Boiler short cycling / aquastat differential
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  1. #1
    Have a Vaillant gas fired boiler (140K Btu/117K Btu DOE/102K Btu Net) with perimeter baseboard radiators upstairs (3 zones - 2300 sq ft)& a heated slab (PEX tubing in concrete) in 60% exposed basement (4th zone - 650 sq ft). Even when outside temp is in teens, the boiler comes on at 185 deg, fires for about 90 seconds to reach 190 deg, it takes 3-5 minutes for the water temp to drop back down to 185 then the cycle starts all over again. The circulator runs as long as heat demand lasts - does not go on and off with the burner. The aquastat is a Honeywell L4080B 112. Upper limit is set at 190. I don't see any adjustment for the differential, so am assuming it is fixed at 5 deg.

    Questions #1: Gut instinct tells me that the boiler is cycling way shorter than it should...any input on what a reasonable or desirable cycle time should be from an efficiency standpoint?

    Question #2: Shouldn't the aquastat differential be more like 30 deg? Water temp drop time and reasonable AM recovery time from 8 deg night setback suggests I have adequate radiator surface area & margin for 160 - 190 deg water to do the job (Note: slab is not setback at night).

  2. #2
    Join Date
    Jun 2003
    Location
    New Hampshire
    Posts
    2,666
    The L4080B is a operating aquastat for a indirect storage tank. I'm pretty sure the differential is fixed at 5 degrees.
    I would use a L4006A. It is a 2 wire aquastat which opens on temp rise. Adjustable limit is 100-240. Adjustable differential is 5-30 degrees. The differential on most high limits is typically 10 degrees fixed. But you can go higher if you prefer on this aquastat.
    Cold start controls like the L8148A with a fixed 10 degree differential are more efficient on fuel usage and will run longer. This is due to the boiler will only start on a call for heat or indirect storage hot water tank call. But that change would need to make sure the boiler would not be shocked with cold water. Bypass piping installed correctly.
    Also L8148A would not work if you have a domestic hot water coil inside the boiler.

    [Edited by oil lp man on 02-02-2005 at 05:40 PM]

  3. #3
    Thanks oil lp man. Your response got me to wondering what sort of incoming water temp might constitute a shock to the boiler? One of my zones is a new addition for my mother, who is currently at my sister's in FL, so I have her thermostat set at 55. That zone won't demand heat very often, but when it does the water will be at 55 or less! I'm a hands-on homeowner, not an HVAC pro, so I'm not clear on the "bypass piping" issue and will look into that before making any changes.

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