Power stealing and OpenTherm gateway

[Woody]

Hi,

Does anybody know what happens to an Evohome R8810 when I put an Opentherm monitor between it and my boiler?

I guess it keeps on working but would like to know for sure before I order the monitor.

[Jack007]

Hi,

In my setup i use a R8810 to operate a Nefit heater (EcomLine exc.).
As the heater doesn't understand OT i have to convert OT to EMS to iRT. This is done by the two modules on the right (small black thingy and the white Bosch box).
The nodo OT monitor sits in between the R8810 and the OT/EMS/iRT converters. It only monitors the heater, so no control.

This setup works great since a few years.

20201105_161730.jpg

[bruce_miranda]

OT Gateways need power themselves.

[Woody]

By the looks of the setup of Jack007 the R8810 still gets its power via the monitor. I'll go and order it and then replace my on-off BDR91.

[bruce_miranda]

Are you referring to the R8810? Then yes that is OT Bus powered. You used the word OT Monitor, which I read as a monitoring gateway (OTGW), that needs power.

[bruce_miranda]

The other thing to bear in mind is if you insert an OT monitor in between the chain, between the OTB and your boiler, the OT monitor plays quite an important job and needs its power. OT monitors don't work as sniffers, they are genuinely man-in-the-middle even in monitoring mode.

[Woody]

Yes, sorry for the misunderstanding. I was aware the monitor itself needs power; my question was regarding the R8810, will that still be bus powered by the monitor. Reading back not a very smart question; how else would any OT thermostat be powered when using a mponitor in between it and a boiler.

[bruce_miranda]

I have an eBus monitor, the biggest difference between the eBus monitor and the OT monitor is that the eBus monitor can sit to the side sniffing and injecting data, but doesn't become a critical component in the chain. So if the eBus monitor dies, things still work. The OT monitor in comparison becomes a critical component in your heating system. You could loose all Heating and HW if your OT monitor loses power.

[Woody]

Is Ebus fully compatible with OpenTherm?

[bruce_miranda]

Not at all compatible. My Vaillant boiler only talks eBus. But lucky for me Vaillant make an eBUS to OT conversion adapter. My Honeywell OTB is connected to this converter. and I monitor things on the eBUS side. Which actually is better than monitoring the OT traffic, because no point monitoring what the boiler doesn't understand any way

[Woody]

Ah, so. Well, then it will be the OpenTherm monitor for me. I'm not planning in keeping it in between indefinitely; just for some time to find out what goes on (and wrong) between Evohome and Remeha.

A particular problem I want to look into is that in my setup Evohome+OT sometimes asks for a lot of heat to heat up a small, 1 radiator room (e.g. bathroom) leading my Remeha boiler to overheat and (temporarily) die. After RTFM this should not be possible, as the boiler continuously checks all temperatures and if necessary modulates back to almost nothing, but still, it happens. All this was reason for me to reconfigure my setup with a BDR91, that so far did not overheat my boiler.

Anyway, that is probably going to be a whole new thread.

[bruce_miranda]

There are a few boilers that when used with OT have no TMax. Maximum is an optional OT command which the OTB does not pass to the boiler. But some boilers expect it to be sent when using OT. The OTB will demand heat of 90C when any zone is 1.5C colder than it's set point. This 90C demand causes those boilers to ramp up indefinitely. Most boilers will clamp that ramp up at the Maximum that is set at the boiler, but some don't. As I said this only happens when OT is used and doesn't happen with traditional On-Off thermostats are used.