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Modcon paired with older cast iron boiler

Voyager Member Posts: 394
edited February 2021 in Thermostats and Controls
I’d like to bounce an idea off you all and get some opinions and suggestions. My church currently has a cast iron boiler, 225K, that is probably 27+/- years old. The system has five zone valve controlled zones and a single loop and single circulator and fin tube baseboard radiators. Pretty simple.

The boiler has gotten less reliable in the last seven years or so with three failures to fire. The first resulted in a burst pipe as the church is not always attended and this failure was not discovered until it was too late. After that, I installed wifi thermostats so I could monitor remotely and get notification of a failure. This has prevented frozen pipes as the last two failures were detected early and fixed in time, but the last failure required a $$$$ repair, and the repair tech suggested we start thinking about replacing the old boiler. And, of course, both failures occurred on weekends late and night and incurred expensive off-hours labor rates.

Certainly, an in kind replacement is the easy decision, but I am wondering if it is the best decision. Here is my thought for feedback. I’d like to move the church to having two smaller boilers rather than the one large one. Redundancy is the main objective, but increased efficiency would be nice also. Since this last boiler repair was pretty expensive and thorough, I expect the boiler will run another 5 - 10 years before it needs a lot more attention. So, my thought was to change to a P/S configuration and install say a 120-130K modcon in parallel with the existing boiler. The modcon would be adequate for the spring and fall and the warmer winter days. The existing boiler would be there to pick up the slack when demand exceeds the modcon’s capacity. And then when the old boiler has the next expensive failure, we would replace it with a second modcon of similar size to the first to provide something close to the current 225K capacity. And we would then have a newer and an older modcon and could replace them every 30-40 years on a stagger.

Obviously, the cast iron boiler wants to run in the 160 - 180 F range and the modcon much less than that. So, it seems that running both boilers at the same time would be troublesome, although I suppose a thermostatic mixing valve could be used to protect the cast iron boiler. I was thinking of a simple arrangement that would run either the modcon or the cast iron under automatic control, but not both simultaneously. However, I have yet to find a controller, at least an affordable one for a small system, which can do this. My thought is to run the modcon until it can no longer maintain temperature setpoint and then automatically shut it down and fire up the cast iron and run it alone.

I can think of a couple of ways to do this, but not sure which would make the most sense.
1. The simplest would be based simply on the ODR temp. When above a certain temp, run the modcon alone and when below a certain temp, run the cast iron. This is simple, but not optimal.
2. What seems more desirable, but more complex, is to monitor the duty cycle of the modcon and if it gets close to 100% and the thermostats are still not satisfied, then fire up the cast iron and shut down the modcon. The challenge here is what is the criterion to switch back to the modcon?

Enough rambling. Any thoughts on this? Is it a completely crazy idea? Should we just buy a new cast iron boiler and leave the plumbing unchanged? Should be save up for two new modcons and plumb P/S and run the modcons completely in parallel? Something else?


    JUGHNE Member Posts: 11,061
    What water temp do you need to heat the building and what temp is the return water?
    Is it all BBs or fan coils?
  • Voyager
    Voyager Member Posts: 394
    Temp needed obviously depends on outside temp, but I believe at 0 F, a typical design day temp for our area, it needs to be 160 F to keep up. Obviously, in the shoulder seasons, probably 110 to 130 would be adequate most days, but that is just a guess.

    All radiators are fin tube baseboard.
    JUGHNE Member Posts: 11,061
    The return water temp is of concern to prevent condensation in the CI boiler. Usually 130 minimum.
    Maybe there is some form of boiler protection for the return water being low.
  • Ironman
    Ironman Member Posts: 7,376
    Install bot mod/cons now. It will be a lot cheaper to do both at the same time than trying to piecemeal things separately.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.