Smart Central Heating

As I've been playing with the scheduling features of the Evohome Controller, I've been thinking about the various headline features that smart central heating systems are marketed on, and why they're needed.

Traditionally your central heating system had one job to do: ensure that your home is at a comfortable temperature. Now there's a second job for it: minimise overall energy use. Arguably, through the use of a well positioned thermostat and TRVs on each radiator you could achieve job number one. The recent explosion of smart central heating systems are all focussed on job number two: minimising energy use.

There appear to be two basic approaches to minimising energy use. The first is to turn off the heating when you're not at home. And the second is to only heat the spaces that you're using when you are at home. Different companies are tackling one or both of these approaches. For example Nest, Hive and Tado are using motion detection and geo-fencing to tackle the first approach, while Honeywell and Heat Genius are also zoning your house and controlling each radiator independently in order to tackle the second approach.

The pre-smart approach to turning off the central heating when you're not at home was to use a programmable timer. When you delve into the detail it feels like all of the smart central heating systems are really continuing that approach. Some of the systems still require you to program them with the times and temperatures that you want the central heating to work to. Even the ones that promote a 'learning' capability are just using motion detection and geo-fencing to work out your schedule without you manually entering it yourself. Some of the learning systems benefit greatly from you manually entering your schedule first, so they've got a baseline to start learning from.

So if your routine is entirely predictable then all of the smart central heating systems can ensure that your home is nice and toasty when you're in it. It's the unpredictable that some of them struggle to deal with automatically. Those occasions when the train is delayed, the meeting overruns, the motorway is congested, and the heating system kicks in when you're not actually there. In this case no amount of smart learning algorithm is going to be able to compensate and delay heating the house until you're actually going to arrive. As far as I can see, only the systems offering geo-fencing can automatically cope with the unexpected. The others will have to make do with manually controlling the heating system remotely via an app or the web.

When it comes to zoning the house there's very little differentiation between the smart central heating systems. All produce wireless TRVs that allow a central controller to independently adjust the heat output of the radiators. Some of these TRVs allow the temperature to be adjusted locally as well, and some have displays to report on room temperature or temperature setpoint.

Honeywell Evohome - Configuration

The first step in configuring the Evohome system is to bind/pair each of the system components to the Controller. Each component has a slightly different procedure for binding, giving the impression that different teams developed different components for the system. For example, the HR92 TRVs require a 5 second push on their button to get them into configuration mode, then select Bind from the 3 options, and off you go. The BDR91 wireless relays however need a 15 second push on their button to clear the previous binding data, then another 5 second push to get the binding process going. The supplied installation manual explains the binding procedure pretty well. On the plus side, the Controller is battery powered so you can carry it around the house, binding it to each component in turn.

As mentioned before, the Evotouch evolved into the Evohome Controller. Be aware that in the Controller software the Controller is still referred to as an Evotouch. One mistake we made whilst initially configuring the system is to set the Controller as a temperature sensor for the living room, rather than use the temperature sensor built into the HR92 TRV. Not referring to it as the Evohome Controller might have contributed to our confusion.

We also had a little trouble setting up the hot water part of the system. The first stumble was the use of the word Actuator, which seems to be used synonymously for both the BDR91 wireless relay and the mid-position valve. We were also prompted to rebind the BDR91 wireless relay for the central heating at one point during setting up the hot water, but in the end we appear to have everything bound and operational.

We wanted to set the temperature for the hot water tank to 55C, to be below 60C at which limescale deposits start increasing, and also above 50C to combat bacterial collonies such as Legionella. Setting the temperature for hot water is straight forward using the Controller. However two other options are also presented, Overrun and Differential, but the explanations of these options are omitted from the supplied manuals. However, the Evotouch manual (doc ref 50056315-001 B) has the following:

Hot Water Overrun
Specifies how long the valve for the hot water is open or the hot water pump is running to extract all the heat to avoid blocking the heat source.

Hot Water Differential
Setting the switching differential of the domestic hot water control.

So, by setting the Differential parameter to 5C, the hot water should stay in the 50C to 60C range. We also put in a 1 minute Overrun, which I think will keep the Grundfos pump running for a minute after the boiler has been shut down.

Other parameters that we tweaked were to do with Optimisation (Optimum Start, where the system learns when to fire the boiler up in advance to hit the temperature requested) and Fail Safe (where the system will run at a 25% duty cycle if the Controller comms fail completely).

I'm pretty sure we'll get into more of the configuration parameters that the system offers as we get used to the way it works. And then, of course, there's the whole scheduling aspect we'll start playing with.

Honeywell Evohome - Installation

We've started installing a Honeywell Evohome system in the house for our central heating and hot water. So far so good, although with some stumbling points along the way.

When we bought the house we inherited a gas-fired Potterton Suprima boiler hooked up to a Sundial Y-Plan system with a mid-position valve to heat up a storage tank for hot water, and water radiators in each room. There was a truly ancient 24-hour programmable timer, and the thermostat for the entire house was located behind a door in the entrance hallway. Consequently the house was simultaneously too hot and too cold, cost an arm and a leg to run, and within a month of moving in the whole system died just as the winter snow started falling.

The Evohome system comes in functionality packs, so we needed the Base Pack (a Controller and one BDR91 Wireless Relay), a Hot Water Pack (a CS92 temperature sensor and another BDR91 Wireless Relay), and a Radiator Multi Zone pack (four HR92 temperature regulating valves). We bought these in from JTM Plumbing, who provided an exceptionally quick delivery service and very keen pricing.

In the airing cupboard we removed the wired thermostat from the hot water cylinder, the 24-hour programmable timer from the wall, and unwired the mid-position valve and Grundfos Alpha2 water pump. To the hot water tank we fitted the CS92 temperature sensor, and ran the cable to the CS92 transceiver which was screwed to the wall. We then fitted the two BDR91 wireless relays to the wall, and proceeded to rewire the various components up according to the Y-Plan wiring diagram on page 47 of the Evohome installation manual (doc ref 50040745-201 A).

There were two issues with the wiring that we needed to resolve. The first was that the Honeywell wiring diagram has an error. The BDR91 for the hot water shows an unlabelled connection, which is actually to the C contact. The Evotouch was the Honeywell product that the Evohome Controller evolved from, and the manual for the Evotouch shows the same Y-Plan wiring diagram, with the connection to the C contact labelled correctly. I can only assume the cut-and-paste between manuals introduced the error.

The second issue was that the wiring for the Suprima boiler requires an always-on power supply, and then a switched live connection to get it to fire up. The boiler can also provide power to the Grundfos pump, presumably to ensure that water is circulating when the boiler is heating water. So we tapped into the power from contact 1 in the junction box, and used contact 8 for the switched live supply to the boiler. We also chose to disconnect the Grundfos pump from the boiler, and connect it to contact 8, as per the Y-Plan wiring diagram.

In terms of wiring, that's the extent of the work. Apart from a small bit of redecorating where we removed the old thermostat from the wall in the hallway, all of the work was in the airing cupboard and out of sight. The only other installation required was to fit the HR92 TRVs, which for us just required screwing them onto the M30x1.5 valves on each radiator. The HR92s separate into two parts, one of which is threaded for installing on the valves, and then the main body then just slides into place on top, with a sliding lock to keep them in place.

Now we just need to configure the system!