Room thermostat controlling rads & boiler. OpenTherm?

[JackKelly]

I'm currently planning an upgrade for our ancient central heating system. We're renovating the livingroom at the moment and I want to install a room thermostat. I want the room thermostat to be able to both turn on the room's radiators and ask the boiler for more heat. I've found several radiator actuators which replace the TRV heads, which I'm planning to use.

To my surprise (I'm new to this), programmable room thermostats only seem to have one output. But I want to control both the boiler and the rads. So does this mean I need to add a relay, something like this:

(of course, I'll add fused switches where necessary).

Can I buy and off-the-shelf wiring box with the necessary relays or do I need to make one myself?

Also, I've been reading about the OpenTherm spec. Our current boiler is currently on its last legs so we'll be upgrading our boiler soon. Is it possible to get an OpenTherm room thermostat to control both the boiler and the room's radiators? From my research so far, I can only find OpenTherm stats which communicate directly with the boiler, with no ability to control the rads.

Thanks loads for the help,
Jack

 

[D_Hailsham]

Controlling the temperature of each room is usually done by installing a thermostatic radiator valve (TRV) on each radiator. One room is left with normal valves as that room has the wall thermostat, which controls the boiler. This is normally suitable for most houses, unless you have a very large house.

A more sophisticated method is to use wireless TRVs which can each control the boiler. The Honeywell CM Zone system does this. This can handle two zones. You can link up to four CM Zone controllers together to control up to eight zones. If you need more than eight zones you would have to install the Honeywell Hometronic system, which is very expensive.

Zones are areas with the same time and temperature settings. Current regulations say that, provided your house is below 150m² floor area, you have to have at least two heating zones, with temperature control, in each zone. The zones can have common times. The room stat/TRV system meets this as each room with a TRV is effectively a zone and the room with the room stat is another zone.

If the floor area is above 150m² then you also need separate time controls. Normally this is done by dividing the house into a downstairs zone and an upstairs one and having separate times for each. There would be a room stat upstairs and downstairs as well as TRVs in the rooms.

Opentherm is a method of communication between a boiler and its controller. The controller can either send info to the boiler, eg. the required flow temperature, or request info from the boiler, e,g the current return temperature. I cannot see any need for this two-way communication from a thermostatic valve. The only signal would be for the TRV to tell the central controller that more heat is required. The controller would then communicate with the boiler.

Not all boilers can work with Opentherm controllers.

 

[ianniann]

Someone is going to say it, so I'll go first. Huh?

What are you trying to achieve? Do you want to switch radiators on and off individually on a timed basis? On the basis of the temperature in that room? What are you trying to do to the boiler when this happens? What do you mean by "more heat"? The level of heat produced by the boiler is controlled automatically (assuming a half decent boiler) to maintain a given temperature of water within the system. You don't have to ask the boiler to do this. More radiators on means more heat lost as it circulates and the boiler will turn up to replace that heat. Your diagram showing one radiator in one room is presumably not the whole system?

There are certainly multi-channel controllers out there. How many channels do you need? Two is common, intended for controlling heating and hot water separately, but you can have more than two.

Opentherm is a standard for communicating between controllers and boilers that allows functions such as the controller changing the flow temperature of the water from the boiler. This allows you to have these complex functions without having to use a proprietary controller from the same manufacturer as the boiler. If you have an Opentherm compatible boiler then you can use any Opentherm compatible controller. Otherwise you'll have to use the maker's own controls to do anything more complex than providing heat and no-heat commands to the boiler.

 

[D_Hailsham]

More radiators on means more water lost as it circulates and the boiler will turn up to replace that heat.

Presumably you meant to write heat?

 

[JackKelly]

Many thanks for the replies. All interesting stuff.

What am I trying to achieve? I'm trying to ensure that we only heat rooms when they're occupied. We don't have a huge house (main bedroom, spare room, study, kitchen, livingroom, dining room). At the moment we have TRVs on all the radiators and we manually turn them on and off.

Here's a typical evening:

8pm = dinner in the kitchen
9pm = watching TV in the living room
11:30pm = bed!

At the moment this means that I have to do this:

At 8pm = turn on the TRV in the kitchen and make sure all the other TRVs are off.
At 8:30pm = turn the livingroom TRV on to give the livingroom time to heat up.
At 9pm = turn the kitchen TRV off and we move to the livingroom.
At 11pm = go upstairs to turn the bedroom TRV on.
At 11:30pm = turn the kitchen TRV off and we go to bed

What I want is a way to automate this!

