Replacing Gravity Supply and Return Lines

OldHouseGuy

About 10 years ago we purchased a relatively small home that was built in 1922 that had a hot water gravity system installed.   I have since had the boiler in the basement replaced with a Buderus gas fired boiler and the installer added a Grundfos circulator.  All of the cast iron radiators and the original iron pipe suppy and return lines are as they were originally installed.   We are wanting to finish our basement and part of this process will be replacing the low hanging (head banging) gravity lines with either copper or perhaps Pex with an oxygen barrier (?).   The primary supply and return lines are 2" iron pipe and the stub ups to the radiators are between 1/2" and 1" depending on the size of the radiator above.  There are currently 11 radiators that heat the house (8 on the first floor and 3 on the upper story).  Each radiator has its own shut-off valve.  The boiler has 1 1/4" supply and return lines that are adapted up to the 2" iron pipe.  I assume that since the system now includes a circulator, I could reduce the main supply sizing down and also would like to consider adding a couple of zone valves to only heat the upstairs or downstairs as required. I would truly appreciate any guidance on the replacement pipe sizing.  Thank you!

meplumber

This would be a great time to add TRV's.

TRV's are Thermostatic Radiator Valves.  Essentially, they turn each radiator into its own zone.  The key is to make the basement loop a monoflow loop, by using closely spaced diverter tees.  Then the TRV's are placed in each radiator.  I would recommend going to the 2107 control with the outdoor reset and warm weather shutdown, but that is optional.



The circulator would run constantly and each radiator would take heat from the basement loop as needed.



You would not believe the difference that these TRV's will make to the home's comfort level.  Also, you would get rid of the head bangers in your basement.



Ask your boiler installer about it.



Good Luck.

Ironman

BTU's

We're gonna need to know how many btu's those pipes are carrying. Measure your EDR (square footage of each rad) and multiply it by 150 to get the btu output.



Also need the btu output or net IBR rating of your boiler. The attached radiation tables will show you how to measure the square footage of your rads.

OldHouseGuy

Radiation

Bob,

Many thanks for the response and the radiation tables.  I have approximately 564 square feet of radiation for the whole house (127 upstairs and 436 main floor).  If I multiply this by 150, I get approximately 84,525 BTUs.  The boiler output is 85,300 BTUs/hr (Input 103,000 BTUs/hr).  As a note, If I do end up finishing the basement, I would like to look at adding radiant floor heating to heat the basement.

Thank you again!

Ironman

pipe size

The simple answer, unless you want the formulas, is this: a 3/4in line will give you about 40k btu's; a 1" line will give you about 80k btu's; An 1 1/4 in will give about 160k btu's. With a 20deg Delta T, each GPM of water will carry 10k btu's. The above numbers are on the conservative side and the pipes could probably be pushed to carry about 20% more, but you don't want to exceed 4ft.per second velocity or you'll have noise issues. So really, a 1" line could carry your entire load with what is presently connected.



If you're adding radiant later on, you may want to go with 1 1/4 on your mains. The radiant loops should be sized with a 10deg Delta T. Which means that every GPM will be giving you 5k btu's.



I'd highly recommend that you do a load calculation on your house, you'll probably find that your boiler is oversized.



I second the motion of installing TRV's. You'd have a much greater comfort level as well as room control on each rad.

OldHouseGuy

Load Calculation

Bob,

Thank you again for the specific answers.  I really appreciate your assistance.  You suggested that 1" lines would probably carry the entire load with what is currently connected.  If I ran 1" lines as the main supply and  return lines, could I branch off in smaller sizes at each radiator?  For instance, as long as the load was less than 40,000 BTU to a radiator could I branch off the main 1" trunk with 3/4" supply and return lines?  If this method would work, how many BTU's will 1/2" line give?  Do you see any problems with going with Pex tubing with an oxygen barrier (since it is less expensive than copper)?  Lastly, I am very interested in doing a load calculation on my house.  Is this easy to do?

Thank you!  

Ironman

Manifold

A 1/2" pex line is good for 1.5 gal. or 15k btu's. If you're going to run pex to all your rads, I'd get a 1" manifold and "home run" each rad to the manifold. With 1/2" or 3/4" connectors available for the manifold you cold run either size you need, though it's rare to find a rad that has over 15k output.



The Slant Fin load calc. is easy to use.

OldHouseGuy

Home Runs

Bob, if I connected the radiators to the manifold using the home run method and I wasn't ready to replace all of the handwheel valves with TRV's yet, wouldn't I require more zone valves by doing this than by branching off a main trunk?  I guess I could put in a large zone valve before each manifold and then run one manifold for the main floor and one for the upstairs?

Ironman

Manifold Actuators

Use actuators on the manifold to create as many or as few zones as you want.