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Heating Tappings on an Weil McLain Model H-4 Boiler

I have come across an old steam boiler, a Weil McLain Model H-4 (estimated install year 1969) that has two domestic cold water tappings on the boilers left side, that a previous installer piped the house domestic cold water into and out of the boiler at these points.

It took some time for me dig up some old literature about the model H boiler, but I can not with 100% certainty answer the following two questions, so here they are:
(1) Does the boiler require the domestic cold water feeds for any reason?
(2) If the two tappings are disconnected and plugged will that result in a failure of an interior coil, tube bundle or tank burning up? I attached some literature that I found, but the manufacturer doesn't provide any details to what is going on inside the boiler. It appears that the domestic water was piped in such a manner to use the boiler as pre-heater to the incoming domestic water.

Without knowing exactly what is going on inside the boiler, I can not advise whether these line can safelty be disconnected from the mechanical system. I welcome any insight or discussion on this matter.

I attached the literature and a couple of pictures, along with a online piping schematic.
Best Regards.


  • SWEISWEI Member Posts: 7,356
    Almost certainly a tankless coil -- can you provide a photo?
  • There is an attachment in the original photo with pictures and a cut-sheet.
  • SteamheadSteamhead Member Posts: 13,997
    edited October 2016
    Definitely an old tankless coil. If it's not in use and it's not leaking, it can be disconnected.

    If the boiler is maintaining temperature all the time, and the coil is not in use, you can change it so it only comes on when the room thermostat calls for heat.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • What I can tell you is that the boiler maintains temperature just fine that is while domestic water is available to the tapping's.

    Without taking off the jacket of the boiler, I have no idea what the tapping's are doing behind the scene.

    So that is the million dollar question, is if these heater tapping were plugged, could damage ensue inside the boiler?

    The cut sheet remarked with an asterisk "*" that heater tappings are 3/4" when required by utilities. I do not know what that means, other then pre-heat function. Agree?

    Based on the cut sheets, the H-4 was a tankless heater, but came with provisions for an additional storage heater. What I think happened is that the storage heater was removed at some point in time, or never provided, and the heater tapping were piped to with a domestic feed, acting as pre-heat for times when the boiler was firing.

    I do see how these tapping's or internal coil would be connected to the house thermostat? Could you clarify your last sentence?

  • SteamheadSteamhead Member Posts: 13,997
    With a tankless coil, the boiler must be kept hot all the time if you want hot water. This is quite uneconomical for most of the year. Many people install separate water heaters and stop using these coils, which eliminates the need to keep the boiler hot all the time.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • Dave in QCADave in QCA Member Posts: 1,759
    Many years ago, I had a house with and H-5 boiler. It was set up in a hot water system and had the remains of a domestic hot water coil. The piping to cold and hot water had been disconnected and the pipes plugged. I assume that it had sprung a leak at some time. The heater coil consists of a steel plate and a copper water coil on the inside that is immersed in the boiler water. It is held in place a series of bolts and a rubber gasket forms a seal between the plate and the opening in the boiler section. All was good in my system until I had the boiler empty for a period of time while I was installing zone valves and replacing leaking radiator valves. After a month or so, when I refilled the boiler with water, I discovered that the rubber gasket on the heater coil plate was leaking. It was impossible to get the bolts out, they just twisted off. So, I ended up having a welder / blacksmith come and remove the bolts. The plate was pitted badly, so he made a new steel plate to cover the opening. I cut a new gasket, got some hardened bolts, and put it all back together. It sealed up fine and the boiler, even though it was 25 years old at the time.

    So, the short answer is, yes. You can remove the piping and install pipe plugs. It will have no effect on the boiler. Also, based on my experience, do not leave the boiler empty for any length of time, as the gasket might dry out and start leaking.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
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