Can anyone tell me what the acceptable hysteresis ie the switch off/on points of a room thermostat is. My wife always complains it's getting cold in that period between the thermostat switching off the heating and coming back on again to meet demand.?
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Mastermixlee
Gas Engineer
Depends on what type of thermostat you have installed. There's multiple types of stats out there but can put them into 2 categories, mechanical and digital.
Mechanical types vary but most common ones are using bimetallic strips which expand and contract at different rates based on temperature, causing metal to bend slightly in either direction accordingly. These types tend to take longer to switch back on again after temperature drop than the digital types which rely on a thermistor type sensor to monitor air temp.
Mechanical thermostats can take longer to switch on as they age.
Id recommended switching to a digital thermostat firstly for increased accuracy.
Though there could be other factors at play. It could be heat loss/heat input imbalance for the radiators to room coupled with boiler anti cycle delays.
Digital stat recommended as first stage as cheapest and easiest thing to change out.
Mechanical types vary but most common ones are using bimetallic strips which expand and contract at different rates based on temperature, causing metal to bend slightly in either direction accordingly. These types tend to take longer to switch back on again after temperature drop than the digital types which rely on a thermistor type sensor to monitor air temp.
Mechanical thermostats can take longer to switch on as they age.
Id recommended switching to a digital thermostat firstly for increased accuracy.
Though there could be other factors at play. It could be heat loss/heat input imbalance for the radiators to room coupled with boiler anti cycle delays.
Digital stat recommended as first stage as cheapest and easiest thing to change out.
Older mechanical thermostat could have a 4 or 5 degree hysteresis. Modern digital stats, which use an NTC thermistor to measure the temperature, usually have a half degree hysteresis.
From what you say, you must have a older stat. The Honeywell DT90E would be a suitable replacement.
From what you say, you must have a older stat. The Honeywell DT90E would be a suitable replacement.
M
Murdoch
You may have the boiler CH water temperature, i.e. the temperature of the water being sent to the radiators, set a bit high for the current relatively warm weather. Try reducing it a bit. The theory is that doing this will make the emitters (radiators) come on at a lower temperature for longer and reduce the fluctuating radiant component heat from the emitters, which makes a big difference to how people perceive the temperature.
Roughly speaking, the most comfortable heating systems are the ones that run continuously with emitters balanced to compensate for the building losses. So-called 'weather compensation' achieves this, when it's correctly set up.
If this doesn't work, see if @Nicola84 has another friend 🙂
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I'm always amazed in cold weather how quickly any open window upstairs ,fills a downstairs hall area with cold air !
Has the problem just started, or have you recently moved in, or what? Many roomstats have an accelerator heater, energised when the stat calls. This switches it off a bit sooner than if the heat had to arrive from the rads, and improves sensitivity. The heater needs a neutral wire, so 4 total, live (from the programmer or main switch), switched live, neutral and earth. Possibly it's never been wired right.
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