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Outdoor reset and setback - neither is always better than the other

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.


  • unclejohn
    unclejohn Member Posts: 1,757
    So you do not have outdoor reset.
  • Brewbeer
    Brewbeer Member Posts: 616
    The description of how the ODR works on your boiler, is different than how it works on mine (Lochinvar WH55). On my boiler, the ODR varies the temp of the water supplied by the boiler to the heat emitters (baseboard find tube radiators), based on the out door temperature. This results in the house being kept at a constant temp, no matter what the out door temp is. Admittedly, it did take a couple of years of fooling around with the settings and observing indoor conditions to get it right.

    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • dm9321287
    dm9321287 Member Posts: 36
    Correct. I run ODR technology with set-back because ODR alone does not work in my circumstances because (i) the curve is wrong for the property and cannot be flattened, and (ii) the heat loss of the building makes it inefficient to run the heating 24x7. Therefore, in my circumstances, ODR is no panacea, but it’s still useful to load limit the boiler based on outside temperature, to avoid inefficient overshoot.

    It’s the most efficient balance for my system. In a garden shed, set-back will be best. In a passive house, ODR will probably be. My house is somewhere between the two, as are most others.
  • dm9321287
    dm9321287 Member Posts: 36
    @Brewbeer, that’s how mine works too, but per my response to @unclejohn, not very well, due to system and property characteristics.
  • Jamie Hall
    Jamie Hall Member Posts: 18,935
    Good information and study. Thank you! Although it reinforces something I've been saying for years -- each situation and setup is different. One size does not fit all!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dm9321287
    dm9321287 Member Posts: 36
    @Brewbeer, I see what you mean about my description, which could also be the root of @unclejohn’s comment.

    When I refer to flow temperature, I essentially mean curve. So, I’m saying if I set to a curve with a low flow temp at freezing point outdoors, that will be too low a curve to hold the house at temp when milder outside. If I set the curve to hold the house at temp when milder outside, it’ll be too warm inside when it’s freezing outside. The inherent flaw in the Viessmann 100 basic ODR is the curve can be moves up and down, but the slope cannot be adjusted. I need a flatter curve. This is why I cannot get a steady temp. However, even if I could, the heat loss cancels out (and some) the lower flow temp efficiency of the ODR. So, in my case, I raise the curve and use setback = optimum efficiency. Horses for courses.
  • unclejohn
    unclejohn Member Posts: 1,757
    From what I read and maybe I misunderstood, is the tstat is what tells the boiler the out door temp but the boiler will not modulate on that info. So do you have a outdoor sensor that tells the boiler what the outdoor temp is and will it react to it. TT has it's own presets as well but you can also set the temps and curves you self.
  • dm9321287
    dm9321287 Member Posts: 36
    We are indeed at crossed purposes, no doubt from my choice of words.

    The boiler outdoor sensor regulates the flow temp. The room stat merely calls for heat (but also relies on ‘Net weather data to influence frequency of fire calls somewhat).

    The ODR curve can’t be flattened or steepened. It can only be moved up and down. In other words, it has a fixed ratio. Therein lies the problem for maintaining a fixed room temp with constant circulation. I have to set a higher curve than needed when it’s cold, or the indoor temp falls when it’s mild.

    This causes me to need to use setback to stop the house overheating PLUS the heat loss of the building makes it inefficient to run the heating 24/7.

    So, in my case, there are benefits in both the ODR and set-back functionality. They just have to be balanced for optimum efficiency as neither is best alone.
    HVACNUT Member Posts: 4,593
    What is the thermostats widest setpoint differential? And does the weather information automatically adjust that differential if it sees colder or milder weather ahead?
  • dm9321287
    dm9321287 Member Posts: 36
    @HVACNUT, I'm guessing here, but the stat fires until 0.1C over set point and re-fires at 0.1C under. I've never noticed that change, but logically, if it holds off firing based on warmer weather approaching, then it must effectively widen the differential. Hard to say for sure, and probably some BS in there from the manufacturers.
  • unclejohn
    unclejohn Member Posts: 1,757
    You may be able to get into the parameters and fine tune the curve. I tell customers I can retune to make final adjustments if needed. Like I said TT has presest curves and adjustable curves.
  • ratio
    ratio Member Posts: 3,177
    Kind of agree with @unclejohn re not really having outdoor reset, regardless of what it's called in the paperworks. Outdoor reset means at x° outside air you supply y° water, at x′° outside air you supply y′° water. IOW, the slope is adjustable. If not, not ODR.
  • dm9321287
    dm9321287 Member Posts: 36
    @unclejohn - by TT are you referring to the room stat? Tado is a (so called) smart stat and as far as I’m aware, all the parameters are held on their servers, not locally. If any adjustments need to be made to the stat, you call them up to change parameters.

    @ratio, I agree with your point. If you can’t alter the ratio, then it’s not ‘real’ ODR. The Viessmann 200 is fully adjustable but on the baby brother 100 that I have, the ODR is more basic and the curve can only be shifted up and down. The ratio cannot be adjusted to steepen/flatten the curve as needed to reflect the building characteristics.

    To @unclejohn’s point, I don’t believe there is any way to access the boiler logic to alter the angle either, but I’ve never asked my engineer, or Viessmann, for that matter. However, there’s no mention of adjustment in the marketing material, and you’d think there would be, if it were possible.
  • RPK
    RPK Member Posts: 91
    The latest version of the W-100 B does have an adjustable ratio. Is you boiler a bit older, or are there differences between versions of the boiler being sold in Europe and North America?

  • dm9321287
    dm9321287 Member Posts: 36
    It is a few years old now (getting on for 10), but it also looks like the US 100 has the same ODR capabilities as the UK 200. UK 'weather compensation' brochure here: https://viessmanndirect.co.uk/files//8e57dbc7-8a10-4065-bcc6-a27700ee752a/weather_comp.pdf
    HVACNUT Member Posts: 4,593
    The OP has stated himself that his test was based on his heating envelope alone. There were a lot a variables and a little tweaking in the test.
    Is .1C equivalent to 38°F? That's what Google tells me.
  • dm9321287
    dm9321287 Member Posts: 36
    @HVACNUT, a temperature of 0.1C is about 32.2F, but I was referring to increments, i.e. if my room stat is set to 20C set point, it will call for heat from 19.9C (67.8F) to 20.1C (68.2F), so (very) approximately, each 0.1C is equivalent to 0.2F. I believe that makes the differential on the stat 0.4F, but I'm certainly no expert.