Is a heating thermostat a simple on/off?

Or does it modulate the boiler so as it get closer to temp the output of boiler is varied?

I think it depends how much you spend.

It’s on-off.

When it reaches the set temp it turns off.

Probably complicated by thermostatic valves too, but it’s a simple switch.

So a boiler is either fully on or off?

So a boiler is either fully on or off?

If it is controlled by a thermostat, which is simply a switch that turns on or off when a set temp is reached, then yes.

Depends.
Basic spring loaded stat with no neutral supply is on or off.
Add a live and neutral to the stat, as well as the switch contacts, and it can start predicting temperature slightly.
Electronic ones are different as they can turn the boiler on before it gets to the ‘turn on’ temperature, and turn them off before they reach the required temperature.

Tell us what you have, and it may be clearer.

Most boilers will have a dial to adjust the flame size this adjusts how quickly it will heat the water up. This is usually totally independent to the thermostat control. The thermostat controls when to turn the boiler on or off depending on the temperature of the room. The thermostat does not modulate the flame size.

I’ve wondered if you can get smart boilers that measure the hysteresis of your room and modulate the flame accordingly?

Also regarding thermostatic valves. You should not put thermostatic valves on the radiators in the same room as the thermostat. The two conflict each other.

Boilers are not just on or off though they cycles to the required temp and then switch off (even if thermostat still on). Our boiler also doesn’t work as hard if the temp outside is warmer.

A stat is on or off. You can get fancy stats that “set back” ie lower the set temperature by a number of degrees in certain circumstances, or predict the heating curve, but they don’t cause the boiler to vary the flame directly. Any such logic (often found in heatpumps where there is often an outside sensor) is in the boiler/heatpump itself.

Rich.

A normal room stat is a switch with a differential to stop ‘hunting’ (switching on and off repeatedly).
A modern boiler will have a modulating burner, varying output to match demand, if it’s a system boiler it will also have a modulating pump as well.

If you spend some folding money you can get optimizing controls that factor in outside temps, however,
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It’s all marketing bollocks…..ffs put on a sweater and save some money.

I’ve wondered if you can get smart boilers that measure the hysteresis of your room and modulate the flame accordingly?

You can. Most electronic stats have something inbuilt called TPI which learns how best to use the heat from the boiler to heat the room.

Most boilers will have a dial to adjust the flame size this adjusts how quickly it will heat the water up. This is usually totally independent to the thermostat control.

No they don’t. Most boilers have a dial that adjusts the temperature the heating circuit goes to, usually max 80deg. Some also have a dial to set the max temp for DHW as well, usually max 65deg. The flame is controlled by the PCB modulating the gas & air flow to the burner. A decent boiler will modulate at around 7:1 which means that if you have a 28kw boiler it can modulate down to 4kw so it isn’t burning the amount of gas required to generate 28kw.

Also regarding thermostatic valves. You should not put thermostatic valves on the radiators in the same room as the thermostat. The two conflict each other.

Not sure on that one either. Usually the only room without a TRV is the bathroom. Most room stats are put in a hallway. Yes the room stat will turn the boiler off when the room gets to temp but as the TRV is a passive control it can’t do anything to make the room hotter until the stat clicks back in to fire the boiler. If you turn the TRV right down then yes the stat will make the boiler constantly run but to be fair all it takes is a bit of common sense to make it work properly and if any rooms get hotter than you want just turn the TRV down a notch or two.

Most boilers have a dial that adjusts the temperature the heating circuit goes to, usually max 80deg … The flame is controlled by the PCB modulating the gas & air flow to the burner

Ah that’s good to know the flame is being modulated. My boiler simply has a flame symbol and dial from 1 to 6. I know this was controlling the water temperature but the manual did not mention anything about modulating the flame when getting the water up to temperature.

Also regarding the TRV, that’s the advice I got from a professional so I assumed it was true and his logic and reasoning seemed good. Yes they won’t cause the room to get hotter but will undermine the room’s thermostat and fool it into running the boiler longer.

My boiler is either 100% on or off. You can set a dial which adjusts the temp at which the boiler switches off based on the returning water temp form the HW/CH, this runs in parallel with any thermostat you have on the HW tank / in a room. The CH and HW thermostats are just on/off.

A decent boiler will modulate at around 7:1 which means that if you have a 28kw boiler it can modulate down to 4kw so it isn’t burning the amount of gas required to generate 28kw.

Are they still as efficient (with all the condensing jazz that modern boilers have) when modulated down that far, compared to full blast?

I remember reading that modulated was not as efficient. But I might be remembering wrong.