Two-Pipe Pumped Return Pressure Setting Question

Mike_Sheppard

Quick question about the pressure settings of a two pipe pumped return system. Reading through the Lost Art again and started wondering about this.

“B” dimension is a big deal in gravity return two pipe systems because for every 1 psi of boiler pressure you need 30 inches of “B” dimension. But in a pumped return there isn’t a “B” dimension and the pressure of the system doesn’t affect the return of condensate to the boiler. So in a gravity return the pipes were sized usually for a 1oz/100ft pressure drop so the boiler could run at a very low pressure due to limited “B” dimension. But a pumped return system could be sized for a higher pressure drop to use smaller pipes, but requiring a higher operating pressure.

So my question is, do you guys ever run across two pipe pumped return systems that have to run higher than 2psi to reach the furthest radiator? Have you seen a system that actually has to run 5+ psi before?

Edit: this is referring to “heating” applications only.

JUGHNE

I have never seen one needing higher pressure other than when the pressure had to lift condensate out of a trap.

Smaller piping to individual runouts because there would be no flow of condensate back thru the supply.

The steam main would have to handle only the parallel flow of the main supply pipe condensate produced and not the accumulative returning condensate, so it could be smaller.
One pipe 2"=386 EDR....where as 2 pipe 2"=648 EDR.
It is the same if pumped or not.
The standard is that lower pressure is just about always better.
IMO

Mike_Sheppard

For example, a system sized for 1oz/100 feet pressure drop would require larger pipes than a system sized for 6oz/100 feet pressure drop. So let’s say, since a pumped return doesn’t have to worry about B dimension, they decided to go for 6oz/100 feet. The install could use a size or two smaller in piping, but would require a higher pressure to deliver the steam to the furthest radiator.

I was just wondering if anyone has actually come across a system that actually needed to operate at a higher steam pressure.

Steamhead

I'll bet that system was gravity return originally. Crank it down!

Mike_Sheppard

@Steamhead

Well I did have a system in mind from this passed winter. It used to be a vacuum system. The vacuum pumps were removed at some point and a feedwater unit was installed. They were running the building at 7 psi with two zone valves. Steam pouring out of the vent on the tank. They had run the vent outside because of how much vapor was coming out of it. I know my question doesn’t fit well with this severely messed up system though lol

AMservices

Chapter 9, lost art, Dan talks about working backwards to figure out the pressure settings in a two pipe gravity return system.
That explains how to find the systems pressure drop and how to set your pressuretrol.
No B dimension with a pumped return.

Mike_Sheppard

> @AMservices said:
> Chapter 9, lost art, Dan talks about working backwards to figure out the pressure settings in a two pipe gravity return system.
> That explains how to find the systems pressure drop and how to set your pressuretrol.
> No B dimension with a pumped return.
>

This is exactly what gave me this question. He used an example showing a higher pressure drop with a pumped return. My question is, has anyone actually seen a system that was designed for a higher than usual pressure?

EBEBRATT-Ed

@Mike_Sheppard

Really doesn't matter. Steam, hot water, or warm air. Undersized pipe, ductwork or oversized pipe and ductwork.

A lot of installers don't play by the rules. We can't even get them to read the install manual and pipe the boiler right, never mind the rest of the system.

Did they always "follow the rules" in the old days? NO Every system becomes a troubleshooting job if it doesn't work.

But back to your question:

A vacuum system converted to just a two pipe pumped return system would have undersized condensate returns. The supply side should be ok. A vacuum system can use smaller returns and they also in some cases used the vac pump to lift condensate with a lift fitting

Do the returns need to be re piped with larger pipe? Probably not, unless the building doesn't heat well or hammers

the_donut

I have the same issue with a hacked vacuum system. Vac pump,was removed at some point. Bfp added. Returns clogged at some point. Second boiler feed pump was added to increase receiver size and runs slave to the first receiver/pump on a float switch. One 100’ section of returns were repiped.

Original returns are small diameter (1/2” running 3-6 princess wall units or Longest return is in the neighborhood of 200’ horizontal and 30’ vertical. From what I have made out so far from tenants and talking to the lead tech, the building doesn’t heat well in the periphery. Building was running at 7 psi to heat evenly. Receiver vent is an open pipe with a check valve.

With 5” of vacuum steam is around 200*F. With 7 lbs steam is 230*F. That 30* swing increases heat output quite a bit. I wonder if the dead men sized radiators in vacuum systems for 200*F steam?

Mike_Sheppard

@the_donut that makes me think back to my original question here. You mentioned they have to run 7psi to heat evenly. What I was wonder was, has anyone actually found a system that was DESIGNED originally to run at 7psi?(other other higher than normal pressure)

Obviously that one was not. But your messed up system sounds just like one I was fighting. I got involved in the job after they replaced all the F&T traps in the basement of this fairly large building. It was probably 8 stories and was quite long. They replaced all the “bad” traps and couldn’t believe there was still steam pouring out of the feed tank vent. They had the system running at 7psi. They had added two zone valves that weren’t there originally and abandoned the vacuum system. That place was a mess. I’m glad they decided they did want to spend any money.

the_donut

I am pretty sure, with all the piping being undersized by one, and after 90 years having sludge buildup in those 1/2" returns is the main reason people crank it. Condensate would stack up after pressure loss and periphery units wouldn't heat well.

I haven't come across many steam systems in my neck of the woods, but I bet if such a system was made, owners would have found it uneconomical to run and switched to hot water or electric baseboard. If it is still around, I imagine someone would have thought seriously about a vacuum pump to lower pressure.