Crown boiler is cycling on & off

Joseph4848

Greetings
This is my first time using heating help so any advice you could offer would be greatly appriciated. I have a Crown cast iron sealed combustion CSC 125,000 BTU gas boiler that is cycling on and off. The on / off cycles last around 13 to 30 seconds. I don't know why it is cycling. 20 years ago I installed this system. The controller is a Tekmar 370 house controller. It has a side arm for DHW. I have in floor staple up heat. It has a primary & secondary loop with variable speed injection. Pressure going to DHW is 15 PSI. Temp in primary loop and boiler is 120 to 135 degrees. When it cycles the gas low water cutoff also turns on & off (as it should). I am an electrician by trade and took two semesters at a local community college to learn how to install the boiler & the radiant heat. This system has been nearly bullet proof all these years. In the last 3 months I did have problems with the DHW temperature being too hot. At first I tried replacing the aquastat in the sidearm & that didn't work. I then replaced my three way mixing valve & that corrected the temperature problem. Other than that I have not had any problems with this heating system in 2 decades. It workes so well. You should know that I have not done any maintenance on this system the entire time. My guess it that it is some sort of carbon buildup. If you think a part replacement is required could you include what you would try first. I am 62 years old & near the end of my career but if I could go back in time & choose a different career I would choose hydronics. It's like a blend of electrical & plumbing. The type of heat it produces is so sweet & comfortable.

EdTheHeaterMan

The on and off is electrical. There is a process of elimination to find out where it is.
1. what exactly is going on and off. The flame, the inducer motor, the circulator pump?
2. look at the system that operates that device that is on and off.
3. find if it is doing its job or if it is defective.
4. if it is doing its job then fix what is wrong. (blocked vent switch ... un block the vent/ Low water cut off... put water in there/ Flame sensor signal weak? clean or replace the flame sensor.

Most likely it is a maintenance item. Look in the manual for annual maintenance and check all those items first. They are usually less expensive things to fix. Then move on the the expensive repairs like replacing a control module

So What exactly is going off and on?

JUGHNE

Flame sensor is the first suspect.
Remove and clean with steel wool, if part of pilot burner, clean that also.

Pictures of burners, pilot, flame sensor and ignitor. Please,

Also wiring diagram.

Joseph4848

Thank you to the good people of heating help. Especially you folks who responded to my post. Boiler is working fine now. 20 years of carbon buildup was the problem. It took me half the day to brush and vacuum. What a messy job, but she’s not only working but more energy efficient. Every year millions of moths migrate thru here and over the years thousands of moths found their way into my boiler thru the coaxel vent. Tomorrow I’ll add better screen material to the vent entrance to keep those buggers out. Many Blessings

Kickstand55

I would consult a professional heating guy to come in and perform a combustion test. You're in the trades, I bet you know someone who knows someone. Carbon Monoxide as you well know is a killer. You mentioned carbon in the boiler, that could mean a rich flame and an increase in CO.
Oh, be sure to place CO detectors around the house and one near the boiler that reads low level CO. We want to hear back from you...soon.