Heating demand when there shouldn't be

[benwillcox]

Hi all, I have a bit of a strange problem with my Evohome system that I'm hoping someone can shed light on, and has only started to happen recently.

For background I've had Evohome installed at this house for around 3 years, there is an oil boiler for heat and DHW, a megaflow stored hot water cylinder, one UFH heating loop with pump and valve controlled by a BDR91, another BDR91 operating the DHW valve, and a third BDR91 controlling the heating zone valve and pump.
There is no boiler control relay, the boiler runs using the switched output from the zone valves.
All rooms are zoned using HR92, and quite a few but not all the rooms have a T87 thermostat.

I've built a monitoring system a couple of months ago using an ESP32 running ESPhome, and monitoring things using Home Assistant. My problem doesn't seem to have been occurring when I originally installed this monitoring system, but it only records data going back the last 30 days so I can't verify whether that is true but I'm 99% sure I would have spotted this previously.

So, on to the issue. I've noticed that quite often in the early morning, or other times of no heating demand, that the boiler is firing for a very short time, and then switching off again, repeatedly.

For an example look at this graph:



You can see that just after 5am there are 3 very short periods where the call for heat to the boiler is activated. This is only for around 40 seconds each time. I'm not convinced that the burner even fires at this time - I'm monitoring the live feed to the burner/fan unit, but there is a 20 seconds lead-in before the actual burner fires, and a 30 second lead-out after the burner finishes so can't tell from this precisely when the actual burner is firing, but the zone valve opens, the pump runs, and the boiler fan starts. But clearly this short time has hardly any effect on generating heat.

Looking at graph 2, you can see that no zones were calling for heat at this time, except potentially the third time. These statuses are calculated using the Evohome data comparing heat demand in that zone with it's current temperature, so won't always be 100% accurate depending on the Eco-saver function of Evohome (I have smart Start optimisation switched off, or whatever that's called)
This is from last night.



I had noticed that previously this was happening a lot more frequently than in this example. What I did was remove the Evohome touch control from it's charging mount so that I could monitor the system status that shows zone demand, and wait for HA to alert me that the boiler was firing.
When it did fire next, when there was no heating demand, the system status showed 0% demand on each zone, but the BDR for the heating zone came on, triggering the boiler, then quickly went off.

Now the strange thing is that the Evohome controller just rebooted itself, despite it showing 100% battery. It actually then did this again several minutes later.
After putting it back on it's charging base it's not rebooting again, I guess this could just be faulty batteries even though they show as fully charged, but in any case probably is nothing to do with the main issue.

If we look at the day previously before I did this, this is what we see,i.e. it was calling multiple times from about 1am. None of the zones are programmed to come on before 6am, and it's nowhere near cold enough for it to be a frost-stat activation:



The only thing I can think that has changed in the last month is that I've added a couple of T87 thermostats to a couple of zones that were previously just using the built in HR92 stats.

Any ideas why Evohome is calling for heat for these short periods all through the night, when there is no requirement for heat?

Thanks,
Ben

[benwillcox]

So to follow up my own post, I had a search through the forums here and found this thread: viewtopic.php?f=4&t=593 which describes the same issue, and links off to this support article: https://resideo.my.site.com/ensupport/s ... uage=en_US
So, I will have to check the heating demand in the status menu and see if I can track down whether it's due to any of those reasons, as I'm still getting demand for heat every 20 mins during the night, for only around 30 seconds each time.

However whilst reading other posts from The Evohome Shop it seems that my configuration might not be optimal. I don't have any of the options in the touchscreen related to max boiler starts per hour, which if I understand correctly is because I don't have a 'boiler control' relay.

My configuration is that I have a DHW 2-port valve and pump controlled by a BDR91 (boiler has separate ports for DHW circuit), a 1 zone UFH loop controlled by a second BDR91 controlling a zone valve and a separate pump, and a third BDR91 controlling the main heating pump and zone valve. Switched output of any zone valve triggers the heat demand to the boiler. All rads have HR92 fitted.

