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Help with Hissing Vents and Cycling On Pressure

DooverdixonDooverdixon Member Posts: 6
First off. This site is awesome. This is my first post but i have been a reader here ever since I bought my first steam heat system (it came with a free house) 2 years ago. You all have been invaluable in getting me as far as i am now with my ill maintained system. I've replaced the presurtrol with a vaporstat and 3psi gauge (pressuretrol as safety), I've added main vents with 2x Hoffman 45's, ripped out and re-piped a leg that was done in pex by some bone head contractor, and replaced all of my Rad vents with hoffman 1a's. And i recently added an ecobee thermostat (mostly because my previous honeywell got destroyed by my current contractor)

With that said its time for me to ask for a bit more specific help. I have a relatively big 1 pipe system (4000sq ft 3 story house) with about 10 rad's of varying size. The boiler is a "newer" dunkirt model 299k btu. All of my fixes so far have made a TREMENDOUS improvement, but I need to do a bit of fine tuning now. So here's my issue.

When the system is working hard (recovering from a few degree setback, or when its quite cold out) i get some hissing from the rad vents. It only starts after the Rads are completely full and warm and happens to all the rads, not just one. My thought was that my system was over pressured, but this happens when the gauge still reads 0.5psi. After the hissing starts the boiler starts to slowly build pressure and will go up to 1.5 psi then start to cycle off on pressure a few time before the call for heat stops.

The only thing i can think the issue would be is that my main vents are undersized??? I took them off today to make sure they were working (blew air through the bottom and it came out the top?). Any help (or suggest for other improvements based on my photos would be well appreciated and repaid with a virtual beer or high-five.


  • Jamie HallJamie Hall Member Posts: 13,403
    Classic. Quite classic. Once all the radiators are filled, ideally the boiler would cut back its firing rate to exactly match the rate at which the radiators can condense the steam.

    Good luck with that...

    If the boiler doesn't cut back, the steam has nowhere to go, and so the pressure will rise. Has to. It really doesn't matter what the pressure is at the point where it begins to rise -- what is classic is that the system goes along at a more or less constant pressure -- you mention slightly less than half a pound -- and the starts to ramp up.

    That's where you want the boiler to cutoff. The next trick is when do you want it to cut back in. There have been some very lively -- and worthwhile -- discussions on this over the years. However, with a boiler like yours, if it is not technically oversized (that is, it's EDR rating and your radiation are about the same) my own preference is to set it to cut back in with a generous differential -- but not wait for it to drop to zero; that is, to go off steaming.

    This may result in a surprisingly short cycle time -- which depends on a whole host of variables. But it wouldn't be unusual to have an on time of a minute or two, and an off time of even a short as 30 seconds (the total length of a cycle depends on the overall system characteristics; the ratio of the on time to the off time depends on how much more heat the boiler can create than the system can use).

    If the boiler never goes completely off a boil, the efficiency loss from doing this is very small.

    If it turns out that the off time is longer than the on, you might consider seeing if it is possible to downfire the boiler -- but you want to be a bit cautious about that, as you do want the boiler to be able to actually fill the whole system in a reasonable time!
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Big Ed_4Big Ed_4 Member Posts: 1,433
    You can check , and feel the main vents if they are hot first . Maybe your boiler is over sized ... Open each radiator radiator vent one notch.
    I have enough experience to know , that I dont know it all
  • DooverdixonDooverdixon Member Posts: 6
    This is great. So as far as the individual radiator vents they are basically all wide open right now. My thought was to get steam everywhere as fast as possible. I'll also see if they are warm first.

    Jamie what you are saying makes good sense. Quick short cycles that dont allow the steam to fully cool would most approximate how these used to work with coal to keep a steady match of condensation/steam.

    Is it safe to assume that main venting is not my primary root cause, and i should focus on getting the cycle timing correct??? If that's the case and I'm currently running a cut in of 0.5psi and cut out of 1.5 psi (1.5 main with a subtractive differential of 16 oz), and they system "hisses" with full rads at just over 0.5psi. Then what's a good jumping off point maybe a main of 0.75 (or even 0.5 if its still pushing to all rads) and differential just to hover above zero, say 10 or 12 oz on 0.75?

