Using air pressure switch to control gas valve in furnace

I have an old gas furnace (circa 1981) being controlled by an electronic thermostat.
I've added a relay that is energized when the thermostat calls for heat. This relay is switching the fan contacts at the thermostat so that the fan comes on the instant the thermostat calls for heat. I've done this as a sort of safety feature, so I'm not relying only on the furnace's internal bimetalic thermostat to turn on the fan.
Something else I was thinking of doing was putting one of these in series with the gas valve relay control:
http://www.generalaireparts.com/general-aire-12500-air-pressure-switch.html
That seems to be normally used only to turn on the humidifier motor when the furnace is actually running. Which is strange, since it would be simpler to power the humidifier transformer from the furnace fan power supply, which would insure the same result (that the humidifier motor runs only when the furnace fan does).
I don't see why that air pressure switch couldn't be placed in series with the gas valve wiring, so that the gas comes on only when the furnace fan is started and develops a positive pressure in the plenum.
So the way this all would work would be:
1) Wall thermostat calls for heat, closes relay that energizes gas valve. This also energizes new relay that starts fan.
2) Gas valve does not come on, because air pressure switch is currently in the open position (no plenum pressure).
3) After a few seconds of fan operation, pressure builds up in the plenum, closing the air pressure switch, which then allows the gas valve to open and flame-rollout happens, furnace generates heat, etc.
4) Wall thermostat at some point tells the furnace to turn off by de-energizing gas relay. This shuts down the flame and the relay that turns on the fan. But the fan will still be running, because the furnace thermostat will still be calling for the fan to operate until it reaches cool-down set point.
Questions?
Comments?
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On 11/7/2014 9:06 PM, HVAC Guy wrote:

Assume that you're already happy with the fan blowing cold air until the heat exchanger heats up. Assume that you can reliably manage whatever ignites the flame when you wrench control from the gas valve.
When I open/close a door rapidly, I can hear the flaps in the kitchen/bathroom vents "flap". Same happens in gusty wind. I don't think I'd want those pressure changes turning the gas off. And that depends a lot on what "re-lights your fire".
The things you consider carefully rarely set your house on fire. It's the oversights that matter...and you don't learn about those until it's too late.
Recommend you not fix it. It ain't broke. YMMV.
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mike wrote:

My position is that that phenomena is highly over-rated. The furnace heats one floor of a small office building (about 2000 square feet) so the "chill" effect (if there is one) is small to non-existent. I've turned the gas valve dial in the furnace down quite a bit from the full-on position, so there is just enough to give a stable flame. This allows more efficient furnace operation and a higher duty cycle (furnace runs longer when heat is called for). So that might make the "chill" effect more pronounced or longer lasting, but it happens fewer times during the day.
From an energy capture point of view, you are dumping some heat out the chimney when you wait until the furnace gets hot before you turn the blower on.

There is no ignitor. This is a constant pilot light (remember those?). The most reliable and cost-effective way ever created by man to get a furnace going. No electricity or failure-prone circuit boards or sensors required.

I highly doubt that after a few seconds of fan operation, when the plenum has reached its maximal high operating pressure, that opening / closing doors is going to cause enough of a pressure change in the plenum.

See above.

And it's the things that are rarely if ever are called upon to function - like the furnace over-heat cut-off, that you absolutely depend on when the fan belt breaks at 3 am.
Adding something in the control circuit (like a pressure switch) is a nice backup. If the switch fails, your furnace fails safely. If your over-heat cutoff fails, you're fucked.
And even if your overheat cutoff works, the furnace is just going to cycle for hours before there are people on the scene and the situation is noticed.

But it has failure modes that are easy to remedy.
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HVAC Guy wrote:

Hi, On a quite old furnace almost eaching EOL soon.
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