Weather Compensation

  • 3 January 2019
  • 30 replies
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Userlevel 7
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hi Jelockwood
​Excellent post, thank you.
I assume that in all cases the modulation is gas valve mark-space rather than analogue burner modulation?
Userlevel 3
woz;29005:
hi Jelockwood
​Excellent post, thank you.
I assume that in all cases the modulation is gas valve mark-space rather than analogue burner modulation?


@woz

I don't know the answer to this. All I can say is they will vary the output of the boiler i.e. the amount of flame to match the required heat output. What method is involved I don't know. As someone who is an IT person rather than a boiler engineer I would have thought this would involve the gas valve being opened to varying levels to pass varying amounts of gas to the burner. The smart thermostat is merely sending commands to the boiler it would be the boiler then varying the output accordingly.

@Oakbank
I would suggest you consider Tado as this is the most compatible solution and has the option for multiple thermostats i.e. zoning, and the option of smart TRV valves.

Note: Google owners of Nest have recently announced the discontinuation of the 'Works with Nest' API making the Nest less useful potentially. The v3 Nest will still support OpenTherm but not eBus and still does not offer matching smart TRV valves.

I have not listed Ecobee as a smart thermostat option incase anyone was wondering because their international support is pitiful.
Userlevel 7
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I strongly suspect mark/space, I just can't see gas valves operating reliably in a progressive mode (if such valves even exist).
This means that there will always be some stepping of output (i.e. less integration of output) especially if the boiler is low capacity. The larger the heat exchanger capacity the more the steps will be smoothed.
I could be talking complete b*ll*x but seems to make engineering sense.
​I suppose it all depends on how small the hysterisis is - too small and valve will operate too frequently, too large and the overshoot will be worse.
jelockwood;29458:
@woz

I don't know the answer to this. All I can say is they will vary the output of the boiler i.e. the amount of flame to match the required heat output. What method is involved I don't know. As someone who is an IT person rather than a boiler engineer I would have thought this would involve the gas valve being opened to varying levels to pass varying amounts of gas to the burner. The smart thermostat is merely sending commands to the boiler it would be the boiler then varying the output accordingly.

@Oakbank
I would suggest you consider Tado as this is the most compatible solution and has the option for multiple thermostats i.e. zoning, and the option of smart TRV valves.

Note: Google owners of Nest have recently announced the discontinuation of the 'Works with Nest' API making the Nest less useful potentially. The v3 Nest will still support OpenTherm but not eBus and still does not offer matching smart TRV valves.

I have not listed Ecobee as a smart thermostat option incase anyone was wondering because their international support is pitiful.
woz;29466:
Isuppose it all depends on how small the hysterisis is - too small and valve will operate too frequently, too large and the overshoot will be worse.

Hey Woz.......can you expand on your comment regarding "Hysteresis" The thermostat my Viessmann boiler uses has the ability to alter the Hysteresis setting. Not sure which is the best setting??
Userlevel 7
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Hysteresis is how much it over or undershoots before it switches off or on.
So for example if you have a room stat with a hysteresis of for example one degree either way and you set it to 20 deg. it would switch the heating off when it reached 21 and on when it fell to 19.

The smaller the hysterisis the more frequently it will switch on and off and the more even the output temperature will be, but it will hammer the boiler if too frequent.

I'm guessing that the recommended setting will be as large as possible with an acceptable evenness of output temp.
I suspect there is a manufacturers recommended setting, but if you find it's either switching on and off too frequently, or the output temp is fluctuating too much (for example it's cooling down too much before it fires again, or getting too hot before it stops) you'd have to tweak it one way or the other.

Gray4276;29475:
Hey Woz.......can you expand on your comment regarding "Hysteresis" The thermostat my Viessmann boiler uses has the ability to alter the Hysteresis setting. Not sure which is the best setting??

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