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Discussion Starter · #1 · (Edited)
Pic below shows tubing from the front side of both Pressure Switches T'd together, and attached at Combustion Chamber and OUT PRES port of gas valve. There is also tubing, that you can't see, attached from backside of both (I'm assuming both) press switches, and T'd together into one line that attaches to the Secondary H.E. I believe it correct to say these pressure switches are looking for an imbalance of neg. pressure between comb. chamber & secondary HE., in which case an imbalance would shut the furnace down.

The pic below is a screen capture from a youtube.com video, and its author made this statement: "what the tube going to the gas valve is for, is to put the gas valve in the same pressure area as the comb. chamber." Since a tubing connected to the OUT PRES port of a gas valve normally measures pressure (psi, not IWC), I'm having trouble reconciling the author's above statement.

Grateful for a different explanation of this author's statement.
 

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The port on the gas valve that the tubing is connected to is the vent port of the gas valve. The vent port on any gas valve or regulator is its reference to the atmospheric pressure it is operating in. This allows it to regulate the pressure to that pressure condition.
 

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Lets go back to "The Fundamentals of Gas Utilization" young Luke.:biggrin2:

What do you need to burn gas. > Enough air for proper combustion.

How does the furnace know it has that?> The pressure switch measures the air flow amount going thru the entire piping system and heat exchanger.

How does it do that?> It uses the heat exchanger as a "orifice". Google Bernoulli's Principle and you will learn that flow thru a orifice equals an amount of air. We use this to measure steam flow in large lines. If you are wondering why that orifice is built into the inducer of those Miller MH/trailer furnaces that is why.

Once the pressure switch has proven the minimum safe amount of air for combustion when the gas valve is set at 3.5" WC what do you think happens to your combustion if the intake pipe starts plugging with snow or debris? > It changes and you have not enough air for good clean combustion. The switch has a span or range so you don't get nuisance tripping.

What should then happen Batman?

Answer Robin> We need to reduce the amount of gas pressure to match the amount of air.

How does that happen?>Pressure from those hoses acts on the gas valve regulator and starts to close it in proportion.

Capiche?

Yours truly.
 

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Discussion Starter · #4 · (Edited)
Lets go back to "The Fundamentals of Gas Utilization" young Luke.:biggrin2:

What do you need to burn gas. > Enough air for proper combustion.

How does the furnace know it has that?> The pressure switch measures the air flow amount going thru the entire piping system and heat exchanger.

How does it do that?> It uses the heat exchanger as a "orifice". Google Bernoulli's Principle and you will learn that flow thru a orifice equals an amount of air. We use this to measure steam flow in large lines. If you are wondering why that orifice is built into the inducer of those Miller MH/trailer furnaces that is why.

Once the pressure switch has proven the minimum safe amount of air for combustion when the gas valve is set at 3.5" WC what do you think happens to your combustion if the intake pipe starts plugging with snow or debris? >
It changes and you have not enough air for good clean combustion.
Already familiar up to this point, Master. Been~ corrected my wrong assumption of the tubing being connected to the OUT PRES port
The switch has a span or range so you don't get nuisance tripping. Do the Coleman-type, dual port trailer pressure switches, and single port 80%-er switches also have a range to prevent nuisance tripping. Never see tolerance specs on their labels if they do..?

What should then happen Batman?

Answer Robin> We need to reduce the amount of gas pressure to match the amount of air.

How does that happen?>Pressure from those hoses acts on the gas valve regulator and starts to close it in proportion.


Capiche?

Yours truly.
Capeesh Master!...(looking a little Green again today I see), and happy to see Batman & Robin once again...
 

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All pressure switches for burner operation have a span. The trip/open pressure may be .74", but it won't close until maybe ", 1.04" or higher.
 
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I would suspect about .4 to .5"WC. A lot of pressure switches on 80% mids are set between .4--.6" WC to trip. Some Rheems are lower. IMO the Industry seems happy with the .4-.5 range so the differential is probably the same.
 

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Discussion Starter · #7 ·
All pressure switches for burner operation have a span. The trip/open pressure may be .74", but it won't close until maybe ", 1.04" or higher.
Coleman tech support once told me that the 0.10" wc spec on the particular furnace pressure switch of our discussion, was its set point, but it would take approximately .2" to close it.

I've been wondering what "set point" really means, and given your above statement, maybe trip/open (in the case of this particular NO switch) is the official definition of set point?... assuming this tech was using the term "set point' correctly.
 

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Coleman tech support once told me that the 0.10" wc spec on the particular furnace pressure switch of our discussion, was its set point, but it would take approximately .2" to close it.

I've been wondering what "set point" really means, and given your above statement, maybe trip/open (in the case of this particular NO switch) is the official definition of set point?... assuming this tech was using the term "set point' correctly.
For furnace pressure switches, "Set Point" is the pressure at which it opens.
 
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