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Thread: Too much cycling

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    DIY Member Handymaner's Avatar
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    Sep 2008
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    Default Too much cycling

    I have recently finished installing a Lochinvar Night. Here's a link to the sizing thread:

    http://www.terrylove.com/forums/show...load-calc-help

    I went with the 85000BTU size. It's cycling too much (according to the smart control 127 hours/640 cycles for space heating in the 2 weeks it's been running). Was at about 13/hour for space heating, now it's up to 19! I changed one setting (which did help). In the default settings, it would shut down if it exceeded the setpoint (around 115 with these mild temps) by 10 degrees. A call for heat would come, the boiler would ramp up rapidly as the cool return water flowed. About the time it would get close to 100% the heated return water would start rising the system temp rapidly, the boiler would overshoot the setpoint temp and before it could get the fire down low enough it would exceed the setpoint by 10 degrees and shut down. Then within a minute or two it would fire up again. I changed the setting so it could overshoot by 20 degrees instead of 10 (the max allowed). This seemed to solve the above problem. The only other setting I changed was to set the max heating temp to 155, which I got from Dana's answer in my heat load calc thread. I figure to set it conservatively, and if I get cold I can always raise it. The outdoor reset ramp is still at factory defaults for now.

    As I worked around the boiler I discovered that the thermostat is the biggest problem. I have a Taco zone controller, which indicates when a zone calls for heat. It seems the thermostat only calls for heat for a few minutes, then again a few minutes later. It's a Honeywell RTH7600. I checked the settings, and it is set to #3 on the heating cycle rate (which is Hot water system or gas furnace (more than 90% efficiency). The only other choices are: Gas or oil furnace (<90% efficiency), electric furnace, or gas/oil steam or gravity system.

    With the mild temps it's mostly running on one zone (70' of fin/tube baseboard), as well as some for the hot tub that I just got hooked back up. It runs at close to minimum fire (20%) most of the time with a delta T of around 5-8 degrees. I'm using the pump that came with the boiler for the near boiler loop and the same system pump from the old system (Grundfos UPS 15-42 F) on speed 3. My dealer thought that might be an undersize pump for my set up (~180' of tube/fin baseboard in 3 loops and a fan/coil in the garage). The hot tub has it's own circulator. I don't understand the relationship between pumping and delta T, do you slow the pump (giving the water more time in the radiators to lose heat) to increase temp drop? I'm sure the cycling will reduce when it gets colder and more heat is required.

    The thermostat is on an interior wall about 10 feet from the nearest baseboard radiator. It's the same one that was on the old boiler, although I did order an identical unit for the basement loop during this install. I plan on giving Honeywell a call on Monday. There is nothing in the manual about cycling.

    Any ideas?
    Last edited by Handymaner; 09-30-2012 at 01:28 AM.

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