SlantFin VHS 120 Boiler short cycling

Discussion in 'HVAC Heating & Cooling' started by jgmaciejko, Aug 10, 2007.

  1. jgmaciejko

    jgmaciejko New Member

    Messages:
    8
    I have a SlantFin VHS 120 propane boiler feeding 7 loops of 275' .5" plastic radiant and one water to air heat exchanger . I also have a woodstove. My problem is that sometimes when I'm burning wood only a couple hot water zones will fire up and the boiler will cycle alot . I added an additional aquastat with a 25 degree differential which helped but I'm wondering if I could add a multistage gas valve?....or what else could I do?
    Thanks
    Jim
  2. CHH

    CHH New Member

    Messages:
    225
    Location:
    Denver, CO
    Just so we're on the same page, I'll mention some stuff you already know. The boiler is short cycling because it puts more heat into the water than can be effectively radiated. The ideal solution is a modulating burner with a wide turn down ratio. Unfortunately retrofitting a modulating burner and it's associated controls is not typically considered to be an economically viable option.

    It seems to me that you have two or three choices:

    1) Replace current boiler with a mod-con
    2) Increase the heat storage capacity of the system
    3) Stop using the wood stove when the boiler starts short cycling

    Number 1 may not be cost effective.
    Number 2 is relatively simple to implement if you've got room for a reverse indirect tank. Basically a water tank is added to the primary loop of the hydronic system so that the boiler heats 10 times (or so) the water volume on each cycle.
    Number 3 is obviously not a real effective solution

    Maybe a hydronic specialist will have other options.
  3. GrumpyPlumber

    GrumpyPlumber Licensed Grump

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    The fact that zones are kicking on leads me to one basic question first...are there any t-stats located near the stove?
    This problem usually happens when a T-stat is located directly above a baseboard, the room cools...t-stat kicks on...baseboard immediately heats the T-stat above (because heat rises) without heating the room...stat kicks off, quickly cools and the zone/boiler kick back on very quickly, this would continue until the actual room were heated.
    It sounds like the stove is somehow affecting one or two T-stats in a similar fashion.
    My guess is the zones that are kicking on are from T-stats that are calling for heat, but somehow either a draft within the house, or t-stat location is causing it to read incorrectly.
  4. GrumpyPlumber

    GrumpyPlumber Licensed Grump

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    Also...if you could, let us know which zones are culprit.
    As for the heat exchanger..I'm assuming it's for a hot air blower/coil.
  5. CHH

    CHH New Member

    Messages:
    225
    Location:
    Denver, CO
    Hey Grumpy - He's got a radiant system, not baseboard.

    I'm not suggesting that it's not a thermostat/sensing problem, just clarifying the system description.
  6. jgmaciejko

    jgmaciejko New Member

    Messages:
    8
    boiler cycling

    It isn't the thermostat which is causing the cycling, just not enough demand for the boiler firing. I was hoping to just retrofit a multistage gas valve but now I'm thinking of picking up a hot water tank to become a heat sink.
    Thanks

    jim
  7. GrumpyPlumber

    GrumpyPlumber Licensed Grump

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    I know...just used the baseboard example to make the point...my thinking that a thermostat was being kicked off by the stove.
  8. GrumpyPlumber

    GrumpyPlumber Licensed Grump

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    Hadn't you said zones were coming on as well when it cycles?
  9. alternety

    alternety Like an engineer

    Messages:
    651
    Location:
    Washington
    A well insulated tank (electric water heater) in series with the boiler will act as a buffer tank and may solve the short cycle problem. I believe using two loops is the preferred method but adds cost and complexity.

    For two loops you put a loop from the boiler to the buffer tank then a loop from the buffer tank to all the loads. Temp of buffer tank controls the boiler. If you get into reconfiguring things you might consider adding an indirect water heater tank to the boiler for DHW. That should have a control system that produces just water for the DHW tank when the temp control in the hot water tank calls for heat. This is because of different temperature requirements and to supply hot water a long as you need it. Providing you do not exceed the capacity of the boiler.

    You can get it complex and expensive. The optimal system adjusts the temperature of the water in the heating circuits based on outside temperature. Note that a radiant loop and a fan coil generally need significantly different water temperatures.

    Need to look at benefits and cost. The boiler you have is condensing but not modulating; rated about 82% AFUE. Modulating/condensing units get efficiencies into the high 90s. Getting rid of short cycling is good for the boiler and longer burns will improve efficiency a bit.

    Note that this is a simplified view of the changes necessary to do it right with different configurations.
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