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question about an issue with my weils Mclain

dumb_1 Member Posts: 2
First,  thank you for taking the time to read this.  I know you get paid

for your time and experience and the fact that you are willing to help

out a home owner that is too stupid or cheap to bring someone in is

really really appreciated.  (end kiss butt here)

Ok the issue.  I

have a Weils McLain PCG series 8 hydro system natural gas with

intermittent  Baseboard heat through the house.  It is old and needs to

be replaced but can't do it financially yet.  In the mean while it has

been working great with some minor maintenance (changed a flooded

expansion tank, replaced the air catches, cleaned the burners / flame

senor / orifice etc).   . 

Here is the problem, when the

thermostat calls for heat (I have 3 zones on 2 circulatory with zone

separators on one of the lines) regardless of which zone it is called

from, after it has been off for at least 20 minutes it starts up without

issue.  When the water temp reaches 180 the honeywell aquastat turns

off the main burners turn off (as it should).  When the water temp drops

past the differential (7 degrees I think) then the issue begins.  The

relighter  (?) a white rodgers 5095 starts to spark on the assembly, but

no gas starts to flow to the pilot assembly.  I can hear the click

click click of the spark and see it but unlike when the furnace has been

sitting for 20 minutes the pilot does not come back on.  The gas for

the pilot will finally comes back on either when the water temp is at

about 140 or after about 20 minutes.  I am not 100% which triggers it, I

think it is the time more than the temp but whenever I hear the

clicking stop and look the water temp is about 140.  The click click

click of the spark wire happens until the pilot relights than it stops

clicking.  This continues until the furnace can rest again for at least

20 minutes with no calls for heat.  But as you can image my rooms don't

really heat up and on very windy and cold days can even go down a

degree or two. 

What I have done:

I have cleaned the flame senor and pilot line and orifice. 

I have taken off the relighter, filed the paint off the furnace where the contact is made to ensure I have a good ground.

I have used my multimeter and checked, I am getting the right voltage on the spark line (it lights cold fine anyway)


have changed the fuse at the fuse box for the circuit (I had an issue

with something else once that the circuit would only sometimes get

enough voltage on the line because of corrosion where the circuit

breaker makes contact with power bar)

I have an idea of exactly

might be happening, but before I start spending money and time doing

shotgun part replacement I wanted to know if anybody has ever seen this

before or what tests I can run to isolate why this may be happening. As I

mentioned before I have a multimeter and I am not afraid to use it. 

Seriously I am very conferable with electric and plumbing systems, but I

may not know all the lingo. 

If you need / want pics of anything please let me know.

Hope everybody is enjoying the superbowl.



  • Big Ed_4
    Big Ed_4 Member Posts: 1,758
    Spark Control ,Pilot valve or connections

    We need to know if 24V is delivered to pilot valve terminals on time of pilot delay to determine which is at fault ..
    I have enough experience to know , that I dont know it all
  • dumb_1
    dumb_1 Member Posts: 2
    edited February 2011

    Big Ed,

    Thank you for responding.  As I said before I get a little lost at the lingo.  Where would I test the voltage for the pilot valve terminal?  When I look at the white rogers relight unit I have 4 wires, the spark line that goes to the flame assembly, T1 which runs from the aquastat and two wires that run from T2, one that runs to the gas assembly and one that runs to the other big thing (sorry forgot what it is called) that controls the power and pretty much everything else. 

    Please let me know what exactly you need me to test.

    It it helps, on the spark line once I pull off the boot I have a 24v  reading. 

    Also it starts without issue on a cold start.  And I do have 24v on the coil on the gas assembly. 

    If it is the gas assembly, why would it work only after 15 minutes.  Is it possible it is the thermocouple not putting off enough current?  How much current should be on the line that runs from the thermocouple to the mercury switch?
  • Tim McElwain
    Tim McElwain Member Posts: 4,477
    The system you have is

    a White Rodgers Cycle Pilot system using a Mercury sensor to sense the flame which then causes a SPDT switch to walk over and send 24 volts to the gas valve coil. The gas valve has a pressure switch built into the valve which sounds like what is wrong with you unit the pressure switch is making and breaking. You nee to check gas pressures on the inlet and outlet sides of the gas valve. The Pilot Relight 5059 control is doing its job as it is simply a relight control and once it is powered and the pilot is lit it shorts the high voltage spark to ground.
This discussion has been closed.