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# Oversized cast iron: lower temperature, lower gpm, smaller pipe?

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Member Posts: 39
Can someone check my thinking here:

The rules of thumb I found for cast iron was
- 1/2" pipe in home run
- temperature drop of 20F
- average temperature of 140F-150F
- 1-2 GPM flow

The equation for flow rate is
Q = 500 * f * dT
Q = EDR * radiator sqft
using a relatively large 100sqft rad and dT=20F
f = EDR/100

so the rad that required 1.1GPM at 150F, requires 0.7GPM at 130F.
Home runs with flow rates of 0.7GPM should probably use 3/8" rather than 1/2".

So if you have oversized cast iron radiators, with outdoor reset and a deltaT pump should you downsize the piping further than a 1/2" for home run setups?

Real world example:
- 55k BTU/h heat loss at design load
- 130F average gives 62k BTU/h
- Mostly 60sqft rads: 0.42GPM target
- Most of the heating season is around 0C, meaning 110F average or 0.18GPM average
- Even 3/8" PEX might be oversized?

The math seems to make sense, but the flow rates and pipe are much smaller compared to what I have seen online. Have I gone wrong somewhere, or does this check out?

• Member Posts: 7,241
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Man..very impressive sir!  I'M no math genius but There are many hot water  systems similar to what your describing particularly homes built in Franklin Sqaure And New Hyde Park Long Island that have such systems that went in around the late 40s- mid fifties it looks like.  The 1 1/2c supply out of the boiler goes in to a very wide Tree or as Dan calls it, a Long Island 🏝 Heating 🕎 Menorah! All Brass.  Ill try to dig out a picture.   I used to save them. I gave one ti Prof Silberstein for the museum.
There was 10-12 3/8" O.D. copper BT bendable tubing soldered in to the top. Some also had mininl stops on them. The Returns had the same Beach was a homerun to a convector.    My inlaws house in Seaford had a combination Homerun system with tiny "tubelloy' pipes.  They look like shiny steel, but to my amazement,  they could be soft soldered.  MAD dog
• Member Posts: 15,835
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The BTU output of the radiation changes based on 4 things. Water temp difference between inlet and outlet of the radiation and the amount of water flowing in GPM, air temp in the space and the radiation output at the selected water temp.

Many old systems especially monoflow systems could use 3/8 tubing for the radiation branches if it fell into their flow rates
• Member Posts: 39
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