A thermostat looks like a small, simple thing, but the type fitted and where it sits on the wall makes a real difference to comfort and running cost. With the heating season starting, it's a sensible time to look at whether an old mechanical dial is still doing its job, or whether an upgrade would pay for itself in fewer draughty rooms and less fiddling with the dial.
This guide compares the main thermostat types on the market, the placement rules that matter regardless of which one you choose, wiring and boiler compatibility, and honest indicative costs for supply and fit.
Mechanical thermostats
The classic dial thermostat uses a bimetallic strip or similar mechanism to switch the heating on and off around a set temperature, typically with a few degrees of swing either side. They're inexpensive and reliable, but that swing means rooms can feel a little too warm then too cool in cycles, and there's no scheduling at all, so the heating is either fully on or fully off based on the dial alone.
Digital programmable thermostats
These add a proper weekly schedule, so heating comes on and off automatically at set times, and hold room temperature more tightly than a mechanical dial, usually within about half a degree. Most have a simple screen and buttons, and don't need a smartphone or home wifi to work, which suits anyone who wants scheduling without extra complexity.
Load compensating and weather compensating thermostats
A load compensating thermostat adjusts the boiler's flow temperature based on how close the room is to target temperature, so the boiler works harder when there's a big gap and eases off as the room approaches target, rather than blasting full output then switching off abruptly. Weather compensation goes further, using an outdoor sensor to adjust flow temperature based on outside conditions. Both can improve efficiency, particularly on modern condensing boilers, since running at a lower flow temperature for longer periods tends to condense more effectively.
Smart thermostats and OpenTherm
Smart thermostats connect to home wifi and a phone app, allowing remote control, geofencing, and often learning features that adjust schedules automatically. Many also support OpenTherm, a communication protocol that lets the thermostat and boiler talk continuously about exact flow temperature and modulation, rather than the boiler just switching fully on or off. Not every boiler supports OpenTherm, so it's worth checking compatibility before choosing a smart thermostat expecting that level of control.
TRVs and cylinder thermostats, not a replacement for a room stat
Thermostatic radiator valves control individual radiators room by room and work alongside a main thermostat rather than instead of it. A cylinder thermostat, fitted to a hot water cylinder on a system with one, controls when the cylinder stops heating once it reaches temperature, separate from space heating entirely. All three, room thermostat, TRVs and cylinder stat where fitted, work together rather than one replacing another.
Placement rules that matter whichever type you choose
- Never above a radiator, near a fireplace, or in direct sunlight, all of which give a false high reading and cause the heating to switch off too early
- Avoid draughty spots like near an exterior door or a window, which give a false low reading and can leave the heating running longer than needed
- Around 1.2 to 1.5 metres from the floor, at roughly average adult chest height, in a well-used room, commonly a hallway or living room
- Not in a room with a radiator that has its own TRV set very differently to the rest of the house, since that room won't represent the average temperature of the home
Wiring and compatibility
Older mechanical and basic digital thermostats are usually wired with a simple two-wire connection. Smart thermostats often need either a compatible wiring arrangement or a separate mains-powered receiver if there's no permanent live feed at the existing thermostat location, which is common in older houses. OpenTherm and load compensating thermostats need a boiler that specifically supports that protocol, so it's always worth checking the boiler manual, or asking an engineer, before buying one expecting modulating control.
| Thermostat type | Typical supply-and-fit range |
|---|---|
| Mechanical dial thermostat | £80 to £150 |
| Digital programmable thermostat | £120 to £220 |
| Load or weather compensating thermostat | £180 to £320 |
| Smart thermostat (with app and scheduling) | £220 to £400 |
| Additional TRVs, per valve | £35 to £70 |
Frequently asked questions
- Will a smart thermostat definitely cut my bills?
- It depends heavily on how the heating was being used before. If schedules weren't being followed closely, or the boiler supports OpenTherm and can modulate properly, the savings can be worthwhile. If the old thermostat was already being used sensibly, the gain may be smaller.
- Can I fit a thermostat myself?
- Simple like-for-like swaps on a two-wire system are sometimes within reach for a confident DIYer, but anything involving mains wiring, a new receiver, or boiler compatibility checks is safer left to a qualified engineer, particularly where the boiler's controls need reconfiguring.
- Do I need both a room thermostat and TRVs?
- Yes, they do different jobs. The room thermostat governs when the whole system calls for heat, while TRVs fine-tune each individual radiator, so a house can have both a smart thermostat in the hallway and TRVs on every radiator.
- What if my boiler doesn't support OpenTherm?
- You can still fit a smart thermostat for scheduling and remote control, you'll simply lose the finer modulating communication between thermostat and boiler that OpenTherm allows, and the boiler will switch fully on and off as it always did.
Not sure which thermostat suits your system?
Tell us your boiler make and current setup and we'll recommend a genuinely suitable option, then fit it properly, across Kelso, the Borders and East Lothian.