Navien CH-240 circ pump blowing 5A fuses

[BrrrPopCH240ASME]

I have been helping a friend with a combi unit (Navien CH-240 ASME) issue and am in need of some advice... I maintain my own tankless system but only for DHW so I know enough to be dangerous! Hopefully this is the right forum for this topic.

Short story:

What started with a small leak at the circulation pump air vent due to a broken O-ring has turned into an electrical issue that is causing 1 (or sometimes both) of the 5A 125V fuses to blow immediately whenever the circulation pump is activated/energized.

Long story:

The original issue was a leak at the air vent assembly (from the bottom of the red cap) on the circulation pump and a local HVAC service quoted $1,800 to replace the entire pump. While the service manual (version 1.0 anyway) does show the pump and air vent assembly as a single unit, the Navien website listed a separate part number (#30014451A) which we ordered online for $30 (incl. shipping). It seemed like a simple enough DIY replacement but with my ignorance of the space heating system I managed to complicate things pretty well.

Naively I assumed shutting the cold water supply valve that forks to the unit after the water meter would be enough to remove pressure from the entire system, so when I loosened the plastic nut on the air vent assembly water began spraying out and we scrambled to close every other valve (including before the water meter) as well. That did cut the majority of the water off and I was able to remove the old air vent (and it's broken O-ring) but surprisingly water was still coming out of the pump until the new air vent was installed and tightened down. We have since learned that the S/H hot water outlet valve will not completely close, so the assumption is that most of the water that came out of the pump during the process was from the heating zones. Since a significant amount of water (maybe 3-5 gallons) came out, I was concerned that we put a lot of air in the system but assumed it would either be replaced through normal operation and work its way up to the top of the cast iron radiators throughout the house where it could be felt as cold and bled out from there. When we powered the boiler back on it operated normally for the rest of the day (~12 hours), but the next morning there was no power in the unit and we found one of the fuses had popped.

So we replaced the fuse with 5A 250V (as 125V were not easily found) and powered the system back on. The system initialization finished without issue but as soon as either heat or hot water is called for the fuse blows again. After a bit of troubleshooting and a fair amount of blown fuses we have isolated the issue to the circulation pump activation. When we removed the circ pump connector from the PCB, the system powered on and initialized normally but when either a call for heat or hot water came through instead of blowing a fuse or throwing an error code for the circ pump not activating (apparently there isn't one) the burners fired on and heated the non-circulating water until a high temp limit was reached and E16 was displayed. We allowed the unit to cool back down on its own before powering it off. So my working theory at this point was there must be an air lock in the system somewhere causing the pump to overwork and draw more amps than it would under normal operating conditions and blowing the fuse.

The next attempt to fix it was by draining the entire system (as best we can given the circ pump continuously being refilled once the drain plug is replaced due to the S/H outlet valve not closing fully) and flushing water through both sides (DHW first, then S/H). After this we got an 02E, the auto feeder valve started to fill the system again and came up to a pressure of 17.4 before blowing the fuse as the refill procedure calls for the circ pump to run for 15 seconds. After replacing the fuse and powering the system back on without the circ pump connected to the PCB, it completed the initialization again with a pressure reading of 16.8 and with a call for heat and hot water triggered the burner activation and resulted in the 16E condition again.

The current plan is to order a new pump ($200 incl. shipping) and replace it as this is clearly the component causing the fuses to blow. My concern however is that if the problem isn't really with the pump itself, a new pump will just blow the fuses too and we will be no better off. I've inspected the pumps wiring (as much as I can outside of the black plastic housing) and there are no signs of wear or damage that would indicate a short. I attached a voltmeter and found no resistance between the two wires on the pump itself, but I'm not able to verify the proper AC voltage at the PCB connector as my voltmeter is only working for DC. The fuse blows too fast to even detect whether or not the circ pump actually engages, so in that sense it seems that maybe a short within the black plastic housing is most likely, but to try and open that we need to remove the pump anyway and given the leaky valve we aren't considering doing that until a replacement pump is in hand.

Hoping someone has had experience with a similar situation before, or can suggest anything I may be missing or not completely understanding about the S/H side of the combi unit. Any feedback (including "it's time to call in a pro") or pointers would be greatly appreciated!

One last note - we did perform all routine maintenance along the way as well, including a descale flush after draining the unit.

One last last note - my friend is a new homeowner and this is her first New England winter with the system, previous maintenance history is unknown but the Navien was installed in 2012.

 

[Fitter30]

What scale Ohm meter when checking out the pump? Check to ground with the highest scale? If the fuse didn't blow with pump disconnected from board u think the pump can't be bad.? Any condensing boiler don't care if its either side NEVER fire one with no water flowing and let it go off on limit. These heat exchanger have very little water in them and can rupture a tube in a heartbeat.

 

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[Bannerman]

Have you removed the pump to determine if you can turn the impeller and motor shaft by hand? Perhaps the shaft is jammed due to rust in the bearings.

