Jumpy injection control

varadhammo

Hi all,

I've got a UPS 15-58 (internal check valve removed) for injection pump, Tekmar 356 controller. When all my zones are open, system flow is 8-9 gpm, it is able to maintain a stable mix temperature. But when asked to hold a stable temperature at low injection flow (i.e. if only 1-2 zones are open & system loop flow is ~2gpm; OR if thermal storage temp is 150+ and target mix temp is low, say 90*), it overshoots and undershoots and can't stabilize the temperature. I hooked a voltmeter up to watch the 356 ramp the pump up and down, at the same time holding my hands on the system loop right before & after the close tees to feel the temperature. What I found is that there is a "deadband" in the range control. For example, say the target mix temp is 110*, current mix temp is 100*. The 356 starts to ramp up. I can see the voltage rising, and I can hear the pump whirring and changing frequencies. But there is NO heat input yet at the close tees. The 356 continues to ramp, still no heat flow. All of a sudden it reaches a certain threshold, and the pump "pops" on, sends a slug of hot water into the system loop. The temp swings up past the target, the 356 ramps the pump down. As the 356 voltage starts to ramp down, there is no *reduction* in the injection flow until the 356 control voltage passes a *low* threshold, at which point the flow stops. Then the whole thing starts again. I've tried the 15-58 on all three speeds and it does this on any speed. It's like there's a deadband where the pump isn't moving any water until it hits the high threshold, then "pops" on, then doesn't modulate down until some low threshold, and it "pops" off.

I spoke with Tekmar tech support and they said it sounded like a problem with the pump; they've seen lots of 15-58's used with their injection controls and they should give a smooth ramping modulation all the way down. Has anyone seen this "deadband" phenomenon? Is it a defective pump? At first I thought the control was just outside of it's rangeability, but now I'm pretty sure this behavior is not normal. Any help much appreciated.

Jake

STEVEusaPA

Do you have a balancing valve for the injection loop. Can you post a pic of the piping or show a diagram?

varadhammo

OK, here are some pics. The flex pipes on the left are coming in from 100' of 1-1/4" PEX underground run from the boiler shed. They run from close tees on the storage primary loop (shown in 3rd pic) at the boiler shed. So the whole underground run is the injection loop.

I have a ball valve on the return (below the injection circ in the piping) for balancing. I've tried playing with that valve some, and I can dial in that valve so it puts the pump into 80-90% output range on the tekmar at low injection flow conditions and it stops overshooting, but then it can't deliver enough flow under other conditions. I need the injection pump to be able to range up to the full system gpm (about 9 gpm), for example if storage temp is down close to system mix temp and all zones are calling. And then also range down to a system flow of 2 gpm and mix temp of 90* with storage at 180*. I realize this is asking a lot of range out of the injection setup. But according to the folks at Tekmar, the 356 should be able to maintain fine-grained control over the pump flow all the way down.

Ironman

You don't have proper injection bridge piping. The lines from the boiler shed should be piped together in the house and two closely spaced Tees should have a 1/2" bridge to your radiant loop. In that bridge is where your injection circ should be, controlled by the 356.

In other words, connect the loop from the boiler shed at the end in the house and put two closely spaced tees with1/2" bulls in that loop to form the injection bridge to the radiant loop.

Go to Tekmar's site and down load theirs essays about injection mixing. They tell you how to size it and pipe it.

What induces flow at the bridge back in the boiler shed where the underground lines connect?

I just got a better look at the pic of the piping in the shed. It appears that the alpha circ is what inducing flow through the bridge there. With it piped that way you'll need to do what I described above plus add a circ that is sufficiently sized to the under ground loop.

The problem is that your trying to use the underground loop for the injection bridge and it won't work that way. The 356 has to keep increasing its power to the circ to get it going because of the amount of resistance in the underground piping. By the time it gets it moving water, it's at too high a volume when on one or two loops are open on the radiant side. By then, it's over-shot its target temp. The inject bridge is designed to be two small, short lines connecting two larger loops, not 200 feet of 1 1/4" piping.

varadhammo

Bob-

I'm still trying to understand why this shouldn't work. I did my calculations, and the 15-58 is sized just about perfectly to deliver design flow through that 200' of PEX. The pump doesn't know or care about the length of the pipe, only the resistance, right? I mean, with a normal "small" injection bridge, you have to throttle the pump with a valve which is like adding a hundred feet of pipe. I guess I don't see why the pump should have any more trouble finely modulating it's flow through this piping than it would through a short 3/4" copper bridge with a balancing valve, assuming the resistances work out in the same ballpark (which they do).

