Ecobee Short Cycling boiler.

[NVR]

Recently purchased an ecobee smart thermostat and noticed my boiler was short cycling, staying on for 10 minutes or less every hour or so.
Investigating further, I noticed the heat is only turned on for .1 or 1/10 of a degree before it the thermostat sends an off-command and shuts down the heat.
By factory default the ecobee has the Heat Differential Temperature threshold setting set to .5 of a degree.
This means if the ecobee is set to 65 the heat will turn on at 64.5, which is a great feature I think, however it will shut off the heat at 64.6, only after .1 degree.
For factory defaults this doesn’t sound good for the boiler or any HVAC system and I wish we can also compensate for when we want it off since it’s not turning off at the set temp.
Currently not sure if this is by design or a bug, if someone can share a portion of their Heaton block.
Each cell is a 5 minute block. In the second pic, I have increased the Heat Differential Temperature threshold to a full degree from the .5 factory default and now the heat stays on for .6 of a degree. This is much better than the factory default but curious to see what others have.

In both cases the heat will turn off .5 of a degree from the desired temp but will run for .1 of a degree with Heat Differential Temperature set to .5, and will run for .5 of a degree if the Heat Differential Temperature is set to 1. The issue again is the factory defaults will only let the system run for .1 of a degree before turning off.

 

[NVR]

Heat Differential Temperature .5 degree (Factory Default)
Desired temp set at 65 degrees
Heat on at 64.5
Heat off at 64.6
Run time in degrees .1 or 1/10th of a degree

Heat Differential Temperature 1 degree (.5 above factory default)
Desired temp set at 65 degrees
Heat on at 64.0
Heat off at 64.6
Runtime in degrees .6 or 3/5th of a degree


Wonder if they intended for a full 1 degree run for factory settings instead of the .1 and this was a coding error from the developers. Firmware 4.2.0.392

 

[J Sigmo]

Sounds like a bug, for sure.

I dont see any reason not to set the hysteresis to be whatever it takes to get the cycle to do what you want it to do. 0.1 degree is far too little for a typical "bang-bang" system.

Ideally, for cooling, you can set a temperature hysteresis as well as a minimum off-time to assure that you don't short-cycle a compressor and potentially wreck the valves.

For heating systems that are all-or-nothing (not some sort of proportional control) it is again nice to be able to set not only a temperature hysteresis, but also a minimum on-time and maybe a minimum off-time as well. That gives you the most control. But just being able to set a large enough hysteresis is usually good enough.

I always found the old mercury-switch thermostats that had huge inherent hysteresis, and then an adjustable-power "heat anticipator" to be an amazingly clever, reliable, and flexible design.

But with modern thermostats, you can do even more as long as the software gives you the adjustments you need, and it does what it is supposed to do.

To get good efficiency and long-life from a typical furnace, you really need to keep it from cycling too often. It might be more comfortable to have low hysteresis, but it's really hard on things.

 

[tangent]

I'd agree ecobee likes to short cycle things. The AC compressor minimum run time to protect equipment doesn't seem to work right either.

 

[NVR]

Ive been in contact with ecobee trying to figure out what was the cause of the short cycling and finally I asked why is it turning off after only an increase of .1 degree in their default settings and they went silent. Prior to bringing this up they were communicating daily trying to troubleshoot.

Then theres these scattered posts online of people complaining of short cycling and not knowing why after a firmware upgrade, also given wrong solutions as they're not aware of the root cause of the issue.


For now the only solution is to change the Heat Differential Temperature from .5 to either 1 or 1.5 and speaking heat wise this will run your heat for .5 or 1 degree respectively.The problem here would be the temp going down 1.5 degrees from your desired setting. So if trying to achieve a comfort level of 65 degrees, it will have to go down to 63.5 before turning on and turning off at 64.5 but you get a full degree of run time before turning off. These are all work arounds to a bigger problem they need to properly address.

Minimum runtime can also be set to 5 or 10 minutes.

 

[J Sigmo]

Short-cycling people's compressors might create quite a bit of expensive liability. That may explain their sudden silence.