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One Tstat to run 2 boilers....time for Relay

JUGHNE Member Posts: 11,021
This involves the other 2 boilers of the 3 I am putting in this schoolhouse.
The 3rd one with the Cycleguard is for the single pipe system.

These, #1 & #2, have the M&M 150S feeder pump controller, these multi boilers feed the 2 pipe system. Common drop header.
Separate boilers that run from 1 Tstat.

Fired all 3 boilers yesterday.
But with #1 power off and #2 on, both ignition modules were trying to weakly spark.

Sudden realization that obviously there are 2 power supplies now tied together (back feeding) with one Tstat..........simpleton dummy, me, not thinking that far ahead. :(

So time for a separate power supply and some form of double pole relay.

I have plenty of 24vac xformers, any good suggestions for DP 24v coil relays?
Maybe RIB makes one?


  • mattmia2
    mattmia2 Member Posts: 9,410
    RIB or there are a bunch of "fan relays" that will do it too that are less expensive but require mounting and some terminals. I think Tekmar makes some controls for the purpose that can do staging and rotation automatically if you want that.
    JUGHNE Member Posts: 11,021
    Lo and behold, just found a RIB on the shelf in the shop.
    It is RIBU2C, 2 SPDT relays in one case, 24vac or 120vac coil.
    It has 12 wire leads coming out.

    I like the RIBs as they have the LED (2 in this case) and long enough leads to enclose all in JB's.

    Been on the shelf here since 2-17-2017, pays to look first. :)
    (I actually have 2 in stock, there are plenty of $20 and $100 dollar bills on the shelf that I may never use up....need one....buy 2 or 3 just in case).
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,214
    In the 2 pipe system at my church I use a 2 stage thermostat to bring on the boilers as needed. The supply valves are orificed to help ensure even steam distribution even with only 1 boiler running. Still need isolation relays, though.
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    EBEBRATT-Ed Member Posts: 15,307
    edited September 2021
    This is something I drew up with years ago. You can run both boilers on lead or both boilers on lag or lead either one and lag the other. You could get fancy and use a time clock to do the switching as well. Lead during the day lag at a lower pressure at night etc.

    Cost is 2 pressure controls, 2 relays and some wiring.

    You could have the thermostat(s) operate the spdt relays and eliminate the switches.
    JUGHNE Member Posts: 11,021
    Thanks Ed, I have to wait for cold weather to see how the 2 boilers might function.
    I will print out the diagram to consider.

    What follows is responding to Steam Whisperer; I typed it up yesterday and forgot to post it.....does anyone else do that or just me? :*

    I jumped both boilers to run on Sunday.
    These feed a 2 pipe system.
    I worked on this system a little over 10 years ago.
    At that time, I put orifices in all the convectors to deliver 80% EDR at 2 PSI.
    For the hour of firing I might have reached 1/2 PSI.
    The FLIR viewer showed some convectors heating about 50% across.
    Returning condensate was cool temp.

    Very early to tell but hoping that I can now simply drill out orifices as needed for more heat if needed.
    Will have to survey about 6-8 ladies (are you too hot or too cold?) to make the best guess. Anyone else been there?

    I do have a HG vaporstat, new in box, which may be used here to stage the 2 boilers by pressure. Need cold temps, also piping is not insulated, abatement done last year.
    Insulation may get applied this year....over 400' worth from 5" down to 2". Most of it in 4' x 4' pipe tunnel.
    A fairly expensive venture for a younger person.

    Floors around the perimeter of the school were nice and warm last year.
    EBEBRATT-Ed Member Posts: 15,307

    "Will have to survey about 6-8 ladies (are you too hot or too cold?)"

    LOL This could be the problem even if their were only two of them :)
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,214
    I imagine you're losing loads of BTU's with the uninsulated pipe, so you probably won't build much pressure. Also, the pressure is not proportional to the heating of the emitters, which you seem to already know. I have found that some systems need to be primed by a period of higher rate firing before they can run at a lower steady burner level. We have a number of orificed systems that run on straight outdoor reset. The burner output is varied directly with the outdoor temperature. This has worked for most systems, but some need that start up boost. We simple place an aquastat on the end of the piping that has trouble heating up and tie it to an manually adjustable firing rate control. WE adjust the aquastat to send a signal to a relay to run the boiler on the manual firing rate control until the pipe warms up. Once the pipe is warmed up, we switch back to the outdoor reset firing rate control. This set up only works with maintaining constant temp in buildings, just like hot water outdoor reset.

    My church runs unoccupied setback of 45F during the week and we have 2 steam boilers. The 2 stage stats bring on the 1st boiler on a call for heat, and if that doesn't satisfy the stat, the 2nd boiler is fired. We ran all last winter on only 1 boiler with the exception a couple of sub zero, high wind, no sun periods where a small supplementary HW boiler took some of the HW HE load off the steam heating plant. We were heating about 15,000 sq ft with just a single 350,000btu/hr input boiler. Total therm usage, including a 2,000 sq ft addition was about 2500 therms.
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    JUGHNE Member Posts: 11,021
    Ed, even if there was only one.....depending upon which day of the week, month or life cycle it was for her.
    JUGHNE Member Posts: 11,021
    Thanks, Steam Whisperer, it is 90 today so maybe a month away from real heating time.