Boiler running but relays not calling

[Peter]

I had a normal Y-plan system which seemed to be running fine. I recently got a new boiler (Intergas HRE OV) and upgraded to OpenTherm. There was minimal changes to the wiring and on the controller I just set the boiler control to OpenTherm.

I seems to heat the house and water fine but I have seem some strange behaviour. It sometimes calls the boiler but has both relays closed. Previously, the boiler would only every come on when one of the relays opens. With no relays calling the valve (3 port) defaults to hot water, so while it's in this state it is unintentionally heating my water (and also the space below my floorboards where the pipes run). I've looked on the controller and on one occasion it showed 3% on OpenTherm and 3% on one radiator. Of course, with the value on hot water only, the radiator wouldn't have done much. Is this behaviour normal?

I've read about similar things in other posts but not quite the same as this. I'm thinking that I should probably put loads of insulation on all of my pipes where I can to reduce loses from this. Fortunately the boiler mostly shuts down and just run the pump in these conditions.

[Richard]

Hi Peter,

So firstly the system should have been converted to 'hot water priority' when going to OpenTherm. Low temp OpenTherm space heating and high temp hot water recovery are not compatible with each other, so unfortunately you do need to convert in this situation for best results.

There are a couple of ways to do this, but the easiest way to do this with a 'vented' cylinder on Y Plan (do not do this with mains pressure un-vented cylinders) is to have the 'closed' port of the 3 port valve to DHW and the 'open' port of the 3 port valve to heating. The central heating BDR91 is then removed from the evohome system completely and the hot water BDR91 is then wired to the 3 port valves grey and white wires so the valve acts like a 3 port diverter rather than a 3 port mid position valve. The evohome system is then rebound on the hot water side as a 'hot water only valve'.

As for the OpenTherm Bridge, it carry on doing the last thing it was told to do until it is told to do something new. Therefore the communications between it and the evohome Controller are essential to ensure correct operation. So if there are any large metallic objects in the 'line of rf', it will not work correctly. This is similar to the HR92's, so all devices need to be checked.

I hope that helps.

Thanks,

Richard

[Peter]

So firstly the system should have been converted to 'hot water priority' when going to OpenTherm. Low temp OpenTherm space heating and high temp hot water recovery are not compatible with each other, so unfortunately you do need to convert in this situation for best results.

There are a couple of ways to do this, but the easiest way to do this with a 'vented' cylinder on Y Plan (do not do this with mains pressure un-vented cylinders) is to have the 'closed' port of the 3 port valve to DHW and the 'open' port of the 3 port valve to heating. The central heating BDR91 is then removed from the evohome system completely and the hot water BDR91 is then wired to the 3 port valves grey and white wires so the valve acts like a 3 port diverter rather than a 3 port mid position valve. The evohome system is then rebound on the hot water side as a 'hot water only valve'.

Thank you. Am I right in thinking that with this setup the valve would never go into the mid-position? It would always default to heating but switch the heating off and go to DHW when there is demand for hot water? If this is the case, would it ever cause a delay if there is a sudden demand for water and heating?

[Richard]

Thank you. Am I right in thinking that with this setup the valve would never go into the mid-position? It would always default to heating but switch the heating off and go to DHW when there is demand for hot water? If this is the case, would it ever cause a delay if there is a sudden demand for water and heating?

Yes, that right the valve would be a 'diverter' valve... The valve would always default to heating without any power and when powered would go to hot water. You would amend your schedule if you need to for your hot water timings but for the short duration of hot water recharge you really wouldn't notice any loss of efficiency with the heating in terms of comfort just as people with combi boilers don't notice anything.