Grundfos 15-55f double dashed lines, zone won't circulate, bleeds ok and electrical OK

[BPP]

First, some photos for reference:
https://imgur.com/a/EStMYRg
https://imgur.com/a/Jn5JwId

Top is old (disconnected) pump. Bottom-right is new (installed) pump. Bottom left is original (installed) pump for other zone (functional).

I have a forced hot water heating system with baseboard radiators. Five individual zones are fed by Grundfos Alpha 15-55f pumps. One basement zone, three 1st floor zones, one 2nd floor zone. Near the end of last winter, one of the first floor zones stopped circulating, and pump showed only the double dash, which manual says means either voltage, air, or stuck rotor (?):

• I bled the zone by opening the return and making sure pump was powered. Water flows out the return with no noticeable bubbles.
• With multimeter, confirmed zone is getting +120V, same as the other (functional) zones.
• Power cycled, pump boots up and starts running at low (1 watt), but goes back to double-dash within ~10s.
• I purchased a nearly identical pump (only difference is front says PC1442 instead of PC1449, couldn't find what that means). When installed, it showed the same double dash.
• I got some suggestions on reddit. 1) Swapped in the electrical connection from one of the working pumps - no effect. Still double dashed line and no circulation. 2) Hooked up garden hose to return spigot, disconnected pipe above pump and pushed hose water backwards through zone. Filled HD bucket 5x (first three times water was dirty), closed without letting air in and reconnected, but no effect - still double dashed lines
• One curious thing - when touching some of the pipes/nuts/Feed-Pressure Regulator, I at times felt the tingle of electricity. Nothing painful but pretty sure I wasn't imagining it.

I am at a loss. Going by what the manual said, it can't be a voltage issue. It seems unlikely to be a stuck rotor since a new pump swapped in immediately showed the same issue. It seems unlikely to be air since the zone flows when bled. What am I missing?

 

[Davie]

Hi BPP,

Is your trouble zone a series loop of baseboards, or is it in parallel? If it's in parallel, you could have an air pocket in one of the rad sections that just isn't moving along, but is creating problems for your pump. I think that's the most likely answer. If there is a way to bleed air from each of the individual rads, try that.

I'm also very curious about your tingling feeling, however.

Have you put a voltmeter to the pipes/nuts/etc. that you think are hot? Touch one of the leads to the pipe, and the other to the ground on an electrical outlet (or the ground on a 3-prong extension cord). If it's more than a couple of volts, I think it's probably worth some more serious investigation.

If it is a significant voltage, the pump's circuitry may be responding to being improperly grounded. (If the electrically energized section of pipe is in contact with the metal body of the pump, then the potential difference across the unidentified and ground conductors would be abnormal--either smaller or larger than normal, depending on the phase of the source. I don't know enough about the Grundfoss, or, indeed, about electronics, to know if this would be an issue, but I am confident that it could be one.)

Even if it's not causing the issue with your pump, I think this is well worth investigating, as it sounds like a potential serious wiring issue/defect/damage, and certainly a safety hazard.