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Sensitivity of Condensing Boiler to Cast Iron Baseboard & Radiator Rust

BradW Member Posts: 2
I'm updating my 1953 gas boiler, and I've got a 1920 house in Norwich CT with wonderful cast iron baseboard heat and one cast iron radiator, none of which I want to replace.  I'm trying to understand some of the factors unique to my situation that might make me swing between a condensing unit and something like a Burnham Revolution at 88% AFUE.  At the moment, my biggest reliability concern is the chemistry involved

Can anyone tell me about the sensitivity to stainless (I'm not keen on using aluminum) condensing boilers to the chemistry of having all that cast iron in the system?  The piping to all the units is copper. I'm concerned that I'm going to re-introduce a lot of new disolved oxygen when I flush the system out--which I will absolutely have to do before reconfiguring all my plumbing--and that is going to free up a lot of new rust in short order.....

I want to do the install myself, and I'm trying to scope out the project.  (I'm a reasonably accomplished home plumber and electrician and a mechanical engineer, so the install doesn't concern me too much.)

Thanks in advance for your really technical discussion to come....



  • Tim McElwain
    Tim McElwain Member Posts: 4,597
    The chemistry can be controlled

    by using a water treatment recommended by the particular boiler manufacturer. There is also a water treatment company called Rhomar which may be able to help you out.

    If you are going to Mod/Con technology, the Revolution I is not Mod/Con as you note. There are other things you will have to consider. First and most important the cast iron baseboard and radiators must operate at 180° temperatures or higher. This does not allow for sufficient Delta T (temperature drop) to cause the condensing feature (140° F or less) to operate on the Mod/Con. You will need to use an outdoor reset feature and find a way to drop more temperature across the system before returning to the boiler. This can be done by increasing radiation somewhere in the system and dropping some BTU in some perhaps otherwise cold areas.
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Operating temperature

    What water temperature are you presently operating at, and is the boiler operating cold start or maintaining a constant temperature via an aquastat?

    Depending on your heat loss and how oversized your radiation is compared to the heat load, you still may operate in condensing mode a good portion of the heating season. It really depends on the specific parameters of your system. For example, the 1954 system with cast iron radiation in my parents house typically runs average water temperature of about 130F in the 15F weather this weekend. In warmer weather the system runs only a few hour a day, with average water temps below 110F.

    There is also a lot you can do to improve the efficiency of your present system. In the case of the example above, the boiler operates cold start and with its high thermal mass sort of modulates water temperature as a function of outdoor temperature. In addition, I have added a reverse acting aquastat to the circulator control to purge residual heat from the boiler after the call for heat ends and firing stops. This aquastat is presently set to continue circulation until water temp drops to about 100F, then shuts down thhe circulator until the next call for heat.
  • BradW
    BradW Member Posts: 2
    Okay, But What Are Users Experiences with Mod-Con and Cast Iron Radiators?

    Tim & Mike, thanks for the info, but I'l still looking for real-world experiences with using a mod-con with cast iron radiators.  After initial fill, and for some time thereafter, new rust will begin to form from the new levels of dissolved O2, but what about the sensitivity of these fancy boilers to flakes of rust being freed-up?  Anyone installed a mod-con in a house with lots of cast iron radiators, or, like my house, with all the baseboards being the old Burnham cast iron types?  I am a K.I.S.S. kind of guy, and it seems to me that the additional particulates in a cast iron -laden system will really cut down on the long-term reliabilty of a mod-con.  Thoughts anyone?    

    Tim:  What logic or details support the 180° temperature statement?   I hadn't heard that before about cast iron baseboards.    

    Mike--based on the age of my current furnace, I have no interest in doing anything other than replacing it with a completely new system.  I used about 1500 ft3 of gas last heating season to heat my drafty (soon-to-be-far-less-drafty) old house.  Nothing less than a fresh start is required.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,841
    Filter it...

    Place a good quality of cartridge filter in a parallel fashion to filter the system water and remove the particulates. The use of an oxygen scavenger will also alleviate the generation of new "stuff".

    Personally, I have a LOT of these gems in the filed on cast iron systems, and have not had one failure. But time may be the one missing factor. But if you consider that these beauts came from Europe, and they have a lot of cast iron and iron systems, seems they would have discovered it long before us.

    If you are really worried about it, go with Triangle Tube. Their water passage ways are large enough to drive a truck through without hitting the mirrors :-)

    Just make certain that the filter and cartridge are of high temperature rating to handle the heat.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Modcon and cast iron radiation

    Installing a modcon with existing cast iron radiation is a fairly common retrofit. You might want to post your question in the main Wall category, as it will get a lot more exposure to experts with such experience.
  • EricAune
    EricAune Member Posts: 432
    Almost a Non-Factor

    I like, and have used, Mark's idea of using a filter.  I have many of this type of install out there with little to no problems related to rust/corrosion.  (I also use TT's)

    I have gone as far as fitting up a temporary filter set up.  I have a union on each side of a ball valve, filter, ball valve.  I install the filter set-up and return after a couple of weeks and replace it with a pre-cut piece with the unions.
    "If you don't like change, your going to like irrelevance even less"
  • Henry
    Henry Member Posts: 996
    Gas Mod/con

    We have installed probably over 100 Lochinvar Knights with very few issues compared to others. It is very common to install them on cast iron systems. I am looking at the gas bill of a very large church in downtown Montreal. The December consumption went from 24,560 cubic meters for 28 days to 6,099 after installing four 500,000 BTU KBN500 Knights. We use 1200 hours of operation as a standard here. Of this, the maximum operating times of the boiler at peak demand is under 200 hours. So, by using proper piping practices, pump selection that will give about 35F D T, good control setup on the boiler with a proper outdoor reset, one can save between 35 and 55% and yes they do condense!

    One important issue you must look at: NFPA 54 specifies in section 4.1 that gas appliances servicing and installations and, gas piping shall be performed only by qualified labour and companies.There are legal issues for DYI. I am sure that your insurance company would also have issues for a DYI. 

This discussion has been closed.