Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

Corn Fired Boiler/ Fuel Oil Back Up Controls Help Needed

blumeri8 Member Posts: 18
Hi All,

I have a corn fired boiler plumbed on a manifold with a fuel oil boiler . Boilers are plumbed primary/ secondary with hydraulic seporator...please see attached...

Trying to build a control system that does the following:

- use a corn boiler supply water temp sensor, (tie strap to pipe) and controller set to a low T to detect corn boiler has failed . Sensing water temp rather than flue temp gives more delay before fuel oil fires. This ensures corn fire (like a pellet stove)is completely burned out...both boilers share same mansonry chimney and cannot be allowed to fire simultaneously
-use sensor described above and low T set point (100F or so) to redirect 24V heat calls from corn to oil boiler while below setpoint
-Also need to disable fuel Oil 110V while above corn supply setpoint. This prevents the oil boilers aquastat from firing the oil boiler during periods of no heat call.
-one zone pump operates during heat call
-corn pump runs constantly with or without heat call while above corn supply set point. Keeps water circulating around the boiler loop. Oil pump off

-fuel oil pump runs constantly with or without heat call while below corn boiler supply setpoint ( corn failed)
-no expectation to have corn refire if fuel oil fails, it is manually lit like a wood stove

The good folks at Tekmar have been helping me for quite sometime and have went as far as they could. We started with a 150 setpoint and pump sequencer 132. I bought the parts and realized a major issue when wiring: that set-up only redirected the 24V heat call. It did not cut the fuel oil 110V in corn mode.

Attached is their latest proposal. It involves two 150 setpoints and 6 relays. It seems like their proposal will work, I'm just wondering if another company has a more "all in one solution "

Recently I've been trying to learn more about the Honeywell T775 series...seems like everything is in one controller(relays, transformer, sensors)....however I'm open to any other suggestions

Right now I own a Tekmar 150, 3 Tekmar relays, and a Tekmar 132( that I need to sell)...( all items are opened so I can't return for full refund)

Trying to determine if I should buy 3 more Tekmar relays and a second Tekmar 150 and go that route OR if there is a much better solution that warrants me to sell all the Tekmar equipment I have and go another route..(right now since it's getting cold , I am going to wire it simple where I have to manual switch from one boiler to the other while I figure this out )

Other final points:
-A1 aquastat purpose is to "heat dump " the corn boiler if it goes too long without a heat call and starts getting hot in low fire mode. Over rides thermostat to turn on zone pump.
-manual toggle switch to be added to prevent relay switch to oil burn while trying to light and bring corn boiler up to temp.
-corn boiler does not necessarily need 110V cut off in oil mode...once corn fire burns out, corn boiler's internal controls ( snap switch on flue) turn off all feed augers and combustion blower

Thanks for the help !


  • ratio
    ratio Member Posts: 2,880
    You might look into a small PLC. This TECO has been used by Wallies I believe. The programming software is freely downloadable, no reason to not try and whip up the programming just to see if it'll fill your need.
  • hot_rod
    hot_rod Member Posts: 14,883
    is there a buffer tank involved? Maybe a sketch of your actual system.

    I used a multi output solar controller which has the choice of set point or differential control and a few virtual relays that you can configure how they operate and what sensors they use. It is a common control used all over the world, mainly solar with boiler backup, but two boilers are the same concept.

    I'm not sure how much support Alpha Thermal offers for each individual customer, at some point the installer needs to develop the logic for his own system. I find it's best to see what the control is capable of before you start piping :) Build the system to what the off the shelf components can can handle.

    Or buy a PLC and build a control from the ground up, as ratio mentioned. A fellow in Vermont builds custom controls for biomass systems, he hang out on one of the woodturning websites, goes by NoFossil.

    I have a wood boiler, 500 gallon storage, LP backup and DHW via a flat plate on the tank running from one controller. The controller is a mix of preprogrammed arrangements, but you also can build a couple relays inside. Consider it a cross between a PLC and ready to go control.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • hot_rod
    hot_rod Member Posts: 14,883
    I prefer ∆T controls to setpoint for systems like this as it gives you more and better control.
    A buffer, even a small 40 or 50 helps both boilers have a defined load, offers some storage, gives you a central control point, and allows loads to be pulled via ODR.
    A buffer should eliminate the need to "dump" from the corn, they turn down much faster than wood fired, so worse case you overshoot tank set point by 10 degrees, maybe running to 190 or so.

    So, when the corn fired boiler is warmer than the tank, regardless of tank temperature it fires circulator, runs to it reaches desired tank temperature. The control could wake up the boiler, or you could maintain the boiler temperature, costing more fuel of course. Depends on how quickly the corn heats, or IF the oil is always ready to come online.

    Oil boiler also runs via ∆T. If corn is offline, or behind, oil fires based on ∆T. With either you can limit how hot they run, and also at what temperature they kick in on the low end. Maybe have oil only kick in when tank drops to 140, or whatever is the lowest SWT you can work with. Possibly limit the oil to 160, allow the corn to top off or catch up.

    Do you want both boilers to be able to add energy at the same time, for instance the corn cannot keep up or recover fast enough?

    If not a double throw relay allows the oil to always be off when corn is fired, even if corn is not keeping up?

    With a buffer, the fossil fueled boiler should only heat the top of the buffer, you want the corn to do the heavy lifting.

    Do you want or need boiler protection on either or both boilers?

    Loads should be pulled from the buffer via ODR, so on all but design days you are suppling just the right temperature to meet loads, near constant circulation once you dial them in properly.

    If you just need two ∆T functions the Resol BX has plenty of outputs. DHW can be easily added from a buffer also and the control could control that function.

    Everything you need to know to design, pipe and control is here, in this NYSERDA training manual. But it is a lot to digest and decide, especially when boiler return options are added. Don't want to overload you with options and info :)

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,281
    I've built a few controls like this out of stand alone controllers and relays. I agree that a Delta T is much better than a simple strap-on to start oil boiler. This way you can know if there is any heat being added to the system via corn. You could also have redundant backup from the stack in parallel with the Delta T control, so both the stack would have to be cold and also the heat output would have to be zero.

    A PLC based control offers more flexibility in programming as well as easily tweakable setpoints, Delta T, time delays, possible internet connectivity, etc etc.

    I've done it both ways, I like the ease of I/O of PLC based controls, but like the stone cold reliability of hard wired relays, timers.

    TECO is somewhat user friendly, and can be had on the cheap.

    I've used Koyo PLC'S for this purpose as well, but that's just what I'm familiar with.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!