Oversized circulating pump repercussions?

[Macman]

A general boiler knowledge question, please. If a circulating pump in a hot water boiler system (residential) is oversized, causing too much flow in the system, what problems could this cause?

 

[Jadnashua]

noise, erosion of the pipes, cavitation, which could burn out the pump.

 

[Sponsor]

 

[Macman]

Not so oversized as to cause those problems. Once you've got enough flow to heat properly, does too high of a flow rate affect heating capability?

 

[Jadnashua]

It takes time to transfer the heat from the heat exchanger...just like waving your hand through a candle flame, it may not heat up as much on a pass through resulting in the water temp being lower than desired. This can affect the ability of the system to distribute that heat as the radiator may not get as hot. See what the pros have to say...

 

[Macman]

That makes sense. Good analogy with the hand and flame thing Jim. Thanks. I'll hope maybe someone else chimes in too.

 

[CattleDog]

The heat transfer to the emitters tends to go up a little bit with flow rate if the incoming water temperature is held constant. I believe it has to do with the turbulance of the flow. There is a trade off with pumping costs at higher flows.

Most hydronic baseboards are rated at 1 and 4 gpm with slightly higher btu/hr output at the 4 gpm. You should be able to find the same specs for panel radiators.

It's a myth that the water goes too fast to give up its heat. More water is going through, and 5 gpm with a delta t of 10 degrees is the same as 2.5 gpm with a delta t of 20 degrees or 1.25 gpm with delta t of 40 degrees. You need to be looking at input and output temperatures along with flow rate.

If you have long runs, moving more water will keep the temperature higher at the last served emitter.

On the other hand, if you have a condensing boiler you want to have the water returning at a low enough temperature to get the added efficiency.

You want the flow rate to be high enough to sweep air bubbles through the system.

Why do you believe your system is overpumped? Do you know the flow rate through your loops or zones?

 

[Macman]

Thanks for the reply. I have been told what you describe...that the water may be going too fast to give up its heat. We are having some issues with the boiler not keeping up during some of the extreme temps we've had over the past few weeks. This only becomes a problem when the temp is below minus 10 deg. F, with significant wind. Tests on temp drop across the boiler, and temps at the baseboard units have not yet been done. However, my gut feeling is that the boiler is keeping the water up to temp. The "going too fast to give up its heat" thing seems odd, but I'm not a heating pro. That's why I posted the question.

 

[Jadnashua]

While the radiator may not be as dependent on flow rates (the heat available can dissipate into the room at whatever rate it can), the boiler heat exchanger also is affected by it's ability to provide heat to the passing water (the surface area is often restricted)...how does that affect system performance? While you may ultimately transfer a lot of heat into the passing water stream, the temperature rise through the heat exchanger would be smaller, giving a lower Delta T at the radiator, which means less ability to transfer heat into the room. Maybe the differences aren't all that great...what's the real-world scoop?

 

[Bill Arden]

I think the concept of the "water going to fast" is a Myth.

The faster the water flows, the lower the temperature difference is between the radiators and the boiler's heat exchanger. This increases heat transfer from the combustion air and the water.

The water temp will change as needed to radiate that heat into the room and therefore the lower water temperature is not a limiting factor since the water would get hotter if needed.

This means that total heat flow increases with flow and the only reason for not using larger pumps is the increased energy that is used and possible issues with cavitation in the pump.

I would focus on sealing up the house and adding more insulation.

 

[Macman]

Okay, let me run this by anyone who's still interested...
Suppose that the heat just wasn't getting off of the fin tube fast enough. If a small fan was placed to blow air across the fin tube, woudn't that force heat off and into the room? Of course that assumes that the fin tube would heat up fast enough to accomplish this.

 

[Jadnashua]

If you look at the spec sheets for hydronic radiators, you'll notice that the heat output varies depending on the incoming water temperature...free air convection becomes more effective when there is a larger temperature delta. It is more capable of transferring that heat when there is a larger temperature differential. So, I don't think it would matter on the radiator if the pump was going too fast or not except for erosion, noise and power. Blowing air across the radiator will mean that the contact air has a greater Delta T (i.e., will be colder), so yes, you should be able to extract more heat. Again, though, keep in mind some boilers really don't like the return water to be too cold, and that could mess things up.

But, there is a limited surface area in the heat exchanger (i.e., your boiler), and again, similar to the specs for any radiator out there, the longer the run, the more exposure you get and the more BTU's you can transfer (and raise the water temp - back to the hand through a candle example). The run in the boiler is limited, rush that water past the heat exchanger, and you'll possibly cause turbulance, and the media (water) won't be in contact with the conducting surface very long as you pump it past so it won't absorb as much heat per volume, but you're putting more volume past it so you may have the same total amount of heat in the circulating water. A lower water temp to the house radiators means those radiators will be less efficient in transferring their heat. If they were marginal at a designed 180-degree input, on a cold day, they just won't keep the house warm, no matter how fast the pump runs if your boiler can't raise the temp high enough. Running the boiler at a lower temp may mean it is more efficient, and if your radiators are oversized, then you may never notice a difference. Smart, modern boilers use a computer and sensors to adjust the boiler output temperature to compensate for the load, raising it when it is cold outside and it detects the return water temps are dropping more than design. An older boiler usually doesn't have that feature.

Too big is a relative thing. How many problems you see, if any, will be based on the extent of how big is that.

 

[Macman]

Really interesting stuff. It give me just a small feel for the engineering puzzles that have to be solved in designing these systems. Thanks for the replies everyone!