This setup is working fine when more radiant zones are open, it just won't stabilize with a low injection flow rate. So is there some reason which I'm totally missing so far, why the length of the injection bridge would affect the pump being able to modulate down to a low rate of flow?

Don't mean to contradict, you certainly have a lot more experience than I. I'm just trying to figure out the "why" before I go having to make any major changes...

Ironman

"The pump doesn't know or care about the length of pipe, only the resistance, right?"
- That's a very confusing question since the length of the pipe is what causes the resistance. The more length, the more resistance.-

"With a small injection bridge you have to throttle the pump with a valve which is like adding a hundred feet of pipe."
- You are confusing two different things here. The purpose of the valve, when needed, is to limit the amount of flow through bridge, not simply to add resistance. The reason for limiting the flow is so the control can keep it low enough to where it doesn't over-shot it's target temp in the secondary loop. The exact problem that your having. You call it "jumping" but the correct term is "searching" and it's caused when something in the circuit is over-sized and letting too much flow through.-

The injection pump, control and bridge are not intended to flow enough water through 200' of piping. It's not a matter of whether a 15-58 circ has the capability, it's a matter of sizing the bridge properly to allow the 356 to operate the circ so it injects the correct amount of water from the higher temp loop to the lower. That's what an injection bridge and its circ do. The bridge is not intended to move the total volume of water from one loop to another.

I do these regularly with that control, in fact I did two today. So I'm quite familiar with the control and the principle behind it. Go read the essays and study the formula for sizing the bridge and you'll see that you don't have it right.

varadhammo

""The pump doesn't know or care about the length of pipe, only the resistance, right?"
- That's a very confusing question since the length of the pipe is what causes the resistance. The more length, the more resistance.-"

What I meant is that the resistance is a function of the length and the pipe diameter, etc. So for example, using coefficients from Siggy's book: c value for 1.25" PEX = .01668, for 1/2" copper = .33352. The total head loss at a given flow is directly proportional to this c value ( HL = alpha*c*length*flow^1.75 where alpha is a factor based on the temperature & viscosity of the fluid). So divide those c values and you get a ratio of 0.05. So 200 * 0.05 = 10. So 200 feet of 1.25" PEX has the same resistance as 10 feet of 1/2" copper. Put identical circulators on closed loops of 200' 1.25" pex and 10' of 1/2" copper, and they will pump the same gpm at the same head pressure, right?

"The purpose of the valve, when needed, is to limit the amount of flow through bridge, not simply to add resistance."

Yes, well one does the other. Add resistance, in the form of piping or in the form of a restrictor valve, and the pump moves up along its curve. You limit the flow by increasing the resistance.

I don't see how you can say "it's just too long, it won't work". Besides, Siggy's "minitube" distribution system is exactly that; a long injection bridge from a boiler loop to a manifold loop, could be hundreds of feet away. It comes down to proper sizing based on the head loss of that injection bridge, not simply the length of the bridge. A short bridge between two loops is the standard setup they give in the essay, but it's not the only way it can work.

http://www.pmengineer.com/ext/resources/PME/1999/08/Files/Images/3753.jpg

Ironman

Has all this instruction that you've given me solved your problem?

Harvey Ramer

I would put the flow check back into the pump. It will help buffer the inertia of the water in the loop.

Also, put the tekmar system sensor in a thermowell, down stream of the system circulator.

varadhammo

Ironman,

No it hasn't solved the problem, I admit. I do appreciate your help, I'm just trying to figure out what the problem most likely is before I go tearing things apart... I do understand the theory of injection mixing and I've read the literature. I just can't understand how the length alone of the injection bridge is the problem. There's something else going on. Thank you for all the help you generously offer.

Ironman

Sometimes, you have to do the what before you realize the why.