Boiler relay activated with all zone valves closed

[bucket123]

Hi, I have a simple S-Plan system with two zone valves. One Zone valve for the entire CH system and another zone valve for DHW; each valve activated by a BDR91. The zone valves are not used to control the boiler; there is a boiler control relay (also a BDR91).

The CH zone thermostat is simply the Evohome controller itself. DHW has a wireless thermostat. A pretty simple and standard setup.

Problem: What I often observe is that, with DHW off, the boiler is sometimes firing for a few minutes but the CH zone valve is not opened. Thus the system is not heating anything except the boiler and the pipes to the bypass valve. Surely this cannot be right?

This tends to happen when the CH is close to its set point and the 'system summary' shows a low demand of say 27% on both the 'boiler control' and the CH zone (a.k.a. 'living room').

There are no comms problems. For example if I whack up the CH set point (so 100% demand) then the CH zone valve always opens which shows the comms are fine. This strange behaviour of boiler firing but no zones open only occurs when the CH demand is low but not zero.

[Bart]

On 27% heat request you should have some circulation through the radiator. Try to remove de valve body and open by hand the valve fully by rotating the plastic wheel ccw. Then put back the BDR91.

[bucket123]

Hi @Bart,

This is not a physical problem.

The problem is in the EvoHome controller. It fires up the boiler by activating the 'boiler control' relay but does not activate the CH zone valve relay.

It is not a communication problem; I've eliminated that possibility.

The problem is in the logic of the controller.

Is this a known problem?
Is there a work-around?
Can my setup of the system be incorrect?

[Richard]

Any chance of a picture of the BDR91's?

[bucket123]

Hi, They're standard honeywell wireless relay boxes. I have three: one controls the boiler, one the CH zone valve and one the DHW zone valve. There is a wireless temperature sensor on the DHW tank and that's it: four wireless devices all communicating with one EvoHome system box which acts as controller and also as the CH temperature sensor.

See: https://theevohomeshop.co.uk/honeywell- ... y-box.html

All communications are fine. The problem is due to the behaviour of the controller. See my previous posts on this thread.

[Richard]

Hi,
How close together are all these relay devices was more why i was asking.. If they are in too close proximity to one another they could be screening the signal hence the zone valve not opening.

[bucket123]

The CH relay is around 50cm above the boiler and the boiler relay around 40cm above that. The DHW sensor and relay are miles away but they're not involved in the problem.

There is no communication issue here. If I change the CH set-point so that the demand is 100% on both boiler and CH then the relays both activate every time.

The problem only occurs if the demand is low (but non-zero) on both boiler and CH; then the boiler often activates for short times without the CH relay.

Also the log on the EvoHome controller shows only very occasional comms issues.

[Richard]

The CH relay is around 50cm above the boiler and the boiler relay around 40cm above that. The DHW sensor and relay are miles away but they're not involved in the problem.

There is no communication issue here. If I change the CH set-point so that the demand is 100% on both boiler and CH then the relays both activate every time.

The problem only occurs if the demand is low (but non-zero) on both boiler and CH; then the boiler often activates for short times without the CH relay.

Also the log on the EvoHome controller shows only very occasional comms issues.

Why do you need 3 relay boxes? If its standard S Plan on a gas boiler with no HR Radiator Controllers, then no need for the boiler demand wireless relay.

[bucket123]

Okay... you need the history.

It is an oil fired boiler. A new boiler was fitted recently to replace a defunct boiler in an old style system with no zone valves: The old system was controlled by a timer, the CH pump controlled via a single room thermostat, and the DHW gravity fed so DHW was always 'on' with the boiler.

The new system could have been made with just two zone valves and with the zone valves wired so that they control the boiler. However, due to many factors, the engineer fitted the DHW zone valve near to the DHW tank. It was extremely difficult to run a cable from the DHW zone valve back to the boiler so we opted to control the boiler via the EvoHome box and a third BDR91 relay ('Boiler Control') which the EvoHome is (or should be) perfectly capable of handling. Indeed, the EvoHome specicially recognises that relay as 'Boiler Control'.

The problem is that the EvoHome controller sometimes activates the boiler control relay without activating the CH zone valve. It only does this at low demand and this looks like a fault in the software.

[Richard]

Okay... you need the history.

It is an oil fired boiler. A new boiler was fitted recently to replace a defunct boiler in an old style system with no zone valves: The old system was controlled by a timer, the CH pump controlled via a single room thermostat, and the DHW gravity fed so DHW was always 'on' with the boiler.

The new system could have been made with just two zone valves and with the zone valves wired so that they control the boiler. However, due to many factors, the engineer fitted the DHW zone valve near to the DHW tank. It was extremely difficult to run a cable from the DHW zone valve back to the boiler so we opted to control the boiler via the EvoHome box and a third BDR91 relay ('Boiler Control') which the EvoHome is (or should be) perfectly capable of handling. Indeed, the EvoHome specicially recognises that relay as 'Boiler Control'.

The problem is that the EvoHome controller sometimes activates the boiler control relay without activating the CH zone valve. It only does this at low demand and this looks like a fault in the software.

OK, fair enough. As its oil I would have advised a 3rd BDR91 anyway so you can adjust the cycle rate to 3 cycles per hour and minimum on time to 5 minutes anyway. Give Honeywell Technical support a call and they can have a look at the device remotely for you (as long as you have the Wi-Fi model).