Blowback Pressure during Start-up

[versatek66]

I had replaced a regulator for a an elderly female client changing from standing pilot to intermittent due to her hard time in relighting pilot. As i tried it myself unit blows out the pilot during start-up what appeared to me was an excessive gas build-up prior to lighting of the main burner. During this time, I also noted metal chips accumulation at the bottom of the combustion chambers. I recon the unit was at least 20 plus years old, I cleaned up the shavings and showed it to my client, also informing her of the possibility of furnace wall thinning. Also I took out the burner pipes and tapped to get rid of loose iron oxide, reassembled and replaced the gas valve. Operated the unit on and off multiple times, blow back was intermittent, flames appeared blueish-white, intermittent yellowish flame and pressure sounds normal. T'stat kick it on/off few times, I close the door and she was happy to have heating.
Fast-forward today, I received a call from her asking me why her gas bill went up so high. She claimed she had the gas co to check for leak and they determined no leak was found. I will be checking the unit tomorrow, I'm asking the salty's out here for ideas, concern and possible solution...many thanks.

[Joehvac25]

I hate to say it but did you check for a cracked HX or anything? 20 years old is kind of pushing it.

[billg]

How does normal pressure sound? Did the gas company check for co? Sounds like one to worry about today.

[versatek66]

I hate to say it but did you check for a cracked HX or anything? 20 years old is kind of pushing it.

No evidence other than the metal oxide accumulation, I have recommended for her to purchase CO sensor just to be safe...20 plus is definitely old , I did advise her to start saving for unit replacement...she just lost her husband and I hate to be the bearer of bad news. My concern is the flame blow back during start up....my theory is the possibilty of enlarged pipe holes due to oxidation causing extra gas build up prior to ignition.

[Joehvac25]

I'd the pilot in a good spot sounds like delayed ignition, did you use an s86 retrofit kit?

[versatek66]

I believed it was, the pilot flame was definitely on one of the hole.. Repositioning might be a good idea...what do you make out of higher gas bill though?

[Joehvac25]

I'm sure it's colder outside, maybe she turned stat up who knows

[billg]

Whenever you change pilot you must do a turn down test. Clock the meter, check gas pressure and check unit for co with a ca.

[versatek66]

Whenever you change pilot you must do a turn down test. Clock the meter, check gas pressure and check unit for co with a ca.

Alright billg, I must have missed this in trade school, What's a turn down test? I will test for others...thanks.

[versatek66]

To billg and joehvac...you guys rock...thanks for your responses and ideas.

[billg]

Here you go.
http://www.baso.com/lib/subdomain/ba...%20sensing.pdf