Millvolt gas valve

Seth Rothenberg

Hello Steam gurus.
I have a new challenge, would be grateful for some (more) advice.
The system is about 30 years old, Crown 890 with millivolt, single pipe.
I now suspect the gas valve itself.

The boiler has been humming this season, until...
Tuesday morning, there was no heat. The thermostat was calling for heat.
I started to check the controls - Pressuretrol, Low Water...
and then the main valve opened and the burners came on.
I did not find the source of the problem.

It was still cycling normally due to the Pressuretrol when I left hours later.
When I got home, it was cold again, so I was even more careful with testing order,
and the same thing happened. The issue evaporated when I touched it.
Today went the same as yesterday.

On the way home, I picked up a new Millivolt generator.
The old one gave readings around 400-410 mv, with pilot on.
The new one gives a reading of 575mv with just the pilot,
490-500 mv while running.
Still, the boiler is ignoring calls for heat until I touch
one lead of the Multimeter to the wiring block.

My question is, should I test anything else before
calling in a professional? Wednesday and Thursday
weather is good, the weekend may be colder.

The millivolt generator I had for 3 years: Honeywell Q313A 1055
The one I put in today: Chatham 233-mv
The Valve: Robertshaw 343-701-400 (presumably original, 30 years old)
The whole control loop except the valve shows resistance of 452 ohms.

Thank you in advance.

Seth

Double D

Make sure you have a clean blue pilot flame. Bright yellow and erratic won't due. Make sure all connections in the loop are snug.

BobC

It sounds like an intermittent connection, tighten up and connection in the area and (with the power off) try tugging on any wire going into a connection to see if there are any bad wires.

I got a call from a friends son a few years ago telling me the heat was out at his house and he was about to board a plane for a business trip. The house was only a couple of blocks away so went over to poke around. I did a visual inspection and decided to open the pressuretrol to see what it was set at, as soon as I exerted pressure on the screw the boiler came on. I told his wife to rap that control if it acted up again and that I would order a new pressuretrol for them.

Next time the boiler doesn't come on (power on for this test) try hitting that pressuretrol with the handle of a screw driver to see if the boiler comes to life. Try the same thing with the spill switch.

Bob

Abracadabra

everything grounded properly?

Seth Rothenberg

Thank you for the replies. I will look for loose connections.

Each time this happened (this year), I was able
to test the Pressuretrol and found the switch
closed as it should be.

If there were a ground missing, what would it be?
The Millivolt generator has 2 wires
that connect to the designated terminals on the valve.

Thanks

JUGHNE

Every wire connection must be clean and pristine, not only at the boiler with all controls but also your wall thermostat. Which may be specially built for millivolt switching BTY.

Tim McElwain

Go to the top of this section look for "Before you ask" click the second here and look for my picture. There is a section there on how to troubleshoot millivolt "powerpile" systems. A good pilot generator tested disconnected with the pilot on should be at least 750 millivolts. The generator you installed is suspect as it is not one of the best, Make sure you get a Honeywell Q313 as a replacement.

Seth Rothenberg

Thanks for the guidance! I took some measurements per Tim's thourough Powerpile guide. Only 2 things I am not sure of.
I measured all controls disconnected from the boiler, except the last 2', and got 2.1 ohms. The last 2' are about 1 ohm.
I measured the wiring a different way and got simiar, 3.6 ohms.

I measured the millivolts with just the pilot 573mV
With the main burner on, 285mV
With short jumper, 244mV-249

voltage drop, all wiring, .108 V
Voltage drop across Thermostat .165 V

Pilot Drop Out .32 V

If I did it right, pull-in 294 mV
That's close to the end of the range 179-320

When I jumpered all controls out,
Voltage dropped from 527 to 400 something
and quickly to 325 mV, with the burner on.

This seems to suggest I need to have the valve replaced.
Have I used the right procedure?

The valve is Robert Shaw Grayson 7000 BMVR 343-701-400
which I understand is discontinued, but there seem to be a number of qualified replacements, for example
700-506 Robertshaw Cooking Heating Gas Valve MilliVolt
MV 54-1011 810-0014

Does it sound like time to have the valve replaced?

Thanks

Chris_L

If it was me, I'd double check all the connections first before getting a new gas valve.

If you have enough slack in the wires, you might cut a bit off the ends, strip and reconnect to the gas valve. I've found that sometimes solid copper wires will break inside of the insulation if they are flexed too much. Not easy to find.

Also, have you tried jumping the two TH terminals on the valve and running the boiler--while closely keeping an eye on the water level and pressure--since you are bypassing the low water cutoff and pressuretrol. (Don't let it run this way without watching it.) If it runs ok then, you have a problem elsewhere.

Tim McElwain

You should have 750 millivolts with the generator disconnected from the valve and the pilot knob held down to keep the pilot lit. Before you replace the valve clean the pilot and recheck the generator.

If you have to replace the gas valve use a Honeywell VS820A powerpile valve.

lchmb

also check incoming gas pressures while your cleaning the pilot. Tim's leading you in the proper direction, without the right reading you may be tossing good money away...

Chris_L

lchmb said:

also check incoming gas pressures while your cleaning the pilot. Tim's leading you in the proper direction, without the right reading you may be tossing good money away...

Agreed. But if he is measuring 573 mV with just the pilot on, doesn't that tell you that the pilot is getting enough gas, and the generator is putting out enough voltage?

I've got two of these on different boilers with two different gas valves, and the voltage is 520-550 mV with just the pilot on, dropping to around 300 mV (or even less) with the boiler running. Both work fine.

lchmb

with just the pilot on you want/should see 750mv. Anything less may work, but it's not getting what it should.. If he's using the generator to hold the pilot open then yes it would be less. Depends how he's testing.

another thing I like to do, all wire nuts get redone. clip and restrip all wires just to make sure it's not a bad connection. A guy named George showed me the benefit of that trick...