Evohome 2 * rads on with either DHW or CH

[finiterex]

Hi all,

I’ve been troubleshooting my evohome for the last few days.

I am hoping you can help, I have one final issue that I don’t seem to be able to resolve. I have a bathroom rad and an ensuite rad that do not yet have evohome TRVs fitted (coming shortly). Regardless of this, the issue I have is that that these rads get hot both when CH demand is on and when just DHW demand is on. Below I’ve outlined what I’ve done to date to troubleshoot.

• Confirmed with plumber and electrician that the S-plan has been wired up such that one BDR91 relay activates the CH valve, the other BDR91 activates the DHW valve. Each valve is wired such that opening activates the boiler. In-short good old standard S-plan.
• After much reading of this forum I’ve ascertained that:
  • Honeywell “optimum” config is that the CH valve is removed, one BDR91 relay acts as an “appliance control” relay, the other as the DHW valve relay. In this configuration the evohome controller would have the BDR91 that’s acting as the “appliance control” relay bound under “appliance control”, the temp sensor and the DHW valve BDR91 would be bound under “Stored hot water”. In this scenario the logic is then:
    • If a evohome rad TRV detects room below threshold temperature it opens and sends a demand to the evohome controller. The controller turns on the BDR91 relay bound to “appliance control”, the appliance control relay triggers on and activates the boiler. As there’s no CH valve the central heating loop is sent hot water which loops into the relevant rad(s) the TRV(s) opened on.
    • If the DHW schedule is in an “on” period and the temp sensor shows temp in DHW cylinder is below threshold, the evohome controller (1) turns on the DHW BDR91 which opens the DHW valve AND (2) turns on the “appliance control” BDR91 which fires up the boiler.

The above logic explains why if the evohome controller is configured as above but the BDR91 relays are installed on the S-plan valves that a DHW demand also turns on the CH – because the DHW demand would open both the DHW and CH valves with the boiler being triggered on by the valves.

• Evohome will work on S-plan but its “sub-optimal”. In S-Plan mode one BDR91 relay would be set to control the CH valve, the other to control the DHW valve – just as my plumber and electrician have set mine up. In this configuration the evohome controller has the temp sensor and DHW BDR91 bound under “Stored Hot Water” with the valve set as “2 two port or three port valves”, and then also the CH BDR91 bound under “Stored Hot Water” as well.
  
  To be fair to my plumber and electrician this is the first evohome they’ve installed, and they are being awesome about working it through. Anyway, I’ve now setup the evohome controller to the S-plan config by:
  • On the evohome controller setting “appliance control” to “none” and “stored hot water” to “disabled”
  • Unbinding the BDR91 relays (hold button on each for 15 seconds)
  • On the evohome controller going into “stored hot water”, setting the valve as “2 two port or three port valves”, binding the temperature sensor, binding the DHW BDR91 and then binding the CH BDR91.

And then I tested it…

1. Test 1 – DHW only
   1. Set both CH and DHW to off on the evohome controller
   2. Ran all the hot water out of the cylinder until only cold water came out and the evohome controller was reporting 12 degrees for hot water
   3. Waited for all the rads to cool down
   4. Set the hot water on the evohome controller to “boost”
   5. The DHW BDR91 was then triggered a few seconds later (green light on, valve could be heard opening and boiler fired)
   6. Hot water temp in the cylinder rose up 51 degrees over about 20 minutes
   7. At the same time the bathroom and ensuite radiators got hot – the plumber has wondered if these two rads are on the DHW circuit, so this would evidence they are (however see below).
      
2. Test 2 – CH only
   1. Set both CH and DHW to off on the evohome controller
   2. Ran all the hot water out of the cylinder until only cold water came out and the evohome controller was reporting 12 degrees for hot water
   3. Waited for all the rads to cool down
   4. Set the heating to “on” on the evohome controller
   5. The CH BDR91 was then triggered a few seconds later (green light on, valve could be heard opening and boiler fired)
   6. After 20 seconds or so the evohome controller started to show various TRVs demand % under “system summary”
   7. I checked the rads the evohome was saying had demand and lovely jubbly they were getting warm
   8. The temp of the DHW on the evohome remained at 12 degrees
   9. Waited for about 20 mins to see how things went, and all seemed good… except…. At the same time the bathroom and ensuite radiators got hot!

