McDonnel 67 LWCO

Don2010

Hello,



I am having trouble with a McDonnel 67 LWCU. The unit drops out the boiler as it should on low water but never resets. The Uni match feeder adds water to the boiler after a 60 sec delay (set on MM), the 67 never resets. This is a 3 month old installation with a Peerless 63 series steam boiler. It runs at about 3 psi, and no other problems until she calls for water. When I manually open the LWCU blow down valve, the boiler comes on, float appears to reset. The 67 was replaced and we still have the same problem. The boiler is clean and the blow down discharge is not dirty.



thanks in advance for any suggestions

nicholas bonham-carter

mysterious LWCO behavior

are you sure that the pressuretrol is not stopping the burner?  that 3 psi is too high, and could toast the main vents.

other than that, check that the wiring is as required by M-M co. in case there is some sort of strange electrical back feed through the valve solenoid contacts.

peerless have excellent tech support [610-369-3208].--nbc

Don2010

McDonnel 67 LWCO

thanks for the reply, no the pressuretrol is ok, I lowered the pressure so that the boiler operating at less than 2 psi now. I checked the lwco switch this morning and drained some water to simulate the problem and it worked ok. The water level dropped, float opened the contacts, auto feed added water after about 1 minute, then as it should float closed the contact and the boiler restarted. I did notice that when I measured the voltage across the open contacts, I was only reading 6.5VAC and thought that was odd since this is fed off of a 24VAC transformer. There are other safety components in this circuit including the blocked vent switch, and pressuretrol. No sure if the blocked vent switch is a varistor or either full open or closed based on temp.



 I am no boiler expert, but my installation contractor is not figuring this out at the moment, so I am trying to figure it out. Pg 26 of the attachment has the wiring diagram.

Any other suggestions appreciated.

nicholas bonham-carter

LWCO anomaly

it must be in the wiring some where. can you measure the voltage at each point along the way of the series of transformer, and safety switches?

i really wish that the mfg's would put in some "idiot lights" on each electrical part to show either correct, or incorrect operation.--nbc

TomM

nbc is right

with very limited knowledge and experience, i would venture to say that the 'blocked vent switch' is just a normally closed bimetallic switch (like a cheap thermostat) that cuts off at 800F or whatever degree its set to cut off at. 

.

Speaking from my rear admiral, I would check the line in the lwco to ground and see if that is 6.5v.  If it is then you have excessive voltage drop somewhere.  Check the voltage at each of the components to find where the drop is.  probably, its before the lwco, which only leaves the blocked vent switch, thermostat, or transformer.  or the wiring between them.  how long is your thermostat connection, and did you use the same type cable as specified in the wiring diagram?  if they check out ok, you also have the pressuretrol and the flame rollout switch wiring to check, whatever the heck that is? (FRS). 

.

 

Don2010

LWCO

The boiler problems continued this week and I have been resetting it early every morning this week. I reviewed all the control wiring diagrams from the manufacturers of the various safety components as well as the boiler installation manual and systematically began ruling out various components including the low water cut-off, blocked chimney sensor, pressuretrol, flame rollout sensor, inducer pressure monitor and I was really stumped. Today I checked the damper and I discovered that the damper was stuck. When I gently moved the damper blade, the damper motor activated and opened the damper, flame on and the boiler started. I downloaded the manufacturer manual for the damper and during my google search discovered this manufacturer is presently recalling the same model damper  due to a manufacturer defect, see Consumer Product Safety Commission link below: Although the serial number of the damper installed in our Peerless boiler does not match the recall, it is exhibiting the same intermittent behavior and has stalled numerous times and remained closed when it should have opened. During this failure, the damper safety  prevented the boiler from starting. During the past weeks, when I responded to the no heat calls, I was able to start the boiler by powering down the boiler or activating the blow down valve, the voltage drop kicked in the damper and  the damper opened properly making it very difficult to troubleshoot. I am not convinced it is a defective damper, the voltage to ground at each safety device to ground is only 6.5VAC, this is a 24Vac transformer. There is a drop somewhere, perhaps miswired and still looking. The contractor has indicated he will investigate further with his electrical sub, waiting on that and  I may end up having to figure this out.



http://www.cpsc.gov/cpscpub/prerel/prhtml09/09008.html



Our damper > Effikal model RVGP-PC-98KF serial 3326768



thanks for your replys

Don2010

problem resolved











The problem is resolved – its not the damper





 





After jumping out the damper interlock, the boiler still

would not start unless the power was cycled. I reconnected the damper power and

interlock and same problem. I finally broke down and read the wiring diagram

inside the Tjernlund UC-1, I already read the manual I downloaded, but that did not have

all the information necessary to figure this out. The wiring diagram inside the

UC-1 has some notes and references to the LED’s and some dip switches.





 





If you look at the LED’s they tell you exactly what is

happening. I power cycled the boiler, it ran and I let the rooms reach set

point and the thermostats stopped calling for heat and boiler shut off

normally. I waited for the inducer post purge cycle (indicated by led’s 4&5

flashing together) to complete. I then bumped up the stat and the boiler sat

lifeless. The amber LED indicated the call for heat and that was confirmed with

the meter on terminals 1&2. I also noticed LED 4 was blinking a code, 2

blinks over and over. I read the fault code and it indicated that the prover

switch was closed before the inducer relay closed.





 





Apparently, there is a pre check of the P1 P2 fan prover

(pressure switch) to verify open upon start-up. There was also a small note on

the circuit board that indicated “ if the natural draft is such that the prover

switch is closed prior to the inducer power relay activating, a fault is

initiated and the inducer motor relay does not close – Bingo! The note further

states that if the natural draft is too great, position DIP switch 9 to the on

position to disable the firmware pre-check of the prover when a call for heat

is received on terminals 1&2.





 





I disabled the prover pre-check and it’s working fine now.

This explains why the problem was so intermittent, when the natural draft was

high, the prover closed and prevented the board from closing the inducer motor

relay and interlock relay to power the gas valve.





 





Now for the real question>>>>> Why would the boiler

start if the LWCO, damper switch or any interlock was momentarily opened and

closed? It somehow reset the UC-1 board?





 





Amen!