90+ gas furnace problems

[hvactech1jm]

alright a senior tech at work has been working on a bryant 90+ gas furnace he said the gas valve was bad so he put a new one on and it did the same thing so he got another gas valve it did the same thing. what it does is the HSI lights up and starts to go out and about that time the gas valve opens and putters and spitts and trys to come on. what i think it is is the HSI is not building up enough resistance to open the gas valve like it should and sending a weak signal but of course im young tech so they dont take my advice. any ideas what it might be.

[BaldLoonie]

Hate to tell you but there's a reason they don't take your advice! You got some learnin' to do. The ignitor doesn't build up resistance to open the gas valve. The valve is powered by 24 volts from the control. Hopefully the senior tech has the training to properly diagnose this then explain to you what he found so if you run across it in the future, you'll know what to do.

[skippedover]

A good tech will, before comdemning the gas valve, check for voltage to the gas valve. In a situation like you've described, I'd run it through a cycle and check when power if applied to the gas valve, all the while watching the igniter to see when it drops out. If the igniter is a silicon nitride unit, the frequency to the igniter might be too low, thus causing the problem. My first inclination would be to check with the manufacturer if I don't have the cycle specs at hand. It could be a bad board but it could also be interference from some other electrical source. As an example of how extreme those problems can be, our company once followed the manufacturer's specs exactly, including installing electrical filters on the incoming electics, replace wiring harnesses, gas valve, control board, igniter and finally, the whole darned furnace!!!! Still had the same problem. Factory finally said, switch the phases on the unit. We switched it at the C/B panel and the problem went away. This was in a very small U.S. Post Office and apparently there was some postal equipment creating electrical noises that affected the main control board. In that case, the SiN igniter received a different frequency through each start cycle, each one less than the previous one until the furnace wouldn't light. The control was then supposed to do a purge and then step the frequency back up to the last level that lit the fire. Unfortunately, with the electrical noise it was receiving, the control would just go into a lockout mode. So knowing the manfucature's specs is hugely important these days. Guess work is a thing of the past.

[jibroni69]

Strange, sounds more like the time delay built into the board is malfunctioning, check the HSI for voltage and exactly when the 120v is dropped from the HSI. I suspect the board is dropping the HSI too quickly, it cools a bit and the gas valve opens. These are 18 second HSI's are they not? Double check the sequence of operations on this particular model and time when the gas valve cuts in, also check the voltage at the gas valve, weak voltage could account for valve opening late. Could be a number of things , but resistance means nothing on this one.

[sarpanch2001]

Hate to tell you but there's a reason they don't take your advice! You got some learnin' to do. The ignitor doesn't build up resistance to open the gas valve. The valve is powered by 24 volts from the control. Hopefully the senior tech has the training to properly diagnose this then explain to you what he found so if you run across it in the future, you'll know what to do.

thats good lesson

[hvactech1jm]

so your saying no matter if the hsi lights up or not it will open the gas valve. i always thought the hsi had to build up a sertin amout of resistance befor the gas valve opened.

[jibroni69]

so your saying no matter if the hsi lights up or not it will open the gas valve. i always thought the hsi had to build up a sertin amout of resistance befor the gas valve opened.

exactly, first thing that is going to happen is the board will check for continuity to safty limits, if this checks out good the inducer fan will start up. Then when the preasure switch closes, proving venting the board will continue with it's set sequence of operations. First it will start to power up the HSI for a set number of seconds, at a set time the gas valve will open and hopefully the gas will be lit by the HSI. The gas valve does not care whether the HSI worked or not. There is no check by the board to test the HSI while it is operating. Once the gas valve has opened the flame rod sensor has about 2 seconds to prove there has been ignition or not. If not the board will shut off gas valve and start sequence over again. After 3 tries it will lock out the furnace and throw a code.

The HSI is really the dummy in the equation, the board doesn't care or check whether it works or not. It has nothing to do with nothing.

[rayr]

SENIOR TECH??????? LMAO. I'd hate to see the Jr. techs diagnostis!!!!!

[hvactech1jm]

no one asked for a smart ass i was just looking for some help.sorry you new everthing the day u started.

[hvactech1jm]

exactly, first thing that is going to happen is the board will check for continuity to safty limits, if this checks out good the inducer fan will start up. Then when the preasure switch closes, proving venting the board will continue with it's set sequence of operations. First it will start to power up the HSI for a set number of seconds, at a set time the gas valve will open and hopefully the gas will be lit by the HSI. The gas valve does not care whether the HSI worked or not. There is no check by the board to test the HSI while it is operating. Once the gas valve has opened the flame rod sensor has about 2 seconds to prove there has been ignition or not. If not the board will shut off gas valve and start sequence over again. After 3 tries it will lock out the furnace and throw a code.

The HSI is really the dummy in the equation, the board doesn't care or check whether it works or not. It has nothing to do with nothing.

thanks for the info man that helps me out.

[tinknocker service tech]

with these new ignitors the voltage to them drops to the point where they wont lite to burners then on the next try the voltage is increased to the last recorded voltage the burners lit and remain there till the power is disconnected the starts the search orver again

it is possible the valve is slow to open and not lite but not likely
the board doesnt care if the ignitor isnt hot enough it just times it

there is something else happening on this unit and time for your tech to become a tech and stop changing parts

[ZZZRSC]

The gas valve does not care whether the HSI worked or not. There is no check by the board to test the HSI while it is operating.

The gas valve doesn't care, but I know of at least one control board that does. It was on a commercial boiler though. A "senior tech" with any knowledge at all wouldn't put in a third gas valve, as the likelyhood of the first two being bad is pretty slim. If there is a drain pressure switch, it has to be closed at this point to allow board voltage to the gas valve, otherwise make the following test: You should have a voltmeter on the gas valve terminals and a pressure gauge on the test port; it should have made up to three 8 second attempts to open to allow the burners to fire. If it opened and the pressure (3.5") was there, the burner should have lit o.k.
Above all let us know what you find, as sequence of operations are always changing.

[stvc]

A good tech will, before comdemning the gas valve, check for voltage to the gas valve. In a situation like you've described, I'd run it through a cycle and check when power if applied to the gas valve, all the while watching the igniter to see when it drops out. If the igniter is a silicon nitride unit, the frequency to the igniter might be too low, thus causing the problem. My first inclination would be to check with the manufacturer if I don't have the cycle specs at hand. It could be a bad board but it could also be interference from some other electrical source. As an example of how extreme those problems can be, our company once followed the manufacturer's specs exactly, including installing electrical filters on the incoming electics, replace wiring harnesses, gas valve, control board, igniter and finally, the whole darned furnace!!!! Still had the same problem. Factory finally said, switch the phases on the unit. We switched it at the C/B panel and the problem went away. This was in a very small U.S. Post Office and apparently there was some postal equipment creating electrical noises that affected the main control board. In that case, the SiN igniter received a different frequency through each start cycle, each one less than the previous one until the furnace wouldn't light. The control was then supposed to do a purge and then step the frequency back up to the last level that lit the fire. Unfortunately, with the electrical noise it was receiving, the control would just go into a lockout mode. So knowing the manfucature's specs is hugely important these days. Guess work is a thing of the past.

Just curious, electrical is one of my favorites.
Was this a single phase or 3 phase system?
120 or 208/240V ?