Opentherm

[Atoss]

Evohome system with an Ideal vogue max, 11 zones, 10 controlled with HR92, BDR91 2 port valve. HW with BDR91 2 post valve.
The other zone has thermaskirt heating, controlled by DTR4 and relay, 2 port valve. I fitted the opentherm bridge, I removed the 2 port for the CH.
It worked initially, but now the openthrrm doesn't fire the boiler, despite the system calling for heat. Any advice ?

 

[kraduk]

I have the same issue with the same boiler. Did you get to the bottom of it?

 

[MeldrewsMate]

HR92 is a TRV control, I assume?
BDR91 is a remote relay?
Not sure what a DTR4 is, but assume a programmable thermostat.


Opentherm will adjust the boiler flow temperature relative to the implied load, so if it thinks that the boiler flow temperature is higher than the desired temperature then it would be silly to raise it further.
Opentherm should be interrogated to determine the system load. If those data indicate the room temperature is rather high (because I am the sort who doesn't believe that two things go wrong at once on more than 1 in 1000 cases) then you might want to investigate the boiler flow thermistor (sensor) or a malfunction of the opentherm bridge.

You have mentioned nothing about hot water performance, however this may be demanded via the SP2 connection, so that part may work satisfactorily.

 

[ericmark]

I have not worked out what OpenTherm can do, which TRV heads don't already do?

We aim for a gradual change where ever we can, this will allow the boiler to adjust the output to match the return water temperature so it can gain the latent heat from the flue gases. In other words, analogue control.

So HR92 is the TRV head, fair enough analogue control. But BDR91 is a relay box, and DTR4 is an on/off thermostat, and 2 port valve again seems to be an on/off device, I can see that to heat domestic hot water (DHW) it takes that long, using an on/off device makes sense, but for the central heating (CH) as much as possible should be analogue.

There is a trade-off, the TRV head ideally should not be against an outside wall, as it can fire up early, due to wall cooling the TRV head, and there seem to be very few wall thermostats which work as a master/slave configuration, I think EPH does, but in general with OpenTherm one device needs to connect to the boiler, and all other devices connect to that one device.

So if for example we have 9 on/off controls, in the main one or the other will be on, so most of the control is done by the TRV head and the return temperature of the water, we can't stop the boiler turning on/off, but want it to do that as little as possible. I simply fail to see where a motorised valve fits in with that? It would need to gradually open and close, and only the TRV heads do that, the motorised valve is either on or off. It would need many motorised valves, so the net result would be a gradual increase or decrease of the return water temperature.

So simply can't see how it would ever work?

 

[Deleted member 325366]

I have not worked out what OpenTherm can do, which TRV heads don't already do?

We aim for a gradual change where ever we can, this will allow the boiler to adjust the output to match the return water temperature so it can gain the latent heat from the flue gases. In other words, analogue control.

So HR92 is the TRV head, fair enough analogue control. But BDR91 is a relay box, and DTR4 is an on/off thermostat, and 2 port valve again seems to be an on/off device, I can see that to heat domestic hot water (DHW) it takes that long, using an on/off device makes sense, but for the central heating (CH) as much as possible should be analogue.

There is a trade-off, the TRV head ideally should not be against an outside wall, as it can fire up early, due to wall cooling the TRV head, and there seem to be very few wall thermostats which work as a master/slave configuration, I think EPH does, but in general with OpenTherm one device needs to connect to the boiler, and all other devices connect to that one device.

So if for example we have 9 on/off controls, in the main one or the other will be on, so most of the control is done by the TRV head and the return temperature of the water, we can't stop the boiler turning on/off, but want it to do that as little as possible. I simply fail to see where a motorised valve fits in with that? It would need to gradually open and close, and only the TRV heads do that, the motorised valve is either on or off. It would need many motorised valves, so the net result would be a gradual increase or decrease of the return water temperature.

So simply can't see how it would ever work?

From what I can gather Hr92 and 91’s are just another way to zone your house. If you have a cold office the heating can just be turned on for that room.

I have experimented with open therm and weather compensation and found open therm is better. Both together have no benefit.

 

[ericmark]

From what I can gather Hr92 and 91’s are just another way to zone your house. If you have a cold office the heating can just be turned on for that room.

I have experimented with open therm and weather compensation and found open therm is better. Both together have no benefit.

Thank you, I don't any more have a modulating boiler, so no longer an option for me. The boiler fires for such a short time, it is clearly too large to heat one room. I have looked at the idea of a heat store, but the installation cost is too high, and it will never pay for its self.

 

[Deleted member 325366]

Thank you, I don't any more have a modulating boiler, so no longer an option for me. The boiler fires for such a short time, it is clearly too large to heat one room. I have looked at the idea of a heat store, but the installation cost is too high, and it will never pay for its self.

Don’t all boilers modulate a bit ? Have you tried turning the flow temperature down low and letting the room heat up gradually ?

 

[Harry Bloomfield]

Don’t all boilers modulate a bit ? Have you tried turning the flow temperature down low and letting the room heat up gradually ?

I think Eric has an oil boiler, they do not modulate.

 

[ericmark]

I think Eric has an oil boiler, they do not modulate.

Yes, oil, but the gas boilers in old house, only one modulated, fitted before the Comb-boiler came out so one boiler for CH and one for DHW.

Late mother's house had a modulating boiler, and yes it did work, set the TRV to the temperature required, and the radiators would keep warm, in the main, but there was no real way to work out what the boiler was doing. The gas meter was outside so not sitting watching that, and nothing on the boiler to say if running at 8 kW or 35 kW, only could see when it switched off.

I looked today at the total oil used per year, assuming central heating only running for 6 months, I am looking at an average of around 3 kW, and this is a large house, so it would seem the boiler would need to drop to 3 kW at times to not switch on/off, all the time, but it could only modulate down to around 8 kW.