Sizing circulators for old gravity-hot-water heating systems

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Sizing circulators for old gravity-hot-water heating systems

When servicing old hot-water heating systems that originally circulated by gravity, but now use circulating pumps, I began to notice a lot of oversized circulators. Here's how to correctly size circulators for old gravity-hot-water heating systems.

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Sebra_wells

When I replace older boilers that are converted gravity system I use the system syzer app from bell and gossett to calculate head pressure for system and add head pressure of boiler and near boiler piping. Then choose your pump based on thses requirements

Steamhead

Thanks, Erin

Erin Holohan Haskell

Steamhead said:

Thanks, Erin

Thank you!

jumper

Do we want flow through system (not necessarily through boiler) to be same as when it was gravity?

Paul48

I've always been curious as to what that was. Typically

Steamhead

Yes. If we over-pump these systems, the water short-circuits through the radiators.

Alan (California Radiant) Forbes

Thank you, Frank! Very helpful!

GW

I’m curious, if the Water flowed with no circ in the old days, I don’t understand why the pump needs to be upsized. Doesn’t the convection process aid the circulation process?

I rolled the dice with an Alpha pump on an old gravity job we did a couple of months ago (old boiler 210k input, new boiler 151k input). The Alpha moves about 11 gpm. Considering the giant rads, I thought the tab-bit lower flow would be fine. And it is, went though the wicked cold snap just fine.

It seems like giant rads can allow for a slightly lower flow. Just wondering

Paul48

Without the circ, each supply riser could only accept so much hot water, always leaving some for the next radiator in line. That's why the mains are so large. When you pump it, the water want to find the path of least resistance....5,4,3,2,1,..........drop the sixth radiator. That's typically what happens. The most remote radiator gets dropped. By over-sizing, you're trying to create head in the full length of the main.

Alan (California Radiant) Forbes

Gary: The newer boilers with smaller heat exchangers wouldn't tolerate gravity flow; too slow to pick up all the heat.

Interesting about the Alpha, but why use one where the ΔP never changes?

Paul48

I often wondered about using a DT circ with the sensors on the most remote supply and return.

GW

Alan here on Mass the utilities have a $100 instant rebate on ecm pumps so all the heating guys use these, they’re cheaper!

If I didn’t get at least 10 gpm I would have put a bigger pump in. Ball valved flanges, all good

GW

Alan yes this was a Bosch KBR, has its own boiler pump. Alpha was just for space heat