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Water radiators with ibc condensing boiler?

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brucewayne
brucewayne Member Posts: 8
in Gas Heating
i heard you need at least over 180 fahrenheit for steam, otherwise the radiators fill with water.

i keep getting alot of offers for condenser boilers with modulation. But my radiators use water


i dont know what to do. This isnt my expertise. I found out how to size boiler at least, my EDR is 175 with 6 cast iron radiators.

thus if average water temp is 170, i need 30k BTU. Anyway, im not sure if these offers are worthy. How do these contractors know if my water temp is hot enough to condense? and that return temp is low enough?

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  • Jamie Hall
    Jamie Hall Member Posts: 23,283
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    Say what? There seems to be some confusion here.

    First: is your system steam or water -- hydronics?

    Second: if it is water, hydronics, then the water temperatures from the boiler will be controlled by the boiler and flow rates. It is not a question of hot enough to condense, it is a question of cool enough return to condense.

    If it is steam, it's a whole different animal and condensing boilers with modulation are completely inappropriate...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    brucewaynekcopp
  • brucewayne
    brucewayne Member Posts: 8
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    Thankyou, all radiators are water. I thought steam radiators were required for condensing boilers.
    This is because average temp of steam radiator is 240 F
  • Jamie Hall
    Jamie Hall Member Posts: 23,283
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    Uh... no. The average temperature of a steam radiator in domestic use is around 212 F to 215 F.

    A condensing boiler, to condense, must have the return water temperature below about 140 F. And condensing boilers don't boil -- you are not using steam -- it's all liquid water, and you have pumps and controls...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • brucewayne
    brucewayne Member Posts: 8
    edited October 2016
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    i read it was average from this guide
    http://www.columbiaheatingsupply.com/page_images/Sizing Cast Iron Radiator Heating Capacity Guide.pdf

    but the outdoor reset would make sure water return is perfect?

    i live in canada though. maybe average water temp differs
    either way, how can i know for sure my water temp return will be compatible with the boiler?
  • Jamie Hall
    Jamie Hall Member Posts: 23,283
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    Ah. I see. I believe you may have misread the heat output of a steam radiator -- 240 BTUh per square foot -- as the temperature. The radiator temperature is given as 215 at 1 psi, which is what we all use as a rule of thumb heat output for steam systems.

    The heat output of a radiator is determined by the average temperature of the radiator, as that handy graph you referenced showed. The temperature, in a hot water heating system, is in turn , set by the input temperature and the output temperature of the water flowing through the system. The "universal heating equation" is GPM * delta T * 500 = BTUh. Which, of course, can be rearranged.

    Thus one can control the heat output of a radiator, in a hydronic system, by controlling the temperature of the water delivered to it; one can control the outlet temperature by controlling the input temperature and the flow.

    Juggling these variables so that one gets the desired amount of heat out and the desired return temperature to the boiler is the job of the various controls on the boiler.

    Has nothing to do with what country you are in!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,546
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    The contractors would absolutely need to perform a heat loss calculation, AND a EDR ( equivalent direct radiation) survey of your existing radiators. From that information they can then determine what water temps are needed at design day conditions which is a coldest outdoor temp that occurs 1-2% of the heating season. If your EDR is more than needed to compensate for the heatloss you will not need the 170 avg. water temperature usually used to determine their output.

    Modulating condensing boilers have what is called an outdoor reset control feature. When used it determines what water temperature the boiler needs to produce in correlation with the outdoor temperature. Since the coldest days only occur 1-2% of the winter then your avg water temp will be lower on warmer days. To get the full efficiency out of a condensing boiler the Return water temps need to be 130 degrees , or less the colder the more efficiency gains are produced.

    IF there has been any envelope upgrades to the home after the original heating system was installed chances are good the EDR is more than adequate to use lower water temps to meet the heating load.

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