gravity system piping

onthepeak

So I was reading the article in the help center on gravity systems and I noticed the recommended piping drawing showing a p/s set-up. It looks like the drawing is showing the two original loops piped together with the supply from one piped to the return of the other and visa-verse to make one big loop. Is that correct? See the pic below.

Jamie Hall

You can do it that way, but you have to consider the delta T and the radiator temperatures. You can also pipe it so that the two old sections are still separate,and flowing in parallel. Then you could, if you wanted,have zone valves for each section separately.

Take your pick. It's a design consideration, and you need to consider what it is you want the system to do, and how it will do it.

JUGHNE

That diagram is on page 28 of "How Come?" and I have never really understood the logic of it. The house is divided into the right and left halves. To me it seems that the right half gets the hottest water and the left half gets left over not so hot. (Think one bathtub of hot water and family of four taking turns....dad always goes last ).
So what I do is to reduce the pipe sizes as far back as possible and practical. So 3" goes down to 1 1/4". That makes the price of all required fittings more affordable and user friendly. Then isolating valves on both supplies & returns are less money. More unions can be installed for servicing etc.
I tie both returns together to create one pipe and both supplies together to create one. Then pipe between them installing CS tees, pump, fill valve, tank, purge valve etc in the connecting 1 1/4" pipe. This gives the original flow pattern to existing radiators. I throttle the supplies a little to achieve some balance if needed. Or you could add zone valves and another t-stat if desired.
If you have much for EDR and pipes you will probably need the #60 tank....How are you sizing your new boiler?

onthepeak

I agree with your assessment of the diagram in the drawing.

So if I am following your description correctly, you are manifolding the supply and return lines creating the secondary loop, and then piping in a primary loop at the boiler. Is this correct? I was also planning on installing a bypass with a thermostatic 3 way valve.

The house has about 10 cast iron rads, and is about 1900sqft. I sized the boiler by doing a heat loss calc.

Is a p/s the best option for a previous gravity system?

JUGHNE

It would depend upon what boiler you want to use; with out p/s all the system water would have to be able to move thru the boiler without too much pressure drop.