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VENTURI with Hydronic wall radiators

cc510cc510 Posts: 2Member
I have an 1940  2100  sf home built very solid with 8 inch cement floors  The old oil boiler has been removed and being replace with 3 separate gas boilers  The old system had a venturi system using steam radiators as the convection.  It had 1/2 inch lines coming of 1 inch feeds and returns I am planning to install 3 separate loops. Should I replace everything with 3/4 pex or can I use most of the existing piping which is a hodgepodge of 1 inch and 1/2 inch. I am thinking  1/2 inch with 3/4 should be ok .  Presently 1/2 inch is going to the radiators. I am wondering how much of the 1/2 inch can still be used. I prefer to keep everything 1/2 inch. or should all the pipes ideally be the same diameter on a closed loop.


  • Paul PolletsPaul Pollets Posts: 3,063Member
    edited November 2011

    First, a 2100SF home does not require 3 boilers. 2nd, the "near-boiler" piping should be replaced with new copper or black iron around the boiler, than converted to Pex or Pex-Al-Pex at least 5-6 ft from the appliance. Old steam piping should not be used because of the rust and possible debris it would send back to the new appliance. The system becomes a "closed loop" and the radiators have to be converted to HW radiators by tapping an air vent into the casting. If the contractor has not properly done a heat loss to estimate the reduction in Btu's from the conversion, you could see a 30-40% reduction in output. Radiators need to be sized accordingly for the source...either steam or HW. Steam radiators are different than HW rads. Take a look in the library and tech topics on this site.
  • cc510cc510 Posts: 2Member
    Venturi with hydronic radiators

    Thank you for your response.  I failed to mentioned that is  a 3 family house  with square footages of 1400sf and 700 sf and 700 sf.  and not 2100sf'   I did size the boiler correctly using the factors necessary.  It almost doubles the BTU versus slant fins. Based on the radiators  it needed 80,000, 50,000 ,and 50,000 btu respectfully.   My real question is whether 1/2 can still be used and to what extent?  These are the size of the loops presently off the venturi system.  The radiators are also presently used with hydronics and not steam.  1/2 inch copper pipes  are attached to the radiators now.  I am trying to cut out as little as possibe.  Presently the returns and feeds are 1 inch.  Thanks  again
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