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cast iron with mod/con

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Brad White
Brad White Member Posts: 2,398
> Hsa any one here tried to mix a mod/con boiler <BR>
> with a cast iron boiler? I want to use the <BR>
> mod/con to supply heat when the demand is low and <BR>
> second stage it with a cast iron boiler that is <BR>
> already there. How would this be piped? <BR>
<BR>
especially in commercial work and is known as a hybrid plant.


One way to do it is that each boiler is piped in parallel. The cast iron boiler would be fitted with some form of return water protection to keep the return above 135F.

The condensing boiler or boilers becomes the "lead" boiler to get most benefit out of it. When indexed HWS temperature goes much over 145-150F (meaning return water temperatures would fall below 130F on average), then the cast iron boiler takes over. The CI boiler is sized for the entire load in the coldest weather (where they do better) and the condensing boiler is sized for the load any time HWS temperatures in the low 140's or less can serve well. Thus a 125 MBH heat loss at zero outside would be served by a CI boiler with 125 MBH output and a modulating condensing boiler of about 80-90 MBH input modulating down to the 20-25 MBH range.

Another way is to pipe the boilers in series such that the Mod-Con always sees the coldest water temperature and in effect pre-heats the return water. Can be problematic in that the load balance between boilers is critical. Each has to be sized so that the final water temperature in coldest weather is at say 180F yet to be effective, the ModCon has to see 130F or less return. You would have to ask if an average water temperature of 157.5 would work with your radiation and if running two boilers makes sense in general.

With your set-up of an existing boiler (which I assume heats the place now), I would put in a ModCon in parallel sized for the heating curve when 140F or lower HWS meets the load. Given the hours of occurrence at those milder temperatures, that is what I would do.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"



-Ernie White, my Dad

Comments

  • [Deleted User]
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    can this be done

    Hsa any one here tried to mix a mod/con boiler with a cast iron boiler? I want to use the mod/con to supply heat when the demand is low and second stage it with a cast iron boiler that is already there. How would this be piped?
  • Ken_40
    Ken_40 Member Posts: 1,320
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    Brad's right...

    But there may be a way to have cake -and eat it.

    Most mod-cons have ranges that go from 15-50MBTU, 30-100MBTU, etc.

    You c.i. boiler probably gets mid 80's AFUE - tops.

    If you've done a real heat-load calc., you may be surprised at the benefits of just having a properly sized mod-con - fawgettabowt the existing unit?

    That would certainly keep the controls strategy unencumbered - as well as the materials costs and labor too.

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  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    No reason that wouldn't work

    Proper sizing is key, as mentioned above, and the method would work best if the load is beyond the range of a single mod-con. The mod-con should see the lowest water temperatures, with the CI boiler coming in when it really gets cold and water temps are up past condensing range. Together they should equal the calced heat loss.

    If the system is designed for a wider than usual delta-T, the mod-con could see low temps over a greater range.

    Depending on the load, you might be able to size the boilers so the mod-con could handle maybe 2/3 or 3/4 of the total load, with a smaller CI boiler coming on line to assist when it's colder.

    The nice thing about this arrangement is that if one boiler breaks down, you still have some heat.

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  • Uni R_2
    Uni R_2 Member Posts: 589
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    What's the application/heatload?

  • Boilerpro_5
    Boilerpro_5 Member Posts: 407
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    No problem

    I installed a Hydrotherm Multipulse with Burnham iron together about 10 years ago and it works great. The Burnham has a ESBE Thermix return mixing valve for temp protection and the two boilers are piped in parrallel with common P/S piping into the main. Check valves are installed in the outlets of each boiler to prevent backflow when only one boiler is running. A tekmar 252 boiler control (now long obsolete) stages the boiler with full outdoor reset in this baseboard hot water system. BTW, the pulse has had only one service call since installed....seems alot better than most of the newer condensors out there and it uses almost no electricity since the fan almost never runs...just on startup and purge.

    Boilepro

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  • [Deleted User]
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    thanks for the help

    The building is a large condo complex about 6 years old. One of the boilers has a bad section and the manufacturer has not been very receptive to helping out with the situation. The other boiler that is there is in tip top shape and i was going to pair it with two Lochinvar KBN 399's. I figured this would be a great way to heat the building on first stage and when the DHW kicks in on priority i want the cast iron boiler to help out. If the manufacturer doesnt come up with an answer soon as far as warrantee issues imay propose this new system to the board.
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