I just finished installing a Monitor MZ25S Boiler and am having trouble with excessive cycling. Itís a NON-MODULATING combi boiler and the DHW side operates fine. However, when warmed up, and operating in heating mode, the burner fires for less than a minute and shuts down. The pump continues running after the burner goes out and the boiler fires again after about 3 to 5 minutes. The burner continues to fire for another 30 seconds to a minute and then stop again. This cycle continues as long as the boiler is turned on. I assumed I was having a flow problem so added a Grundfos UPS15-58. The MZ25S has an integrated Grundfos UPS15-42 so I am now running both pumps. Even with both pumps my flow meters are only showing about a combined flow of 2 gpm.

My house is small so I only have one zone servicing the main floor. The main floor is 750 square feet and I have about 950 feet of pex ran in four loops which average just under 250 per loop. The loops are attached to my subfloor with aluminum heat transfer plates. Also integrated into the boiler is about a 2 gallon expansion tank and actuator to switch between the DHW and heating modes. I have added a y-strainer and Taco Vortex air separator. As far as I can tell nothing is plugged.

The boiler is a 95000 BTU 95% efficient unit- with an output BTU rating of about 90,000. My plumber ran a combustion analysis which he said was normal and the CO and CO2 were within manufacturers specs. I know the boiler appears oversized, but itís not as much as it appears. I have a full unfinished basement with uninsulated concrete walls, single pane windows and an exterior access uninsulated door. I am planning on trying to heat the basement with heat loss from the main floor loops and heat loss from my boiler and manifolds. I may add a supplemental hydronic heater if necessary. My main floor has significant north facing windows with poorly insulated 4 inch walls. The ceiling has about R-40 insulation. As such, I am guessing my heat loss would be in the 50k to 60k range. Because of space limitations with my house, I wanted a combi boiler and the Monitor was the smallest I could find.

I canít figure out why my boiler is cycling so much. I am assuming its overheating, but that doesnít seem to make much sense. I have been careful to bleed the system thoroughly. My vent is somewhat long for the boiler at about 45 feet including additions for 90ís because I had to go out my roof from the basement, but I doubt that is the cause. Monitor only officially rates their exhaust venting for up to 30 feet, but unofficially one of their techs told me previously he does not see problems until they exceed at least 60 feet. The 60 foot range seems more reasonable as the vent is 3 inch pvc and all other manufactures seems to permit at least 60 to 70 feet with 3 inch pvc on similar sized units. Any thoughts would be much appreciated. Luckily its been a warm fall, but my luck is running out. Thank you.