thermocouple 101 question

allenh

There are 3 identical boilers. The one with the problem is the middle one. They are all gas 203 burnhams side by side into same flue pipe.

Standing pilot was going out intermittently - more likely for low temps or windy days. I do wonder if the gas company has reduced pressure / volume lately here in New England.

Last week swapped out the millivolt thermocouple - the one with the red and yellow wires that go directly to the gas valve. Same problem a couple of days later. Yesterday when comparing the boilers on each side I noticed that they had a different thermocouple. Bingo.? The one that the others have has a little box that connects 2 wires to the gas valve, 2 wires from the pilot flame and 2 wires back to the high limit box.

So the question is... what exactly is this little box in this thermocouple all about?

This boiler had local gas company do some work for effiency years ago and right afterwards the pilot going out started. I wonder if they changed the therm out then to this other type.

D lux_2

what do you have

Do you have a thermocouple or a millivolt ? Millivolt has wires with spade termails t couple has a screw on the end

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Kevin_12

themocouple

the other boilers may have an ECO switch which is a energy cut off. in case gas valve sticks on then the limit will open(probably a fusible link) and drop out the thermocouple and hopfully shut down the stuck on gas valve.

Randy Tibbits

It sounds like you have a 750 milli-volt powerpile. Check the milli-volts I like to see at least 600. Also make sure the pilot assymbly and pilot orfice is clean. Our rule is to clean both when changing a thermocouple or powerpile.

andy_5

I would have to agree with Randy.Most pilot outages have to do with dirty pilots.It also sounds like someone installed ECO's to the system as an added safety.You should have a minimum of 425 mv's on the pilot generators.If you have good mv's and the pilot still keeps going out,may be a bad gas valve.

Unknown

It is probably wired incorrectly

the little box is a junction block that connects the 750 millivolt pilot generator to a back up high limit (Energy Cut Off ECO). These are what they call auxillary limits installed on some powerpile boiler systems. The ECO should be wired into the wire leads from the pilot generator. That way if the systems exceeds the setting of the ECO it will break the flow of millivolts and shut the pilot safety down, thus stopping all flow of gas. If this is not wired correctly it will keep causing the pilot to go out.IT SHOULD NOT BE WIRED IN SERIES WITH THE BOILER HIGH LIMIT. It is also important to have the boiler high limit set at least 30 degrees less than this back up limit. Most of the ECO limits are set at around 230 degress F. I find that setting the boiler high limit at around 180 is best.
What is probably happenening is that your boiler high limit is wired in series with the ECO and when the boiler reaches boiler high limit setting it is knocking the pilot off. I have seen this many times and it just requires a small wiring change.

If you will send me an e-mail I have a wiring diagram that shows the correct wiring.

I have a feeling that when the gas company worked on your system they followed conventional wiring practices and moved some wires. These systems are not wired the conventional way.

Unknown

Randy & Andy those ECO's

where installed by the manufacturers in order to satisfy the ANSI standard for dual seated (redundant)gas valves. Powerpile valves could not meet that criteria so the manufacturers got permission to add an extra limit wired in series with the millivolt output from the generator to the pilot safety circuit.

On steam systems they added a second pressuretrol wired in series with the same circuit.

You are also correct that most pilot outages are a dirty pilot and that should be done up front. Good idea while you have gone to the trouble of removing the pilot to install a new generator or thermocouple.

Randy Tibbits

It's puzzling that the problem is happening on the boiler without the ECO. And I wonder why the gas co did'nt install an ECO on the middle boiler, unless that is the same model but newer that had a factory installed ECO.

allenh

Looking at records all this stuff started when the gas company came into the apt and just modified this one boiler. This one is the middle one and getting to the back left for the high limit switch is going to take a really small person if at all possible. They also added a flame roll off switch and wired that in series to the valve.

I installed the same kind of thermocouple that the boilers on each side have. This is now the one with the little block. It had spaded lugs with a yellow and a red wire going to the gas valve. From the valve the red and yellow wires go to the "little block" which has 2 more sets of wires coming off it. One set goes down to pilot. The other set was disconnected hanging in the breeze. I put those wires on the little block. And those wires go back to the high limit switch box which is on left side of boiler.

Those wires that were dangling in the breeze - that went to the high limit - were not connected in the front. I wonder where in the high limit box they go and what their purpose was for. And how did the boiler even work without them? It has to be some sort of secondary shut off as mentioned by a few of you. There was no set of wiring documentation in the cellar which makes me very mad. Hopefully some enterprizing person will publish every controller some day.


So now the pilot has stayed on for 2 whole days in a row.

Unknown

Enterprizing person

I may not have all of them published but I have quite a few.

Unknown

Allen

Depending on when these boilers were installed, they would have either been equipped with an L8148L High Limit/Relay control mounted outside of the boiler vestibule in a tee on the supply outlet of the boiler or an L4080L High Limit installed inside the vestibule directly into the boiler. The L8148L had a Manual/Auto switch for manual operation during a power outage. The L4080L did not have a switch and a bracket with Manual/Auto switch was provided on the inside jacket panel. Both controls had an ECO in the aquastat well with a pair of black leads that would be coming out of the well and were connected to the black junction block incorporated in the powerpile generator wires. As Tim stated, this ECO was there to shut the boiler completely down in the event of an overheating condition either in Auto or Manual mode.

If the middle boiler in your situation is equipped with a flame rollout switch and the others are not, it may very well be that this switch and its related wiring is creating a bit of resistance causing the gas valve to drop out. We haven't produced boilers using these controls since the mid 1980's but as far as I remember, the valve (probably VS820C) needs a minimum of 530 millivolts to stay operational. The size of the pilot for proper operation of these devices is also rather critical. All of our pilots are unregulated meaning that if incoming pressure drops, so will the pilot size. The pilot assembly for these boilers had a rather large 2600 btu pilot and tee shaped flame spreader to ensure proper operation. If this has been modified or changed, this can also affect operation. Hope this helps.

Glenn Stanton

Burnham Hydronics