Two customers rang us within a fortnight of each other. Small tents, near enough the same kit, same complaint. No pressure. Nothing out of the drippers, nothing out of the flush valve at the end of the run, and both of them convinced something was blocked.

Nothing was blocked.

Both had bought a pump that could not open a valve sitting inside their own whip kit. We went through it properly afterwards, datasheet by datasheet, and found a number we had been getting wrong on our own product pages for a long time.

What The Drippers Want

FloraFlex micro drippers are pressure compensating. There is a small diaphragm inside each one that flexes to hold the flow steady, which is why the dripper in the far corner puts out the same as the one sitting next to the feed tee.

That only happens above 15 PSI. Below it the diaphragm stops regulating and you are left with holes in a pipe. The pots nearest the pump get fed, the ones at the back get whatever is left over, and people start pulling stakes out looking for a clog that was never there. Twenty five to thirty five is where they are happiest.

So 15 PSI is the floor. About 1.03 bar.

The Valve Nobody Reads The Datasheet For

Most whip kits have a small Netafim DNL in them. It is a non return valve, and its job is to hold water in the line between feeds so the system does not drain back and fill up with air. Sensible part. Does a real job.

The datasheet quotes two pressures for it, and they are nothing like each other.

Netafim's own figures. The colour of the cap tells you which one you have.

The 0.2 is where it closes. The 0.8 is what it takes to get it open in the first place. We had been reading the first number and repeating it as though it were the second, and so has half the internet.

0.8 bar is roughly 8 metres of head. Before you have lifted a single drop out of the tank.

Which Pumps Actually Clear It

Head is quoted at zero flow. These systems draw so little that the pump sits close to that figure anyway, so it is a fair guide here.

The Q4003 is the one that stings, because it is the pump most people buy for a small tent and it is the pump we used to recommend for exactly this job. Eight metres of head works out at 0.78 bar. The valve wants 0.8.

It is short before the water has left the reservoir. So the valve never opens and nothing gets into the ring.

Not weak flow. No flow.

Which is also why neither of those customers had anything coming out of the flush valve at the far end. Water was not getting past the whip kit to reach it.

Netafim DNLNeeds 0.8 bar to open Aquaking PumpsOnly the 3 bar clears it Pressure GaugeStops the guessing

The valve, the pump that has to open it, and the thing that tells you whether it did.

The Flush Valve Has A Minimum As Well

While we were in the datasheets we looked at the FloraFlex Lateral Flush Valve, which sits on the tee opposite your feed and is supposed to purge air and debris at the start of a cycle.

It is rated 8 to 25 PSI, and 2 to 5 GPM on its low flow setting.

Two GPM is 454 litres an hour.

A nine plant tent moves about 68 litres an hour on 1-way assemblies, and 34 on 2-ways. So depending which you run, you need somewhere between sixty and a hundred and twenty plants before that valve so much as reaches the bottom of its own flow range.

Under that, the diaphragm never moves. It does not flush and it does not let the trapped air out, and the trapped air is exactly what gets diagnosed as a blockage a fortnight later. On a small system a hand valve does the job better anyway. No threshold to meet. Open it, the air and grot come out, close it.

So How Much Head Do You Actually Need

Nine metres will crack the valve and get water moving. Twelve gets the drippers regulating. Nineteen to twenty six puts them in the 25 to 35 window they actually want, which is why the 3 bar pump comes with a regulator: it pushes about 43 PSI and the emitters should not sit above 40.

How To Tell Without Guessing

Put a gauge on it. Forty two quid, goes inline on the feed with a tee and a cap combo, and it tells you in one look whether you are in the window or wasting your evening.

We started stocking one because we got tired of guessing down the phone.

What We Changed

The whip kit now comes in a version with no DNL in it, six quid cheaper, for anyone running a small pump. There is a manual flush kit for the far tee. And the drip system builder knows the difference: tell it your room size and plant count and it picks the right pump, the right whip kit and the right flush arrangement, then puts the lot in your basket.

If you already have one of the old kits and a small pump, take the DNL out. That on its own will get you watering.

And if you are still not sure what is going on in your lines, the other usual suspect is air, which we covered in air locks in drip irrigation.