Question about Mod/cons on old gravity systems

steve_25

Hello everyone. I am a plumber in northern New Jersey. Sandy has flooded my basement and I need to replace the boiler ASAP. It's a cast iron boiler on an old gravity system with a B&G 100 on the return and it's all one zone. I was planning on updating the whole system in the spring with a modcon, separate zones and primary secondary piping. So now that I have to change the boiler immediately here's my question. If I were to just replace the old cast iron boiler with a modcon using the same supply and return pipes and a new B&G 100 would the system work for now? I am willing to sacrifice efficiency for safety until I can update the rest of the system. My current boiler sits in the middle of my basement and feeds 2" horizontal runouts that feed 1 1/4" risers at the front and back end of my house. Each set of risers supplies one large cast iron radiator per floor. It's a rowhouse with three floors for a total of 6 radiators. Any advice you could give would be greatly appreciated. Thanks.

Al

SWEI

lacking more info

Do a heat loss calc, taking into account any envelope upgrades (insulation, windows, etc.) to properly size the boiler.



If you choose a fire-tube based mod/con (Triangle Tube, Lochinvar WHN, Dunkirk VLT, plus a few others -- we really ought to have a list here somewhere) you should be able to avoid primary/secondary piping and direct pump a gravity system.

Ironman

Mod/Con

What brand of mod/con?



If it has a high head heat ex., then it must be piped p/s. With rare exception, any mod/con is better off being piped p/s. I don't see where it's that much extra to pipe it that way, in fact, some models come with it already done at the boiler. ECR (Dunkirk Utica), Buderus to name a few.



With your type of system, it may be difficult to get the flow the boiler wants balanced with what's needed for system. Even with a fire tube heat ex., I wouldn't use a series 100 to pump through the boiler. The Delta T must be within the manufacturer's design or the boiler's control will limit the firing rate or possibly even lockout.

SWEI

good point on the pump

Definitely need to look at the circulation rate, taking into account the calculated demand.  There's so little friction loss in those distribution pipes that you should be able to downsize.



Thought about TRVs?  Pairing those with a smart circ can provide fantastic comfort.