Controlling UK Combi Boiler

I am mid installing cbus in my new home and have been considering the benefits of integrating this with the central heating system. I have yet to install the central heating/hot water system in and i'll be getting some costs from plumbers over the coming weeks. Is it worth integrating and using a cbus thermostat? Also how easy for the plumber will the integration be? Are there any boilers which offer better integration?

Thanks
Louis

 

UFH heating control with c-bus

Not wanting to hijack your thread, but add to it.

The 4 zone c-bus thermostats are possibly ideal for zoned underfloor heating (UFH), locating a stat in each room. However I am guessing development in Aus has led to all the product litrature pertaining to use with AC vs heating.

In principle the operation should be the same, collateral supporting this and any casestudies would boost my confidence, the same goes for OP requirement.

Many UFH systems may be connected to, and part use solar thermal panels and other sources, i.e. wood/pellet burning stove with a back bolier, into a thermal store. Control of thermal stores for efficient use, making for cost savings along with a few trees, is a serious requirment with many dedicated electronic control systems- perhaps C-Bus should stand alone, and we should be looking to IP intergration.

 

Better integration is a combination of physical plumbing , boiler control capabilities and the sensors.

There are several systems of boiler control aside from simply a 'demand' based system ie one that turns the boiler on full when heat is called for from a thermostat.
Some boilers can maintain a specific flow temperature rather than just off / very hot which can help provide more stable room temperatures. This can also be created externally by manifold mixers, used for under floor systems, as they need to run much cooler.

Modulating boilers will automatically reduce their heat output when needed so that they can be proportionally on, eg producing say 60% of their rated output, which cuts down on boiler on/off cycling. Some boilers allow this modulation to be controlled externally using say a 0-10V signal which can then be used to control flow temperature. Some boilers offer weather compensation which allows them to raise their temperature when its colder outside.

Some thermostats can generate proportional heat demands ie they ask for more heat/cool when the difference between setpoint and current temperature is large cf when it's small. This can bring rooms up to temperature faster. Also some thermostats with inbuilt schedulers can learn how long it takes to bring a room up to heat. Using this they can calculate exactly when to turn on in advance such that the room achieves its setpoint at exactly the programmed time, particularly useful for office type environments or to assure your house is just right when you get up in the morning. This is called optimised start and can save energy.

In an ideal situation in a room the radiator would be on at a stable temperature that just produces enough heat to maintain that room at the required setpoint, rather than constant hot/cool cycles that can lead to temperature cycling and overshoot. i.e. ideally the flow temp in the room should be at this temperature. The problem however is that flow temperature is usually a constant across several rooms unless the system is zoned with manifolds providing individual flow temperature control. Thermostatic radiator valves (TRVs) get close to accomplishing this as they proportionally restrict the flow of the hot water through the radiator to create a lower temperature.

Any feedback system that is simply on/off will exhibit a hot/cold cycling behaviour and an important consideration is the response time between the change and the sensor feedback. As a radiator gets hot it takes some time for the sensor to notice the change as the heat has to travel across the room through the air (which is not a good conductor). Hence it is important to have well sited thermostats. There is also the switching hysteresis of the thermostat ie how sensitive it is to change and reporting that change. In a mechanical thermostat this is the temp change between off/on, on an electronic system it can be much finer - not sure what it is for C-Bus and the external sensors ??

One particular issue occurs with simple installations when you just have one thermostat located in your coldest room eg the hallway but that becomes satisfied. If another room should then need heat then it can't get it and so remains cold. If that thermostat , or indeed any other does demand heat then without TRV's on radiators then all other rooms will heat too. You can use simple on/off electric valves in room zones to avoid this.

Heating control can be as simple or complex as you wish.. and there are systems out there that will manage this. For most people using a radiator based system, TRV's and well sited thermostats work well and the fancier boiler control or plumbing changes are not worth the effort.

You can aggregate multiple heat requests into a boiler and if you use the C-Bus thermostats then you have flexible capabilities for manual overrides, timing, temp changes and later expansion too.

One surprising thing I don't believe the C-Bus thermostats offer is assignment of an individual (external) temperature sensor to each zone so you can't for example buy the four zone thermostat and three extra temp sensors and have individual thermostatic control of four rooms. You would need a thermostat for each room... correct me if I'm wrong Clipsal....??

K

 

Isn't it just one external sensor for the whole four zone thermostat though rather than supporting three or four sensors - one per zone ?

ie Each zone can be individually timed controlled but not temperature controlled.

K