Gas and Oil Home Heating Furnaces - Frying new furnace control board

Hi,

Since I can't seem to figure out what my furnace problem is, I'm thinking of installing the new control board I bought last week as a back up in case I actually needed it.

This is the new age form of diacnostics my car dealer uses. Don't fix anything just throw new parts at it until it works. It's also the diagnostic tool my furnace service guy used on me 2 years ago when he charged me 600 for a new control board. The control board could actually be the problem But it has not been diagnosed as such.

My real question is whether I run much risk of frying this new board while installing it or could an undetected existing problem with say one of the safety devices kill the new board when the furnace is turned on?

My reasoning is that if the new board goes into the same mode as the old one and emits 4 flashes on the LED at least I know the problem is not the board.

My alternative is paying for a professional service call for some pro diagnostics and providing my own pristine board if called for. This would save me big bucks on markup on a board from the service guy.

What is your opinion?

ohyeah

I seriously doubt you have some problem that fries boards.

Isn't 4 flashes an open limit or auxilary switch?

Tell us what the furnace does and doesn't do. Does the exhaust motor come on in a matter of a number of seconds after you turn up the thermostat? Does any sparking or glow occur a number of seconds after that exhaust blower has ben going?

Normal sequence of operation (roughly)

- Thermostat calls for heat
- One loud click on the control board
- draft inducer fan starts
- pressure switch closes
- igniter glows
- gas flows, burner lights
- big fan starts
- burner shuts off
- big fan shuts off

This is sequence on my non working furnace

- Thermostat calls for heat
- One loud click on the control board
- followed by three fast clicks on the control board
- draft inducer fan starts
- big fan also starts simultaneously
- LED continuous 4 flashes = THERMAL PROTECTION DEVICE OPEN
- No igniter glow
- No gas flow/burner start
- To stop, turn thermostat down
- draft inducer fan stops
- big fan stops a minute later

Is the control board just doing its job and reacting in a split second to a problem it senses (3 fast clicks).

Or is the control board itself bad and malfunctioning when it receives the call for heat?

Is there a wiring diagram on the furnace? Do you have a multi-meter & know how to use it? If yes to both, get out the meter & check all high temperature safety switches. With no power applied & one wire off of the switch, each switch should have continuity.

>> With no power applied & one wire off of the switch, each switch should have continuity.

- "With No power applied" - That means to me with furnace off. Correct?

- & one wire off of the switch - That means pull off one wire and leave one on Correct?

- each switch should have continuity. That means to me to place one probe on one switch terminal (without the wire) and place the other probe on the other switch terninal (which has a wire) and you should have continuity.

First I'm checking the pressure switch. With the above test conducted there is no continuity. That has to mean the switch is open/ Right?

It's my understanding the pressure switch _should_ be open (off) when the power is off. There should not be continuity.

When the furnace is on and draft induce fan starts, the pressure switch senses negative pressure (through hose) and closes.

So there is not continuity through the pressure switch while the power is off.

ohyeah

Correct on the continuity thing. On the pressure switch MOST operate as you state. The switches you should be checking are not pressure but thermal switches. Depending on the furnace, there could be several around the burner area as well as one or more on or near the fan. They are usually small (less than dime size) discs with two spade terminals.

Grady,

>> They are usually small (less than dime size) discs with two spade terminals.

I found 2 one on each side of the burners mounted on the gas tube coming down from the gas valve. They look like what I've seen called "fusible links"
Each has a ceramic base with 2 terminals. On the business end what looks like a resistor, one of those firecracker looking things about 1/8 in. in diameter sticking out the end. Each one of those has continuity across the terminals.

The only other device I see is limit switch on the back wall (flush). It has continuity across the terminals also.

Those are the only other devices I can find.

What else should I check?

ohyea

Those do sound like fusible links. Continuity is good. If your wiring diagram shows no other limits (high temp switches) it sounds like you have covered the bases. It may be board time.
Hint: Often all limit switches have the same color wire going to them. It just makes a big loop.