Smart thermostat causing boiler to short cycle or something else?

[u0362565]

Hi all,

When I use my hive thermostat in the way I think your supposed to. I.e. set it to a desired temperature then don't touch it. The combi boiler seems to cycle on/off in short succession as can be seen in the attached image of the heating history. Does this indicate what's called short cycling? In a way it makes sense to me that the boiler is doing this if what is happening is:
-temp drops below set point
-thermostat triggers boiler
-room temp is raised beyond set point in short period of time
-thermostat triggers boiler to turn off
-temp falls
-repeat

But I'm suspicious of how frequently this is happening and how quickly the temperature is apparently changing.

Is this how it's supposed to be?
Thanks for any advice

 

[ShaunCorbs]

What do you have the temp set to 15 ?

 

[Chuck]

It's possible that the thermostat is too close to an emitter (radiator).

If that is not the cause, and if by 'in short succession' you mean every ten minutes or so, then the system is probably working as intended; boilers have had built-in 'short cycling' protection for years / decades and it doesn't seem to bother them in practice.

Boiler cycling is noticeable in the spring and autumn when very little energy is needed to maintain the temperature at the set point because the external temperature is, on average, only slightly below the set point. If the power required is below the minimum that the boiler can supply continuously (typically 5–7kW) it will switch on and off to achieve a lower average.

It is possible to improve, i.e. reduce, the cycling behaviour, e.g. by installing a buffer tank or a thermal store, but these are quite large and fairly expensive, which is why they are not usually installed if a gas boiler is the only heat source.

 

[u0362565]

What do you have the temp set to 15 ?

I can't remember now but yes looks like 15C

 

[u0362565]

It's possible that the thermostat is too close to an emitter (radiator).

If that is not the cause, and if by 'in short succession' you mean every ten minutes or so, then the system is probably working as intended; boilers have had built-in 'short cycling' protection for years / decades and it doesn't seem to bother them in practice.

Boiler cycling is noticeable in the spring and autumn when very little energy is needed to maintain the temperature at the set point because the external temperature is, on average, only slightly below the set point. If the power required is below the minimum that the boiler can supply continuously (typically 5–7kW) it will switch on and off to achieve a lower average.

It is possible to improve, i.e. reduce, the cycling behaviour, e.g. by installing a buffer tank or a thermal store, but these are quite large and fairly expensive, which is why they are not usually installed if a gas boiler is the only heat source.

Thanks Chuck, it's not near a radiator. So I think you must be right. As it gets colder then this behaviour should reduce our at least the frequency of switching as boiler will likely have to stay on longer when the thermostat drops below the set point..

Thinking out loud, would it not make more sense to be able to set the thermostat to a range rather than a single value e.g. 16-18C?That way you'd never get this effect. When triggered at 16C you'd know the boiler would stay on until 18C so you'd get fewer on/off cycles. This might be better for the boiler too. Although you said it doesn't harm the boiler to come on/off frequently. I guess it does this for HW demand but perhaps different components involved..

 

[Basher]

Thinking out loud, would it not make more sense to be able to set the thermostat to a range rather than a single value e.g. 16-18C?That way you'd never get this effect. When triggered at 16C you'd know the boiler would stay on until 18C so you'd get fewer on/off cycles. This might be better for the boiler too. Although you said it doesn't harm the boiler to come on/off frequently. I guess it does this for HW demand but perhaps different components involved..

You may know that difference between the cut-in and cut-out temperatures of a sensor is called its hysteresis.
With some 'smart' thermostats you can set those two temperatures seperately. I have heatmiser thermostats on our ufh system, and you can set the cut-in and cut-out temperatures separately to what you want.
I have read that the Hive has no hysteresis at all, and you can't change that! If that is true it would surely contribute to the effect you are describing!

I have seen a post on another forum of a member frustrated with this exact issue with his Hive smart stat: View: https://www.reddit.com/r/HiveHeating/comments/r6dnmr/hive_thermostat_has_no_hysterisis_at_all/

 

[u0362565]

I see yeah we have air conditioning units in work and they can set a range which is what made me think of it. I have to say the Hive thermostat isn't great, I wouldn't buy one again. It can turn the heating on and set a schedule that's about it.

 

[Basher]

You could always change the Hive for something else, though quite what I'm not sure
You might find this of interest (post #10 onwards)

Are there any thermostats with a good range of user adjustable hysteresis?

Hi Sorry if the title is a bit vague. I'm struggling to think of the best terms to describe or explain this, and as such I'm not finding out much when googling either.... What I want to know is.... If a room thermostat is set to say 16c, presumably it will heat to above and below this...

www.diynot.com

 

[Chuck]

Thanks Chuck, it's not near a radiator. So I think you must be right. As it gets colder then this behaviour should reduce our at least the frequency of switching as boiler will likely have to stay on longer when the thermostat drops below the set point..

Thinking out loud, would it not make more sense to be able to set the thermostat to a range rather than a single value e.g. 16-18C?That way you'd never get this effect. When triggered at 16C you'd know the boiler would stay on until 18C so you'd get fewer on/off cycles. This might be better for the boiler too. Although you said it doesn't harm the boiler to come on/off frequently. I guess it does this for HW demand but perhaps different components involved..

A few decades ago, on-off control with a wide hysteresis band, which is what you are describing, was how most domestic thermostats worked. The problem was that the room temperature would ramp up noticeably during the period the heating was on and then ramp down during the period it was off. The result was that occupants tended to set the average temperature higher than necessary in order to avoid feeling uncomfortable, which increased fuel consumption by ca 10%. The problem was aggravated when 'radiators' were used as emitters because the radiant heat from them, which is a major component of perceived 'comfort', would swing from high to low quite rapidly as the boiler came on and off.

Modern thermostats control the temperature using strategies based on so-called 'PI' or 'PID' algorithms that give much tighter control of temperature and, by using a cycle time of 10 minutes, avoid the swing from too-hot to too-cold associated with on-off control. This allows occupants to keep the average temperature lower and still feel comfortable all the time.

If you really don't like the 10-minute cycle time, most smart thermostats have an 'installer setting' that can be changed from 'gas boiler' to 'oil boiler'. This increases the cycle time to 20-minutes. Alternatively, replace the boiler with one with a lower minimum output than your current model.