How your central heating system works
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There are several different types of central heating system but we're talking about the commonest type, a wet central heating system with a boiler and radiators. It doesn't matter whether it's a gas boiler or an oil boiler or even a solid fuel boiler. Most are gas, so we'll assume it's a gas boiler.
Very simply, you have a boiler which heats the water, pipes which carry it round the house, and radiators which transfer the heat from the water to the air in the rooms. You have a pump, which forces the water through the pipework to the radiators and back to the boiler, and you usually have some form of timer and room thermostat. The timer and thermostat don't matter to us. You know that you have to have the heating timed to be on and the room stat turned up high enough to be calling for heat.
Central heating systems with a cylinder
Most central heating systems also have a hot water cylinder. System water is diverted from the boiler, passes through a heat exchange coil in the cylinder and then flows back to the boiler. The coil in the cylinder acts like a small radiator but instead of heating air, it heats up the stored tap water in the cylinder. Some hot water cylinders work differently but most are like this.
The flow of system water from the boiler, through the cylinder, is controlled by a timer, a cylinder thermostat and (usually) a motorised valve. They have an indirect effect on radiator balancing since a "call for heat" from the cylinder diverts some of the pumped flow of system water away from the radiators. The radiators then have to share the flow with the cylinder, but we'll come to that.
When the tap water in the cylinder is up to temperature, the motorised valve closes the flow of heating water to the cylinder. All the pumped flow then goes to the radiators.
In an ideal system, the radiators can all be heated evenly even while the hot water is heating too. In reality, some radiators may heat more slowly when the cylinder is also heating. This is not a real problem as it only takes an hour or so to heat the cylinder, even with the radiators running. When the cylinder is up to temperature, the flow to the cylinder is closed off. All the flow is then diverted to the radiators which should get hot evenly.
Central heating systems with a combi boiler
If you have a combi boiler you're very unlikely to have a hot water cylinder. The combi heats the tap water, directly, on demand. While it's heating tap water, none of the flow goes to the radiators. When the demand for tap water ends (when the hot tap or shower is turned off) all the flow goes back to the radiators.
Combis take about 15 minutes to fill a bath and during that time the radiators won't be heating. If someone has a really long shower, and stays in for half an hour, the house may begin to get cold!
In a well designed heating system...
Pushing water round a central heating system takes energy from the pump. The longer the pipe runs are, the more resistance there is to the flow. Tight waterways through radiators add to the resistance, so do the radiator valves and the bends, elbows and tees in the pipes.
Larger diameter pipes and shorter pipe runs, however, reduce the resistance and increase the flow.
The pump also has different power settings, or speeds. If the system is right, the pump should be capable of running at any of its speeds without causing separate problems. A low pump speed, though, may not be enough to make all radiators equally hot. If the radiators do get equally hot at a lower pump speed, that's fine. If not, try a higher speed. If, at a higher speed, the pump starts to pull lots of air into the system (air entrainment), that's a fault in the system. You will need to turn the pump back to a lower speed, bleed out as much air as possible and arrange to have the air entrainment problem sorted out. In a well made and maintained central heating system that doesn't happen.
In a well constructed heating system, pipe sizes and routes are designed so there is a balanced flow to all the radiators. If it's done right, they all get hot in about the same amount of time. In some systems, it's possible to balance the radiators together with the hot water cylinder circuit so that they all get hot evenly. When we balance radiator systems, we tend to do it with the hot water cylinder side turned off. This makes it easier and more realistic. Unfortunately, there are not may really good systems around. Even if you're the lucky one, there may still need to be some radiator balancing, and that's where radiator valves come in.
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