If the impeller can be turned by hand, you might try activating the pump briefly without the PCB but instead, directly from a 110VAC outlet.

 

[BrrrPopCH240ASME]

Thanks for your responses! I will reply to each as well but want to update on the progress we've made so far first.

We picked up a new pump and installed it today, and thankfully it all went smoothly. When we powered the unit back up it ran through the self-check and energized the new pump without blowing any fuses. So we have confirmed the DHW side is fully functional again at this point!

My main concern now though is that there is a good chance the first pump failed due to an air pocket on the space heating side, and without fully draining and flushing both heating zones we may end up burning out the new pump in similar fashion... Is that a reasonable concern at this point or should any remaining air be able to work it's way up to the top of the radiators where it will manifest as cold spots and can be bled from there? I have forced hot air and have never had to maintain a forced hot water heating system, so not sure what the best way to go about it would be. We are planning to call around for estimates tomorrow to try and see what it would cost to have a pro get the heat side back to a safe working condition from here, but are also still open to suggestions if it is something that should be simple enough to try ourselves.

 

[BrrrPopCH240ASME]

What scale Ohm meter when checking out the pump? Check to ground with the highest scale? If the fuse didn't blow with pump disconnected from board u think the pump can't be bad.? Any condensing boiler don't care if its either side NEVER fire one with no water flowing and let it go off on limit. These heat exchanger have very little water in them and can rupture a tube in a heartbeat.

I don't remember offhand so will have to break out the voltmeter to confirm the settings and test it again...

We knew the pump was what was blowing the fuses, we just weren't sure whether it was definitely bad or just stuck and unable to operate normally due to an air lock... Now we know it is dead, but still aren't certain exactly what killed it and whether it will happen again to the new pump (see prior update).

Thanks for pointing out that it isn't a good idea to let it heat the water when it isn't circulating causing the high limit error - when I first disconnected it from the PCB it was to try and determine if it was the pump or the 3-way valve was causing the fuse to blow and I was fully expecting to see an error condition that pointed to the pump not activating. But apparently there isn't a way for the unit to sense that, so instead it fires the burners until it hits a high limit. We only ran tests that way a few times (making sure it cooled down in between) and will not be doing that again anytime soon for sure.

 

[BrrrPopCH240ASME]

Have you removed the pump to determine if you can turn the impeller and motor shaft by hand? Perhaps the shaft is jammed due to rust in the bearings.

If the impeller can be turned by hand, you might try activating the pump briefly without the PCB but instead, directly from a 110VAC outlet.

We did remove the old pump and I am planning to disassemble it at some point to try and figure out exactly what caused it to fail but haven't done so yet. I will try to manually manipulate it as you've suggested as well and let you know what I find. The main concern now though is trying to get the heat back in working order without burning out the new pump too.

 

[Fitter30]

We did remove the old pump and I am planning to disassemble it at some point to try and figure out exactly what caused it to fail but haven't done so yet. I will try to manually manipulate it as you've suggested as well and let you know what I find. The main concern now though is trying to get the heat back in working order without burning out the new pump too.

Pump is pumping boiler side water not dhw but it does heat both. Take a few pics of boiler, piping and ex tank. Don't get to close to see how it's piped. Air goes to the highest point. Turn pumps and heater off wait 20 minutes then bleed from the top.

 

[BrrrPopCH240ASME]

I had thought the same at first but the flow diagrams in the service manual show that the 3-way valve and pump are used for both S/H and DHW. We were able to confirm that too as the fuses would blow regardless of which side called for heat.

We are only able to bleed 4 of the 6 radiators, but 3 of the ones we can bleed are on the top floor at least. So we have already removed all the air from each of those, and then opened up a valve from the cold water supply that cuts back to the S/H return in order to put some more water back into the heating zones with the unit powered off. The idea there was to both refill water that had been lost and also try to circulate any air through without the pump running.

Next we are planning to run heat to that one zone first for a bit and, like you suggested, repeat the bleed to confirm we are moving (and removing) any remaining air from the whole system.

Pics attached as well.

 

[Fitter30]

I had thought the same at first but the flow diagrams in the service manual show that the 3-way valve and pump are used for both S/H and DHW. We were able to confirm that too as the fuses would blow regardless of which side called for heat.

We are only able to bleed 4 of the 6 radiators, but 3 of the ones we can bleed are on the top floor at least. So we have already removed all the air from each of those, and then opened up a valve from the cold water supply that cuts back to the S/H return in order to put some more water back into the heating zones with the unit powered off. The idea there was to both refill water that had been lost and also try to circulate any air through without the pump running.

Next we are planning to run heat to that one zone first for a bit and, like you suggested, repeat the bleed to confirm we are moving (and removing) any remaining air from the whole system.

Pics attached as well.

Your using all radiators both zones? What water temp are u running?What is the water temp through the boiler in and out? Both zones calling. Using outdoor reset what temp at 0°.