The Efficient Home Forum

Our S-plan plus system has worked well for a few years with a BDR91 relay controlling the 2-port valve to the radiator circuit, while other "traditional" controllers manage the UFH and DHW valves. When there is no call for heat from Evohome the CH 2-port is closed, so no water can circulate through the radiators if UFH fires the boiler.

I'm thinking of rejigging things to add Evohome control of the DHW and single-zone UFH via a new relay each. AIUI when either of these needs heat Evohome will trigger the relevant BDR (to create a flow path), plus the one that it thinks is connected to the boiler (to generate and pump heat). It also assumes that there is no CH valve, and that the path to radiators is open all the time (hence "standard" design of latching the CH 2-port open). So presumably it ensures the HR92 are fully closed if choosing not to supply heat to those zones.

I realise it isn't the "official" wiring approach, but I'm missing why it would be wrong (as in unintended effects) to take a BDR to each zone valve. When UFH or DHW needs heat Evohome will effectively trigger the boiler twice (via relevant zone valve plus the CH valve which it thinks is "appliance control"). But as long as the HR92s are closed then nothing unexpected will happen.

I prefer to keep electrical control of the CH valve as I think it is better for the valve mechanics: less constant strain on the spring and less likely to sieze open.

So does Evohome ensure the HR92 are closed? Am I missing something else?

Hi
I'm certainly no expert, just some experience of 2 systems I've installed, from my experience the guidance is to have a CH motorised valve (if you have one) permanently open so as to allow a flow path to any of the HR92s which should fully close when no demand from them (ie demand from DHW only). Both installed systems have had a bypass valve for if the pump is still running when all of the valves are closed (i.e. overrun or fault).

I have also used a 3rd Motorised valve (Not CH, Not DHW) for heating a specific Zone (Lounge - as the valve was already in place) so that sounds similar to your set up with your UFH.

All seem to be working well for me.

Thanks

It does indeed appear to work that way. I've now replaced separate controllers for UFH and DHW with Evohome components, including relay to 2-port valve. When either of them needs heat relevant relay plus the CH one light up, and everything works as expected due to the HR82s being closed.

OptiMiser wrote:Our S-plan plus system has worked well for a few years with a BDR91 relay controlling the 2-port valve to the radiator circuit, while other "traditional" controllers manage the UFH and DHW valves. When there is no call for heat from Evohome the CH 2-port is closed, so no water can circulate through the radiators if UFH fires the boiler.

I'm thinking of rejigging things to add Evohome control of the DHW and single-zone UFH via a new relay each. AIUI when either of these needs heat Evohome will trigger the relevant BDR (to create a flow path), plus the one that it thinks is connected to the boiler (to generate and pump heat). It also assumes that there is no CH valve, and that the path to radiators is open all the time (hence "standard" design of latching the CH 2-port open). So presumably it ensures the HR92 are fully closed if choosing not to supply heat to those zones.

I realise it isn't the "official" wiring approach, but I'm missing why it would be wrong (as in unintended effects) to take a BDR to each zone valve. When UFH or DHW needs heat Evohome will effectively trigger the boiler twice (via relevant zone valve plus the CH valve which it thinks is "appliance control"). But as long as the HR92s are closed then nothing unexpected will happen.

I prefer to keep electrical control of the CH valve as I think it is better for the valve mechanics: less constant strain on the spring and less likely to sieze open.

So does Evohome ensure the HR92 are closed? Am I missing something else?

So just really two points to clarify. We don't have S Plan systems and HR91/HR92's. As per page 46 of the latest evohome installation guide you will see that when HR91/HR92 are fitted to all radiators, there is no motorised zone valve for the radiator circuit. This is critical in how the evohome system operates, as anything else you wish to do relies on this configuration method.