    Or did i completely miss something in your thorough and well written reply (btw, thank you for that)
  • nicholas bonham-carternicholas bonham-carter Member Posts: 8,193
    I would say from your house footprint, and the resulting piping size, that the main vents are too small.
    Try putting a Gorton#2, (or 2), on each return.
    If you have a low pressure gauge, you should see the air escaping at a back-pressure of a couple of ounces or less.—NBC
  • DooverdixonDooverdixon Member Posts: 6
    Thanks for that.  I can T them in with my existing 45s right?  They are only a year or so old and most likely in good working condition.  

    Is over venting a thing?
  • nicholas bonham-carternicholas bonham-carter Member Posts: 8,193
    Yes, put the added vents on tees.
    No worries about over venting, except for the cost. You want to fill the supply piping with steam first, by allowing the air to escape with minimum resistance. Then, when full of steam, all the risers to the radiators begin to fill simultaneously.
    The greater aggregate of resistance of the radiator vents will facilitate this.—NBC
  • ChrisJChrisJ Member Posts: 11,249
    edited October 16
    I test my main venting by heating everything and then shutting it down for 5 minutes and restarting. This tests the backpressure when the piping is still hot and not condensing much steam. This is also how my system behaves in use when it's below 0F out, it often only has 5 minutes off between cycles.

    On my own system I don't like to see more than 0.25"WC during main venting. This would be 0.009 PSI. Systems with longer piping may have more, but it shouldn't be anything near 1/4 PSI on a residential system.

    The object in my opinion is to allow steam to free flow through the mains with no resistance. This makes sure little to none starts heading out to radiators before the main vents close.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • DooverdixonDooverdixon Member Posts: 6
    ok, ive done a bit of work and some homework this am. I adjusted the vaporstat so it kicks off at about 0.75psi and on again about 0.25. Unfortunately it doesnt seem to be helping with the hissing and radiator vent noise much. I tried to feel the Mains and they seem to get warm about the same time (or slightly before) the rads start to heat up. I think regardless i'll put gortons #2 on each main to help (cant hurt). But now im wondering what my other options are.

    Once its actually up to temperature it still hisses even when the gauge reads low pressures (0.4psi). If the new main vents wont help that really only leaves the boiler being oversized right?? I did some homework and found out the EDR of my 10 rads. Using the info from "lost art" i came up with ~450 Sq ft of radiation so an EDR of 108,000 and based on the attached chart from my boiler specs my NET capability is around 184,000? So, im kinda overfired (unless i did something wrong). Could this be causing the hissing? if so, what, if anything should I do about it?

    Thanks again everyone!!!!!!

  • DooverdixonDooverdixon Member Posts: 6
    I forgot to add that im in denver (5000 ft) so it looks like i need to subtract 20% off the btu to adjust for altitude. So maybe 147,000 is rated capacity?
  • Gary SmithGary Smith Member Posts: 328
    make sure the pigtail below the pressure gage and pressuretrol is clear (not clogged). Maybe the gage and pressuretrol are not seeing the real boiler pressure.
  • Jamie HallJamie Hall Member Posts: 13,403
    20% off or not, you are significantly oversized. There's no need to convert to BTUh; the boiler is rated at 767 square feet, which includes the pickup factor. Even if you take 20% off of that, you still have around 620 square feet -- feeding, if your numbers are right and I have no reason to doubt them -- 450 square feet of radiation.

    About 50% oversized, and that will cause short cycling once the system is full. Can't help it.

    However, that doesn't affect the hissing vents. Sorry. Those Hofffmans are usually pretty good about that...
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ethicalpaulethicalpaul Member Posts: 1,858
    Your first post hinted at the answer, but I wanted to ask here anyway. Is the hissing that is concerning to you hissing air, or is it hissing steam?
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • DooverdixonDooverdixon Member Posts: 6
    Hissing air.  Not steam.  They kinda puff while the system is ramping up, then turn to a hiss after the rads start to get warm.

    Is under firing the boiler a good idea?  Sounds like it won't help ha.

    If the Hoffman's 1A's are to blame, it seems weird that ALL of them do it! 
  • Big Ed_4Big Ed_4 Member Posts: 1,433
    Take out and cap a burner tube or two . if you can find the right tech...
    I have enough experience to know , that I dont know it all
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