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Thread: Hydronic Radiator Siziing

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    DIY Junior Member Ingolf's Avatar
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    Default Hydronic Radiator Siziing

    I have a hydronic geothermal system with a maximum temp of 110 F. So far I have only installed staple up heating under the main floor using heat transfer panelsand PEX piping. The next step is to install some radiators in rooms where I can not do the under floor panels. Most of the specs for radiators seem to be based on temperatures of 180 F with a temperature drop of 20 F. I have no problem getting specs for 110 F but I am wondering if at 110 F I can get a 20 F change in temperature, I notice that the temperature drop for the floor panels I am currently running is less than 5 F. I see there are all kinds of complicated formulas out there but I am wondering if any one has some practical experience on sizing radiators for the low temperatures generated by geo thermal systems.
    Last edited by Ingolf; 02-21-2011 at 07:47 PM.

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    In the Trades Tom Sawyer's Avatar
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    Here's your problem with the system. If you supply 110 degree water to either baseboard or radiators and they do give you a 20 degree delta T, the water coming back to the boiler will be cold enough to cause unwanted condensation and probably thermal shock so....... you either need tempering valves to isolate systems or 3way zone valves or VFD circulators.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The smaller the temperature difference between the radiator and the room, the more you need to heat the space. On your geothermal system, the return temp probably isn't an issue. I've no experience with sizing that, so can't be much help.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member Ingolf's Avatar
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    I do not have a boiler, I have a buffer tank which is heated by a heat pump drawing heat from a lake loop plus I do not think that a temp of 90F (110 - 20) would be considered cold. The industry standard for radiators seems to assume entry temp of 180F with a return temp of 160F to yield 10,000btu/h at a flow rate of 1GPM. I need to know if I can get a drop of 20F with the entry temp at 110F and will the btu/h be at 1GPM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    As I said, the smaller the difference between the radiator and the room, the smaller the heat exchange. I doubt you'll get anywhere near that drop across the radiator and as a result, a much smaller transfer of heat into the room. The radiator usually has a chart that says x BTU at 180, y BTU at 160, etc. Extrapolate that down to your 110, and it will be an approximation, but my guess is it is not entirely linear, and would be less than a linear extrapolation. If you could warm the space with say 10' of radiator with 180-degree input, you might need 40' with 110-degrees. A call to the manufacturer might help. The lower the temp, the less convection AND radiation will occur, and that is why I don't think you'll find the curve is linear. That is one reason when they do radiant floors, they cover the entire floor since you can't run it too hot (plus, it's just more comfortable to not have cold spots on the floor!).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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