Hi. Could this be why my new radiator is only getting warm. Would it help if sloped the outlet pipe back in to the one pipe??It's simple physics. With a one pipe system, the water is pumped around the one pipe loop, meaning that because the radiator inlet and outlet pipes are connected to the same pipe they receive the same pressure from the pump, as a result, the pump can't force the water through the radiator in the same way a two pipe system does.
One pipe systems rely on the gravity principle of hot water rising and cooler water sinking to circulate through the radiator. This means to work the radiator ideally should sit directly atop the one pipe. The one pipe can of course be re-routed so that it passes directly below the new radiator.
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If there is a slight horizontal distance away from the one pipe, (ie it's not directly above) through a wall for example, the pipe must be inclined upwards and the horizontal distance kept as short as possible.
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What you can't do is have the two radiator pipes from the 'one pipe loop' running horizontally and through bends before reaching the radiator because the pump pressure on the flow and return pipes is the same it isn't able to force the water through the radiator.
If you re-route the one pipe loop so that it passes directly below the radiator, with the pipes going directly vertically in and out of the radiator above, all will be well.
The outlet pipe is the nearest in the pic, that outlet pipe is warm as It looks like it’s taken the least resistance for the water to take (it’s a smaller tee height)