OpenTherm control behaviour

[kimber.kimber]

Morning all!

So I finally installed the evohome system yesterday. TRVs on all rads, set up as per the Honeywell diagram with a two port switching valve for hot water only, and an ABV. I'm using a Ideal Vogue 32 system boiler, with opentherm control.

After some initial issues with binding, I completed a factory reset and cleared the binding in all of the hr92s. System seemed to fire up properly, the rads got nice a hot, and the TRVs shut off as they should.

However, looking at the boiler this morning, it's stating a 20% efficiency, which surprised me somewhat! I wanted to check a couple of details to see what people think.

When there's no heat demand, the pump runs for quite a while, which I think I was expecting, but also the boiler intermittently fires for short bursts. I've taken a couple of photos of the boiler graphs to show what I mean. I've tried reducing the boiler temp to move it towards the condensing point, bit that has has no effect.

I should point out that currently not alll of the rads are plumbed in, and the hot water isn't switched over from the Aga, so that technically the boiler is oversized for the system as it stands today, but I wouldn't have expected that lower efficiency.

Thanks in advance.

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[kimber.kimber]

So for example, now non of the zones are calling for heat, yet the boiler pump is running and then intermittently cycling the flame as per the pictures abo e.

On of the rooms is near its 16 degree set point, but we've just checked the rad and it's cold, so it can't be requesting heat.

Pipe after the auto bypass is warm, so it must be running through that.

[paulockenden]

Something is calling for heat. It's a real pity that Evohome doesn't let you know what's causing it.

The only solution is Domoticz with an HGI80 (or equivalent) to see the lower level data.

Or, I suppose, there could be a problem with the OpenTherm/Boiler interface.

[kimber.kimber]

Thanks Paul. Is it worth trying another factory reset and rebind? Just wondering if it's hanging on to something wrongly. But I'm pretty sure I managed it right last time round.

[StephenC]

So for example, now non of the zones are calling for heat, yet the boiler pump is running and then intermittently cycling the flame as per the pictures abo e.

On of the rooms is near its 16 degree set point, but we've just checked the rad and it's cold, so it can't be requesting heat.

Pipe after the auto bypass is warm, so it must be running through that.

Interesting - I get that as well. With Opentherm connected, the pump in my Viessmann 111-W storage combi runs for a long time when it finally does ramp down the flow temp.

I have a towel rail that stays warm just as you say (equivalent to a auto bypass pipe) with no other radiators with HR92 warm at all.

It's almost as if one zone/HR92 is calling for a minimal 1%-10% heat, but not opening the valve enough to actually let any water through!

Didn't ever see this with BR91 Relay that i know of. Could this be radiator balance issues, even slight?

[Alan C]

There was a post in another thread from someone who works for Honeywell that may help. Although we have no method of seeing which zone is calling for heat, I understand that Honeywell's technical section can see this through their connection to the Evohome (assuming you have connected it to Wi-Fi).

Regards,

Alan.

[kimber.kimber]

Interesting - I get that as well. With Opentherm connected, the pump in my Viessmann 111-W storage combi runs for a long time when it finally does ramp down the flow temp.

I have a towel rail that stays warm just as you say (equivalent to a auto bypass pipe) with no other radiators with HR92 warm at all.

It's almost as if one zone/HR92 is calling for a minimal 1%-10% heat, but not opening the valve enough to actually let any water through!

Didn't ever see this with BR91 Relay that i know of. Could this be radiator balance issues, even slight?

Interesting thought on the balancing. The rads haven't been balanced yet, due to the few more rads that need adding. Unfortunately it's going to be about a week before they are added into the system.

[kimber.kimber]

There was a post in another thread from someone who works for Honeywell that may help. Although we have no method of seeing which zone is calling for heat, I understand that Honeywell's technical section can see this through their connection to the Evohome (assuming you have connected it to Wi-Fi).

Regards,

Alan.

I might put a quick call into Honeywell on Tuesday to see what they say. Interestingly, the efficiency has now gone up to a whopping 33%!

[kimber.kimber]

Just one thought. Does the main controller need to be configured on one of the zones? At the moment, it's not, and HR92s are doing the control in all zones.

[DBMandrake]

However, looking at the boiler this morning, it's stating a 20% efficiency, which surprised me somewhat! I wanted to check a couple of details to see what people think.

Well, how exactly is the boiler determining an efficiency of "20%" ? 20% of what ? Without knowing precisely what this is trying to measure I think you'll find its a red herring.

When there's no heat demand, the pump runs for quite a while, which I think I was expecting, but also the boiler intermittently fires for short bursts. I've taken a couple of photos of the boiler graphs to show what I mean. I've tried reducing the boiler temp to move it towards the condensing point, bit that has has no effect.

As Paul says, if the boiler fires for short bursts there is still some demand from one or more zones. How are you determining that there is really no demand ?

Are you familiar with how TPI works ? If not have a quick read about it as it will help to understand what is probably happening. On a TPI system just because zone temperatures may be up to temperature (or even slightly above) does not mean there is no demand. There is a +/- 1.5 degree proportional band around each zone's set point where there is potentially some demand, even if it is only 10%. (Which would result in the boiler firing for 1 minute out of 10 minutes)

There is also some biased rounding of the measured temperature for display, so even though you might have a room set to 20 degrees and the system might report a measured temperature of 20 degrees, in reality it may only be 19.4 degrees for example, because the measurement (for display purposes only) is biased towards the set point by up to 0.5 degrees and then rounded to the nearest 0.5 degrees. (Thus saying 20 in the above example) In that situation there is definitely demand as the temperature is 0.6 degrees below the set point, even though you don't realise that's the case.

Currently the only way to see the true measured temperatures without biasing towards the set point and rounding is to monitor the system using Domoticz, which as Paul says will also allow you to see the normally invisible "heat demand" request from each HR92, which is what actually calls for the boiler to fire.

If you adjust all the set points so they are at least 2 degrees below the current measured temperatures and wait a few minutes you should find the boiler goes off and stays off. If that is the case most likely the system is working normally. Unless the weather is really warm its normal for the boiler to run intermittently to maintain the set temperatures as a small amount of heat must be put into the house to equal the heat lost just to maintain the status quo.