Yeah, absolutely, those look closed or open very, very little. You can see it says 0 at the top and 5 at the bottom, which are litres per minute. I'd open it to 1 litre per minute to see what happens. 

If you have rads connected to the same heating circuit it might affect them, which is why I suggest opening to 1 litre per minute for now.

Yes, your interpretation is correct, and those flow meters are indicating a zero or close to zero flow.

Assuming you have a buffer or low-loss header, this would mean that your heat pump will essentially just be circulating water through the buffer and back to itself, with a minimal heat loss (and so a low dT).

If you don't have a buffer or low-loss header, you may instead have a bypass valve, or some other component which is able to act as a kind of short-circuit for the heating fluid (this includes another manifold, or radiators etc).

You may be seeing zero flow here because the flow-regulating valves are closed right off, because the circulation pump feeding the manifold is not pumping (for whatever reason), or because the zone-control thermostats (if you have them) have closed those circuits.

From checking your other post, it seems as if you're running open loop, so there shouldn't be any thermostat-controller actuators on the heating circuits. Then again, you also didn't flag any concerns with your heating not actually working, so I'm a bit suspicious.

Was this photo taken while the heat pump was on, and actively heating?

@davidb, turn the red bit (see photo) anti-clockwise. 2-3 finger turns should drop it to around 1l/min.

@editor so simple, thanks.  Reevaluating what I was seeing, the valve was only just cracked open. It meant the flow pipe was warm a meter away from the manifold.  I have set both to your suggested 1l/m but curious to know why that figure.  Is that a simple industry rule of thumb?

@davidb there isn’t really a universal 'rule of thumb' number for UFH flow rates, which is why I was careful not to present 1 l/min as the answer. In your case it was more of a safe, visible starting point than anything else.

UFH flow rates are ultimately a function of heat output, pipe spacing, loop length and design temperature difference, so in a perfect world you’d calculate them loop by loop. But when you’re troubleshooting a system that’s clearly doing almost nothing, the immediate goal isn’t optimisation, it’s simply to get meaningful circulation and see how the system reacts. Going from effectively zero flow to 1 l/min is a big enough step that you’ll feel it quickly in pipe temperatures and see it reflected in system behaviour, without suddenly dumping a load of flow into the circuit and destabilising everything else.

As you’ve already seen, with the valves barely cracked open the flow pipe warming a short distance from the manifold makes total sense... there just wasn’t enough movement to carry heat back. At 1 l/min you’re now firmly into 'this should actually do something' territory, which gives you useful feedback.

There’s also a practical reason for not jumping straight to higher numbers. If your UFH is on the same primary circuit as radiators, or shares a buffer/LLH, opening the manifold too far can absolutely throw balancing out elsewhere. UFH likes low and slow by nature, so sneaking up on the flow rather than swinging it wide open is usually the least disruptive way to go.

On @steelbadger’s point, he’s right to flag the bigger picture. Near-zero flow at the manifold will lead to low system dT because the heat pump is effectively circulating against itself via whatever path offers the least resistance, whether that’s a buffer, bypass or another open circuit. That doesn’t automatically mean anything exotic is broken, but it does mean the UFH loop wasn’t participating in heat delivery at all.

Given you’re running open loop, no actuators closing things down and the pump appears to be running, the simplest explanation was also the right one here: the flow limiters were basically shut. The key now is to see how the system behaves with some actual circulation... how the UFH responds, whether the return starts to lift and what happens to overall dT and COP once the heat has somewhere useful to go.

I’d live with 1 l/min for a bit, observe, then adjust incrementally. 

@steelbadger yes, HP running “normally” and, if you have seen some of my other thread, I have no stove burning.  I have the system in active mode - so control via WC curve only.  With only a trickle through the UFH pipes, I am not surprised I haven’t been reaching set temperature.

Update - I have been checking the UFH return manifold, waiting on a rise in temperature.  Response was fairly slow so I figured the heat was being absorbed by the slab.  However I then realised the flow had returned to nearly closed.  Strange, I had it set clearly at just over 1.  I readjusted the flow control only to find it wasn’t operating, regardless of how much I rotate the adjuster.  It looks like I have a hardware failure.  I have no idea what that will take to fix as I am on a Scottish island and my installer on the mainland.  Add to that we have had no ferry service for 2 days due to an easterly gale!  If there are any tricks I can perform to get the flow going, please take to your keyboard.  Thanks to all for their comments.  BW. 

@davidb, if the flow has drifted back to near-closed and the adjuster now isn’t doing anything (bearing in mind that I'm not a plumber or heating engineer) that, for me, points to a failed or stuck flow regulator on the manifold rather than anything the heat pump or controls are doing. With UFH, a trickle like that will happily fool you into thinking things are running normally, while in reality the slab is being starved of energy. Under those conditions, not hitting set temperature is exactly what you’d expect.

I think that the slow return temperature rise you saw also fits. Some absorption into the slab is normal on start-up, but if the flow control is intermittently or permanently closed, you’ll never get sustained heat transfer... you’re effectively trying to heat a concrete mass with a dribble of water.

Before you resign yourself to waiting for ferries and installers, there are a couple of low-tech things worth trying. Gently tapping the flow meter body and valve stem can sometimes free a stuck regulator.

Fully winding the adjuster open and closed a few times can also help if it’s jammed with debris or scale. That solved a similar issue for us a couple of years ago.

@editor thanks for the tips which I will try before calling my installer on Monday. Again much appreciated.