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Using a Mod-Con with Cast Iron Radiators and Finned Tube Radiation

I am considering a new natural gas fired modulating condensing boiler (Embassy Onex) to replace my oil-fired cast iron (originally perhaps coal fired, circa 1936 gravity hot water) boiler.  Is this recommended? Using a mod-con with cast iron radiators?

I like the instantaneous hot water feature and of course space savings as well as anticipated $$$ savings.

Currently each of the zones has its own pumps/thermostats. Should I do a primary secondary with zone pumping, switch to primary pump with zone valves or is there another alternative?  Goal is to take out large 3" headers and reconnect black iron risers with pex to a 1 1/4" header. I addition, split one zone (the cast iron) into multiple zones with their own thermostats.

Thanks for any advice.


  • CMadatMe
    CMadatMe Member Posts: 3,086
    Cast Iron and Fin-Tube

    I recently designed a system as this for one of my contractors. We used a Viessmann Vitodens 200. The boiler control can run 2 heating curves plus domestic hot water. We used the boilers reset for the fin tube curve and a motorized modulating 3-way mixing valve that plugs into the control  for a second heating curve for the cast iron zones. We zoned pumps. I would rather use a low loss header/hydro seperator for mod-cons vs primary/secondary. This eliminates the temperature drop between zones when multiple zones call and it promotes condensing. Primary/Secondary is great when you have a cast iron boiler and need some boiler protection. 

    Make sure you do a heat loss. If you decided to primary/secondary its a good idea to figure out each zones loss and make sure that the amount of radiation can cover the loss when multiple zones are open. You will with primary/seconday have a supply water temp difference in each zone when multiple zones are calling.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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