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question about controlling a boiler

Hi all,

just installed a new high efficiency condensing modulating boiler with outdoor reset high efficiency pumps etc...nice system and works great.  Installation was in a condo building with 18 units, all of which have their own zone valve obviously.  the old boiler that we took out was just a hot boiler running a 180 degree loop through the building at all times, and no one ever wired back any end switches from the zone valves.  all that I have is a bank of transformers in the boiler room each of which powers the thermostat and zone valve for three units (honey well zonevalves, 40 va transformers, and mostly non programmable heat only thermostats).  is there any way, with access only to the transformers (powered separately from the boiler) to have the boiler only come on when there is a demand for heat?  pulling my hair out trying to figure out some control or relay setup that will work and haven't come up with anything.  Granted we are talking about a small number of days when there won't be at least one person calling for heat, but it would be nice to know that on the random 65 degree day we get every now and then in the middle of heating season, the boiler wasn't trying to fire for no reason.  right now the t contacts on the new boiler are just wired together and it heats the system loop to the outdoor reset temperature.  system is working well, and is much more efficient than the last one, just want to get that little extra bit ya know?


  • furnacefigher15
    furnacefigher15 Member Posts: 514
    Current relays

    I'm not 100 percent certain, but you may be able to find a current sensing relay that you could use to see when there is load on the transformers secondary. Might need to use a amplifier loop to make it work.

    .3 amp load with a wire wrapped ten times around the current sensor = 3.0 amps of amplified current.

    Any time there is current draw over .3 amps , I would assume that to mean there is demand.

    I've never tried this for your particular application, but it may work.

    In my experience though, in applications such as the one you are dealing, I would think it best to not over engineer it.

    Just let the system pump run with a DP valve in the loop, or a smart pump on delta t or delta p, and let the tekmar do the rest.
  • hydronicsrock
    hydronicsrock Member Posts: 3

    Yeah, I thought about a current sensing relay, just never had much experience with them other than on humidifiers and that experience is limited.  I might try that, do you ever have any problems with longevity on those relays?  just don't want to end up with a callback nightmare.  Thanks for the reply.
  • furnacefigher15
    furnacefigher15 Member Posts: 514
    Rib relay


    Here is an example of what I would consider using.

    The above, and likely most current relays are pilot duty only, meaning they have very limited amperage carrying capacity, on the output wires.

    So of you plan on switching anything more than a dry contact, and you'll need to incorporate another relay (and power source) to handle the real load.

    As long as the control wiring and application is done correctly, the setup would offer good reliability.

    But, at the same time, the more stuff you put into the controls, the more likely you are to have trouble.

    If efficiency is the name of the game, then not running the boiler w/o demand would be key.

    If efficiency is less important then reliability, then I would let the boiler run as is.

    Also, the other thing to consider is sometimes common area heating is not controlled, b/c some of the piping has low air temp exposure, like in a entryway, or in a heavily ventilated trash room where there is exposed piping.

    If that's the case, then continuous flow and heat availability is critical in preventing pipe freeze and burst situations.
  • hydronicsrock
    hydronicsrock Member Posts: 3

    for the link.  That looks like it should work, The only thing I would be switching is the demand terminals on a lochinvar knight xl.  The control board does supply twenty four volts to those contacts but there shouldn't really be any real load on the relay.  Efficiency is definitely my priority, and really there are not any other external controls on this boiler.  all of it from the outdoor reset to the pump control is done through the boiler control board itself, so this relay would be the only external control i put on the system.  As for the heating of the common areas, we found out a long time ago when we started working for this condo association that the common area heat is running off of the unlucky units who's heating lines happened to be close to a common area and therefor easy to hook on too.  gotta love those early seventies buildings.  Thanks for your help I think I will try one of those current sensing relays and see how it works.
  • furnacefigher15
    furnacefigher15 Member Posts: 514

    Happy to help.
  • Zman
    Zman Member Posts: 7,467
    Please keep me posted

    This is a very common problem. I think you are looking at some interesting and viable solutions. Please post a followup.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
This discussion has been closed.