Aquastat/Gas Valve Not Talking To Boiler?

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Old 11-11-08, 02:48 PM
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Aquastat/Gas Valve Not Talking To Boiler?

Howdy, I've got a Burnham P-204 -W forced hot water boiler that's been giving me problems. I started it up for the first time this season & right away I knew I needed a new thermocoupling. No sweat. I swapped it out, fired her up & I thought that was that. I used the rear zone of a two zone system for the initial test. The problems started when I tried to fire up the front zone. I turned up the thermostat, the zone valve opened & the circulator engaged but the gas didn't fire up although the pilot was still lit. I then tried to repeat using the rear zone again but this time the gas didn't come on either. I tried each several times & no luck. I've got a Honeywell type L8148L aquastat relay, a Honeywell VS820C gas valve & two Taco 571-2 zone valves. It's a milivolt system. Maybe it's the gas valve? I don't know. Thanks DAS
 
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Old 11-11-08, 04:00 PM
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I think you need to use your voltmeter and see if you are getting voltage at the gas valve, etc.
Doug
 
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Old 11-12-08, 05:17 AM
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Thanks gilmorrie. I failed to mention in my original post that the boiler burns natural gas & has a Taco 007-53 circulator. Also, it's an older system circa 1983. I have a multimeter but am ashamed to say I don't know how to use it. If it's a milivolt system what should the meter be set at & what should the proper reading be? DAS
 
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Old 11-12-08, 08:53 AM
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Those Taco zone valves need to see 24V AC to open. You can Google the valve models, and find a spec sheet. You can also Google "multimeter" or "DMM," and find instructions for use. But basically, you put your meter on the AC scale that covers 24V, and touch the two leads to the terminals on the valve.
Doug
 
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Old 11-12-08, 09:16 AM
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Well... I got my multimeter & figured out the basics. If I'm reading it correctly (& I think I am) I'm not getting any voltage from the aquastat end of the burner wire to the gas valve. Everything else in the aquastat (power, thermostat & circulator) is juiced. The zone valves are fine. My multimeter has 3 places to the right of the decimal point (in both AC & DC) so it should read millivolts. Does this mean I need a new aquastat? Like I said in my original post, the burner lit up once then quit never to ignite again. It's a 25 year old boiler system. Is the age of the aquastat a factor? DAS
 
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Old 11-12-08, 04:12 PM
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I'm not real good with gas ... but... I don't believe you can use a multimeter to check a millivolt gas valve.

When you said you replaced the 'thermocouple', you meant 'powerpile', correct ?

You're sure that the water temp hadn't gone up to High Limit during the initial test ? You tried again after the boiler cooled and still no go ?

Do you know how to use the OHMS scale on your meter ?

If so, there are some readings you can take that will tell us if the aquastat is signalling the valve to open or not ...

I forget ... does the 8148L have an AUTO / MANUAL switch on it?
(and you're sure it's an 8148 L ? )
 
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Old 11-13-08, 06:38 AM
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Hey NJ Trooper,
Question: What's the difference between a 'thermocoupling' & a 'powerpile?' I ask because the wiring schematic on the back of the cover refers to a powerpile rather than what I thought was a thermocoupling. In any case, you're correct, it's a powerpile.
I'm not sure if the L8148L aquastat has a AUTO / MANUAL switch. The only one I was able to locate is on the side of the aquastat facing the wall (small black up & down switch) & I didn't want to mess w/what I couldn't see, at least not until I posted this thread.
I double checked the aquastat model # L8148 'L' after not finding the 'L' designation on the Honeywell list of aquastats. It's definitely an 'L' & not any of the other indicated letters. It may be 'cuz it's an older model? I don't know.
Last & most important, It's working now & I'll tell you why. It was the water pressure sensor (presumably on the back of the aquastat). Isn't it 'achem's (sp?) razor' principle that says go for the simplist explanation first? Well due to a long history of air in the system, when I first fired her up I heard the familiar gurgle & added water from the supply line ignoring both the water expansion tank & the pressure guage. I figured the guage was old & not giving accurate readings (I had this problem w/a steam system). In any case, for some reason when I again started it up to troubleshoot, the boiler fired up. It fixed itself! (yeah right). I went upstairs for 15 minutes or so & then returned only to find the pressure release valve spitting, the pilot out & hot water on the cellar floor. D'oh!!!
I wicked love this website. Is there any way I could support it w/a donation or something? Thanks again!!! DAS
 
