gas furnace pressure switch bad?


I got a call the other evening from the tenants in our rental house saying the air was blowing but there was no heat. I checked and they hadn't mucked up the programmable thermostat, so I figured a service call was in order.
A technician from the HVAC company we use came out to the house for a service call yesterday and determined the pressure switch was bad.
I don't dispute the professional's diagnostic, but I would like to know what the pressure switch on a gas furnace does, since the office assistant who actually called me couldn't tell me. What kind of questions should I be asking the company?
And should I ask to talk directly with the technician who made the service call, since I wasn't able to be there myself for the call?
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Kyle wrote:

If it's a newer furnace with a draft inducer fan (90% eff, e.g.) a small pressure switch monitors the draft conditions and won't permit gas valve opening unless draft is correct.
Jim
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Speedy Jim wrote:

Hi, Usually the switch is sensing pressure thru a little plastic hose which some times gets contaminated. Make sure the ais passage is clear. Switch itself does not go bad easy.
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Actually a furnace does not have to be a 90% to have a pressure switch. All 80's have them too. Pretty much any furnace with a draft inducer has one. It is actually sensing lack of pressure IOW vacuum to insure that the inducer is working properly and the unit is venting properly. Tony is right about the tubes stopping up, usually with condenstation, however the switches can and do go bad. On the plus side, usually they are not terribly expensive and are easy to replace. Larry
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On Feb 8, 12:34 pm, snipped-for-privacy@webtv.net (lp13-30) wrote:

Thanks, guys, for the replies. The HVAC man is coming out Monday to replace the switch. I will be sure to ask him (or have our tenant ask him) if the air passage is clear or if it needs to be blown out as well.
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