Evohome Opentherm and bypass circuits...

[darpania]

In the middle of having my evohome kit installed and have a bypass circuit question.

Our boiler is an Ideal Logic+30 and we are controlling it with an R8810 (supplied by EvoHomeShop at very short notice after another "supplier" repeatedly let us down... Note to self buy any further Honeywell kit from Richard)

We have 10 radiators and a towel rail.

Plan is to put HR92s on all but the hall radiator and use that as a bypass circuit in parallel with the boilers internal one.

But everything that I'm reading says HR92 on all rads and I'm assuming that a bypass circuit is far less necessary in an Opentherm setup, as the boiler won't be having to guess about when and how much to fire up

Is this right?.....or?

Thanks for Any and all help

[Richard]

If using OpenTherm is acts very similar to weather compensation with regards to how it controls the boiler.

OpenTherm when it has demand but at/near 'set point' will turn off the boilers gas valve but keep the pump controlled by the boiler 'running'. It is important at this stage to have somewhere for the boiler water to go. This OpenTherm action is not to be mistaken with 'pump overrun' which happens at the end of heating demand.

IMHO 'controlled areas' need HR92's on all radiators, however my recommendation with OpenTherm is an airing cupboard radiator or small towel radiator left open circuit. If this is not possible then a correctly adjusted automatic bypass valve should be fitted at a good distance away from the boiler.

I hope that helps.

Thanks,

Richard

[darpania]

Really helpful.. thanks Richard!

[neugen]

If using OpenTherm is acts very similar to weather compensation with regards to how it controls the boiler.

OpenTherm when it has demand but at/near 'set point' will turn off the boilers gas valve but keep the pump controlled by the boiler 'running'. It is important at this stage to have somewhere for the boiler water to go. This OpenTherm action is not to be mistaken with 'pump overrun' which happens at the end of heating demand.

Hello Richard,

So this means that as long there is at least 1% heat request from a zone, the boiler pump will be running ? (leaving aside the gas valve, which could be on or off)
Or is it Evohome's controller aggregated heat request percentage that drives the boiler pump ?

This could mean that the pump can run for a very long time, right ?

Can you please elaborate about the 'pump overrun' ?

Thank you.

[Richard]

'Pump Overrun' is a period of time after the end of heating demand that the boiler pumps water around the system to cool the boiler...

The latest evohome firmware allows you to see where the demand is coming from and any demand percentage on the OpenTherm Bridge or Boiler Relay will mean the boiler has heating demand. So yes, the pump controlled by the boiler could be running for a large period of time until the room temperature is satisfied...

[neugen]

Many thanks for the reply

[billytkid]

Ive just installed opentherm on a Valliant and I've noticed that sometimes when there is 0% pull the heating is still pulling (i saw 42 degrees the other day), is there a delay between evohome requirement and opentherm activity?
thx