas valve voltages......which leads me to believe that you first tested the pressure switch for a number of (only a few?)seconds, saw it was correct, and then transfered your meter onto the gas valve terminals. Leave the test meter on the pressure switch terminals, since we already know that when the gas valve does not open, that you are not getting volts to it.

Thanks ECMan51 and SeattlePioneer... Still troubleshooting trying to avoid a $130 plus controller. I checked the pressure switch again as you suggested ECMan and it is holding the voltage steady all the way through the ignitor heat up and well past the two minute mark. I also tried jumpering 24 VAC directly to the Normally open lead, thinking that it might be making just enough contact to read voltage but not enough contact to pass enough amps. No change. If I disconnect the normally open lead, I can hear the relay clicking in the controller. This is kind of confirming that something is wrong with the controller. I'm going to pull that off and take a look and see if there is anything visible/fixable, bad solder joint etc.

Ignition control are not designed to be repaired. I recommend against doing so.


They control a wide variety of different functions within your equipment. Take some comfort that ALL those systems will be renewed with a new ignition control, reducing the probability of further problems for years to come.

Well, nothing visible, some dust that I blew out of the controller, but that's all. Put it all back together, and problem still exists. Still bothers me that light tapping on the pressure switch seems to be working. I'd hate like hell to replace the controller to find that the problem is still there. I even blew out the sensing lines to the switch, just to eliminate that possiblity.

Again gentlemen, thanks for all your assistance. Going to have to bite the bullet on the controller and pray. I'll post back to let you know how it went. I'll order the controller on Monday.

It can't be the pressure switch - positively - if you are sustaining that voltage through that whole time period of the igntion sequence. Boards and modules do fail quite often.