As far as I can tell, my best bet is to use programmable room thermostats in each room. These thermostats will control radiator actuators. Ideally I'd like the room thermostats to also ask the boiler to turn on so the boiler and pump turns off entirely when there's no demand for heat in the occupied room(s)

Why do I want the room thermostats to communicate with the boiler? Consider the situation where all the programmable room thermostats are above their target temperatures and so their radiators of closed. Without a way for the room thermostats to communicate with the boiler, the boiler's pump will continue running, wasting electrical power. Granted, the boiler will monitor the return temp and won't fire very often but it will still continue to fire because the CH circuit will be losing heat through the pipework and the bathroom radiator.

In my absolutely ideal system, the room thermostats would send detailed temperature readings to the boiler so the boiler can modulate optimally. And, in an absolutely ideal world, I and my wife would be able to use our smart phones to view and modify the room timers and target temperatures (say, for example, we go out for a drink instead of coming home at 8pm and hence we want to delay the heating until 11:30pm. Or say we're watching a film in the livingroom and we want to delay the bedroom's radiators until midnight).

What's the ultimate aim? To minimise gas consumption whilst maximising comfort.

 

[ajg68]

Sounds like a lot of hard work to me!

Why not spend your efforts on insulating the house & getting weather compensation with your new boiler?

 

[JackKelly]

Sounds like a lot of hard work to me!

I don't mind investing the time running the cables. But I'm eager to keep the equipment cost low. Some home automation systems are absolutely extortionate.

Why not spend your efforts on insulating the house & getting weather compensation with your new boiler?

We will do these things as well. We're currently insulating our living room to the Energy Saving Trust's Best Practice targets.

Will my strategy work? (using a programmable room thermostats to switch relays which, in turn, switches the radiator actuator and the boiler)

Thanks for all the replies

 

[ianniann]

Presumably you meant to write heat?

Thanks. Edited to fix.

 

[ianniann]

OK. I understand the issue now. Essentially you want a multiple zone heating system, with potentially every room being a separate zone. This isn't usually done in homes for the simple reason that the cost far outweighs the potential savings. I'm sure that 90% of the people who reply will tell you this.

So you need to investigate multiple zone heating systems and controls. Typically these are expensive both because of the number of separate controls and valves, and also because of the logic to cross-link everything. Very often there will be multiple pumps and even multiple boilers. To allow each zone to be controlled separately the piping usually has to be run separately to each zone from a central manifold. All very pricey. You can visit the Honeywell Hometronic page if you want to see what these systems are like.

You are basically trying to bodge together a very simple multi-zone system. The radiator actuators themselves should be simple enough to time. So then the only problem you have is controlling the boiler. New boilers (half decent ones anyway) are designed to automatically modulate their output to match the number of radiators on or off. Better boilers will also modulate the pump. This works fine on the basis that at least one radiator is always on to allow some heat to be dissipated, but even so there should be a bypass in the emergency event that all radiators are closed off. If you could see your way to providing a zone which is always on at the times when you want heating then your job becomes a standard installation plus remote radiator actuators. Otherwise you need to link all your separate room controllers to control the boiler. I'm guessing most gas fitters wouldn't go near it. The permanent on zone could be a central hallway or perhaps a towel rail radiator. It should really allow for a decent amount of heat dissipation, you'll have to check with the boiler instructions on how much guaranteed circulation it needs.

 

[PEDANTICVINDICTIVEMAN]

I have a headache and your not helping

 

[JackKelly]

Really excellent reply, thank you. Yes, I suppose I am trying to achieve a zone-per-room system on the cheap

My current thinking is something like this:

Radiator actuators replace all the TRV heads (actuators are around £30). The TRV actuators are cheaper than zone valves plus they have the advantage that if they go wrong you can just unscrew them and replace them with a normal TRV head.

HeatMiser Network Programmable Room Thermostats in each room (around £50 each) to control the radiator actuators and to send a signal back to the boiler

HeatMiser produce a box which allows their room thermostats to be controlled over the Internet. Costs about £300. Alternatively...

HeatMiser publishes the protocol used by their network room thermostats and there are a few folks on the net looking into hacking together a home-brew control system to replace the £300 system.

 

[mickyg]

I won't bother telling you what to do, as you seem to be quite clear on that but here's some products that might be worth a look, not generally available in UK but can be imported.

Danfoss ra plus-w (z wave)
Danfoss ra plus and ra-k programmable
Honeywell Rondostat

I recently completed a system with 2heating zones, WC and 5 rondostats for the very same reason as you, that the customer only used certain parts of the house at certain times of the day.

mick

 

[D_Hailsham]

What am I trying to achieve? I'm trying to ensure that we only heat rooms when they're occupied.

Do you keep all internal door closed, except when you are moving from one area to another? If you don't, you are wasting your time trying to control the temperature in each area individually.

 

[Boilerman2]

If you use your wiring diagram expect electrocution (switching Neutrals indeed!!!)

 

[JackKelly]

Yup. Internal doors are kept closed and draught proofed. I've insulated in the void between the ground floor ceiling and first floor floor where possible too.

Good point re switching neutrals boilerman, thanks