Weil Mclain CGA 5 Boiler. Low Boiler Temp Problem

JJC1184

When my thermostat calls for heat the boiler fires up normally and the circulator runs. Eventually I reach the high temperature limit (180 f), the burner shuts off and the circulator runs while the thermostat is calling for heat boiler water cools off to room temperature. It seems the low temperature limit on the aquastat isn’t working but I tested it and have continuity. The transformer is delivering 24V. Could it be the relay? Any thoughts or similar experiences/solutions are appreciated!! Thanks Jim

EBEBRATT-Ed

So is the circulator running all the time? What should happen if you have no domestic hot water off the boiler is when the thermostat is satisfied nothing runs. In a call for heat circulator starts and burner fires. If the boiler reaches the limit setting of 180 the burners stop and the circ keeps running if the stat is not satisfied. When the temp drops the burner refires. This continues until the stat is satisfied and everything shuts down.

Sounds like the relay may be sticking or a short in the thermostat or wiring. Disconnect the stat wires at the relay and use a jumper to start and stop the boiler to test.

This is assuming no DHW, 1 zone, no outdoor reset just a standard normal system

JJC1184

Ed, thanks for the quick response! Yes, my thermostat calls for heat and the boiler fires and reaches the high temperature on the aquastat. The circulator continues to run until it reaches thermostat setpoint. When the outside dips below 40 degrees outside the thermostat setpoint (typically 71 degrees) is never reached and the boiler water dips below the low setpoint on the aquastat and goes to room temp while the circulator continues to run. Aquastat is supposed to have a 30 degree differential). In order to get more heat I need to shut the system off on the thermostat and then restart it and the boiler fires again to the high aquastat setpoint and repeats the cycle. I will get a new relay tomorrow. If that doesn’t work I’m thinking a new aquastat next. I like the thermostat test which I will try today. Thanks again, Jim

bburd

"Aquastat is supposed to have a 30 degree differential". Why?

If the differential is adjustable, try reducing it to say 10° and see what happens. There might also be poor contact between the aquastat sensing bulb and the boiler.

JJC1184

bburd….good question on the 30 degree diff. This is what the spec sheet said on the Weil McLain site. Unfortunately it is not adjustable

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bburd

Try setting the high limit aquastat a bit higher. Perhaps 200°. For some reason once the high limit trips the burner will not refire when the boiler temperature drops below the differential. As pointed out above, this could be a problem with the relay or the aquastat itself. A little electrical investigation with a multimeter would tell you which.

JJC1184

Changed the relay and as the thermostat calls for heat, the aquastat reaches the high limit (now 200 degrees), turns off the burner and the circulator continues to reach the thermostat set temperature. The aquastat should redirect the boiler at a 30 degree diff from 200 degrees but does not do that and the boiler water cools off to room temp even as the thermostat is calling for heat. I’m going to change the aquastat next.

JJC1184

redirect = refire

JJC1184

just a note, the boiler water cools off from 200 degrees to room temp as the thermostat is calling for heat and the circulator is running.

bburd

Sounds like a bad aquastat.

hot_rod

So how are you getting the boiler to fire up again, now?

Is it a single zone system, one thermostat?

Where is the circulator getting powered from? The boiler, or an additional zone pump relay?

mattmia2

https://forum.heatinghelp.com/discussion/comment/1873890#Comment_1873890

none of this exactly explains why it fires after the t-stat turns off and back on again but not if the temp of the water just drops. some pictures of what is there and some testing with a meter are needed here.

JJC1184

Hot Rod, I turn the system off on my Nest controls, wait 30 sec and then turn it back on and it fires if the thermostat is calling for heat. This is a single zone system. The power is coming from the boiler controls.

mattmia2

what if you do it with a jumper instead of the nest? this could be a nest problem.

JJC1184

mattmia2, I’ll post some pictures. I continuity tested the permissive switches and the Aquastat. The switches are good. The Aquastat switch opens at set temperature and shuts the burner off as it should. When the Aquastat temp reduces 30 degrees (design diff) the burner should fire again if the thermostat is calling for heat with circulator running. The burner does not kick on with the boiler water temp cools 30 degrees and if the thermostat is calling for heat the circulator keeps running until the water temp goes and stays at 70-80 degrees. To get the burner to fire again, I need to turn the system off and back on again.

JJC1184

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mattmia2

try a jumper instead of the nest. nests can be flaky especially if the limit opening cuts power to the t-stat

EdTheHeaterMan

This may or may not be a Nest Problem. I think not.   That is because the relay is still operating the circulator and that is a function of the thermostat.  The thermostat will have no effect on the aquastat opening and closing to make the burner circuit open the gas valve to start the flame after the limit temperature drops below the differential and makes contact with the burner circuit again.   

There is a vent damper in the mix here and perhaps the end switch on the damper is the problem or perhaps the damper motor is not closing all the way when the limit opens the circuit, so when the burner is re-energised the damper is not in a position to reopen as a result of a bad cam switch inside the damper motor… I would guess that is the place to look next.

Verify before ordering a damper motor, it may be the actual damper getting stuck inside the housing that is the problem and the new motor will have the same problem.  Then you need to buy the complete damper (Because they don't sell the damper less motor) and you will end up with 3 good damper motors and one bad damper housing.   

Shooting the parts cannon at the problem can get expensive if you don't know how to use a multimeter and don't understand the sequence of operation.  

mattmia2

check the limit controls with an ac voltmeter with the system running. the aquastat, the damper end switch, the thermostat. you should read 0v if the safety is closed and 24vac if it is open.

i was also thinking vent damper earlier.

that relay is switching next to nothing and isn't soldered so i would be surprised if it were to fail

EdTheHeaterMan

Here is what is inside the vent damper to make sure that the burner can not operate if the vent damper end switch (Proof the damper is open) is not closed on the call for heat

So if you follow the wire from the thermostat connection R on the Boiler control relay transformer to G on the Boiler control relay transformer. That circuit is completed when the relay pulls in and there is nothing else the thermostat needs to do.  As long as there is a call for heat from the thermostat, R to G energises the relay coil from G to C (not shown) on the boiler control relay for a complete path to the source (R to thermostat to G to relay coil to C).  That is a complete circuit on its own, that is not part of the limit circuit.  

The boiler relay also makes the circulator contacts close and the limit circuit contacts from R to Y on the boiler control transformer close (not shown).

But if you look at the damper connected to the ladder diagram all the "Not Shown" contacts are there.

Now look closely at the path for 24 VAC from R on the boiler control relay transformer goes to G on the boiler control relay transformer thru the relay coil and back to the source at C on the boiler control relay transformer.

Once the relay is energized R also goes thru the CR1 relay contact to Y on the boiler control relay transformer, then it goers thru all the other limits on the boiler until it goes to the #5 pin on the Molex plug (that is connected to Y Wire). From the Y wire on the damper the cam switch closes when the damper blade is open.  That cam switch connects to the BL wire on the damper to the Molex connected to #2 pin that connects to the gas valve with a return path back to C on the boiler relay transformer. This will operate the main burner. 

By following the Sequence of Operation with a multi meter, you can see the exact location where the 24 VAC is not getting to the gas valve. 

So my question is: when you have no flame and the call for heat is keeping the circulator on, is the vent damper open or closed?