Gravity boiler to forced water conversion

Brandon123

I have a customer that has a super old 1960 gravity boiler system, 2 pipes up and 2 pipes down, 1 thermostat. My question is if I replace this how I would normally with a circulator pump and tie the 2 supplies/returns together and pipe the boiler as necessary for a new one, what issues am I going to run into? I have never removed and converted from a gravity. First one I have ever seen in person.

mattmia2

Biggest one is the mass of it(the water, the pipes, usually the cast iron radiation), if you use a conventional boiler you will need some sort of return water temp protection to keep the boiler from sustained condensing(or use a mod con boiler).

Radiators may have orifice plates in the valve unions on the upper floors if it is more than one level to slow down the flow to those radiators. You will either have to remove them or move them to the 1st floor if they exist. Gravity circulation favored flow at the highest points, pumped circulation favors flow near the boiler.

You would do a heat loss of the house and size the piping and circulator and boiler off of the heat loss, the piping to the boiler will be much smaller than the gravity supply and return mains.

Probably will need to use the radiator valves to balance it.

If it is an open expansion tank convert it to a bladder tank, if it is a compression tank you can either convert it to a bladder tank or arrange the near boiler piping to separate the air and put it in the compression tank.

Use a magnetic dirt separator to protect the wet rotor pump from the 70 years of much that will get stirred up by the circulator.

Not a bad idea to run some cleaner in it and flush out as much muck at possible before/during/right after the install.

EdTheHeaterMan

Condensation of flue gas would be my biggest concern. You can try several different piping designs, but I find this one to be bulletproof when you need to add a small water-content boiler to a large water-volume system. Primary/secondary with a bypass for adding heat to the return to get it up to 140° quickly.

Don't put the water flow into the tee at the run in opposite directions and exit at the branch, or put the water into the branch and exit thru the run in opposite directions

Here is a job similar to your question:

Old boiler with two supply and two return not circulator

Remove old boiler and start from this point

This is the primary/secondary piping with the bypass just like the first diagram.