Replacing Iron Pipe Mono-Flow System for Finished Basement

Hello, I just discovered this site – fantastic so far and have ordered a couple of the books recommended for radiant heating. I am getting conflicting advice from different contractors about what to do and am hoping that some input here will help me sort it out!



Situation: I have a 60-year old house in upstate NY, single story ranch, about 1800 ft2, with a single loop black iron pipe radiant heat (water) system. It is a mono-flow system and works very well. The natural gas boiler is only 1 year old, a Well-McLain Gold CGa. The existing pipes hang below the floor joists in the basement, and the mono-flow Tees tap off and go up through the floor to the radiators which I understand are a type called convectors. The main loop is 1-1/2” or perhaps 1-1/4” and the Tees to the radiators are ¾”. There are 8 radiators in the house.



I want to finish out the basement with 2 bedrooms, a full bath, and a large living area. In the process I need to relocate / replace the radiant heating pipe so that it is up between the floor joists. Additionally I will need to add a second loop for the basement heat. Several contractors and professionals I have spoken to all recommend Pex tubing – so far so good. Beyond that I am getting all kinds of mixed inputs.



The first suggestion was to keep the supply line 1-1/2” black pipe and then split off into two ¾” Pex runs to return back to the boiler. Each Pex run would go in series through 4 radiators but both runs would be controlled by the same thermostat / circulator pump. I purchased my Pex and started getting it run, ready to connect to the system once the weather got warm enough.



I wanted some help actually connecting into the boiler so I asked a major HVAC contractor in the area to take a look. His suggestion was to go in a single loop in series through all of the radiators, still using the ¾” Pex. I asked him if there would be a problem with the first radiator being too hot and the last being too cold and he said no because the flow would be fast enough through the system to keep the heat evenly distributed.



Then I had the guy who installed my new boiler last year come out and take a look. He works for a small general contractor that has a good reputation in the area. This guy said he had never replaced a mono-flow system with pex running directly in series through the convector radiators and was very, very leery of such a proposal. He thought perhaps a separate supply and return loop for each radiator would be a better proposal. (putting each radiator in parallel rather than in series with the water flow). But even with this suggestion he was uncertain that the heating would be evenly distributed through the house. He seemed very cautious about proceeding at all.

This separate supply and return loop with the radiators in parallel sounds like the right approach. Do I need to be concerned about heat load balancing from the first radiator in the system to the last with this configuration?



Thanks to all in advance!