Are condensing boilers compatible with cast iron radiators?


Old 07-29-11, 09:43 PM
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Are condensing boilers compatible with cast iron radiators?

I am considering replacing my 71 year old oil fired boiler with a condensing natural gas fired boiler. The distribution system is copper tubing ( originally gravity powered) with cast iron radiators and cast iron convectors. I have read that condensing boilers have narrower passages than cast iron boilers, which might get clogged with the sediment or rust from the old radiators, and that reverse power flushing (with or without chemicals) of the system and radiators individually would be needed. But this flushing might cause the radiators to leak, requiring costly replacements.
So, since I have cast iron radiators, should I chose a cast iron or condensing boiler? and if condensing, which brands would be best for my system? Thanks
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Old 07-30-11, 05:50 PM
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Yes, modcon boilers are fantastic with cast iron radiators. If you already have forced hot water radiators (as opposed to converted from steam), switching to a modcon is really no big deal. Cast iron rads are high-mass emitters, and typically the older large-diameter piping provides a good volume of water for the boiler to work against. It will cruise along rather than short-cycle as some low-mass, low-volume systems are prone to do.

Go ahead and flush the system. It is probably not necessary to flush individual rads, and certainly not with high-pressure water.

Put a wye strainer (like so Wye Strainers, Water Safety & Flow Control, Watts) on the boiler return. That will catch any crud. Some modcons like the Triangle Tube Prestige and Firetube Knight have very open passages as well.

But first, do a heat loss calculation to size your boiler. Yes, even modcons need to be sized. Also figure out the total BTU/hr output of your radiators at various water temperatures (Heat Loss calculation provides guidance for this) and you can estimate what kind of outdoor reset curve you can run. Chances are it will condense for a good portion of the year, giving you maximum attainable efficiency.

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