pipe sizing

kevin_35

I was at a basic heating class and we were going over pipe sizing. Every time we sized a system the supply and return was the same size. So my question is why do most residential boilers come with an 1-1/2 supply and an 1-1/4 return? I had asked one of my instructors that question and he said he had asked one of our boiler manufacturers and he didn't get a straight answer. Do you know?

bigugh_4

Darn good Q

I wonder if it has to do with the expansion of the water as it heats up through the boiler. It is the only logical thing I can think of! Take a moment and calculate a volume of water at say 120*F and then at 180*F ans see if it is perhaps the reason.

Dave_74

Pipe sizes

In my experience, the supply and return connections for most residential boilers are the same size, not different sizes. You typically use the same size pipint at the boiler outlet and return.

Any expansion of water at the outlet due to being hotter than water on the return side can be neglected.

On larger boilers, typically scotch marine and firebox style, you do sometimes see larger outlet than return pipe sizes. This is more a function of trying to reduce the pressure drop due to water leaving the boiler through the fitting.