evohome turn on/off every 5mins when in away mode - let's debug together my system

[mazzy89]

I have a weird behavior of the DHW. maybe it's some feature I do not know.



I was away from home and my system was set to Away mode. As far as I know the DWH once in AWAY mode is off.

Then the water temp went down as you can see on the graph during the day (date 11/12). However, while the system was still off because I was still away for the day, after midnight the temperature slowly increased bringing my water to around 50 Celsius.

Now I do not get how that is possible while the entire system was scheduled to be off and for the time being, it remained off as far as I konw.

Beside that the way to heat the water was very weird. Usually the hot water is turned on and it is heated in less than 35 minutes. Has the DHW some logic during away mode operation that I'm not aware of?



Interesting thing is that once it got to 50 C (which is not my setpoint. my water setpoint is 55C) it stopped.

I've read here and there that there is a mechanism built in the DHW called freeze protection but i'm not sure how it is related here.

Another thing very weird I've noticed somehow connected w/ what I'm describing here is that the BDR91 which control the heating system (not the hot water) was turning on and off every 5 mins as you can see here. green ON and red OFF



Now the setpoint of all the zones during the AWAY mode was set to 18.5C.

Why turning on and off so rapidly? interval every 5mins

[mazzy89]

Here is what I think about what is happening with the hot water. just assumptions though. need you guys to help me to validate.

the 3-way valve does not close completely and hence when the heating is on some portion of the water is also heated. slowly this is why the water temp increase slowly.

not sure why the controller goes crazy and turn on and off itself every exactly 5mins

[mazzy89]

I have tested my system during normal operation (mode auto - I'm at home) and I've noticed that when the DWH is off and the heating is ON then the water temp drops and the water of the heating system does not go flow into the DWH system so I think that my 3-way valve is fine.

I think the reason of this must be looked in why the controller decided to turn on/off the system every 5 minutes.

Considering also that my mim on time is 4min.

[DBMandrake]

I have a weird behavior of the DHW. maybe it's some feature I do not know.



I was away from home and my system was set to Away mode. As far as I know the DWH once in AWAY mode is off.

Correct, Away mode disables hot water completely.

Then the water temp went down as you can see on the graph during the day (date 11/12). However, while the system was still off because I was still away for the day, after midnight the temperature slowly increased bringing my water to around 50 Celsius.

Now I do not get how that is possible while the entire system was scheduled to be off and for the time being, it remained off as far as I konw.

You say your system was scheduled to be "off" but later you say your Away mode is configured to 18.5C.

Clearly that is not "off". It's important to understand that neither "Away" or "Heating Off" modes are "OFF". They are just different set points. By default Heating Off is 5C and Away mode is 15C but you can customise it to any value you like.

When you set your system to Away mode you are just doing the same as turning your hot water off and setting all zones to 18.5C in your case. If any of your rooms falls down below about 20C (and definitely by about 18.5C) the boiler WILL come on and radiators WILL get hot. Your system is not "scheduled to be off" as you describe it. It is in an away mode "set back" where set points are reduced but it will come back on again if room temperatures fall low enough.

If your hot water BDR91 is staying off the whole time, the hot water schedule is staying off and your hot water temperature still goes up slowly when the heating came on (due to your high Away set point) there must be a leak in your zone valve - there's no other possibility, a hot water cylinder won't heat up unless there's hot water going through the indirect loop.

The fact that it goes up very slowly shows that either there is only a small leak, or that the boiler was operating at a low duty cycle at the time, which is quite likely.

A leak is very easy to test - schedule your hot water permanently off, turn the heating down and use some hot water to get the reported cylinder temperature down 10-15C then turn the heating up but leave the hot water scheduled to permanently off for a few hours. Also keep an eye on the BDR91 that controls the hot water.

If the relay stays off but the water still heats up you have a faulty zone valve or other plumbing issue.

If you see the hot water BDR91 coming on at all when you have hot water scheduled off you probably have a binding problem where the relay has been bound to more than one function such as being bound as both a hot water relay and a boiler relay, in which case it would come on when it shouldn't. This could result in the hot water being heated whenever the boiler is running or there is a central heating demand.

Beside that the way to heat the water was very weird. Usually the hot water is turned on and it is heated in less than 35 minutes. Has the DHW some logic during away mode operation that I'm not aware of?



Interesting thing is that once it got to 50 C (which is not my setpoint. my water setpoint is 55C) it stopped.

I've read here and there that there is a mechanism built in the DHW called freeze protection but i'm not sure how it is related here.

There is frost protection for hot water but it won't kick in until the temperature of the water in the cylinder gets down to 5C - clearly not the case here.

Another thing very weird I've noticed somehow connected w/ what I'm describing here is that the BDR91 which control the heating system (not the hot water) was turning on and off every 5 mins as you can see here. green ON and red OFF



Now the setpoint of all the zones during the AWAY mode was set to 18.5C.

Why turning on and off so rapidly? interval every 5mins

This is absolutely normal behaviour for a TPI system.

If there is only a small heat demand it will turn on and off in 10 minute cycles. In your example 5 minutes on, 5 minutes off. This will be trying to maintain your 18.5C "Away" temperature.

By the way that horizontal bar looks like a home assistant graph. I'm new to Home Assistant so I'm curious what type of sensor you're using to monitor the BDR91 output in Home Assistant ?

[mazzy89]

Thanks DBMandrake to jump w/ me in this debugging session.

so just to be precise. my system was in Away mode. the setpoint of all the zones set to 18.5C and my DWH off.

> If the relay stays off but the water still heats up you have a faulty zone valve or other plumbing issue.


I thought as well about the 3-way valve issue but in normal condition, everything works fine. I've in fact run the test w/ me at home, hot water is turned completely off and heating on, and the hot water is never heated so the 3-way valve does the correct job.

this is for instance the graph of the last night



over the night I turn off the DHW and leave the heating system in auto mode. of course during the night my radiators are warmed up so the heating works normally and the water as you can see stay down. so all good. if there was leaking in the valve I could see it again.

Hard to call the plumbers in this condition when the issue is not easy and perfectly reproducible.


> This is absolutely normal behaviour for a TPI system.

about the turning ON and OFF every 5mins. does this not destroy the heating system and all the engine faster? can't be somehow regulated?

> By the way that horizontal bar looks like a home assistant graph. I'm new to Home Assistant so I'm curious what type of sensor you're using to monitor the BDR91 output in Home Assistant ?

yes it is home assistant and I use this amazing project https://github.com/zxdavb/evohome_cc

[mazzy89]

What does it mean when you say

> that the boiler was operating at a low duty cycle at the time, which is quite likely.

how does this affect water heating and heating? could you elaborate?