Evohome opentherm way overshooting target temperatures

[Andyheroes]

Good day,

Im having a hard time with my evohome installation. I have a intergas HRE 36/30 boiler and i have a evohome system with 8 zones. Im having an issue where my system is far overshooting the target temp.

WhatsApp Image 2021-10-11 at 11.34.38.jpeg (52.59 KiB) Viewed 782 times

. As you can see on this graph the target temperature was set to 19.5 degrees and it shot up to almost 24.

In another room in the house

overshoot 2.jpg (108.94 KiB) Viewed 782 times

the same situation can be seen. This is also when only one zone is open. it seems it just keeps overshooting. Ive had this system about 3 years but never really paid a huge amount of attention to this but ive recently become a father and my wife is getting pissed off that she dresses the baby in clothing for a specific temperature but with overshoots like this she is rather unhappy.

Any advise as to where i can improve things? I thought the theory of opentherm was to deliver specific temperature water to a radiator to maintain a temperature as opposed to crazy hot water to shoot the temp up... or is this expected behaviour?

[BrummyGit]

Your post is quite interesting as I had this in the last week as well. I also use Opentherm on my boiler.

I am in the process of requesting an upgrade to the latest firmware as I appear to have been stuck on 02.00.17.03 but I wondered if mine was due to one of 2 issues:

• I had a radiator leak and therefore manually closed the valves on both ends. The HR92 was removed and left on a shelf with the heating for that zone turned all the way down to off. I wonder if the removed HR92 was causing delays due to it not being responsive in some way. The radiator has been replaced and the HR92 is back in operation.
• The other thing I noticed when researching this was that the HR92s need to be calibrated and I have recently needed to replace a number of batteries. I could not be sure that I had fully opened the gear wheel on each one before it went into "CYCL" mode after being restarted, so I have removed each valve, fully opened the black wheel, and refitted the HR92 body to calibrate.

It's a bit early to tell so far if this has helped.

[Richard]

Hi,

Apart from the graphs, there isn't really any detailed context behind the situation.

So firstly, why is the boiler so big? The HRE 36/30 is only capable of 7.6kW low end modulation, so won't really do you any favours in an 8 zone (8 rad? 8kW load at -3 deg C?) house this time of year. OpenTherm will be doing its best, but probably working as on/off load compensation in this situation. However saying that, the zones should be throttled by the HR91/HR92 so suggests something else is going on.

As its an Intergas combi boiler, have you range rated it? What flow temperature for the central heating you have you set the boiler to? I would suggest setting this much lower, maybe 55 deg C. evohome will run for longer periods, but will give much much better heating efficiency and comfort performance than running at say 75 deg C!

Where are the radiators located verses temp sensors? Behind a sofa or bed won't give you very good sensor data. Are you using HR91/HR92's as the sensors or a wall mounted thermostat and the HR91/HR92 as the actuator?

If you are using the radiator mounted sensor, you are going to see a small temperature spike before the convection current of the radiator starts to pull the air through with a system like Home Assistant. It could be the the HR91/HR92 is not fitted or calibrated properly. Also what radiator valves do you have them fitted to?

If you want complete room temp accuracy, use an external sensor like a DTS92, T87RF or one of these (https://theevohomeshop.co.uk/honeywell- ... ostat.html) in the room and the HR91/HR92 as the actuator.

Some pictures of the affected rooms would help the situation, including locations of the radiators and evohome radiator controllers.

[MisterBoy]

If the OP is around, I would love to know how you are recording and graphing the temperatures in your rooms?

I've seen similar although I do not run OpenTherm... but regardless I thought EH would modulate flow to the radiators via the HR92 and keep adjusting on a fairly regular basis. I do use a separate thermostat located the opposite side of the room and to overshoot by 2C represents a lot of heat. If the radiator is on and it reaches the target temperature it will get warmer even if it immediately closes the valve, in practice it seems to not do so until the temperature is already high.

Isn't EvoHome supposed to learn how rooms behave so it will shut off early, etc?

I keep having to take my jumper off as my room gets too hot which is crazy with energy prices right now.

[Richard]

If the OP is around, I would love to know how you are recording and graphing the temperatures in your rooms?

I've seen similar although I do not run OpenTherm... but regardless I thought EH would modulate flow to the radiators via the HR92 and keep adjusting on a fairly regular basis. I do use a separate thermostat located the opposite side of the room and to overshoot by 2C represents a lot of heat. If the radiator is on and it reaches the target temperature it will get warmer even if it immediately closes the valve, in practice it seems to not do so until the temperature is already high.

Isn't EvoHome supposed to learn how rooms behave so it will shut off early, etc?

I keep having to take my jumper off as my room gets too hot which is crazy with energy prices right now.

He's using Home Assistant - https://www.home-assistant.io/

evohome does modulate, but its normally too late if the radiators or the boiler is oversized in the first instance. Ultimately, evohome cannot fix unbalanced and incorrectly designed heating systems.

[Debruce]

Hi OP did you ever sort this out?

I changed my Evohome system from BDR91 control to Opentherm on a Valiant 831 boiler by replacing the relay with the OT bridge and VR33 and now I am getting big overshoots with my system. I'm mapping using Home Assistant and I can share some of the graphs but I can see cycling and huge overshoots just like OP. I can see on the Evohome interface the temperature going 1.5 - 2.0 degrees over target which it was definitely NOT doing before with BDR91 boiler control. I can see the boiler switching on and off and water temps being varied by the OT bridge but the control of the room heating now seems terrible.

For information I changed to OT on 16th December so it's had circa 3 weeks so far and doesn't seem to be settling down through learning. Also the overshoots are on multiple rooms, which are all controlled using HR92 TRVs on every radiator except two towel rails.

I was wondering if this was a consequence of changing an established system from On/Off control to OT confusing the system or whether there is something else at play.

Either way I am definitely not getting the fine and stable control of room temperatures that OT is supposed to provide.

[Richard]

Have you checked the demand percentages in the SYSTEM SUMMARY menu in the installer settings? If you have a rouge HR92 calling for 100% demand, it would upset the others as the OpenTherm demand on the boiler would be 100% (max flow temp).

[Debruce]

I will keep my eye on the demand but generally I haven't noticed any one HR92 having excessive demand values. Mostly at this point in the day the demand is very low. I checked just now and everything is showing 0% and boiler is off however I will keep my eye on it throughout the day and see what figures I get.

[Debruce]

Been monitoring this today and it's definitely not a rogue valve... at least not one that's showing high demand on the SYSTEM SUMMARY page.

I see a lot of oscillation in zones backwards and forwards across the setpoint temp.