Y87RF setpoint needs to be 2C over the current temperature for BDR91 to come on

[bruce_miranda]

My UFH is controlled by a Y87RF thermostat which is configured as a Zone Valve. But the weird thing is that the BDR91 will come on only if the setpoint is 2C over the current temperature. Why is that? All the other radiator controller will make the BDR91 come on even if the setpoint is only 0.5C over the current temperature.

[DBMandrake]

Did you wait at least 10 minutes after increasing the set point for the BDR91 to come on before deciding that it didn't work ? (And I assume you tested it with all other zones turned off)

Remember that the BDR91 will do TPI (time proportional integral) modulation of the boiler. By default this is a 10 minute cycle. A low "demand" such as a zone that is within 2 degrees of the set point will not turn the relay on constantly.

Depending on where you are in that 10 minute cycle if you only create a partial load it can take many minutes before there is any reaction from the BDR91 as it waits for the beginning of the next 10 minute cycle even if it was previously constantly off.

Another thing that it could be is that due to the room being consistently slightly over temperature for a long period of time due to other heat sources, there might be "integral windup" in the proportional band. This can cause it to over compensate (in the off direction) for a while when you turn the set point up to only slightly above the current temperature. However if you set it to say 1 degree above current and just leave it for a long time (30+ minutes) you'll probably find that it does switch on eventually, it may just take longer than you expect.

This can be exacerbated by the "minimum on time" setting - which defaults to 1 minute. With a 10 minute cycle time this means that any "demand" less than 10% of the boiler output will be ignored and the boiler will not fire at all even if a zone has a small but non zero demand for heat. if you set minimum on time higher this will get worse so if you have a high minimum on time and your boiler doesn't really need it, consider setting it back to 1 minute.

I find if my living room is the only zone in the house with any demand (in the late evening it is the only zone active) and the radiator has gone completely off, then it will sometimes let the room drop to 0.5 - 1 degree below the setpoint before the boiler actually fires again - this is because the heat "demand" called for by the radiator valve is not enough to meet the minimum on time requirement.

I don't think there is any solution to this minimum on time problem other than switching to OpenTherm, which does not have the minimum on time limitation as it uses flow temperature control instead of TPI. (Not an option for me on my ancient boiler though!)

I would reduce your minimum on time to 1 minute if you have increased it, then set that zone to 1 degree above current and just wait for at least 30 minutes to an hour to see what happens...

[bruce_miranda]

I will try this and see. I do have Opentherm.

I used to see this behaviour even when the Y87RF was operating in stand alone mode i.e. not integrated with the Evohome. The setpoint had to be atleast 2 degrees higher than current temperature for the thermostat to tell the BDR91 to come on. This seems to have just carried over to the Evohome too. Hence I thought there must be a setting in the Y87RF somewhere that states how much below the setpoint should the temperature get before it fire up again.

[top brake]

The proportional band is -1.5 degC from set point so setting a 2 degC overide will make it switch on. This is normal for all TPI controls. If you set an overide say 1 degC above current the controller will think about it and may not fire for a while or at all if the trend is increasing beforehand. Normally if you want to go into comfort from setback you'll increase at least 2 degC perhaps more

[bruce_miranda]

So the HR80s work differently to the Y87RF. The HR80 will make the boiler fire even if the target is set 0.5degrees more than the current. But good to see an explanation for the Y87RF.

Is there anyway that value can be changed?

[DBMandrake]

So the HR80s work differently to the Y87RF. The HR80 will make the boiler fire even if the target is set 0.5degrees more than the current. But good to see an explanation for the Y87RF.

Is there anyway that value can be changed?

I might be wrong, but I believe that when you use a separate wall stat for a zone (instead of just HR80/HR92 on its own with internal sensor) that the wall stat does not calculate the heat demand value, it simply reports the measured temperature back to the evohome which then passes it on to the HR92/HR80, and that it's the HR92/HR80 which handles the proportional band and makes the decision about what "heat demand" is sent back to the Evohome and then relayed onto the Boiler relay.

(BTW when you use the internal sensor in an HR92 it is not used directly - the temperature reading is first sent to the Evotouch controller then a bit later relayed back to the HR92 for use in calculating the valve position and heat demand - which is the reason why the temperature reading on the HR92 actually lags behind the reading on the Evotouch instead of being ahead of it!)

With a zone valve zone there is no HR92 so the job of calculating the proportional band and heat demand for the zone must fall to the evotouch itself.

Did you try the test I suggested where you set it to 1 degree above the current temp and wait an hour to see if it does come on at all ?

Also, do you see the same problem if you use the Evotouch's built in temperature sensor ? (Temporarily relocate the controller to that room on its stand and rebind a different sensor to the old zone if you were previously using it for another zone)