pumps on gravity systems

cj cosgrove

I have a friend with an old gravity system and was thinking about installing a circ.pump to help heat his house.Has anyone reading this ever installed a pump in an exsisting gravity system?If so,what size pump would you put on a 2" return line?Is this even a good idea?Will it only act as a restriction?

Reynz_2

Interesting Article

http://www.oldhouseweb.com/stories/Detailed/830a.shtml

Mike T., Swampeast MO

Many, many, many gravity systems are converted to forced flow. Look over in "Hot Tech Topics" at this site for info on sizing the pump and near boiler piping.

2" is kind of small for mains in a gravity system unless it's a VERY small system. Sure it's not 2½"?

For the near boiler piping, the rule of thumb is cut pipe size in half, then drop one size. So for 2", you would use ¾" for 2½" you would use 1". A B&G 100 circulator is very common on gravity conversions as it's designed for high flow with low restriction.

Plumbob

Actually the complete original article is right here, look in the menu at upper left and click on "Heating Q&A".

The short answer is that changing from gravity to forced circulation is not hard but there are one or two issues to consider. It is not purely a matter of hacking in a pump. You will have to re-balance the system. Then there's the expansion tank... Also, the return line may not be the best place for the pump.

Why should the pump "act as a restriction" unless it is too weak? The whole point of a pump is to give you faster flow and a more responsive system.