Magna3

Harvey Ramer · February 6

What do we think here. Bad sensor?

HVACNUT · February 7

Did you happen to see my mother in-law sneak by? It doesn't take much.

ScottSecor · February 7

It's been year or two, but I thought there would be an error signal with a bad sensor? Screen turns red or has a red indicator? The newer ones could be different.

Harvey Ramer · February 8

https://forum.heatinghelp.com/discussion/comment/1886678#Comment_1886678

Haha! She gets hot 🔥 does she?

Harvey Ramer · February 8

https://forum.heatinghelp.com/discussion/comment/1886720#Comment_1886720

There was ne error signal. The pumps just went into limited speed. Minimum speed actually. This is a twin head pump on a geo ground loop. Must have been that way for a while. Long enough to burn out 2 10 ton compressors. I disabled external limiting till I can get them fixed.

109A_5 · February 8

Could be the sensor, but if it is a common 10K thermistor the wires to the sensor may be shorted.

https://www.workaci.com/resources/calculators/thermistor-curves

Harvey Ramer · February 8

https://forum.heatinghelp.com/discussion/comment/1886827#Comment_1886827

Good point. It is a 10k sensor.

ratio · February 8

Note that there are several common flavors of 10K thermistors. They'll read identically at 77°, of course, but the further they get from that the more they vary.

Thermistor 10k zoomed max.png

Thermistor 10k.png

hot_rod · February 8

https://forum.heatinghelp.com/discussion/comment/1886860#Comment_1886860

Good point. I've talked with sensor manufacturers and they suggest using the sensor replacement that the control can with. Not all sensors are the same profile.

On many of the Grundfos Magna pumps one sensor does flow and temperature or flow and pressure. These vortex shedding sensors are right in the pump body. An early Magna example in my shop.

Screenshot 2026-02-08 at 10.48.59 AM.png

Screenshot 2026-02-08 at 10.47.16 AM.png

hot_rod · February 8

Harvey Ramer · February 8

@hot_rod is there any reason I would need to keep the sensor? The pump is controlled via 0-10v signal from a GEO Flow controller operating on the ground loop delta-t.

109A_5 · February 8

The manufacture put the sensor there for a reason. I would suspect some aspect of the operation is not working as efficiently as it should or could be.

As far as the the sensor type curve if you remove the short circuit I suspect the actual sensor may be fine.

I'd look for the wires being crushed somewhere.

Harvey Ramer · February 8

https://forum.heatinghelp.com/discussion/comment/1886888#Comment_1886888

The sensor allows various modes of operation using the onboard control. I'm not sure that it performs any essential function outside of that... If it does not function as a critical safety, I would just as soon do without it in this scenario.

hot_rod · February 8

The Grundfos VFS Vortex Flow Sensor is mainly giving the control flow and temperature data to the pump readout. I agree the 0-10VDC operates the pump. I'll bet it runs fine without it.

We used the same Grundfos sensors in our Solar Pump Stations. The Resol controls would take the Grundfos VFS input

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