gas blows out pilot

[pames1]

Hi,I've been working on a 16yr old coleman MH furnace and everything works fine until the full gas valve opens all the way and it blows out. The gas co. came out twice and it has 8 " of water column. Put on a new valve[$186.00 later] and it does the same thing.Ive been doing this work for 10 years and it is baffling me.I checked the flu/lower limit everything. m#7956a856 s#068968827 .Thanks in advance Paul

[Robert]

Hi Paul,

Running Nat Gas or LP ?


Would need 3.5" wc manifold pressure for NG and 11" wc manifold pressure for LP .


Check that and main burner orifice and burner for any blockages.



Thanks,
Robert

[pames1]

N Gas is what we are on.I had already blew out the main burner and orifice.No spyders or bugs just some dirt.The tech came over last sat. and for 2 hrs tried to adjust the valve and got it down to 2'' water colume before it would not blow itself out.He was younger but also said he had never seen anything like this before.Two days later i went back to check it and the unit was down again.I called the company about coming back out because we purchased the valve from them but they said it was not their responseability since they did'nt diagnose the unit to begin with.I told the owner he would have to see it to believe it but he wasn't interrested.Should i get another gas valve because the original valve did the same thing. Thanks Paul

[Robert]

Hi Paul,

First get the input gas pressure at 7" to 8" wc if not there already. Then get manifold gas pressure at 3.5" wc.

If you live above sea level, those pressures will need to be derated.


Then, if it is still doing it, get a reputable hvac tech/contractor to properly test entire system/cycle to diagnose problem area.


Thanks,
Robert

[pames1]

Can you bench test a gas valve and I thought I was an hvac tech.What for cycle are you talking about.When the valve opens it burns till full open and blows the pilot out.

[Robert]

Hi Paul,

Gas Valve test = Input pressure correct, then 24vac to gas valve solonoid terminals, then manifold pressure correct.

IF input and voltage is correct, then manifold should be. If not, gas valve is bad.


IF it is a step valve, will have lower manifold pressure and then increase to proper level.

If it comes in correctly and has power, it should exit correctly.


Cycle = heating cycle.



Thanks,
Robert

[pames1]

Home owner said he ckecked the flu but I guess he didn't do a very go inspection.Pine needles found in the flu cap was the problem all along.Thanks for all your help. Paul

[Robert]

Hi Paul,

Thanks for update. Always inspect yourself when working on hvac units for customers.


Saves a lot of aggravation and time.


Take care and best wishes,
Robert