Outdoor rest overshooting target/surging, replace pump, works great, month later same issue…..

pushinluck

We have a open system with outdoor rest and pex w/ fins under sub floor, the system was working fine and then I noticed ticking from pipe expansion which was new so after investigation this is what I found.

Tekmar outdoor reset …. pump is a grundfos variable speed UP-15 (with hi, mid, low setting) pump is on mid setting but same thing occurs on all three settings. Pump % increments are 10% 30% 50% 70% 90%

The overshoot/surge goes like this...\

design temp mix temp pump% time(min)

90 90 10% 1
90 89 10% 2
90 88 30% 3
90 87 30% 4
90 86 30% 5
90 85 30% 6
90 84 30% 7
90 90 50% 7
90 100 50% 7
90 105 30% 7
90 105 10% 7
90 100 10% 8
90 95 10% 8
90 90 10% 9
90 95 10% 9
90 90 10% 10

I first suspected a pump problem, The pump at low% made a abnormal noise, kind of like when an electric motor is locked and not spinning, that low humm of resistance. But when the pump reached 50% power the sound changed and the temp shot up, then when the power drops from 50% to 30% there is a sound that sounds like skreeching for a moment. So I replaced the pump and everything worked great!

This lasted a month until the ticking from pipe expansion returned. It is the exact same problem, overshoot the same and pump sounds are basically the same.

At this point I am unsure what to do. voltage/amps from the reset? I did not change the housing just the pump. I’m at a loss

thanks for any help.

Ironman

Is it a stainless steel or iron pump?

Zman

This sounds like injection mixing with a tekmar 356? Because the controller does not have the ability to vary it's signal infinitely, the overshoot you are experiencing is pretty common. The trick to this is to size the circ correctly and then add a balancing valve so you can dial in the resistance on the loop. What you want to do is add some resistance to the loop so the controller will stop "hunting" for the right temp. If you have not already, put the circ on speed 1.
http://www.tekmarcontrols.com/images/_literature/e0021_06.pdf

pushinluck

The circ pump is an iron pump. I thought this might be the problem as well but the first iron pump lasted almost 5 years and I woul have expected the second one to do the same. Do you think it could be a problem with the iron housing?

pushinluck

Zman

I'm not sure this is the issue as everything has worked smoothly for 5 years. It will hold the design temp within one degree no problem. But now all of a sudden I have lost 2 pumps in 2 months.

DZoro

What did the original pump body look like inside? Open systems tend to build up calcify the insides of pumps and piping. Thus restricting flow and sensing characteristics. That can change your operating system drastically.
D

Ironman

pushinluck said:

The circ pump is an iron pump. I thought this might be the problem as well but the first iron pump lasted almost 5 years and I woul have expected the second one to do the same. Do you think it could be a problem with the iron housing?

The pump should be stainless or brass on an open system.

You're probably building up deposits in it which are hindering the pump from starting or maintaining the target speed. Hence, the 356 sends more power to it to compensate and then over-shoots it.

As Zman said, proper sizing of the injection bridge is also critical.

Is this system directly connected to an outdoor wood boiler? Some pics would be helpful.

Zman

I think you have 2 problems.
The overshoots are absolutely caused by the injection loop balancing and pump size. It probably always did it, just no one noticed.
An open outdoor boiler is constantly boiling off water and concentrating the TDS's. This makes for a chemical cocktail that will eat anything. I would recommend checking at the very least the TDS and PH of that water. You may need to flush it several times and use a cleaning agent to get the water back within a workable range.

pushinluck

SOLVED

thank you for all your help

So I removed the new pump from the housing. The calcium deposit on the interior of the pump housing were minimal but I remove the housing anyways. Under closer inspection, I noticed a small amount of calcium buildup on the plastic check valve at the outlet end of the pump. Using a small screwdriver I tested the resistance needed to open the check valve against the resistance needed to open the check valve on a new unused housing. There was a noticeable "catch"in the check valve when applying a very small amount of force. Seeing this as a likely problem causing the over shoot i soaked the check valve in cleaning solution we use for cleaning of the domestic hot water side of the heat exchanger. After about an hour the check valve was free of any build up. Replacing the old housing and the old pump, the system is now performing perfectly with the clean check valve.

Some pictures:

Ironman

If you'll install a 5"x12" 30 or 40 plate heat exchanger where indicated, and pressurize the house side, you'll solve the open loop issues.
You'll need a relief valve and expansion tank, also.