On a one pipe system, does it matter which side the return and intake valves on a rad are?

[JonnyH12]

I have one radiator in my living room, where the return valve gets hot, but the water does not seem to flow up through the intake valve - so the radiator gets very slowly but from the bottom.

Is it possible that the radiator has been fitted the wrong way round, where the return should actually be the intake? Or is that not a thing on a one pipe system?

 

[Stuart981]

You could try taking the radiator outside and flushing it out with a hose pipe.

 

[just pumps]

A lot of tall vertical radiators tend to be one way flowing.

 

[stem]

If the radiator has a specific flow and return as per @just pumps then yes.

If the radiator is not flow specific and a TRV is fitted, it is usually fitted to the flow end, some TRV's can be used at either end, but some do have to be on the flow.

Also, is the lock shield valve fully open? It should be. The balancing rules applied to two pipe systems don't apply to one pipe systems.

However in general, one pipe systems are notoriously slow to heat up. The water is pumped around the one pipe loop, and it then relies on natural circulation of water (hot water rises, cooler water falls) for it to travel through the radiator. Because of this principle, the flow is pretty much the same through any radiator on a one pipe system, so larger radiators, especially doubles, sometimes don't get sufficient water through them fast enough to get them piping hot.

Unlike a two pipe systems where positive pressure from the pump is connected directly to the inlet and forces the water through the radiator, a one pipe radiator sits directly on top of the same pipe so the pressure from the pump that is exerted on the inlet and outlet are the same (apart from a tiny pressure drop produced by the section of pipe between the inlet and outlet) hence the slow flow.

The radiators at the furthest end from the boiler also receive cooler water as much of the heat will have already been removed by the previous radiators.

 

[JonnyH12]

You could try taking the radiator outside and flushing it out with a hose pipe.

Done this recently. That wasn't the issue.

 

[JonnyH12]

If the radiator has a specific flow and return as per @just pumps then yes.

If the radiator is not flow specific and a TRV is fitted, it is usually fitted to the flow end, some TRV's can be used at either end, but some do have to be on the flow.

Also, is the lock shield valve fully open? It should be. The balancing rules applied to two pipe systems don't apply to one pipe systems.

However in general, one pipe systems are notoriously slow to heat up. The water is pumped around the one pipe loop, and it then relies on natural circulation of water (hot water rises, cooler water falls) for it to travel through the radiator. Because of this principle, the flow is pretty much the same through any radiator on a one pipe system, so larger radiators, especially doubles, sometimes don't get sufficient water through them fast enough to get them piping hot.

Unlike a two pipe systems where positive pressure from the pump is connected directly to the inlet and forces the water through the radiator, a one pipe radiator sits directly on top of the same pipe so the pressure from the pump that is exerted on the inlet and outlet are the same (apart from a tiny pressure drop produced by the section of pipe between the inlet and outlet) hence the slow flow.

The radiators at the furthest end from the boiler also receive cooler water as much of the heat will have already been removed by the previous radiators.

The radiator is an old (antique old) cast iron column rad. How would I know if it has a specific flow and return?

The issue is not with the temp dropping - I have that under control. Most of the rads in the house are slow to heat, but that is to be expected with the setup. With this radiator though, the hot water is just not traveling up the intake pipe.

There is a smart TRV on there, but they are on all the rads in the house, and this is the only one that has this issue. They are proper one pipe TRV's.

The lockshield valve is open fully, and that pipe is blisteringly hot if I put my hand to it.

 

[stem]

The radiator is an old (antique old) cast iron column rad. How would I know if it has a specific flow and return?

In that case it would probably have been installed on a one pipe system originally. I believe it's generally the new trendy type of tall radiators that have this requirement.

With this radiator though, the hot water is just not traveling up the intake pipe.

So obviously, something is stopping the flow. I'm taking it for granted that you have bled the radiator of air, but have you tried closing the TRV and lockshield, removing them from the radiator and opening the valves one at a time to see if water is flowing out at a decent rate. If so, that leaves you with an obstruction in the radiator.

 

[Madrab]

Is this old column significantly larger than the others? The volume of water in these old cast iron rads is quite significantly more than the standard steel and you may find that the system's needing balanced again. One pipe systems are very sensitive to change and it's not usually as simple as just adding a rad that may different than the others and (edit) expecting it to just work, it can drastically alter the hydraulic balance in the system or alternatively it just won't work, as seems to be the case here.

Has there always been a rad in that location and are the flow and return short verticals from the main? If you turn off all the other rads, at the TRV's, does it warm up then?