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Over sized boiler solution?


We got a new boiler a few years ago and ever since the steam vents on the radiators have made a loud hissing like the steam is being forced out way too fast. I invested in a copy of "The Lost Art of Steam Heating" (excellent!) and did the math--it looks like the boiler is over sized for the number and size of radiators attached. It also switches on and off every few minutes until the thermostat is satisfied, which seems to match this diagnosis.

Is there anything at all I can do to compensate for this? I've seen references to "down firing" the boiler so it doesn't make as much steam, but I'm not sure how I would do this?

Many thanks!


  • will smith_4
    will smith_4 Member Posts: 259
    Hissing vents

    It isn't steam that you hear from the vents (that is, if the vents are working), -it's air. When the system is off, it becomes open to atmosphere as the radiator/main vents open, and when the boiler cycles, the steam races out to the system wherever it can (ie, wherever a vent is open) Before you think about down-firing the boiler, which entails changing the operating gas pressure, check your main vents and make sure they are all working. If they don't work, the radiator vents are the only source of venting in the system, and will tend to be noisier than usual. Gas valves can only be derated to a point, and to do so,you have to have a manometer on the outlet of the valve while it's firing to adjust it.
  • jpf321
    jpf321 Member Posts: 1,568
    edited November 2010
    make sure...

    That the vents are sized correctly for the rads..if the vent is too small they will hiss and also build pressure faster. What type of vents are installed now? Als make sure that you have adequate venting installed on your mains.
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • BobC
    BobC Member Posts: 5,478
    edited November 2010

    As others have said make sure your radiator valves are sized correctly and make sure your steam main(s) are vented quickly and completely.

    You didn't say if you were running oil or gas; if it's oil you could probably down-fire 20-25% by having a smaller nozzle installed, if it's gas you can lower the gas pressure or eliminate some of the orifices but I'm not familiar with that procedure.

    My oill fired boiler has been down-fired and I've optimized the venting on the main and radiators, the vents still hiss but NOT as much as when i started.

    good luck,

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • haaljo
    haaljo Member Posts: 112

    You got Dan's book so you know the importance of pressure. What are you seeing for pressure on a decent gauge? (ie 3 pounds). And you are short cycling as well. 'The way you "down fire" is to bring in a tech that knows about steam and has the instrumentation to set appropriate firing rate and Ptrol setting.. If you have gas you can clock the meter.

    With that information and the burner/boiler nameplate ratings you will be halfway there. Break a leg!
  • pdjohnson
    pdjohnson Member Posts: 2
    Vent sizes

    Thanks everyone for your responses. I have installed a larger main vent to vent the main quickly (a Hoffman 75) and it is working properly. The loud hissing in the radiators doesn't happen until the heat has reached the main vent and it has shut off. On the radiators themselves I have installed Vent-Rite #1's which I think are OK (these are all normal home-sized radiators). They are all opened up all the way.

    Also, it's a gas system. Can anyone recommend someone who is good with steam systems in the Boston area who can help me down fire it properly? I understand that it won't fix the problem completely, but it would be great if we could make an improvement.

    Thanks again!
  • nicholas bonham-carter
    down firing

    don't forget to check the pressure with a decent gauge. the fact that the rads hiss as soon as the hoffman main vent closes, would indicate to me high pressure.--nbc
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