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# Discrepancy In Tables re: EDR to BTU output

Member Posts: 1,785
edited May 2018
I had always heard that if you knew a cast iron hot water radiator had 60 EDR e.g. and you wanted to know the BTU output of that radiator at a particular average water temperature, you multiplied the EDR x AWT and got the btu. So if my SWT was set 130º, my AWT would be 120º x 60 EDR = 7200 btu. Now from two sources I'm seeing that the BTU output is much lower than that.
From an old heatinghelp post from a very knowledgeable wally:

"The output is a ratio between average water temperature and the room ambient. The potential output naturally and rapidly diminishes as the two temperatures come close together. The chart on that link is for a 70 degree room. A short synopsis is:

AWT / BTUH per EDR
100F / 10 BTUH
110F / 30 BTUH
120F / 50 BTUH
130F / 70 BTUH
140F / 90 BTUH
150F / 110 BTUH
160F / 130 BTUH
170F/ 150 BTUH"

So for every ten degree AWT rise, btus go up 20; or for every 5 degree AWT rise, btus go up 10. I found the attached article which corroborates this. So there I was thinking I had almost a 2 to 1 emitter btu to heat loss ratio, now I have to re-calculate. So my 60 EDR btu output @120º AWT goes from 7200 to 3000btus. So which is correct?

• Member Posts: 17,829
The chart. And the 3000. When you stop to think about it, it has to be so: suppose your room temperature is 70, and for some reason your water temperature is also 70 (yeah, I know, extreme example -- bear with me). Since the radiator will be at the same temperature as the room it cannot transfer any heat to the room, and the heat output will be zero -- but by the simple formula you would expect it to be 70 times the EDR. Big difference.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,785
Yet the Wall posts are full of people saying for to calc EDR's btu output @180 SWT, meaning 170 AWT you multiply EDR by the 170 (though I've also seen 150) = btu. I need to know actual btu output of the radiators vs heat loss to help design system for mod con.
• Member Posts: 17,829
Kindly look at the table. At a water temperature of 180, the radiator will deliver 170 BTUh. So the statement is entirely correct for that temperature.

To really nail down the BTUh output of a hot water radiator, you need to consider the average temperature of the water (and the radiator) -- which will usually be less than the temperature of the water leaving the boiler, but more than the temperature of the return water.

A reasonable assumption for your design work will be to assume that the radiator temperature will be the average of the temperature of the water entering the heating loop and the temperature of the water returning from that loop. For example, if your source water temperature is 160, and your return water temperature is 140, you can assume your radiator temperature will be about 150, and the radiator will deliver 110 BTUh per EDR.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 374
This also applies to any fluid. Low pressure steam (<14 psia) can be as cool as hot water and it’s btu output will vary with vacuum.
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• Member Posts: 1,785
edited May 2018
Ok so now that my EDR-to-BTU calc error has been corrected, looks like I'll be raising the SWT by at least ten degrees and even with that I'll have to settle for about a 1:1 ratio between gain and loss, as opposed to the 2:1 ratio I thought I had. I'll probably present my new calculations on a new thread and see if they pass muster.