Correct temperature setting for hot water heat/furnace

Discussion in 'Boiler Forum' started by edronline, Sep 23, 2012.

  1. edronline

    edronline New Member

    Messages:
    1
    Location:
    Pittsburgh, PA
    I have a 2 zone Burham hot water heating system. It is currently set for 180 degrees F, but I notice there is a lot of short cycling. On very cold days the burner will turn on until the water heats up. After that, the zones will cycle the furnace off and on very rapidly- - usually on for a minute, then off, then on, then off, etc. etc.

    What should I do to the furnace temperature gauge to stop this short cycling? Should I increase or decrease the furnace temperature gauge? Or is this just a "function" of the Burham system -- that it constantly tries to "top off" the temperature no matter what the temperature is.

    I put all kinda of insulating pipe wrap on the exposed basement pipes, but I don't think this should make any difference.


    The thermostats are ok -- typically I set them for 60 overnight, and it is asking the furnace to get each zone up to 65 or 66. The thermostats aren't cycling off and on.

    Thank oyu.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,059
    Location:
    New England
    Short burns will kill the unit. MOre description on how it is hooked up might help. The unit is either not installed properly, the zones are too small, or it is way oversized. Now, you'll probably say it's the same size as the old one, but then that means it is probably 3-4x oversized.

    The load needs to be bigger to keep it from short-cycling. How you can achieve that will depend on what you have and how it is currently arranged, but if it is truely way oversized, it's tough. You may need to add a buffer tank to create a bigger load.

    Most boilers have a min/max setting - yours may be set too close together. Also, you may need to consider an outdoor reset control that will adjust the boiler's temp based on the outside temp (thus load). It's highly likely that for most of the winter, you could heat the house with lower temp water. There's a limit on how low you can go based on the specific boiler or you'd be adding to its early demise. Some can handle low temp returns, some can't and will corrode or the shock will warp or crack them.

    As an aside, double check the operating pressure and make sure it is within range.
  3. Dana

    Dana In the trades

    Messages:
    2,866
    Location:
    01609
    If it's actually hitting the 180F high-limit you may get some relief by bumping that up to 215F, but if you're looking at 1 minute burns it probably won't be enough. It needs more hysteresis between the high & low, which may or may not be separately controlled on your system. Ideally you'd get at least a 5 minute minimum burn (10 minutes even better). If there is no separate low-limit aquastat, note the boiler temp when the burner kicks on those short cycles, as well as the high temp when it kicks off. If it's less than a 20F hysteresis you can probably stretch that to at least 3 minutes by bumping the high limit up, and dropping the low limit to 140F or so (even if it takes retrofitting a heat-purging economizer control such as the Intellicon 3250 or Beckett Heat Manager), which would improve efficiency measurably. With 1 minute burns it's wearing out the boiler and cutting efficiency by at least 10% below the AFUE test numbers, and in your case it's probably by more than 15%.

    Got a model # for boiler, and it's approximate age? (The model number of the aquastat might also be useful.)

    Having multiple zones makes the problem worse- even if the boiler isn't oversized for the radiation with all zones calling for heat, it's probably WAY oversized for any one zone, particularly the smaller zone. Unless the radiation in the zone is sufficient to deliver ALL of the boiler's output to the rooms, it will cycle. What is the boiler's output BTUs on the name-plate (the DOE number, not the I=B=R number, if it has both)? And, how much/what type of heat emitters are on each zone?

    Outdoor reset would only make the short-cycling worse, since outdoor reset controls lower the average output temperature of the boiler during moderate weather, increases it for the deep cold. At lower water temps the radiation delivers even less heat to the zones (which is why raising the temps might keep it from cycling during a sustained call for heat, or would at least lengthen the burns somewhat, since a larger fraction of the boiler output is being emitted by the radiation.) Outdoor reset is great from a comfort point of view since it delivers more even room temps, but without a high hysteresis it'll kill the boiler's efficiency quicker than dumb controls with hotter high-limit would.

    The Intellicon & Beckett controllers "learn" the system and exercise the available thermal mass of the system with as high a hysteresis as possible, and figures out from the behaviors of the system how to anticipate the end of a call for heat, cutting the burners early and letting the system purge heat from the boiler at the finish. On a new call for heat if the boiler's temp is above the programmed low-limit it inhibits the burners until the radiation pulls enough heat out of the boiler to drop it to the low limit. This approach maximizes the burn length while minimizing cumulative burner on-time. It may not fully cure the short-cycle, but it'll make a difference, and will deliver higher efficiency than an outdoor reset controller on YOUR system. On high-mass systems where short-cycling is not an issue outdoor reset can be somewhat more efficient than a heat-purging economizer. Outdoor reset really comes into it's own with modulating-condensing boilers, where the raw combustion efficiency of the burner sees huge increases when the water coming back from radiation can be kept under 125F while still keeping the place warm, but even a mod-con boiler's efficiency get's clobbered if it's short-cycling on zone calls. But that's not your boiler, and cool water returns could shorten your boiler's service life even quicker than short-cycling.
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