OpenTherm control behaviour

[HenGus]

Getting information on how a condensing boiler should respond under Opentherm control is still something of a challenge.

1. Atag has confirmed that the boiler is responding as they would expect if it was being controlled by their Atag One controller.

2. My hot water is limited to max boiler set temperature.

3. Initially, when the boiler comes on, with just heating selected, the boiler flow temperature will rise to the max boiler set temperature and stay there until all the zones are near their set temperatures. During this process, the boiler audibly modulates downwards, and gas usage reduces.

4. Some time after, the boiler temperature flow/return temperatures start to fall. I have seen a return temperature of 45C.

5. The piece of the jigsaw that has me slightly baffled, is how Evohome copes with optimum start when there is no heat; i.e.; when the motorised valves have switched to hot water priority? On simple observation alone, it copes - presumably by upping the boiler flow temperature when heat to the CH is available. I am not sure how it learns from this situation, but I suspect the same applies with the old 'S' plan when a hot water demand kicks in.

6. Zone temperature overshoots have not been a problem. I did see one zone at 1C higher than the set temperature this morning, but that was shortly after the system switched from hot water to heating.

[bruce_miranda]

It's best to set the HW time to either outside of the CH period or at the very beginning of CH. But when you can set what max boiler is for HW and CH, it's not really a big deal. Otherwise a way would be to shut the CH valve completely when there is HW demand but I think that's a big extreme.

[HenGus]

I've done that but it doesn't get around the problem of the HW zone sitting in the differential area overnight, and then dropping into a heating demand situation when the first person up turns on a hot tap.

[kimber.kimber]

Ok, so it's been running for about a week now, and I thought I'd just drop in with my gas readings. I may have been a little harsh on my earlier post. Currently sat at a £2 per day saving, compared to the old boiler!

Can't wait to knock the Aga off. Recon that will save about another £2 a day!

IMG_6950.jpg

There's still some setting to do, as I get some overshoots. Also flow temperature is quite often runs higher than the limit set in the boiler menu. That said, I think I might wait until autumn to iron out all the bugs.

[bruce_miranda]

What boiler is this?

[kimber.kimber]

What boiler is this?

Ideal Vogue S32

It's not doing the hot water yet, but it should be switched over by the end of next week.

[bruce_miranda]

How far from the boiler CH set temperature have you seen it rise to?

[kimber.kimber]

Between 5-10 degs. Why do you ask?

[bruce_miranda]

What is your boiler set to and what is it reaching? Because it should not go beyond the temperature set by the knob on the boiler. 1-2C might be acceptable but 5-10C over the set values is worrying. And this is even before you add HW to the mix, where your boiler will run at full wack.

[kimber.kimber]

I see. I'll double check it tomorrow. TBH I'm not 100% sure of the value, as I've adjusted the max temperature a couple of times. I'll keep an eye on it over the next couple of days and put some figures up.

Interestingly on the boiler there are two ways to control the max temp. A knob on the front and a setting within the menu system. The menu setting limits the max temperature that can be set with the knob on the front.

[kimber.kimber]

So I was right. I just fired a zone up to double check. Boiler set to 60 degrees, heat demand for one radiator only, flow at 75! I don't quite understand how it can demand more that the max set point of the boiler.

IMG_6954.jpg

IMG_6955.jpg

IMG_6956.jpg

IMG_6957.jpg

[bruce_miranda]

Check both the knob and the internal setting for max CH flow temperature. If the flow is indeed reaching 75C when you have set it to 60C then you have a problem.

[HenGus]

Check both the knob and the internal setting for max CH flow temperature. If the flow is indeed reaching 75C when you have set it to 60C then you have a problem.

Not quite the same issue, but I recall an earlier thread about the Vogue with a Lyric OT thermostat where the owner was having boiler flow temperature problems. For what it is worth, I have seen the occasional overshoot of the max flow temperature on my Atag but only by a degree or so. Out of interest, does any one know what calculated flow temperature is (T-set)? There is no explanation in the installer's guide or on the web. It must mean something as it is reported under Boiler Flow Temp, Boiler Return Temp and T-Set.

[bruce_miranda]

1-2C is quite normal and every boiler does that. But 10-15C indicates that the boiler is not stopping to heat the water even after it has reached that max.

[StephenC]

Could be the same as the Viessmann Opentherm issue where they have not followed the Opentherm specs completely. Same issue as what you're seeing, the boiler controls are completely ignored.

Viessmann boilers hand over max ch flow temp to the controller when it detects an Opentherm controller (or bridge in EvoHome case) connected - the caveat is that it appears only Viessmann controllers have the parameters available to configure this value for Viessmann boilers. EvoHome doesn't as of today.

Hopefully it's just a config issue for yours though and not this issue again.