The EVOHOME Shop wrote: So just really two points to clarify. We don't have S Plan systems and HR91/HR92's. As per page 46 of the latest evohome installation guide you will see that when HR91/HR92 are fitted to all radiators, there is no motorised zone valve for the radiator circuit. This is critical in how the evohome system operates, as anything else you wish to do relies on this configuration method.

For me the key design point there is that Evohome assumes there is an open path to the radiators when the boiler is on. That can be done by:

1. removing the valve
2. latching it open
3. signalling it to open via relay and in turn power up the boiler via classic S-plan design

I understand that the preferred / idealised diagrams use option A, but where things are already in place options B (run risk of valve seizing) or C (exercise valve as normal) appear to be equally valid.

OptiMiser wrote:I understand that the preferred / idealised diagrams use option A, but where things are already in place options B (run risk of valve seizing) or C (exercise valve as normal) appear to be equally valid.

Latching open works fine, but C is highly not recommended, because treating a zone valve like a boiler demand will lead to premature failure of the zone valve. Either remove it or electrically disconnected and latch it open.

The EVOHOME Shop wrote: Latching open works fine, but C is highly not recommended, because treating a zone valve like a boiler demand will lead to premature failure of the zone valve. Either remove it or electrically disconnected and latch it open.

It's the first time I've seen that risk mentioned.

In a pre-Evohome system the valve might be turned on and off frequently as a classic wall thermometer struggled to keep the target temperature. With Evohome the switching might be a bit less, but generally similar to get the overall same amount of heat into the house. I'd expect the main thing to make a difference to valve behaviour would be load compensation via something like OpenTherm which would keep the valve switched open longer (with lower flow temperatures). Is it the long hold open times that are known to cause problems?

OptiMiser wrote:It's the first time I've seen that risk mentioned.

Who else with 8 years of evohome experience that you have asked before?

Operating a zone valve 6 times an hour, will cause excess wear and tear on the synchron motor and mechanism. At the end of the day, evohome wasn't designed to operate like your option C, so why would you want to?

The EVOHOME Shop wrote:

OptiMiser wrote:It's the first time I've seen that risk mentioned.

Who else with 8 years of evohome experience that you have asked before?

Operating a zone valve 6 times an hour, will cause excess wear and tear on the synchron motor and mechanism. At the end of the day, evohome wasn't designed to operate like your option C, so why would you want to?

I'm not sure if the first bit is rhetorical. I haven't considered it / asked since installing installing Evohome almost exactly 6 years ago, so everyone would have had 6 years less experience then. ISTR the general advice was to try and follow the Honeywell designs, but no-one mentioned potential valve problems.

Most of the reading about V4043 valves I've done over the last 25 years or so has suggested that being energised for long periods in hot areas is the main cause of failure. That fits with the only time I've had one fail about 15 years after install on the CH circuit in a DHW airing cupboard. Hence my query about whether load compensation leading to long hold-open times might be the risk. The one we're discussing runs in a much more friendly temperature location.

So that's where I was starting from, and I appreciate the advice that the extra switching cycles of Evohome have been know to cause problems.

As to why do it the wrong way now: I think we're agreeing there is no detriment to the way that Evohome works. For now it's simpler to leave that part of the wiring as is, accepting that if the zone valve does fail I'll then have to do the same work that I could do now. I so also still prefer to be exercising the valve rather than leaving it latched open - but can't find any V4043 tech sheet to say how many cycles the valve is designed for to help me figure out if this is sensible or not. Finally, the option of popping a traditional programmer back in place if a selected emergency heating engineer is unfamiliar with Evohome or to take ~£1000 of Evohome kit with me if I move house is attractive.

The first bit was a small joke reference to the fact you said it was the first time you had ever seen the risk of TPI'ing a motorised zone valve. Think nothing of it.

The 'issue' with having a zone valve for heating between the boiler and the HR91/HR92 is that evohome doesn't know it is there. So if it does fail, the evohome system doesn't work. There is no need for it to be there. Electrically disconnect it and latch it open.