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Runtal Radiator sizing, EK Resolute RT oil boiler

H2OBandit603 Member Posts: 48
edited August 2023 in Radiant Heating
Consider this an intellectual exercise for now since the actual heat load number doesn’t matter.

If you have too little radiator you stay cold because the boiler is unable to deliver the btu to the rooms.  Does this work in reverse? Are you penalized with too much radiator when running an EK boiler?  Wouldn’t this allow a much lower outdoor design temp?

I understand it’s non condensing and no provisions will be made to lower the output temperature below its normal operation with a buffer tank or mixing/diverting valves.

Can you really have too much radiator in this configuration?

For sake of argument let’s say the radiators are proportional to each other in relation to heat load.

Help me understand what’s going to happen.
NH well driller and pump installer, 3rd generation
What one man can do, another can do.


  • Jamie Hall
    Jamie Hall Member Posts: 23,443
    Wel, as you are aware, a radiator -- any radiator -- has a heating output (power) which is more or less directly proportional to the temperature difference between the radiator and the surroundings. (as the temperature of the radiation gets really high, beyond what we are worried about (I hope!) it's not linear any more but that's a side problem). So... if you have a radiator of a given size, and desire a certain power output, you will then be able to figure out the required size of the radiator. In your thought about the undersized radiator, if you could feed it hotter water it would radiate more -- and you might be warm enough (this is one of the areas where steam to hot water conversions gets people in trouble -- the hot water in the radiator is nowhere near as hot as the steam). Conversely, as you lower the temperature or the water and the radiator, you get less power from a given radiator.

    So your bigger radiator needs lower temperature water or you'll be too warm. Now there are basically three ways to do that -- you can lower the average temperature over time, by stopping and starting the flow through the radiator, or you lower the actual temperature of the water by mixing the hot supply with colder return. Or, if the boiler itself can modulate, you can reduce the output temperature of the boiler.

    Now for a non-condensing boiler, such as you mention, only the first and second options are possible -- and there is a situation where you can get into difficulty: if the boiler is really too small, it may not be able to deliver enough power to the radiation to keep the return temperature in the operating range, even if you turn it on or off. Without using a buffer tank or mixing valves, the boiler is going to be very unhappy.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 5,864
    Well, on a heat demand, the zone valves won't open until the boiler reaches I believe 140°. Not positive on that temp but just for arguments sake.
    Once the zone valves open, the return temperature sensor monitors that water temperature and adjusts it's output temperature accordingly. 
    So if the return temperature drops too low, the zone valves will close while the burner continues to heat the water. It might take a few cycles of on/off with the zone valves until the return water temperature rises enough to keep them open.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,884
    edited August 2023
    Can you really have too much radiator in this configuration?
    Nah. It costs more, but lets you use less warm water. 
  • Roger
    Roger Member Posts: 333
    Good comments all around.
    Another way to think of it is radiation is always grossly oversized during the shoulder seasons  when heat is still required although it’s “warmer” outside. We never really think of that is causing any issues.
    You’ll need to have proper circulation and flow, or else the temperature drop through the radiation will be very high and the downstream rooms will not heat as desired. All of this points to proper system design.
    The majority of the energy savings come from low mass and thermal purge operation. A small amount of additional savings can be squeezed out with adequate radiation (designed for lower than 120F return temperatures on design day) using a thermal purge condensing boiler, but it is only a few percent more. And the domestic hot water system will need to thermal purge as well or else the boiler will finish hot several times a day (this happens with an indirect tank with an internal coil, thermal purge works well with a heat exchanger). 

    Energy Kinetics, Inc.