3/4 pipe feeding 60,000 btu hydro air coil

I have a hydroair coil that has 3/4 connections but says it can put out 60,000 btu. I was taught as a rule of thumb that 3/4 can give you 39,000 btu. How do I get 60k out of it without increasing the pump size, if we run 1” to the coil and then reduce it down to 3/4 at the coil aren’t we just limiting the flow to whatever we can get through the of the coil? Seems similar to cutting a road from 3 lanes down to 2. You can only get the amount of cars through that can fit through a 2 lane road because of the restriction and it causes a traffic jam.
Comments
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It can put out 60K if you flow 4 GPM and have a DT of 30 between the supply and the return. You might get 5 GPM from 3/4 copper if the length of the 3/4 is not too great. Remember, the total resistance to flow is based upon pipe size AND length of the pipe.
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A 3/4” line carries about 4gpm which = 40k btus at a 20* delta T. That’s the normal design for most systems with radiators. You can very easily get 60k btus from a fan coil using a 3/4” line and allowing a 30* delta T if you have enough air flow.
And no, reducing the connection from 1” to 3/4” at the coil won’t hinder flow that much since it’s the resistance (head) of the entire length of pipe and its fittings and components that matters.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.2 -
The coil should have a spec sheet that shows output and pressure drop at various flow rates. Make sure you take that into account when sizing the pressure drop for the circulator sizing.
The spec sheet will show output at air flow options also, as @Ironman mentioned
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
If you need more output from the coil, any heat emitter really, increase the SWT supply water temperature, and the AWT average water temperature across the coil..
Looking at this Idronics 29 graph from under .5 gpm to 2 gpm increases output quite a bit, but it tapers off at the higher flow rates, above 2.5.
This confirms that adjusting flow to change output is very non linear.
Whereas increasing the temperature as the load increases tracks very closely. An outdoor reset as an example. As outdoor temperature drops 1°, SWT can increase 1°
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream2 -
Why do you think you need 60K BTU/h? The coil might be capable of it if piped properly.
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Agreed with the others and I will add that I have a fan coil in my shop with only 1/2" connections but fed with 3/4" copper and a Grundfos 15-58. According to my QuickSetter, it'll flow approximately 4.5 GPM and with the fan on high it'll pull 36-37 degrees out of the water so it's throwing 80,000+ BTU with 170* supply water and will raise temp in a 1600 sq ft building from 60 to 70 in about 10 minutes.
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