mixed radiant and radiation

Mike T., Swampeast MO

1¼" and 1½" rad connections scream, "Gravity!"

They also tend to shout, "Ground floor and close to the boiler."

Head loss in gravity systems is measured in milinches (thousandths of an inch). In modern systems, head loss is measured in feet!

In a typical residence, ground floor gravity radiators had to be piped with less than one INCH of head loss. For each floor above (based on 10') you had to INTENTIONALLY increase your head loss by about 1.2 INCHES to achieve balance. (These numbers are based on 30° delta-t at very specific temperatures. For say 20° delta-t cut them by about a third! Lower average temperatures reduce them as well but not to the same extreme.)

I daresay that nearly any residential standing iron gravity radiator can be adequately supplied via ½" or 5/8" pipe or tube of any description and any reasonable length.

Since it sounds like you're "home-running" a system, here's what I would suggest.

1) USE REVERSE RETURN AT YOUR MANIFOLDS!!!!

2) Measure the length of each run from radiator to manifold. Consider multiple manifold locations to avoid excessively long runs.

3) Find the mean (average between longest and shortest) run for rads served by each manifold.

4) For runs above the mean, use 5/8", for those below use ½".

5) For exceptionally large radiators slightly below the mean distance use 5/8".

6) For exceptionally small radiators slightly above the mean distance use ½".

7) For an exceptionally large radiator well above the mean distance give HIGH consideration to ¾".

8) For an exceptionally small radiator well below the mean distance give HIGH consideration to 3/8".

9) Oversize the manifolds and associated piping by at least one size above "standard". This oversizing of the mains will come in VERY handy when condensing, modulating boilers with variable speed pumps become the norm.

10) Since this is a comprehensive job, suggest that TRVs are installed on EVERY radiator. The homeowner will be THRILLED with the comfort and efficiency!

Just Tim

mixing radiant and cast iron radiators

I am fairly well versed in standard(old) hot water heat and the methods, my boss signed me up to refeed the existing cast iron radiators and feed a new radiant zone in the base. I am using an Ultra condensing boiler with ODR and zoning Base, 1st and 2nd seperately. I am not too comfotable with the answers I get about feeding the old radiators (1 1/4 & 1 1/2) with 1/2 and 5/8 pex. I have checked my ibr manuals and looked online but am no engineer. Is there a simple way to figure the BTU cap. of pex al pex for a given temp/length? I cannot find one, I have asked the house engineer who designed the system and cannot get a response. I am the one the customer will call if the house does not heat right and would rather know what I am doing than follow blindly.
Tim