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2 boiler steam control

The system would be a 2 boiler Dunham steam heating system.

There will be long periods of low temperature settings, to prevent freezing pipes.

When in use the inside temperature will be set up to 70 degrees, ahead of time.

Playing around with the SlantFin heat loss app, with different temperature values shows that the temperature can be maintained by one of the 2 boilers when the inside temperature is less than 45 degrees above the outside temperature.

Is there any alternative to running the boilers staged on pressure, such as a control which cuts out the lag boiler during maintenance periods, but allows the lag boiler to run during recovery periods, (even with the same outdoor temperature).--NBC

I should add that I am planning for this to be a vacuum system.

Adding on to this to make it more clear for all concerned- when the system is simply maintaining the set temperature, whether it be a setback of 50 degrees, or an occupied temperature of 70 degrees, one boiler operating in a vacuum, can handle the load, when the outside temperature is only 40 degrees less than the set point. This is because the system will be operating in a vacuum, and the steam from one boiler will be adequate, from the heat loss survey.

When the outside temperature is lower than 40 degrees from the indoor setting, then the lag boiler will need to fire. An outside sensor could prevent the lag boiler from firing, when the outside temperature is more than 40 degrees below the inside.

However, when coming out of setback, both boilers would be needed and the WWSD method would not work.

The ideal control would compare the indoor and outdoor temperature, and fire one or both.

If this were a regular atmospheric system, then the staged pressure method would control the lag boiler, and cut it off when a few ounces have been attained, but in the case of sub atmospheric operation, the pressure will be low enough, never to shutt off the unneeded lag boiler.--NBC


  • Jamie Hall
    Jamie Hall Member Posts: 22,125
    Back in the bad old days...

    I would have done that with a set of thermostats, vapourstats, relays and a few switches.  Never actually done it -- but something very like it with municipal pumping stations for water supply or wastewater.

    Nowadays I suppose it would be done with a nifty little PLC chip.  Or an app.  But you would still need the thermostats and vapourstats and some way to tell it what you wanted -- again, I suppose a little membrane pad, or maybe an iPhone app.  Don't ask -- I got lost somewhere around MS-DOS and never really caught up again.

    What you need to start with, though, is to set down in a straight logic manner exactly what you want to have happen.  A set of statements like "if [this set of input conditions] then [this particular boiler operation]" kind of thing.

    You'll probably want to arrange the controls so that either boiler can be the lead/maintenance underway/it's warm out boiler.

    You'll also probably want to arrange it so that if the lead boiler doesn't run the lag boiler does, unless locked out for active maintenance -- and if that happens you get an alarm condition which requires a manual acknowledgement.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England