No Ignition

steve_26

Have a gas furnace by Aireflo owned by Lennox.M# AF80MPAO75B3B.

The problem is the gas valve will not open due to 24 volts is delayed from the control board at the end of the hot surface igniter cycle.

Initially the board was defective and changed .the old one had no signal to the igniter or the blower delay.The new board solved that.

 On a call for heat draft inducer starts,pressure switch makes, hot surface igniter starts to glow, with volt meter on valve, by the time 24volts is present system shuts down  This a new board. It appears not enough time is allowed for gas valve to open. 25 yrs in service i have never seen this  on any gas unit. Has anyone come across this on this AireFlo furnace? unit is about 7 yrs old but was out of commision for 5yrs  and stored just re-installed.

Tim McElwain

The fact that it sat so

long may have caused the valve seat to stick shut. What conditions was the furnace stored in during this time?



What is the make and number of the gas valve? Also what is the make and number of the circuit board?

steve_26

no ignition

I forgot to mention the gas valve was changed after the board was diagnosed as N/G . The board was from Lennox they were both changed on friday 23rd.

I can get the valve to fire if I put an external 24v to it while the hot surface igniter is still engaged.It seems to be timiming out to slow, not enough time for 24v out of the board before the flame sensor shuts it down.

It may be a fualty new board.

Tim McElwain

Again what is

the make of the board and number and gas valve information Honeywell, United Technologies, Fenwal if you give me that I can give you the operational setup and timing. I am assuming you matched up board for board and valve for vale when you replaced them. This by the way is one of the problems with using used equipment or equipment that has been sitting for sometime unused.



Also what are your microamps off the flame sensor? Is there a separate sensor or is the hot surface igniter being used as a sensor?

steve_26

Board #

The number on the board from Lennox is 1012-968-I0939

the number on the box it came in is 1012-968-I0938

Honeywell valve #VR8345M 4302

Hot surface igniter with a separate flame rod.

Rod has no signal due to no flame.

I can get the valve to open with a 24v external  source. hook up spare transformer during the hot surface initiation and everything works and stays on. won't work thru the board.

I picked up another board  the new one may be faulty. timing out to slow before the flame sensor shuts down the circut.

Tim McElwain

That is a United Technologies

board. I would check to see what the TFI (Trial for Ignition) timing is it should be around 11 seconds. The Honeywell valve is a Universal replacement gas valve, what was the original valve on the unit? The difference in the number on the box as compared to the number on the control could be the difference. Timing on these is very critical. Usually the flame rod is sensing a flame and producing a microamp reading immediately, hook up the meter on the flame rod and watch to see if you get any signal before it shuts down.



Also make sure you have a good ground on the sensing circuit. I suggest running a dedicated wire from the board to the special copper connectors on the gas valve marked for ground.

steve_26

Honeywell

The old valve is a Honeywell # VR8205-S2858 .

two terminals on the old valve.

If I remove the wires off of MV/PV-MV there is a jumper for PV I get 24v.             

As soon as I put the wires back on the valve I loose the 24v

Tim McElwain

Make sure you use the adapter

that came with the VR8345M valve and not the adapter from the S8205S.

steve_26

got heat

Thanks for input. But got it going.

The board was replaced and HSI would finally glow . would not glow with the old board. Then it would not fire the gas valve tracked it down to a voltage drop on the  t-stat wiring but not a dead short. fixed that and got ignition only to watch it drop out. found the flame rod from the factory touching the burner inside and shorting.

bent the rod so as not to hit anything and the heat is on. all's well.

thanks for input.

Tim McElwain

I am confused

in your origianl posting you stated that the igniter was glowing???? Whats up with that?



"The board was replaced and HSI would finally glow . would not glow with the old board."

steve_26

Board

It did not glow until I replaced original board

then had the problems one after another.