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New boiler. condensing at lower temp with radiators?

weil_fail Member Posts: 78
edited August 2019 in Gas Heating
I have a couple questions, but I'll give the specs of my system first:
currently, I have a 175000 btu/hr weil mclain cast iron boiler that heats a ~4000 square foot house, with some oldschool standing radiators (160-180°). I also have an early 2000s gas water heater that is getting up there in age. the boiler has already had some issues and needed some replacement parts, so I'm not sure how close it is to end of life, but not too far off. they are both currently gas and vent to the roof.
I want to move their location (for some renovation) and vent out the back of the house, so I figured it's probably time to replace both the boiler and water heater with a newer boiler and a indirect tank.

- am I correct in understanding that a condensing boiler does not get good efficiency above ~140°?
- do condensing boilers typically just run 140° at high duty cycle, as opposed to high temp with more frequent cycles?
- if so, is it advisable to run 140° condensing boiler through old-school radiators? is that a good idea, or will heat transfer suck?
- I currently don't have thermostatic valves on the radiators, just manual knobs. is it easy to retrofit old galvanized pipes with thermostatic valves? is it risky to cut off stuck fittings and re-thread? should I both with thermostatic valves?
- any advice on how to do this project best?


  • hot_rod
    hot_rod Member Posts: 18,177
    The best way to start is a room by room heat load calculation. then determine the radiator output at various supply temperatures. Then you can calculate how often you would be at condensing temperatures.

    It's rare that a building is at design condition throughout the entire season. So you will have days or weeks where you could operate with low SWT and condensing conditions.

    This journal will give you some ideas on how to operate "legacy" systems at low SWT.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • weil_fail
    weil_fail Member Posts: 78
    edited August 2019
    hot_rod, do condensing systems constantly pump water and vary their water temp on the fly, or is it set up at install? if so, how do condensing systems control temperature? do they do internal calculations based on how frequently the thermostat is asking for heat, or do they have some additional sensor for either room temp or outside temp?
  • hot_rod
    hot_rod Member Posts: 18,177
    The boiler water SWT is modulated based on the outdoor reset control. That is built into the control module on most mod con boilers. So the SWT should never exceed what is required to meet the het load at any condition. Mild, low load days present low SWT operation. SWT ramps up as load increases.

    ideally the heat emitters could be sized so the system never requires more that a 130F supply, for example. Then the boiler would condense across the entire modulation range.

    The thermostat on the wall, or TRV on the heat emitters regulate the actual room temperature and initiate the call for heat and possibly pump operation.

    Delta P circulators are ideal for zoned systems and they too respond to the changing loads.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream