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To an answer to How does water travel through a modern column radiator a commenter asked

Does this mean that you could completely open the exit valve, but half-open the input valve, and the water would never travel up, instead just travel from valve to valve without filling the radiator?

RedGrittyBrick
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  • I always assumed that a building radiator worked like a car radiator in that the water enters through the inlet then must flow up & down all the segments because each pair only had an opening at the top or bottom. I never realized that there were openings at both top and bottom. #TIL... – FreeMan Jan 24 '22 at 15:24
  • I'm not sure there's a simple answer. Buoyancy of the hotter water is a thing, but it's not going to be that great if there's not a large temperature difference between the top and bottom of the radiator, and the flow rate of the water in the radiator is going to "fight" it, so that matters. On the other hand, the thermal conductivity of the steel or iron the radiator is made of is about two orders of magnitude higher than the water, so heat can "go around" the water too. – IceGlasses Jan 24 '22 at 16:27

2 Answers2

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No.

Firstly hot water rises.

It is true that the water wouldn't travel up if there were an air pocket in the upper part of the radiator. In that case the water would exit the lower exit valve without passing through the upper part of the radiator. The top part of the radiator would feel cold. The radiator would not adequately heat the room.

If the radiator is full of water then you should remember that it acts as a heat exchanger, hot water entering the radiator is cooled and nearby air and the room are heated by conduction, convection and radiation. Cooler water is denser and sinks, incoming hotter water is less dense and rises.

Secondly remember that when first fitted, radiators are full of air and, with the inlet and outlet valves open, you open a bleed valve at the top of the radiator to let the air out so that the radiator can fill. You close the bleed valve when water starts to seep out without air bubbles in it.

This process requires the water in the heating circuit be at higher pressure than local atmospheric pressure.

In older systems this pressure was achieved by having a header tank in the roof space about two meters above the highest radiator.

In newer systems (or also older systems in some parts of the world) The pressure is achieved by pressurising a closed loop heating system from mains-water into a small pressure vessel.

Also the water pump works by pushing water around the system and this raises the pressure.

TL;DR: Your heating system is pressurised and hot water rises.

RedGrittyBrick
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  • So if the radiator is in equilibrium then the inlet T will be equal to the outlet T, and water will go from inlet to outlet… – Solar Mike Jan 23 '22 at 11:23
  • "Equilibrium" in that case implies incoming water is the same temperature as the surrounding air and outgoing water, so no heat is being delivered; so it's not a case where it matters what the water does in the radiator. When the system is delivering heat, the incoming water is warmer than the air around the radiator. – Ecnerwal Jan 23 '22 at 13:39
  • AIUI If your boiler/heater/furnace is set to heat water to 60C, unless your room is at 60C there will be heat transfer (assuming your room thermostat is stuck on or set to >60C other wise there is no flow as the pump is off and the boiler/furnace is off too). Since most rooms/homes are not perfect insulators and the exterior temperatures are not yet 60C generally, there will always be some transfer of heat energy from the heating system into the room. At equilibrium this rate of transfer in from the radiator matches the rate of heat loss through gaps, windows, doors, walls etc. – RedGrittyBrick Jan 23 '22 at 13:48
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Normally, when you pipe a cast iron radiator for use with hot water instead of steam, the supply pipe is usually piped to the upper tap and the return to the lower port. This way the hot water enters at the top and circulates to the bottom as it cools. The inlet and outlet can both be piped on the bottom but for better heating, "top in bottom out" is preferred.

With the inlet and outlet both piped in the bottom tap there still will be hot water flow to the upper part of the radiator since the hot water entering the rad flows upward due to convection and is lighter than the cool water. Also, natural convection is always at play in a radiator. Water in a radiator does not just blow through the radiator at a fast pace but slowly enters and exits allowing for convection within the radiator.

d.george
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