The Efficient Home Forum

Hi,

I have an Evohome system, purchased from the Evohome Shop, configured as an S-Plan system with 2 BDR91s (CH and HW), 2 zone valves (CH and HW) and HR92s on all radiators. The system is working perfectly, but being slightly paranoid, I wanted to check a couple of things with the experts on here:

With the system configured in this way, it is not possible to adjust the cycle rate and minimum on time. Does anybody know what the defaults are for this configuration? For the Honeywell Sundial systems, the default is 6 cycles per hour and 1 minute minimum on time. I'm not sure if the same is true for Evohome. In any event, do I need to worry about these settings? I have a Worcester Bosch Greenstar CDI30 gas boiler. The reason I started thinking about this is that the Honeywell zone valves take about 12 seconds to turn on, so this would reduce the minimum boiler on time to 48 seconds in theory.

What benefits would I gain (apart from being able to configure cycle rate and minimum on time) by disconnecting and opening the CH zone valve and configuring the CH BDR91 as a boiler demand relay instead?

Thanks very much for any advice/reassurance!

Regards,
Graham

Hi Graham,

Not relevant to your question but I'm just researching HR92's and BDR91's for a potential install.

Do you use a Evohome controller to sync the HR92's to the BDR91 or can you do it without?

Thanks
Giles

Hi Giles,

Yes, I use an Evohome Controller. I don't think it's possible to sync HR92s to a BDR91 directly.

Regards,
Graham

grahamgibson wrote:Hi,

I have an Evohome system, purchased from the Evohome Shop, configured as an S-Plan system with 2 BDR91s (CH and HW), 2 zone valves (CH and HW) and HR92s on all radiators. The system is working perfectly, but being slightly paranoid, I wanted to check a couple of things with the experts on here:

With the system configured in this way, it is not possible to adjust the cycle rate and minimum on time. Does anybody know what the defaults are for this configuration? For the Honeywell Sundial systems, the default is 6 cycles per hour and 1 minute minimum on time. I'm not sure if the same is true for Evohome. In any event, do I need to worry about these settings? I have a Worcester Bosch Greenstar CDI30 gas boiler. The reason I started thinking about this is that the Honeywell zone valves take about 12 seconds to turn on, so this would reduce the minimum boiler on time to 48 seconds in theory.

What benefits would I gain (apart from being able to configure cycle rate and minimum on time) by disconnecting and opening the CH zone valve and configuring the CH BDR91 as a boiler demand relay instead?

Thanks very much for any advice/reassurance!

Regards,
Graham

Hi Graham,

An evohome system with HR92's on all radiators using BDR91(s) for on/off switching, doesn't need the heating zone valve on an existing S Plan system (unless the boiler is capable of two flow temperatures and is running on OpenTherm which Worcester Bosch can't natively anyway with 3rd party controls).

The main advantages of configuring as a Figure 4 system are less wear and tear on heating motorised zone valves, less failure points (if heating zone valve fails evohome won't know) and ability to adjust cycle rate and minimum on time for oil boilers and heat humps.

I hope that helps answer you question.