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Replacing steam boiler on system w motorized valves

I was asked to help size a replacement Boiler for a large home in Fairfield county. The boiler is a twenty plus old Smith 19 series Boiler that is very over sized for the home. The attached load is 753 EDR. There is a Boiler Return trap which I cannot identify. The only markings are an S and the number 2143H . It has a top tapping that is tied into boiler supply and a lower tapping that dumps into return side of the boiler. There are 3 motorized valves on the run outs to the 3 areas of the house. They are close to the boiler and the valves connections sizes are 1-1/4 “ ( reduced from 2” with bushings ) . The owner says the system has worked well but now the boiler is leaking . I also noticed that these valves are not dripped at all. My approach is to right size the boiler, add a boiler feed pump , drip the valves into the return and eliminate the return trap.
The owner doesn’t want to switch to hot water , which I suggested . The contractor wants to simply put in a boiler of similar size and leave everything else as is . I am looking for some expert advice.


  • Danny Scully
    Danny Scully Member Posts: 1,408
    I’m glad the owner is smarter than you @Prisco Panza_2 and doesn’t want to convert to hot water :lol: only teasing. Anyway, why do you think you need a boiler feed pump?
  • pecmsg
    pecmsg Member Posts: 4,206
    Pics would help...……
    Change to HW because the boilers oversized?
  • Jamie Hall
    Jamie Hall Member Posts: 21,573
    There are things you ca do to improve the way the system operates -- start by replacing the motorized valves with full port 2 inch. Dripping the valves (both sides) would also be a good idea. Then make sure the boiler return trap is working properly; get someone in to identify it properly (or take lots of pictures and post them). Chances are that if it has a live steam feed from the boiler it is critical to system operation.

    Then right size the new boiler, with the proper controls to keep the pressure low.

    Why do you think you need a boiler feed pump? I'll grant that they are better than a condensate return pump, but not much, and if the system is working now without it, it won't work any better with it. Let gravity be your friend.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Prisco Panza_2
    Prisco Panza_2 Member Posts: 12
    All my readings indicate that anytime there are Motorized valves a boiler feed pump is a must. Is that correct ?
  • pecmsg
    pecmsg Member Posts: 4,206
    Without being there too see the piping it’s hard to say but ALL no they don’t.
    JUGHNE Member Posts: 10,804
    New water line level compared to old level is to be considered.
    Mark the old water level on an adjacent wall before tear out.
    EBEBRATT-Ed Member Posts: 13,851
    @Prisco Panza_2

    I agree with your approach. Use a boiler feed tank, keep the motorized valves the same size, I would not replace them with oversized valves. Size the boiler correctly, run the pressure low pipe it in accordance with the MFG requirements drip the zone valves on the side water will collect at and skim the boiler.

    Zone valves require a feed tank, you will not have leftover steam pressure to push the condensate back to the boiler
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,193
    If the system has a boiler return trap it is probably a naturally induced vacuum/vapor system. Since these systems run on only 1/2 psi of pressure, the return lines will only back up about 14 inches with no pressure on the returns. Since most newer boilers have a much lower waterline than the old boilers, usually it is easy to get greater than 14 inches on the return, so a direct gravity return is ok. However, the zone valves could present a challenge since the boiler has to run at higher pressure than normal to compensate for the pressure drop of the valves. Check your loads on each valve and see what the pressure drop is to meet the load necessary. Whatever that pressure drop is in psi, multiply that by 28 inches and add another 18 inches or so for the pressure in the radiators. If the valves are a 2 psi drop, thats 56 inches and then 18 for the system, that's 74 inches. You'll need 74 inches from the water line to the lowest dry return to run the system by gravity. If you can get the valves down to 1 psi drop that would only be 46 inches, which is doable in many situations. It's all a matter of geometry. Also, if you are able to reduce the radiation to the current heat loss by using orifice plates in the supply valves , that usually will reduce the pressure drop across the zone valves, But you will be adding some pressure drop at the plates ( I would use 1/2 psi sizing for the plates). The other advantage is that the boiler size usually can be cut about in half versus using conventional sizing by the radiation load plus pick up factor. I'd certainly try to make it gravity return if you can. Using 2 inch zone valves will probably easily get you down to low pressures, and the cost is not bad for the valves. A 2 inch valve can handle about 1200 EDR at higher pressure drops ( maybe 2 psi).
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,575
    Maybe those solenoid valves were installed later as a “fix” for imbalance, and bad traps, etc. are they really needed?—NBC
  • AMservices
    AMservices Member Posts: 605
    I'm also consulting a customer looking to replace a 2 pipe steam system with 5 zone valve.
    21 radiators, 800k boiler. Only needs 500k at most.
    Trying to get them to loose the zone valves and install TRV'S on radiators. Then control the boiler with a tekmar 279 steam control.
    And, of course, replaced steam traps.
    A new steam boiler will perform the same as the old one if you dont fix the problems in the system.
    Step 1 replacing a steam system is finding the system size.
    Step 2 is finding how the steam is getting to the radiator and how the air is getting out.
    I would avoid a feed pump if theres not one there now.
    Even when there is a pump I question if they need it.
    Do what you can to keep the pressure low.
    As @The Steam Whisperer said, the boiler return trap is a indication that this was a vapor vacuum system that was designed around low pressure.

    Keep it simple
  • Pumpguy
    Pumpguy Member Posts: 612
    I haven't seen any mention of equalizer lines on the downstream side of the zone valves.

    When a zone valve closes, and all the steam on the downstream side of that zone valve condenses, there will be a very deep induced vacuum as a result.

    This is due to the change in volume from steam to condensate is 1 over 1600.

    The high induced vacuum, also called vacuum of condensation, causes the pressure in the closed off zoned header to be much lower than that in the return line.

    As we all know, all fluids flow from high pressure to low pressure. So, with a lower pressure in the header than in the return line, the condensate will be held captive in the header until the zone valve opens again, or the pressure eventually equalizes with that in the return line.

    The usual solution is to install an equalizer line just downstream of the zone valve. The equalizer line , typically 3/4", would come off the top of the steam header, then run horizontal through an uncovered cooling leg 6' or more in length, and then connect to a 3/4" thermostatic radiator trap, then drop down into a return line.

    This can also be accomplished using a dropped 3/4" swing check valve in the equalizer line in lieu of the thermostatic trap.

    Now with the equalizer line in place, any vacuum of condensation that occurs in the header will equalize with the pressure in the return line and the condensate will gravity drain the way it should.

    The attached file illustrates the principle.

    Not sure if this would apply to this particular type of system, but thought it worth a mention before this thread comes to an end.
    Dennis Pataki. Former Service Manager and Heating Pump Product Manager for Nash Engineering Company. Phone: 1-888 853 9963
    Website: www.nashjenningspumps.com

    The first step in solving any problem is TO IDENTIFY THE PROBLEM.
  • Harry_6
    Harry_6 Member Posts: 133
    I was wondering when someone was going to address the induced vacuum problem - always has to be dealt with when you zone steam, depending on how the returns are managed. If you don't equalize and a zone closes, the induced vacuum will suck condensate back up from the returns. Icky things can happen. Or at the very least it will suck air backward through the returns, and that too is undesirable. And if it required a boiler return trap, then it was almost certainly designed to run below about 3/4 psi. With no pictures it's hard to tell.