As The Evohome Shop have pointed out in other threads, as per the evohome manual there usually wouldn't be a zone valve for the heating circuit as all zones are controlled by the HR92. However I can't see how one would therefore control the heating circuit pump in this case? I can't have the pump wired in parallel with the boiler demand as it will start up when the UFH or DHW is calling for heat. Presumably the usual configuration would be to have a single pump serving the main heating circuit, UFH and DHW so this wouldn't be an issue, but I think in my case I have no option to have a zone valve so the system can operate this pump independently?
Would there be any advantage in adding a fourth BDR91 as a boiler control relay? I can't see how it would offer any advantage other than it would enable the options to adjust max starts per hour etc (which might be worthwhile?).

Thanks
Ben

[Richard]

Hi Ben,

If you have HR92's on all rads, you should be following the Page 46 diagram (https://thesmartthermostatshop.co.uk/pr ... xd523tng7x) which would be slightly modified when you add a single zone for UFH.

I am a little confused how this is operating properly currently, and without the option to change the minimum run time and cycle rate, the system will not be optimal at all.

As you have probably seen elsewhere, there is only one way to do the evohome system correctly and any systems running outside of this design cannot guarantee operational effectiveness.

If its not installed right, it's wrong unfortunately. As you can see when I start asking for pictures in most posts, the thread very quickly goes dead...

[benwillcox]

Thanks for getting back to me. I did refer back to that schematic, however it doesn't take into account my separate pumps for DHW, UFH and heating, so I was a bit unsure how to adapt it to operate those pumps individually without using zone valves and their microswitched outputs.

But after some more thought I've now added a 4th BDR91 as an 'appliance control relay' and rewired it so that only this relay is sending the call-for-heat signal to the boiler.
I removed the motorised valve head and wiring for the heating zone and manually opened the valve as advised elsewhere, and have retained the heating control BDR91 so that it activates the only the heating pump. This comes on whenever any of the HR92 are calling for heat.

This has given me the additional control options to set max boiler starts per hour and the minimum on time, which has stopped all the weird behaviour with the boiler firing at the early hours of the morning. It now comes on correctly as the first zone calls for heat in the morning, and fires no more than 3 times per hour based on the new settings. So it would seem that this is solved and working more effectively and hopefully efficiently.

I also plan to remove the motorised valve head from the UFH zone too as this seems to also be redundant, and will just use the UFH BDR91 to operate the UFH pump only, I didn't do this initially as wanted to observe the operation of the system with the above changes but now all seems well.

Thanks,
Ben

[Richard]

Thanks for getting back to me. I did refer back to that schematic, however it doesn't take into account my separate pumps for DHW, UFH and heating, so I was a bit unsure how to adapt it to operate those pumps individually without using zone valves and their microswitched outputs.

But after some more thought I've now added a 4th BDR91 as an 'appliance control relay' and rewired it so that only this relay is sending the call-for-heat signal to the boiler.
I removed the motorised valve head and wiring for the heating zone and manually opened the valve as advised elsewhere, and have retained the heating control BDR91 so that it activates the only the heating pump. This comes on whenever any of the HR92 are calling for heat.

This has given me the additional control options to set max boiler starts per hour and the minimum on time, which has stopped all the weird behaviour with the boiler firing at the early hours of the morning. It now comes on correctly as the first zone calls for heat in the morning, and fires no more than 3 times per hour based on the new settings. So it would seem that this is solved and working more effectively and hopefully efficiently.

I also plan to remove the motorised valve head from the UFH zone too as this seems to also be redundant, and will just use the UFH BDR91 to operate the UFH pump only, I didn't do this initially as wanted to observe the operation of the system with the above changes but now all seems well.

Thanks,
Ben

I'm afraid it is a case of adapting the system to work with evohome properly, not the other way around. Multiple pumps can cause issues, but not normally something that can't be solved with a bit of thought (additional relay's, etc) to prevent back feeds.