ANYONE KNOW THIS RIDDLE

[B.O.B DOLE]

Replaced a thermocouple on a water heater (main) tried to light pilot but it
went out every time replaced the cover and still it went out
then i ran the hot tap tried again then it fired, turned the tap off and pilot stayed in cant work that one out can you.

 

[Bahco]

I think you need a corgi BOB

 

[Agile]

But he IS a CORGI !!!

Tony

 

[Paul Barker]

NO, can't help.

I had a funny one, Glow Worm Swiftflow 120, 6 months ago tested tc, faulty so replaced, pilot didn't establish, shorted OHS, pilot established, removed short, pilot remained established...

Boiler has remained working until the other day when low pressure sensor cut in. Had to go out last night and explain use of filling loop, wish I'd explkained that last time (not my install you understand, new people in old house with existing system, old people pass on useless knowledge and miss out necessary stuff like filling loops). Wish I'd thought to educate them. Anyhow to start it again, I again had to momentarily short OHS and on letting go it remained established.

I suppose I better get an OHS in stock.

low pressure senser now leaking a tad but I told customer to call me at end of week if it hasn't stopped of it's own accord, I assume the change in pressure has had an effect on it that might right itself.

 

[weargas]

gotta look a little closer at pilot flame picture I would say.

 

[Agile]

Sorry to have to do a bit of educating but...!

When you have a problem with a thermocouple you need to measure the voltage out of the thermocouple ( on load ) and it should be about 12-17 mV.

Next you need to measure the voltage across the o/h stat. Its more accurate to measure the voltage to earth from each terminal and see the difference. It should not exceed 1 mV otherwise change the o/h stat.

Tony

 

[ChrisR]

NOw you're going to have to say, exactly what bits you carry to measure the on-load output of a tc?

 

[Agile]

Just my standard Fluke DVM!

In the example above with an o/h stat in series with the TC then you can just measure the voltage from the terminals of the o/h stat to chassis.

If I wanted to measure a TC output with no other device I have an old gas valve with an output at the TC connection point.

Tony

 

[Paul Barker]

That's useful Tony.

I uae a plunger style tc tester, but I have sufficiently acurate test equipment to measure dc voltage and current at those levels. It's actually the dc current that holds the solenoid open, do you kjnow what range that should be in?

 

[Agile]

You cannot measure the current in the boiler circuit because the resistances are so low that putting a current meter in series will increase the loop resistance and thus reduce the current.

You can get a comparative measurement, which is still useful, by connecting the TC to a fixed resistance of about one ohm, or even an old magnetovalve stripped from a gas valve and measuring the voltage across it.

Measuring open circuit voltage from a TC shows its producing a voltage ( about 20-30 mV ) but does not take into account its resistance so it may still not be working.

Tony

 

[kevplumb]

You cannot measure the current in the boiler circuit because the resistances are so low that putting a current meter in series will increase the loop resistance and thus reduce the current.

You can get a comparative measurement, which is still useful, by connecting the TC to a fixed resistance of about one ohm, or even an old magnetovalve stripped from a gas valve and measuring the voltage across it.

Measuring open circuit voltage from a TC shows its producing a voltage ( about 20-30 mV ) but does not take into account its resistance so it may still not be working.

Tony

jeez
it really breaks my heart
but i have to agree
load them the voltage goes down BIG STYLE

 

[ChrisR]

Only some boilers have exposed OHS connections to the TC and Tony's spare gas valve would of course not connect to all thermocouples.

Proper answer is to carry split wire/interceptor adaptors for each of the sizes; 8 x 1, 9 x 1, 10 x 1 and 11/32". That's to enable measurement of voltage under load of the particular gas valve.

Another approach might appear to be to hold a resistor across the tc while measuring, but I just measured a gas valve tc input at about 0.012 Ohms..!
(L is about 6uH , btw.) At that resistance in-field test lead contacts aren't good enough - you have to screw things together.

 

[Paul Barker]

OK so what this proves is you need a tc tester like mine.

 

[ChrisR]

The regin one -? That won't tell you if the output is getting weak, or how you might improve it by altering the pilot flame, or what happens to the output when the main burners light.