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Old 11-13-08, 04:56 PM
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Occam's Razor

Thermal Couple

I can't find an explanation of the powerpile... but the one thing is that a thermocouple has a metal tube, and a powerpile has wires ... I think the output of a powerpile is higher than a thermocouple ... but I admittedly ain't a gas guy ...

I'm pretty sure that switch is your AUTO/MANUAL switch.

I don't believe there's a pressure sensor on the aquastat.

There MAY be one elsewhere on the boiler though.

I'm sure the L model is obsolete ... probably for years ... I would imagine if needed, it could be replaced with the J model.

so, what did you do to fix ? apparently the expansion tank needed service for starters ...
 
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Old 11-17-08, 05:57 AM
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Hey NJ Trooper,
Sorry to not get back sooner-busy weekend. The only mystery for me in this latest boiler issue was why did it fire up that last time when the pressure valve released & all was revealed? I can't put my finger on that but it did & I'm grateful for it. The boiler is working like a charm & I even replaced the old mercury thermostat w/a brandy new Honeywell digital programable one. The water expansion tank seems okay but I'll keep an eye on it. Thanks for the info on the 'J' model aquistat. I can see the possibility of replacement. As for the pressure sensor, there has to be one somewhere 'cuz why would the pilot go out when the boiler shut off. Aren't safety control systerms usually the cause of pilot shut down? What's up w/the pipe from the aquastat into the boiler? Maybe that's where both it & the water temperature sensors/systems are? I don't know but thank God it worked otherwise I might be posting this from the stratosphere. Thanks again! DAS
 
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Old 11-17-08, 01:05 PM
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Powerpile vs. thermocouple

A Powerpile ( I think that is a registered Honeywell name) consists of several thermocouples contained in a single housing. The thermocouples are wired in series to produce the higher voltage of the Powerpile. A single thermocouple produces about 30 to 40 millivolts whereas a Powerpile produces about 750 millivolts.

A thermocouple cannot produce enough voltage to operate a gas valve although it can hold the gas valve open. The Powerpile CAN produce enough voltage to open the gas valve.

It is extremely unlikely that you have a pressure sensor on your system, that connecting "pipe" on the aquastat is merely a means of mounting the chassis of the aquastat and holding the thermal bulb in a closed-end tube in contact with the boiler water.
 
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Old 11-18-08, 07:10 AM
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Thanks Luana for the explanation of a powerpile vs. a thermocoupling. Now I know why the new Honeywell 'powerpile' cost so much!

If there's no pressure sensor in my system, can you explain why it would kick on out of the blue with a lot more pressure than the 12 psi indicated in the owner's manual, heat up the water to the point where you couldn't touch the pipe & then cut out completely (including the pilot) when the water pressure valve released? That's the one mystery in all this. Like I said, everything seems fine now & the boiler is firing up & shutting down as it's s'posed to do. Thanks DAS
 
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Old 11-18-08, 04:28 PM
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Most excellent ! Thanks Luana !

That metal tubing on the thermocouple actually holds a wire ... in reality, a 100% shielded coaxial cable ...

So, if the thermocouple is 30-40 mv, and a typical powerpile is 750 mv, there's gotta be more than several ! I would say at least 20 thermocouple junctions in a powerpile ... and it doesn't need to use shielded cable ... because signal is large enough to overcome electrical noise ... (or that naughty ham radio operator next door...)

So, the aquastat then is simply the switch that closes the circuit from the powerpile to open the gas valve...
 
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Old 11-18-08, 04:34 PM
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smithdog, I would do a very close inspection of the wiring ... make sure there's no exposed section that's 'shorting out' ... I would also inspect the relay contacts in the aquastat... maybe there's a fried spider between the contacts whose ghost now is gremlinizing your boiler.
 
