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Now I know why 180 is the magic number
Robert O'Brien
Member Posts: 3,556
<a href="http://www.heatinghelp.com/article/232/TheWholesaler/1763/TheCarbonClub">http://www.heatinghelp.com/article/232/TheWholesaler/1763/TheCarbonClub</a>
Why is Delta T 20 degrees? Why not 25 or 15?
Why is Delta T 20 degrees? Why not 25 or 15?
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20 degrees
I believe it is for ease of calculation. 1 gpm at 20 degree deltat is 10,000 btuh. Anybody else have any other ideas?0 
20 degrees
That is an easy one.
It is an understood principle of engineering.
Most engineers cannot count to 21 if barefoot and with their pants on.
Most engineers wear pants.
So 20 was established as the ideal number.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"
Ernie White, my Dad0 
I still do not understand this.
"1 gpm at 20 degree deltat is 10,000 btuh."
I know that if the flow rate is too low (laminar flow, for example), you get less heat out. And if I run 4 gpm through my Slant/Fin, I get a little (5% to 10%) more heat out than if I run 1 gpm.
But if I were to put 74 degree water in and tried to get a room temperature of 69F, I would not be able to get a 20 degree delta T no matter what the flow rate. Yet if I had enough (an unreasonably large amount) of baseboard, I could perhaps heat the room with one degree deltaT.
To go from the theoretical to the actual, my former contractor put a Taco 007IFC in my baseboard zone. This zone has way too much baseboard for 180F water, intentionally. So I run between 110F and 135F water in it, and it has no trouble putting 6500 BTU/hr out with only a few of degrees of deltaT; when it is in the mid 30s outside, the deltaT is around one degree, maybe two degrees. On the Wall here, we are mostly agreed that my circulator is too big; I calculated that the flow rate is 2.4 gallons/minute. If I opened the windows just right, I might need 10,000 BTU/hour, and the deltaT required would go up, but my guess is it would not go up to 20F.
Let's see. 1 gpm gets 10,000 BTU/hr with 20F deltaT. So 2.4 gpm should get me 10,000 BTU/hr with 8.3F delta T., or 6,500 BTU/hr with 5.4 deltaT, assuming that it is the volume of water that matters. Is this calculation correct? I am not sure it is.
Because if I increase the flow, the heat out does not go up. The heat delivered by the baseboard depends on the area of the baseboard (length x area of a foot of fins) times the temperature difference between the water in the tubing and the incoming air temperture; it does not depend on the deltaT Now with any piece of baseboard, water temperature, and flow rate, there will be a deltaT. But if I employ an excessive flow rate, the only difference, besides wasted energy in the circulator, noise, and wear on the piping, would be that the cold end of the baseboard would be warmer (more uniform) than otherwise; the return temperature to the boiler would be higher, but adjusting the reset would take care of that. At the other extreme, with too low a flow rate, the water would cool off in the piping before it got to the baseboard and deliver no heat to the desired space.
See? I don't get it.0 
I can
explain it fairly well, just not right now. We had a thread on this a few years ago, maybe I will search for that.
For the moment, I will leave you with this: All the while you are measuring deltaT, most people do not have an accurate idea of their flow rate, accurate enough to make a difference. This leads to some wide gaps in assumptions.
EDIT: I did a backsearch and found this thread from 2006:
http://www.heatinghelp.com/forumthread/97197/lowdeltaT
There may be others..."If you do not know the answer, say, "I do not know the answer", and you will be correct!"
Ernie White, my Dad0
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