The heat source, circulation, emitters and flow/return loop that make up a typical domestic wet system.
A wet central heating system moves heat around a house using water as the carrier. Getting the basic loop clear in your head makes everything else, from controls to balancing, much easier.
The heat source, usually a boiler, raises the temperature of the water. A circulating pump then pushes that hot water around the pipework.
The hot water travels out along the flow pipe to the emitters, which are normally radiators. A typical finned panel radiator releases most of its heat by warming air passing over its surfaces (convection), with radiant heat as well. The balance varies with emitter design, and the water cools as it gives up heat.
The cooled water returns to the boiler along the return pipe to be reheated, and the cycle repeats. So every radiator has a flow connection bringing hot water in and a return connection taking cooler water out.
Branches differ in resistance and in the heat they have to give. Balancing sets the flow each radiator needs, usually at the lockshield valves. The nearest radiator is not always the one getting too much, and a balanced system does not mean equal flows or every room warming up at the same speed.
A sealed system uses an expansion vessel and relief protection; an open-vented system uses a feed and expansion cistern and open vent. Both need suitable circulation and controls. Identify which one is installed, and the energy-efficiency guidance that applies.
Fitting the circulator
A circulating pump has to be mounted the right way round. The Grundfos ALPHA2 instructions, for example, say the motor shaft must always be horizontal, within 5°, whichever way the pipe runs; a vertical shaft is not allowed. The pump can sit in a horizontal or a vertical pipe, and its control box is turned to one of the positions the instructions permit. The arrows on its housing show the direction of flow. To take it out without draining the system, switch it off, unplug it and close the isolating valves on both sides of it, then loosen the fittings slowly: the water inside can be scalding hot and under pressure, and the housing stays hot for a while.
Venting and setting an ALPHA2 pump
The Grundfos ALPHA2 clears air from itself through the system. If air makes the pump noisy after filling, use speed III briefly for quicker venting, then return to the recommended setting once the noise stops. This vents the pump only; the rest of the installation still needs venting.
Its factory AUTOADAPT setting uses proportional-pressure control: as heat demand and flow fall, the pump head falls too. Constant-pressure mode holds head instead, while settings I, II and III hold speed. Choose the mode for the actual heating arrangement.
Grundfos asks for the pumped water to be warmer than the air around the pump, so condensation does not form in the control box and stator. Its technical data allows the air to be slightly warmer than the water if the pump-head plug points downwards; follow the actual model's temperature limits and allowed positions.
Automatic night setback needs the pump in the flow pipe, with a boiler that controls the water temperature automatically. The pump senses the lasting fall in flow temperature, so a pump in the return cannot use this feature.
Panel radiator types
Panel radiators are named by their water panels and rows of convector fins. A Type 22, also called a K2, is double panel, double convector: a front and a rear panel with two rows of fins between them. A Type 21 has two panels with one row of fins, and a Type 11 one panel with one row. A Type 22 gives more heat than a Type 21 of the same size but stands further from the wall.
Fan convectors in kickspaces
A kickspace heater is a small fan convector that fits in the plinth space under kitchen units. In Myson's Kickspace a low-limit thermostat keeps the fan off until the heating water reaches 43 °C, so it does not blow cool air; a short silence when the heating starts is normal. It also needs a good flow of hot water through its small heat exchanger. The instructions require a two-pipe, pumped system with at least 15 mm pipe from the boiler, never microbore or a one-pipe circuit, a separate circuit where other emitters could starve it, and effective balancing.
Frozen condensate pipe in winter
Water condensed from the combustion gases drains through the condensate pipe during condensing operation. A frozen section can block that drain and shut the boiler down. Thawing it does not prove there is no other fault.
Only try to thaw it where you can reach the pipe easily and safely at ground level. Use warm water or a suitable warm pack, never boiling water or a flame, and watch for spills freezing underfoot. Once it drains again, follow the boiler’s reset instructions; get help if the pipe is hard to reach or the fault comes back. To stop it happening again, use the route, size, insulation or frost protection specified for the installation.