Iwaki EHN Pump Not Pumping? Air Locks and the Auto-Degassing Head

A sodium hypochlorite dosing point, an Iwaki EHN pump left running overnight, and in the morning the tank level has barely moved. In our experience the causes fall into three groups: gas in the head, a problem on the suction side, or a control setting that means the pump isn't really stroking. The first two come down to bleeding air and checking piping; the third means looking at the panel. We'll go through them in that order and spend most time on how the EHN auto-degassing head actually works.

Why an Iwaki EHN Pump Stops Pumping: Air Lock Comes First

The Iwaki EHN pump is a solenoid metering pump with a very small displacement per stroke. According to the brochure, standard heads move from a few hundredths of a milliliter up to just over one milliliter per stroke. A small pocket of gas in the chamber gets compressed as the diaphragm moves forward and expands again as it pulls back, so the check valves never open. That's an air lock.

Gassing chemicals such as sodium hypochlorite and hydrogen peroxide are the worst for it. It's also common after the pump has stood idle, after a tank change, or after air has leaked into the suction line.

Standard heads: the manual bleed valve

The standard VC, VS, PC, PS and PP heads have a bleed valve; turning the bleed knob releases air from the chamber. Leave the pump running, open the knob, wait until what comes out is steady liquid rather than bubbles, then close it. Run a tube from the bleed port back to the tank so nothing splashes.

Watch for two exceptions. The FC head has no bleed valve, and neither does any Iwaki EHN pump with K in position 6 of the model code. If those air-lock, you have to release gas from the discharge side.

The YN controller's priming function

On an Iwaki EHN pump with the YN controller there is a start-up function: hold the up and down keys together and the pump runs at maximum stroke rate. Use it together with the bleed valve after installation or a tank change. It's much faster than waiting.

How to Use the Iwaki EHN Pump Auto-Degassing Head

Bleeding an Iwaki EHN pump by hand every day isn't realistic on hypochlorite duty, so there's a dedicated NAE auto-degassing version, marked NAE in position 8 of the model code.

How it gets rid of gas

Gas that enters through the suction port travels through the head to the fitting. Because the discharge side is under pressure, the gas heads for the lower-pressure auto-degassing valve and leaves gradually along with some liquid. Once the air lock clears, the pump goes back to normal discharge.

At that point a small amount of liquid keeps coming out of the auto-degassing valve, and that's normal. Plenty of people see the port dripping, assume the valve has failed and take it apart, then struggle to reassemble it. The right setup is a return tube from that port back to the tank, with no kinks and not lifted too high.

Installation and selection points

The NAE handles liquid at 0-40°C, a narrower range than standard heads, so watch tanks that sit in the sun in summer;
Models are EHN-BN11-NAE, BN16-NAE, CN16-NAE and CN21-NAE, with maximum output from 30 to 110 mL/min and maximum discharge pressure of 1.0 or 0.7 MPa;
Besides the auto-degassing valve, the NAE also has a manual bleed valve for relieving pressure in the discharge line, so use it before disconnecting tubing;
The separator rod is titanium by default, and the BN11VC-NAE can also take a Hastelloy C276 equivalent or SUS316 rod.

The NAE isn't a cure-all. If the suction line leaks or the foot strainer is blocked, it still won't prime, and the piping still needs checking.

Suction Side and Discharge Check Valve

Beyond air locks, the suction side is the next place an Iwaki EHN pump loses prime. Keep the suction tube short, compress the fittings properly and don't let the hose get squashed. Sediment at the tank bottom can block the inlet, so clean it out regularly. For liquid temperature, standard heads are rated 0-60°C and the VC head 0-40°C; outside those limits the chemical gasses off more easily.

The discharge side matters too. The brochure requires a discharge pressure of at least 0.12 MPa (0.05 MPa for BN31 and CN36), and below that you need a check valve or back pressure valve. The standard CAN check valve fits at the end of the discharge tube to prevent overfeeding, backflow and siphoning. If crystals jam it, chemical can't get out.

Iwaki Metering Pump Not Priming? Check the Panel Settings

If the light is on but there's no stroke sound, don't open anything yet. Look at the controller.

STOP function. An external contact signal stops the pump until the signal is released. It's often paired with a tank level sensor, so a sensor mounted in the wrong place or a shorted cable keeps the Iwaki EHN pump stopped.

EXT mode. In EXT mode the Iwaki EHN pump runs from external pulses. With pulse division set to n:1, it takes n pulses to make one stroke, so a large n looks like no dosing; if the flow meter sends no pulses, the pump doesn't move. The YN can also run from a 0-20 mA analog signal with two set points, and if those are set the wrong way round, stroke rate can be low even with a normal signal.

FCM flow checker. With the optional FCM, if no flow is detected in the suction line, the pump alarms and runs at full speed, 360 spm, to clear air or a blockage. In PA+AL mode it stops after a period of full-speed running. That's protection, not a failure. Fix the suction problem, then restart.

Check stroke length as well: the BN drive adjusts from 50-100% and the CN from 40-100%, and output drops a lot with the knob at minimum.

Conclusion

For an Iwaki EHN pump that won't deliver, air lock is first on the list, then the suction side and the discharge check valve, then the STOP, EXT and FCM settings. On standard heads, bleed by hand with the bleed valve. For hypochlorite, peroxide and similar chemicals, choose the NAE auto-degassing head and plumb its return line properly. Follow that order and most problems are solved without taking the pump apart.

FAQ

My Iwaki EHN pump auto-degassing head keeps dripping. Should I replace the valve?

No. After the NAE clears an air lock, the auto-degassing valve keeps passing a small amount of liquid by design. Route the return tube back to the tank.

Where do I start when an Iwaki EHN pump won't prime?

Listen for strokes. No strokes means checking the panel and external signals. Strokes but no chemical means bleeding air and checking the suction line.

A standard Iwaki EHN pump on hypochlorite keeps air-locking. What now?

Mount it lower, shorten the suction tube and use the bleed valve often. For long-term duty on this kind of chemical, switching to the NAE auto-degassing version saves a lot of effort.

Can low discharge pressure cause dosing problems too?

Yes, it's related. Below the brochure's minimum discharge pressure, flow swings and the pump overfeeds. Fit a check valve or back pressure valve at the end of the discharge tube.

The FCM alarmed and my Iwaki EHN pump stopped. How do I get it running?

Check the suction line for air, blockage or an empty tank. In PA+AL+RE mode the pump resumes at its set rate once flow returns; after a PA+AL stop, fix the problem and restart it.

Still not dosing? Send us the model, chemical and an installation photo through our contact page and we'll help work it out.