What’s inside a hydronic heating system? The parts, in plain words
The short answer
A hydronic system is a closed loop of water and a handful of parts. A heat pump or gas boiler warms the water, a small pump pushes it round, and a manifold shares it out to loops of pipe under each part of the floor, or to radiators on the walls. Around them sit a few small parts that keep the loop safe and steady: an expansion vessel, a safety valve, a pressure gauge, and sensors and actuators that open and close loops when the thermostat asks. Fewer parts, sized properly, works better than more.
If you’ve ever lifted the bonnet of a car, you’ve seen one of these systems already. A pump pushes coolant from the engine to the radiator at the front, a plastic bottle takes up the coolant as it warms and expands, and the radiator cap lets pressure off if it climbs too high. Hydronic heating is the same idea run for comfort: the heat source warms the water, and the floor or the radiators give that heat to your rooms. Here’s each part in turn, and what it’s for.
The heat source: a heat pump or a gas boiler
Most systems we build now run on an air-to-water heat pump, a box outside that collects heat from the outdoor air and moves it into water, giving about three to four units of heat for every unit of electricity. A gas boiler burns gas to heat the water instead. We install both. On running cost a heat pump usually comes out ahead in Perth, so it’s what we suggest first, but if you’d rather have gas we’ll design and install a gas boiler. More on the machines on our heat pumps page.
The water doesn’t need to be hot. A floor runs on about 25 to 40 degrees, cooler than your hot tap, and radiators on a heat pump on about 45 to 55. That matters, because Phil’s rule is that it’s much easier for a heat pump to make 30 or 35 degree water than 50 or 55, and the cooler it runs the less it costs. Some systems add a storage tank beside the heat pump. Phil calls it a thermal battery: it holds warm water, so the heat pump can make heat while there’s spare solar power and rest when there isn’t.
The pump and the pipes: why there are so many loops
A small circulating pump keeps the water moving. People sometimes ask why we don’t run one long pipe through the whole floor. Phil’s answer is that the longer the pipe, the bigger the pump has to be to push water through it, and by the far end the water has given away most of its warmth, so the last room gets lukewarm water. Splitting the floor into loops of no more than about 100 metres each means a small pump does the job and every part of the floor gets water at much the same temperature.
The pump has to be sized to the system. Too small and some rooms stay patchy; too big and it burns electricity all winter for nothing. The pipe itself is the long-lived part, rated for 50 years under its standard’s test conditions, and the joins are where any weakness lies, which is why we choose a pipe maker on the quality of its joints. More in which pipe is used for floor heating.
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The manifold, flow meters and actuators
The manifold is the box where every pipe loop starts and ends, usually in a cupboard near the middle of the house. Each loop usually has a small flow meter on it, a little gauge that shows how much water is going through, so each loop can be set to what its room needs. Getting those settings right is called balancing, and it’s the difference between a floor that’s evenly warm and one with a cold bedroom at the far end.
Actuators are small electric heads that sit on the manifold’s valves. When a thermostat calls for heat, they open the loops for that zone; when it’s satisfied, they close them. Phil’s advice is to keep the zones few and large, by how the house is lived in, rather than putting an actuator and thermostat on every room. A heavy floor takes hours to warm and doesn’t swing room by room, so the extra hardware adds cost without adding comfort.
A spare room can simply be turned off at the manifold, like a tap, and turned back on when someone visits. As Phil puts it: “the simpler the system, the better, right? The less things that can go wrong.”
The small parts that keep it safe and steady
Water grows a little as it warms, and in a sealed loop it needs somewhere to go. That’s the expansion vessel, a small tank with a cushion of air inside that squeezes as the water expands and pushes back as it cools, the job the plastic bottle does in your car. Beside it there’s a safety valve, which lets a little water out if the pressure ever climbs too high, and a pressure gauge that shows the loop is holding steady. A slow drop in pressure is one of the things a service check about every two years is there to catch.
Where the heat source or a storage tank runs hotter than the floor needs, a mixing valve blends the water down to the right temperature before it reaches the pipes. It’s one of the reasons an ordinary hot water system can’t stand in for a proper heat source: it runs at 50 to 65 degrees and has none of these parts built in.
The sensors and controls
The thermostat on the wall is a sensor first. A good one feels both the air and the radiant warmth in the room, from a spot where people spend time, rather than measuring the floor. Better systems add an outdoor sensor too, so the water temperature follows the weather: a little warmer on a cold morning, cooler on a mild one. We go through the choices in how to control hydronic heating.
What matters most is that the controls are designed as one system. Phil has seen plenty of off-the-shelf controllers stuck side by side with no real connection between them, each switching on and hoping the others follow. His broader point applies to every part on this page: you can buy the best equipment on the market and still not have a working system, because the whole thing is only as good as its weakest link.
Other questions people ask us
Do any of these parts need regular maintenance?
Very little. A check of the heat pump or boiler about every two years keeps it at its best and catches a pressure drop early. The pipes in the floor need nothing. More in what maintenance hydronic heating needs.
Can my existing hot water system heat the floor?
In theory, but not well. A hot water unit runs much hotter than a floor needs and lacks the expansion vessel and valves a heating loop requires. Heating parts also aren’t approved to touch your drinking water, and the unit’s warranty would likely be void.
Do I need a thermostat and actuator in every room?
No, and we usually advise against it. Most homes run best on a few zones set by how the house is lived in, such as living areas, bedrooms and bathrooms, with any spare room turned off at the manifold.
Where do all the parts go?
The heat pump goes outside with room to breathe, and the manifold in a cupboard central to the loops it serves. The pump, vessel and valves usually sit together near the heat pump or the manifold. Our article on how much room the plant needs has the detail.
Where to go from here
If you’re building or renovating in Perth or the south-west, the 5-minute quote gives you your options in writing within 3 working days, each with its price and a modelled running cost, and you don’t need plans to start. If you’d like the whole picture before you choose anything, floor heating, explained shows how the parts come together in a real house.