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Undersized Boiler Problem

versh Member Posts: 5
Two contractors have independently confirmed that my existing gas-fired boiler is undersized, by a factor of 25% to 30%. The boiler services 15 radiators (7 first floor, 8 second floor). My main problem is lack of heat in master bedroom (2nd floor, furthest from boiler).

One contractor has proposed inexpensive approach -- install petcock valves at 4 radiators which will effectively provide 80% shut-off to the selected radiators. Three rooms are generally unoccupied (empty-nest kids bedrooms) so I can do this easily for these rooms; the 4th could be one of two living room radiators as this room is used less frequently (despite it's name).

Granted this is a workaround solution -- doesn't get to the root cause of the problem and doesn't help if I need to heat the bedrooms. But -- it is a low-cost band aid.

The approach seems logical to me, but I am a homeowner, not a heating expert.

Comments and opinions would be greatly appreciated !


  • Mark N
    Mark N Member Posts: 1,115
    undersized boiler

    Read the post about the article by boilerpro. Read the article. Do some of you rooms have 2 rads in them? Shut off the smaller rad at the hand valve. See if it helps. That solution costs nothing. Check the vent on the rad in your bedroom.

  • ttekushan_3
    ttekushan_3 Member Posts: 958
    edited December 2009
    actually a good approach but

    put the petcocks on all the radiators. This will get the mains to act as a true manifold. Make sure the mains are very well vented first. Then begin turning down the petcocks on the warmest rooms first, then a little on the next warmest rooms, then back to the warmest, until you reach the now-formerly "cool" rooms. Touch up the formerly cool rooms until they are evenly heated.At this point your boiler will match the amount of steam radiation it can "see." The radiators will be "virtually downsized." This is actually the most efficient way to run any heating system. You will find that fuel consumption is good. The rating on the boiler in "square feet of steam" is your new effective EDR of the system. When a new boiler is installed, this is what you should use as your guide. Once you've balanced the system, you might want to lock those petcocks in position in some way.If the unlikely event that the system can't keep up with heat loss in the coldest weather, insulate the place and add extra storm windows. Its the right thing to do!-Terry

    An additional note, your system shouldn't really need any more than about 6 or 8 ounces of steam pressure to do the trick. That's not a misprint. Install a Vaporstat that can control the steam pressure from 0 to 16 ounces. This goes in place of or in electrical series with the provided Pressuretrol. You can get a 0-3 lb. gage and set the pressure accordingly. If pressure builds so quickly that the boiler begins to cycle on and off on pressure, you will need to open all petcocks slightly. You'll get the feel of it.
  • jpf321
    jpf321 Member Posts: 1,568
    how was the undersize arrived at?

    can you tell us the method which your contractors used in order to determine that the boiler is undersized?

    I present several different possible options:

    #1 - simply counted radiators on each floor of  house (but didn't measure them)

    #2 - looked at boiler 0-30 pressure gauge when boiler was on for a while and noticed a very low pressure, perhaps under 3 or 4psi

    #3 - performed a heat-loss calculation based on construction/insulation, house N/E/S/W orientation, coldest day of year, expected wind speed and direction, and desired indoor temp

    #4 - looked at your last year's few heating bills

    #5 - felt the radiators after the system had been on for a while and noticed they weren't getting hot very fast or all the way across.

    #6 - noticed the thermostat go off before all rooms were getting heat

    #7 - looked at the boiler specification plate and told you that most of your neighbors have larger boilers

    #8 - a) counted radiator sections and tube/column number and determined the sq footage of effective direct radiation (EDR) as well measured the pipes (length and diameter) and b) determined as well the proper venting needs for each element in the system (mains, risers?, rads)

    If they did #8 and coupled it with #3 perhaps then you are in good shape .. if they did any and all combinations except #8...you should keep looking for other contractors (now that you know what to look for).

    A boiler's job is to fill the pipes and the rads with steam. The first thing that needs to happen is to determine exact how much space there is to fill (#8a) and then determine how best to get the cold air out of the system and the hot steam in (#8b). If your boiler is rated to fill that amount of space (#8a) + 15% (for heat loss from pipes in walls and such) + 15% for speedy heating .. then it is properly sized. If they didn't do #8, they can't tell you anything about the sizing.

    You may find useful the boiler sizing chart posted by another member, Rod, within the last couple of days. http://www.heatinghelp.com/files/posts/1085/Boiler%20Sizing%20Chart.pdf

    Do follow-up and let us know how it's going.
    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
  • shutting off unneeded radiators

    don't forget that you can shut off individual radiators in many cases, on a 1-pipe system by turning the vent upside down. this will tell you whether the boiler is able to provide even steam delivery to the rest of the radiators, [master bedroom].  if not, then there must be some other system problem--lack of venting, high pressure, etc.

    i am not sure where these "petcocks" will be installed.--nbc
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