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bigmatt 09-03-2009 12:00 PM

Furnace Pressure Switch
Hey everyone,
I started up my furnace this week and the combustion blower would come on but the glow plug wouldn't light and the furnace wouldn't kick on. I traced this back to the pressure switch that tells the computer that the blower is running. Furnace works as long as I have the pressure switch bypassed.

The furnace is an Armstrong Ultra 90 sx, propane forced air.

Here's my question. The pressure switch is a Tridelta model #pps10089-2610 which has two port connections, one going to the combustion blower and one going to the Burner box. Is there a reason I have to stick with a two port sensor? The reason I ask is that the single port sensors are much cheaper ($120 vs $40) and more readily available (I have only found one 2 port sensor online)

Thanks in advance,


hvac122 09-03-2009 12:51 PM

Yes, you must use the 2 port. Have you checked all of your pressure lines and connections for blockage?

bigmatt 09-03-2009 01:55 PM

Yes, I checked all the lines and they are clear. The pressure sensor actually works correctly if I unscrew it from the furnace and lay it on its side. I think that's just because the weight of the diaphram helps the contact move a little bit more to complete the circuit.

Just out of curiosity, why the two ports? I assume it's because the combustion fan doesn't develop enough pressure. So they have to use vacuum (from the combustion box) on the other side of the diaphram to make enough of a pressure difference.

Any suggestions on a quality two port sensor? I'd rather spend a little more money now than replace it again in a few years.


beenthere 09-03-2009 02:11 PM

May be nothing wrong with the pressure switch.
Its a differential pressure switch. It needs to check that teh 2 pressures are correct.

Its a safety device!!!!

Could be a heat exchanger problem.

Either you should check it. Or have a pro check it.

Could have a crack or hole in it. Or could be clogged with soot.

Get the correct dual port one. And if your furnace still doesn't work. You may have serious trouble.

yuri 09-03-2009 02:38 PM

You must get the CORRECT rated one as they are rated in "WC. That is a critical safety device and not to EVER be bypassed (except to test). The draft should be checked with a manometer as the pressure switch may be doing its job. Poor draft and you get flame follout eventually and other dangerous events.

Yoyizit 09-03-2009 04:29 PM

You can probably check it with two flexible vertical tubes partially filled with water to the correct Water Column depth to give the correct differential pressure.

How old's the furnace?

beenthere 09-03-2009 04:48 PM

It can be done.

But its hard to read a .3" water difference with a water tube, unless its an inclined manometer.

Yoyizit 09-03-2009 04:55 PM


Originally Posted by beenthere (Post 323366)
It can be done.

But its hard to read a .3" water difference with a water tube, unless its an inclined manometer.

There you go, bringing up Reality! :laughing:
Now he needs rigid tubes and a level. :(
And if the meniscus bothers him a drop of (hand) dishwashing liquid will remove the water's surface tension.

bigmatt 09-03-2009 06:14 PM

Or I can go to our R&D building and "borrow" the manometer.

And I didn't realize the pressure switch was used to determine the furnace integrity. I thought it was just there to verify the blower was flowing before it turned on the pilot. Either way, I would never leave an appliance jumpered if I'm not right there in case something goes wrong. It's currently turned off and so is the gas supply.

The furnace is about 10 years old and I don't believe this is the original unit. There are two sets of mounting holes right next to each other under the pressure switch. This tells me the PO swapped it out and the holes didn't line up so he drilled a new set.

The one that is currently in there is a 0.2 wc switch in place. Is there any way to tell what is supposed to be in there? Just in case the PO replaced it with a lower valued unit to make it easier to close the circuit.

Thanks for all the great information,


yuri 09-03-2009 06:19 PM

You should order the OEM, original from an Armstrong dealer irregardless of the cost. Safety comes first. Propane can sometimes be a problem as it may not burn as cleanly as natural gas. This may foul up the heat exchanger or secondary heat exchanger and reduce the draft and cause a serious problem. I would recommend you get a propane tech to check the firing rate and venting etc. Propane is finicky and you need an experienced tech to do it properly. I have seen an overfired unit warp the burner box and plenty of plugged heat exchangers cause flame rollout, burnt wiring and fire, it's no joke!

beenthere 09-03-2009 06:25 PM

I don't know what the proper switch pressure is for your unit. But, if you order an OEm, then you'll know you have the right one.

Some techs will put in lower valuie switches because they don't know what they are doing.
And its very dangerous for the customer, and can lead to CO being put into the home.

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