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Old 11-18-08, 05:46 PM
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Bummer. Dang thing quite again! Typical. The wiring seems okay, the thermostat clicks on the aquastat relay & the circulator comes on but no gas, at least not consistantly. It's one of those dreaded things, sometimes it works & sometimes it don't but it's getting mighty cold here in the northeast! Is the thermal sensing bulb at all a likely suspect? Ohms are units of resistance correct? How would I go about testing if the aquastat is signalling the gas valve? Burrr DAS
 
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Old 11-18-08, 07:53 PM
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One of the first things I would do is first determine if that switch is an auto/man switch ... use a mirror if you can't see it ... switching that switch to manual should run the burner ... if it doesn't then we'll break out the ohmmeter (yes, resistance)...
 
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Old 11-19-08, 05:49 AM
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Hey NJ T., No luck. You were correct, the mirror revealed an auto/manual switch which, when switched to manual, did nothing. I got out the multimeter & set it to ohms but the meter manual says the power to the tested unit must be off & I'm not familiar w/the resistance function anyway. Basically, does all this mean that in the end I'm in it for a new aquastat &/or thermal sensor bulb? 'Cuz if so I'll swallow the $175+ish & get a new aquastat &/or bulb. They're 25 years old in any case. I just want to make sure that they're the cause & not some much more expensive thing like the gas valve (4-5 hundred at my local hvac store). It was mighty cold here last night! Thanks DAS
 
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Old 11-19-08, 07:00 PM
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Correct, you don't want to use the ohms scale on a powered circuit. So we need to disconnect something ...

You're sure it's a millivolt valve ?

Is this your boiler ?

Series 2

Maybe the newer ones have a 24V valve ? rbeck, do you know?
 
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Old 11-20-08, 06:15 AM
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Hey NJ T.
I think I fixed the problem. The pilot flame was too weak to ignite the main burner but got enough gas to remain lit. I adjusted the pilot valve & the boiler, which was set on manual, ignited instantly. Occam's Razor principle. The new powerpile must have required just a bit more gas than the old one. Anyway, that's my thinking although I'll be a little nervous over the next couple of days & heating cycles 'cuz I was dead wrong re the pressure valve thing. Still, this explains the conundrum better & I didn't have to shell out for a new aquastat & temp sensor which wouldn't have corrected the problem anyway! Thanks so much for you're assistance. I'll probably be back. I've got two other boilers-steam systems. Being a property manager sucks. DAS
 
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Old 11-22-08, 04:35 PM
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So I figure some manufacturer has programmed an obsolescence factor into this system somewhere. I have a nearly identical system: (Burham P-204 W, 2 TACO 555-102 valves, TACO cartridge circulator and a Honeywell L8148L) and yesterday I had the same symptoms: pilot OK, circulator humming, thermostat on but no flame. Boiler built in 1987 and installed in 1989.

I did notice 1 or 2 drips from a valve beneath the circulator. Never ever had this before. Since I added a whole-house water filter last week I assumed I may have introduced air into the system so I opened that valve and drained some pretty nasty smelling water. Still no luck.

I switched the TACO 555-102 from Auto to Manual and back and a few minutes later the boiler came to life. Woke this morning to the same problem and switched the 555-102 from Auto to Manual and back and again the system started up. Not sure what the difference is between Auto and Manual on this TACO valve but moving that switch seems to cure the problem.

Also, since it seems to run fine all day once that Auto/Manual/Auto routine is completed, it doesn't seem to stay OK following an overnight period of being off for 8 hours.

The 110 VAC is fine and the 24VAC side of the transformer reads between 24VAC and 27 VAC depending on reading time.

I'm stumped. Any thoughts would be appreciated.

Rick
 
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Old 11-22-08, 06:14 PM
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Definitely not planned obsolescence as this basic system has been around almost forever.

Your problem is more likely in the "end switch" of the Taco zone valve. Replacement of the power head will likely cure your problem.
 
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Old 11-24-08, 11:01 AM
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Seems to be running OK for the past few days. Its probably waiting for the depest, darkest, coldest part of January to act up again. Thanks for the tip on the valve head.

Rick
 
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