Just over a month ago, I posted here on The Wall, seeking advice on how to get outdoor reset to work with setback. I got some helpful input from some knowledgeable and experienced professionals, but my reason for raising the question in the first place (there apparently being differences of opinion on the matter) remained. I was left none the wiser. I said I'd feed back in on the subject, if I ever worked it out.
Well, a month later, this is what I've learned from tweaking, and measuring gas consumption (the motivation being, to run the system as efficiently as possible - using the least fuel - not to necessarily run it as comfortably as possible).
1 - There is no flow temperature that will keep the house at a steady temperature in both cool and mild outdoor weather. The correct temperature for mild weather is too high for cool weather (the indoor temperature keeps rising), and vice-versa (the indoor temperature starts to fall when it's mild, if I set the flow temperature based on a cool day). This is a flaw in the 'static' curve of my particular ODR control - see more below.
2 - Setback uses less fuel than constant circulation, even though the flow temperature has to be set higher to recover from the setback. This is probably due to the relatively poor insulation of my house.
3 - Too high a flow temperature results in overshoot (the panel radiators continuing to emit heat once the set temperature has been reached). This therefore results in a longer period of the stat not calling for heat (because the higher the overshoot, the longer the time it takes for the house to cool back to the point at which the stat calls for heat again), resulting in occupants noticing the temperature drop and feeling cold. This is exacerbated in rooms with higher heat loss than that in which the stat is located.
In summary, in my personal experience of using my particular set-up, and having taken daily readings of consumption and compared consumption on similar temperature days, I can very confidently conclude in my property set-back partnered with ODR is more efficient than constant circulation ODR (by about 25%), and no less comfortable.
The trick is to find the adequate temperature to recover from set-back in an acceptable time, but to not go any higher (to avoid overshoot). I therefore gain savings from both set-back and ODR, but not the full savings possible by either. The sum of the savings I get from combining the two, seems to be greater than the savings I'd get from using one over the other, but in any other circumstances (e.g. a tightly insulated property), this may not be the case, and I would strongly recommend collecting empire evidence at the property before concluding on the best way forward, and bearing in mind your lifestyle (in my case, I prefer to set-back at night, and am happy to set-back during the working week, when everyone else is out).
- Viessmann W-100B 27KW mod-con gas boiler with 'static' ODR (curve adjustments up and down, but angle cannot be adapted)
- Tado smart thermostat (v1.0), with geolocation, optimised start and built-in weather data compensation (does not provide feed back to boiler modulation, however)
- Thermostatic rad valves throughout.
- 2300ft2, 3 storey 115 y/o, partially double-glazed, solid walls, no underfloor insulation, good roof insulation
- SE UK (temperate maritime climate, rarely below 0C/32F)
I hope this helps anyone else who, like me (for some time), might be going around in circles trying to figure out the best approach for their hydronic system. There is simply no one-size-fits-all, and don't let anyone tell you ODR is better than set-back or vice-versa. Both have proven benefits in particular circumstances. Think of them as being at either end of a control spectrum. Find where your system and property lie on that spectrum and like me, reap the benefits of both.