I wanted to bleed my radiators by myself for the first time and I was wondering what is the reason that bleeding radiators saves energy?
Most of the guides on bleeding radiators mention that this is important for saving energy but none explain the exact reason why. I understand that by bleeding you increase the surface area through which the radiator can heat the room and probably also the surface area through which the hot water heats the radiator.
For maximum efficiency, many modern heating systems (condensing gas boilers, heat pumps) are designed to output heat at a low water flow temperature. The unit can generate heat more efficiently at (for example) 40C than at 70C. The heat emitters (radiators etc) are sized to achieve a certain deltaT (difference from input to output water temp) at that flow temperature - ie the amount of heat supplied to the room.
Air in radiators reduces the usable surface area of the radiators. That reduces the heat flux provided by the radiator. It means the water is returned hotter back to the heater, reducing the deltaT. In extremis the heating system may be unable to keep up with heat losses from the room.
If the surface area of your emitter is reduced, the unit will have to work harder to maintain the same temperature in the room. If it can't successfully heat the room, it'll either work longer (increasing distribution losses as mentioned) and it'll extract less heat from combustion (a higher return temp means less heat is exchanged with flue gases, meaning more is wasted in the exhaust). Or, if the unit has some kind of dynamic adjustment, it'll decide to heat the water hotter, which reduces its efficiency.
If you get less heat because of air in the radiators, the losses in the system can add up to a greater %.
Depending on where the pipes run, some heat might be lost in useless places. This is worse if it runs longer to provide the same heat. Also worse if you return warmer water through one of those useless places.
You also need energy to pump that water. If you need to pump more water, you need more energy.
Not really an answer but I suspect it gets repeated so much because most people don't understand how this works and directly equate "colder radiator" with "lost energy".
A radiator works best if the water inside it slows down and lingers to emit the heat into the room. Flow speed depends on the cross section of the pipe and the volume per time being pushed through, a radiator is equivalent to a much wider pipe section.
A massive airbubble in the radiator decreases the effective cross section of the radiator. meaning that less heat transfer happens in that radiator and that you are returning hotter water back to the boiler which means more heat loss from the return pipe.
Another factor is that you lose quite a bit of surface area that could be used to transfer the heat into the room.
Net energy consumption won't be affected very much if some radiators have some air in them. But if you look at the efficiency of the system's ability to heat the house, and to heat each room appropriately and quickly, THAT efficiency is reduced significantly if parts of the system are essentially disabled by being full of air.
In a ideal system each radiator is sized so that it provides the right amount of heat to the room it is in, each room has a suitably sized radiator (accounting for the particulars of the room eg north/south facing, windows, etc) and the boiler is sized to provide the correct amount of heat flow to all the radiators. If there are zones, the boiler is capable of working efficiently with any zone or multiple zones.
There are not a lot of ideal systems. There are lots of things that can throw the ideal out of whack. Someone can open a window. A room with large windows could have the curtains left open in the sun. The occupant of a room could turn the radiator off, or there could be thermostatic valves on radiators that turn them off automatically.
If one radiator is partly or totally full of air, that's no different to the radiator being partly or totally turned off at the valve by a user or by a thermostatic valve. It does not make the entire system more efficient or less efficient. Except at heating the particular room it's in ... it's less efficient at that, because its capacity is reduced by up to 100%.
If ALL the radiators have a little air in them, the entire system has less capacity to emit heat, so it will heat the house more slowly. That isn't necessarily more or less efficient from an energy-consumption perspective but it's certainly less efficient in heating the house quickly. On a very cold day it may not be able to keep up.
If ALL of the radiators have so much air in them that the house just can't be heated, then the boiler will cycle rapidly and may have trouble finding a stable temperature. The problems with that are bigger than efficiency.
You've hit on some major reasons why you should bleed them so I won't repeat them. Another reason is to balance the heat in all the rooms. If the thermostat is in a room with an un bled radiator, it will take longer for that room to heat up and kick the thermostat off and the boiler will run longer and the other rooms will be hotter than the operator wanted.
It messes up with the water circulation in general in the system. People have mentioned efficiency, but that depends then on the piping layout. If the house and piping is isolated from the outside all of the heat still ends up in the house. But the thermostat on the radiator wont work right if there is air at the top either and that could result in a hotter house overall instead of the temperature you meant the house to be at, the thermostat area of the radiator remaining cooler than the rest - and that would cost more money.
A radiator produces heat. That's the least efficient possible use of energy. To make a radiator more thermodynamically efficient, we would have to turn it into an engine that turns some of the energy into work, so that less of it is is wasted as heat.
A radiator system full of air bubbles will not move heat where you want it, though, or not very well. If the system doesn't circulate properly, the wrong spaces will be heated. If you're heating unoccupied spaces, while people are cold in occupied species, that's a waste of energy. That's what is inefficient: low benefit for the same cost/energy.
Returning to my first point, though, it can't be the case that you're getting less total heat out of the system for a same energy input though, just because there is air in the lines.
Given all the conflicting answers here and the fact that some seem to be partially correct but not completely, I've thought about this, come up with my own answer and then had someone I know with a graduate degree in thermodynamics and an esteemed multi-decade career as an engineer in the HVAC industry confirm my understanding.
The fundamental thing to realize here is that the point of a home heating system is to move heat generated at the heating source to the living areas of the home. In the case of a forced-water hydronic system, the water in the system is the 'carrier' of the heat which moves through the pipes of the system and delivers it to the radiators. Efficiency in this context is basically defined as what percentage of the heat generated at the source to the radiators.
At a very simple level, the presence of air in the pipes means there is less water. That is, the capacity of the system's ability to hold, and there more deliver heat is reduced. In addition (as my expert explained) the "air will go into solution with the water which reduces the heat transfer properties."
The secondary issue is that if there's enough air, it can create gaps in the system which makes moving the water more difficult. That is, instead of cycling water in a gravity-neutral way, the pump needs to work to move water from (typically) the lowest point in the home to the highest point. If this isn't intuitive, note that a siphon can move water with only the force of gravity but only if the tube is sufficiently (i.e.: nearly completely) full. While this extra work is undesirable from a wear and tear perspective, it's the reduction in flow that is going to really have an impact of the efficiency.
In a nutshell the air in the system means heat being produced at the source is not distributed as quickly and/or as evenly as it could be. Here's an analogy, consider a bus-system that travels around a city and we can define efficiency as the maximum number of people that it can move each day per bus. If we were to remove half of the seats in each bus and change nothing else, the efficiency of the system would clearly be reduced. The hydronic system is like a bus-system for heat and air reduces the amount of heat that can be delivered over a given time span. This means more heat loss at the source (such as an unfinished basement because it remains there longer) as well as in other non-living area such as in voids that the pipes pass through.
Two reasons:
Water has a 4x higher heat capacity than air. If your boiler puts out 1000 units (joules, whatever), and your system is 50% air, you end up with 625 units in your system to be circulated around your house.
Water is more efficient at transferring heat than air, by like 24x. If you have 50% air in your system, the radiator will only transfer about 52% of the energy out.
Obvs there are transfer efficiencies that aren't accounted for here, just using some numbers as examples of why you don't want air in your radiator system.
Bleeding may safe money, if you happen to live in an appartment that shares the central heating with other appartments, because the sensors that are used to distribute the heating costs assume equal efficiency. If your radiator is less efficient because of air inside it, you're billed more heat than you actually use.
Bleeding does not save energy, though - as long as you heat your room to the same temperature, it doesn't matter whether you use one radiator or two or half of one or none (use a server rack instead, perhaps :-), the energy required stays the same.