So, at this point I decided to give up and drink cider. I’m stumped. I believe the evohome is now configured correctly for S-plan valve control but as to why the bathroom and en-suite rads are getting hot whether it’s just CH on or just DHW on I really cannot fathom. I thought it might be that the BDR91s were “double bound” i.e. the CH one is still doing “appliance control”, but as only the DHW BDR91 comes on when only asking for DHW (CH turned off on evohome), and only the CH BDR91 comes on when asking for CH (DWH turned off on evohome) I don’t see how this could be the case.

Does anyone have any idea what I can look at next to try and sort this out?

Photos of install below – the BDR91s and the temp sensor are about 300mm apart, the amount of metal in the way could be better but I don’t seem to be getting any comms errors.

 

[JohnD]

People who are lucky enough to have a HW cylinder can have the plumbing arranged so that heated towel-rails, and sometimes bathroom radiators, are heated whenever the cylinder is heated, typically during and after a bath or shower, which is the ideal time. This is a superior design.

You can identify such an arrangement by inspecting the pipework, or by experiment and feeling the pipe warm up.

Worn-out motorised valves can also let hot water leak through when they ought to be shut. You can identify this by feeling the pipes next to the valves.

 

[finiterex]

Hi John,

I could understand if the two rads (bathroom and ensuite) were coming on only when DHW was demanded but they are coming in with either DHW or CH demand.

 

[JohnD]

I've arranged mine to do that.

TRVs are useful so the rooms don't get overheated in summer.

You would have to inspect the plumbing to see if it is by design or due to leaky valves or other fault.

I think it's relevant if only your bathrooms are getting the unexpected heat.

On mine the take-off is just before the 3 port valve, and visible. I do like to add descriptive labels.

IMO the behaviour of yours is not a problem, unless you can find other evidence of a fault.

 

[ericmark]

I would agree, S plan normally one or the other, there are normally indicators on the motorised valves to show open or closed, or the manual control lever goes slack.

I note my motorised valve will not latch open once the central heating has opened the valve, they default to electric controlled, but daughters house the valves would remain latched open, that was Y plan not S plan, but it had a faulty micro switch inside valve, and son-in-law had knocked valve unlatching it, and it caused central heating to fail.

I however agree with @JohnD that once the TRV's are fitted, it does not matter, however my system C plan fitting a TRV to the bathroom radiator would stop the domestic hot water working as they are piped into the return from the hot coil, so if radiator turned off, then no DHW.

 

[John E Turner]

I have a very similar version as your described. It is a new house and has been plumbed so that the 2 bathroom towel rails are across the flow and return of the boiler. This means that whenever the boiler is ON then so are these towel rails even if the CH or HW valves are closed. They effectively provide a bypass across the boiler. I have manual TRVs on these so that I can turn them off in the summer if necessary. To change the arrangement you would need to change the pipework.

 

[JohnD]

I have a very similar version as your described. It is a new house and has been plumbed so that the 2 bathroom towel rails are across the flow and return of the boiler. This means that whenever the boiler is ON then so are these towel rails even if the CH or HW valves are closed. They effectively provide a bypass across the boiler. I have manual TRVs on these so that I can turn them off in the summer if necessary. To change the arrangement you would need to change the pipework.

Anyone with a top class system doesn't need to change it.

 

[ericmark]

Anyone with a top class system doesn't need to change it.

Oh how true, my first and second house never thought about the central heating as it worked. Parents old house and this house, the central heating was in a mess, so worked out what was wrong.

Parents house I complained about over temperature when the wall thermostat stuck on. And it was pointed out, for that to happen, there needs to be two faults, both the wall thermostat and the TRV need to be faulty to get over heating.

 

[dilalio]

Possibly reverse circulation through the 2 rads when DHW is running. Check that cylinder is final connection on return pipe to boiler.