Question on pumps in series in high-head system

[700renegade]

Hi all,

I've been asked to help my neighbor work out an install of his new Aquatherm OWB. My own system is much simpler so I had a lot of reading to do to get up to speed.

He's run the underground pex as one continuous series from Boiler > House ( sidearm > main furnace > sidearm > workshop furnace ) then 200' underground to a new 700 sf outbuilding w/ in-floor hydronic > return to boiler. With fittings losses etc I figure his 1" equivalent length is 750' total.

Best guess on max demand is 230,000 BTU. Assuming 180* out, 110* return ( dt = 70* ) I need 7 GPM.

This will take 2 high-head pumps like the Grundfos UP 26-99 I ordered.

I figure for 80 or 90% of the heating season he could probably run on 150,000 BTU or less, which means <5 GPM which I can get with one pump running.

My thought to save electricity is this.... run the main circ on the boiler continuously. Inside the basement mid-way through the system put the second circ, but trigger it to run only when the main furnace fan is running ( simply tap off the furnace blower power supply ). This would bump my flow from 5 to 7 GPM only at the times his main heat load is demanding.

How much headloss is there to pump thru a un-powered pump? Is there a problem with doing this that I am not anticipating?

Is a return temp of 110* a system killer? ( his last load is the hydronic side loop so it should function that cold, no? )

 

[WoodNotOil]

How much headloss is there to pump thru a un-powered pump? Is there a problem with doing this that I am not anticipating?

I asked Taco this question and the answer was that it added little or no head as it just acted as a continuation of the pipe and would cause no damage to the pump in the long term. As for the rest of your questions, I am not sure and will let someone else chime in. Good luck!

 

[heaterman]

I hate it when that happens........A person can just about pee as fast as he'll be able to get water through that kind of head.

A single 26-99 will do about 5.4gpm and develop 29.4 feet of head doing so on 750 equivalent feet of tube. A B&G;PL-55 will give you 7.4gpm pushing a bit over 47 feet of head. Taco's 1400-50 will do just under 10gpm at about 50 ft of head. Those last two are 5.0 amp motors which will add up to some dough running 24/7. So.................you could say it's do-able I guess......... Something about Mr Murphy and the law of unintended consequences comes to mind here. ................save money using 1" tube and wind up paying an extra $$.$$ per month for life.

Now, here's the rub. No circ will live very long under those conditions on an open system. There's an inverse ratio that comes into play which is simply that the higher the head the circ is working with the higher the suction pressure needs to be to prevent suction "boiling" or cavitation. Any way you slice it the circ(s) will be operating far past the sweet spot on the pump curve and they will die like flies. He may want to consider using a HX with side B (his 750' loop) working at 15-20PSI. This will at least help the circs to last a bit longer, however it won't do anything for the excessive electrical consumption. There is a pretty substantial penalty to be paid for undersizing the piping and it sounds like your friend will find that out one way or another.

 

[700renegade]

Heaterman - Funny thing about you mentioning "Mr. Murphy" as that is actually his real last name.

When we first had a discussion on this last summer he thought 1" pipe sounded "kinda big". Thank God he didn't try running 3/4".

His Aquatherm OWB is a closed + pressure system ( 10 psi ??? ) so perhaps that will help the cavitation issue. Not to get off on a tangent, but why is operating the pump with high head a problem so long as it doesn't cavitate? The electrical consumption actually decreases as the pump gets nearer and nearer to the point of dead-heading.

Any thoughts on the 110* return water temp. The bad news is it will be cold - good news is there won't be much of it (gpm)

 

[heaterman]

Heaterman - Funny thing about you mentioning "Mr. Murphy" as that is actually his real last name.

When we first had a discussion on this last summer he thought 1" pipe sounded "kinda big". Thank God he didn't try running 3/4".

His Aquatherm OWB is a closed + pressure system ( 10 psi ??? ) so perhaps that will help the cavitation issue. Not to get off on a tangent, but why is operating the pump with high head a problem so long as it doesn't cavitate? The electrical consumption actually decreases as the pump gets nearer and nearer to the point of dead-heading.

Any thoughts on the 110* return water temp. The bad news is it will be cold - good news is there won't be much of it (gpm)

The issue with a high head or actually any pump operating at the either end of its curve is that the efficiency goes way down. Every circulator has a sweet spot where the wire to water energy transfer is at its greatest. That's what you want to try and design for. Most wet rotors draw pretty much constant amperage regardless of the gpm being moved. It'll vary somewhat but not as much as one would think. If he can elevate his system pressure to at least 15PSI it will go a long way toward eliminating cavitation and helping the circ survive.

As to the 110* return........as long as his boiler firing capacity is able to maintain 140 or so it shouldn't be an issue. I hope he has enough heat emitter hooked up to get the temp drop you are talking about.

 

[700renegade]

Ok I need help with this. Why would having the system pressurized to 15 psi make any difference at all regarding pump cavitation? The 15 psi exists on both the feed and return bungs on the boiler. The pump sees the exact same head condition regardless of the static pressure in the boiler itself, right?

 

[heaterman]

Ok I need help with this. Why would having the system pressurized to 15 psi make any difference at all regarding pump cavitation? The 15 psi exists on both the feed and return bungs on the boiler. The pump sees the exact same head condition regardless of the static pressure in the boiler itself, right?

Short answer:

It's the static pressure that keeps your suction head positive which will prevent cavitation

Long answer: see below..........

OK, I have my service calls done and I just finished my supper so here goes. (Wiki I'm not)

First you have to understand that a typical wet rotor is a circulator not a pump. True, it's a centrifugal design but when compared to your well pump that's where the similarities end. You could hook up a 26-99 to a shallow water well, prime it and let it spin all day and it will not "pump" water.
Hydronic circulators are just that, machines that induce circulation via pressure differential in a closed loop. Think circulation not pumping. They circulate by creating the afore mentioned pressure differential and one of the basic rules of physics is that high pressure goes to low. All a hydronic circulator is designed to do is create that differential. Now think about where the lowest pressure in the system is going to be.......At the impeller inlet? RIGHT! Where is the highest pressure in the system going to be....at the impeller outlet? RIGHT AGAIN! The drop in the pressure as the fluid flows through the whole piping system and back to the impeller inlet is what is referred to as the "head" or pressure loss in the system.

Using a hypothetical system loop of 400 feet of 1" pex, 12 90* ells, a couple ball valves and a couple side port tees for good measure our 26-99 will flow 6.54 GPM, and develop 23.85 feet of head. To determine the differential pressure caused by the circulator simply divide the head by 2.37 to arrive at PSI drop at the impeller inlet. Which would be 10.05. You'd have to look up Grundfos' specs to see what NPSH they recommend for that circ but let's just say it's 2PSI. This would be added to the differential pressure to arrive at the static fill pressure in the system. (At least that's the way it was explained to me.) If the fluid has to be elevated say from a basement to a second floor, that also has to be taken into consideration because it's the static fill pressure that gets it up there in the first place. All the circulator does is move the water around once it's there.

Hence my recommendation of 15PSI fill pressure.

 

[Redox]

Long answer:

http://en.wikipedia.org/wiki/NPSH

Chris