Adding fin tube to a gravity feed system

SteveSilca

Good evening,

I own a Chicago 2-flat and recently renovated the upstairs kitchen.  The 2nd story is heated by an old gravity feed boiler with two supplies and two returns.  The mains are 2" in size.  In the process I removed an old cast iron radiator (fed by 1 1/4" pipe) and split the supply and return lines to one 3' long and one 5' long fin tube radiator in different parts of the kitchen.  Of course, I ran thick wall 3/4" copper to each but there were several 90 degree elbows to get the pipe to the riht place.

I quickly learned that fin tubes don't work well with gravity feed boilers - they never had any heat in them and I was constantly bleeding the air out but still getting no flow.

At this point I found Dan Holohan's amazing Q and A article on  Old House Web which answered many of my questions. 

I looked into getting some old radiators and re-installing them where I had installed the fin tubes.  2 problems:  First, they were extremely expensive.  At $17 per section, I was looking at about $500 for a 3' and a 5' long radiator.  Second, in re-routing the 3/4" pipe to the two new fin tube radiators, I fear I may have created a path with too much restriction to flow and heat convection.  In other words, I know that a gravity feed system has larger diameter pipes that convey the heat better, and I fear that by splitting the 1 1/4" pipe down into two 3/4" pipes, sending them through 10-15' of pipe and several elbows, and not being 100% careful about pitching the supply and return lines slightly up or down, even if I re-install radiators they may not work tremendously well with my new piping and the gravity feed principle.

So, here is my question.  Having read Dan's article I understand that I can install a circ pump on the supply side to help pump water through the fin tubes.  I understand that it should be a high flow, low head pressure pump.  The complication is there are TWO supplies and TWO returns on my old boiler, so would it potentially disrupt the entire system if I put a circ. pump on just one half of the system? FYI - my two new baseboard heaters would actually be the ONLY heaters on that half of the system - all the other radiators are piped by the other two supply/return mains.  So, if I installed a pump on that half, it would feed just the fin tubes, but would it upset the other half of the system?  Note - I am not talking about "tapping in" to one of the main supplies and adding a circ pump to these new fin tubes - I know that this would steal the gravity action from other radiators on that supply main.  In my case, I would be putting the circ pump right into one of the two supply mains.  My only question is whether or not it would affect the other radiators on the other half of the system which would still rely on the gravity feed action of the other supply and return mains. 

Thanks for your help - I'm a pretty handy plumber but I've entered uncharted waters here with this old boiler!  Hopefully Dan H. himself sees this and posts a reply.

MIke_Jonas

I'm not Dan

But I work on converted gravity systems every day (I live at 35th and Oak Park Ave).



You are correct, trying to circulate just the fin tube will upset the entire system. You may have removed enough pipe that the gravity side will no longer work as The Dead Men intended it to, anyway.



It sounds like it's time to reconfigure the near boiler piping and circulate it all. While the gravity is simple and quiet, installing a circulator and piping it correctly will allow you to enjoy some other benefits, like trvs', that you wouldn't otherwise be able to use with gravity.