Today we’re talking about those unassuming little twisty knob things you probably have on the side of your radiators. I’m guessing you don’t give them much thought, and if you’re like many of my clients you don’t really get on with them so you’ve turned them up to MAX and that’s that. Don’t worry – I get it…
OK, so how are they supposed to work? The principle is sound – they are self-regulating thermostatic valves (TRVs for short) that control the flow of water through the radiator according to the difference between the room temperature and your desired temperature. A standard TRV usually has a little piston moved by wax in a cylinder that expands when it heats and contracts when it cools. This expansion/contraction action on the piston will close/open the valve controlling the flow of water depending on the setting on the plastic dial. The numbers on the dial correspond to an approximate temperature, with * being a frost setting, 1 being around 16degC, 2 about 18/19degC, 3 corresponding to around 21degC and 4 being roughly 23/24 degC.
The idea is that you set a temperature appropriate for your room – we would suggest 1 for spare rooms, 2 for bedrooms and kitchens, 2-3 for living rooms and offices and 4 for bathrooms. Once the setpoint is approached, the wax progressively expands to close off the valve controlling the flow of water through the radiator, thereby regulating the temperature of the room.
As is so often the case, reality gets in the way of a finely-honed plan. The first issue is with the valves themselves, which tend to get a bit “sticky”. We would recommend occasionally unscrewing the head from the valve and applying a touch of lube around the brass pin that moves up and down. It is also worth winding the dial all the way up and down a few times every so often to keep everything moving smoothly. Sometimes, however, the valves just pack up, in which case replacement is straightforward.
Another problem is the position of the valve relative to the radiator. At first glance it seems obvious to site the valve on the pipework connecting the radiator to the central heating circuit, however this causes problems with the valve determining how warm the room is, because – duh! – it’s right next to a hot radiator. The result is that the valve thinks the room’s up to temperature before it actually is, prematurely shutting the radiator off. A short while later when the radiators has cooled down it realises the error of its ways and opens the valve back up again, just for the cycle to start again. Eventually after a couple of hours of indecision the radiator settles into a state of equilibrium, however by this time it’s likely that everyone has gone to work/school and switched the heating off anyway. It’s this behaviour that often results in homeowners getting frustrated and turning the valves up beyond the point that they are correctly regulating the temperature.
In order to deal with the above issue, at least one manufacturer developed a separate temperature sensor on the end of a lead, however this is an ungainly solution and liable to damage. A more sensible improvement is to fit a valve that points the head horizontally rather than sideways – this provides an extra degree of separation between the valve head and radiator, allowing for more accurate sensing of the room temperature. Modern smart TRVs also often have a sensor at either end which work best in this orientation, allowing for a more accurate extrapolation of the temperature.
Another action that can be taken is to reduce the temperature of your boiler or heat pump (this shouldn’t be an issue for low-temperature heat pumps). Gas and oil boilers can usually be turned down to around 40degC, although how low you can go will depend on a number of factors such as how cold it is outside and your patterns of heating. Low radiator temperatures also work best if you run the heating system for a longer period of time to allow the heat to build within the structure of the house – this then acts a bit like a storage heater. Moreover, lower heating ‘flow temperatures’ as they are known are more efficient for boilers and particularly for heat pumps, with savings of up to 15% in the former case and perhaps 30%-50% for heat pumps, depending on the starting point.
Perhaps the ideal solution is to separate the thermostats from the radiators altogether – this would be a significant and disruptive change in most cases, involving replumbing the system into an arrangement with manifolds and zone valves (much like for an underfloor heating system) and fitting wall-mounted electronic thermostats away from the radiators. This is something to consider in a new-build or major refurb project, but probably a bit too much work for those looking for quick wins; if you are looking at this amount of disruption then it may be worth considering underfloor heating anyway, at least on the ground floor.
In summary, some simple changes can significantly improve the control and efficiency of your heating system, however the best option requires some commitment to change!