Carrier roof top Model # 48TJD014 Lockout

[ekadiri]

I perform maintenance on several carrier roof top units and have come across a recurring problem that I am having trouble pin pointing the problem. Model #48TJD014---541-- (All units have the same model) The units cycle in heat mode for a few days than stop, control board indicates ignition lockout. The inducer motor runs constantly however flame does not ignite. If I turn power off and on, heat turns on immediately (inducer, flame and blower) and it works fine for a few days. I changed all limits (2 high temps and hall effect sensor on inducer fan) and it is still locking out every few days. Three out of the seven units are doing the same thing. Please advise, and I missing something, is this a board issue - if so, why three units at once.

Thanks

[Dallas Duster]

Are these units last in line for gas supply? Is there enough gas pressure when all units are demanding gas?

[toptech]

Clean the flame sensor's and verify microamps before and after.
Also verify that the heat exchangers on these trouble units are not cracked and causing flame disturbance.
If building gas supply in high pressure, check inlet gas valve pressure after the regulator. Anything above 14" will prevent gas valve from opening. Verify that the regulator vents are clean.
Last verify the resistance values of each troubled units gas valve and compare to units without trouble.
This should keep you busy
Good Luck

[james80031]

Have you cleaned the flame sensors? One of our customers has several Carrier 48t rooftop units, and slight build up on the flame sensors throws them into a similar fault. Pull the flame sensor and lightly clean it with a scotchbrite pad. Don't touch it with your bare fingers, and don't scratch it or the buildup comes faster.

[james80031]

Clean the flame sensor's and verify microamps before and after.
Also verify that the heat exchangers on these trouble units are not cracked and causing flame disturbance.

you beat my answer by one minute.

[toptech]

you beat my answer by one minute.

LOL, still a good answer!!

[ekadiri]

I have not looked at the flame sensor since no spark is being produced. These units are first in gas supply and have been working for the past ~8 years. I did look at heat exchangers, there appears to be no cracks. Like I said, these units work fine after I reset them for about a week and than fault up again. I will try to head out there today and clean flame sensor and spark rods, test voltages and post reply later on tonight. Thanks for the fast replys.

[james80031]

I have not looked at the flame sensor since no spark is being produced.

Once they have the same fault occur 4 times in a row, they won't even try to produce a spark. The flame sensor is a thermocouple that produces the 1mV needed to keep the gas valve open. If the build-up "insulates" the rod from the flame just enough to where it does not have the necessary heat rise within 5 seconds or so, it assumes there was no ignition, and tries to cycle again. After 4 times, for safety, it will not try to light again unless you kill power which clears the fault until the next time it can't light 4 times in a row.

[yellowirenut]

After you have checked and cleaned the flame sensors look at whre the units are..has anything around them changed?...i had one last year do the same thing, turned out it was strong winds being channeled around some newly installed units actual blowing the flame into the roll-out switch

[timebuilder]

Have you cleaned the flame sensors? One of our customers has several Carrier 48t rooftop units, and slight build up on the flame sensors throws them into a similar fault. Pull the flame sensor and lightly clean it with a scotchbrite pad. Don't touch it with your bare fingers, and don't scratch it or the buildup comes faster.

I'm hoping you already made these checks. I would NOT, repeat, NOT scratch a flame sensor rod with anything but a business card. Scotchbrite works great for cleaning metal, but I have been told by dozens of folks to not use anything abrasive on flame sensors.

If complete, let's go on.

This is a common scenario on these units. Usually, the lockout code is for a rollout switch fault. Sometimes, you will actually have a borderline rollout switch. Usually, not.

That leaves two other possibilities. One involves wind forcing an actual rollout by pressurizing the inducer exhaust, and therefore, the heat exchanger tubes, making an actual flame rollout happen. If you have a lot of wind, you might want to make up some baffles that are more effective than that little hood that Carrier provides. Perhaps the posters here who work on the Canadian prairie can help with that advice.

However, most often, I have found this is a board issue. So, check the items listed above, the rollout switches, and the prevailing wind, and then, you may want to replace the board on such a unit.

If I had enough information about the boards and their logic, I would be tempted to make up a test jig and do some repairs, but I think that no one at Carrier nor their board manufacturer is going to help with that.

So, all I can tell you is what has worked with, ah, "several" of these units.

[timebuilder]

Once they have the same fault occur 4 times in a row, they won't even try to produce a spark. The flame sensor is a thermocouple that produces the 1mV needed to keep the gas valve open. If the build-up "insulates" the rod from the flame just enough to where it does not have the necessary heat rise within 5 seconds or so, it assumes there was no ignition, and tries to cycle again. After 4 times, for safety, it will not try to light again unless you kill power which clears the fault until the next time it can't light 4 times in a row.

The flame sensor is an electrode used to expose an AC current to the flame and the grounded burner assembly. The board looks for a millivolt or two of DC current that is the result of flame rectification of that AC current.

If you have a lockout fault, the board will not retry the startup. And, as noted above, the board needs only to "believe" that a fault has happened, regardless of a good flame DC signal or a complete rollout switch circuit.

[toptech]

[QUOTE=timebuilder;2257342]I'm hoping you already made these checks. I would NOT, repeat, NOT scratch a flame sensor rod with anything but a business card. Scotchbrite works great for cleaning metal, but I have been told by dozens of folks to not use anything abrasive on flame sensors.

Hmm? I'm screwed then!!
For the last 15 years I've used a bastard file, sandpaper, 3"flapper wheel to those suckers!!
Hell, ive dragged the really bad one in a pinch across the concrete!
Customers have not complained..
Someone please post a link to the resourse that states "cant scratch em!
Gracias

P.S. If its true don't tell anyone about this post

[CDNtech]

One involves wind forcing an actual rollout by pressurizing the inducer exhaust, and therefore, the heat exchanger tubes, making an actual flame rollout happen. If you have a lot of wind, you might want to make up some baffles that are more effective than that little hood that Carrier provides. Perhaps the posters here who work on the Canadian prairie can help with that advice.:

Ok, I may not be from the prarie but I am Canadian, and I couldn't agree with you more, I've had many issues with roll outs on carrier units, some missing the little baffle and some with, up here carrier actually carries a "stack" you can install in place of the baffle. Usually the same scenario ekadiri describes, unit locks out, reset it and works okay for a couple days, then locksout again.