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Thread: Prevent HVAC coming back on so soon

  1. #1
    Automated Home Guru jpdw's Avatar
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    Default Prevent HVAC coming back on so soon

    Hi...

    Might seem a strange question, but is there a way to enhibit the HVAC from coming on for say, X, minutes after it last went off?

    I'm trying to avoid short cycling... I think I can effectively do the same thing by using the Run-On Period to make sure I have a certain amount of overshoot.

    I'm battling with a loft conversion room that seems to cool down too quickly when the difference between inside & outside is >16-17C (like at the moment). I'm happy to have some overshoot but would like to "enforce" an off-duration between boiler firings. I figure that during the night a variation of 1-1.5C wont really be noticed (or do our baby daughters any harm) and less frequent cycling might be more efficient on the gas (due to system losses) than tracking the setpoint so closely. [if any of that makes sense!]

    ... this is all a workaround for a bigger radiator and lots of additional insulation!
    Jon

  2. #2
    Automated Home Legend Karam's Avatar
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    With on/off control you will of course get an oscillation about the setpoint level whose symmetry depends on heat input rate relative to heat loss rate. This will change depending on outside temperature amongst other factors.

    What this means is that if no other automated adjustments are made then when outside is warm the average temperature will typically be higher than the set point (since overshoots will dominate) and when outside is cold the reverse. Of course various things can be done to compensate - for example the external temperature related auto set point adjust feature in the Cortex HVAC.

    So in your case you are in effect asking to increasing the asymmetry of the oscillations towards the 'under setpoint' region - meaning decrease the average temperature. So you might as well just adjust the set point downwards.

    I suppose an alternative would be to have higher hysteresis - such that you increase overshoot above setpoint as well as below it. This could mean the average temperature remains around the setpoint but you have bigger deviations around it (normally not comfortable). This would also mean boiler on/off periods would be stretched (which is what you wanted). There is no method to change the hyteresis of the HVAC on/off controller but you could change the resolution of the temperature sensor which is serving it (you would also have to change the polling period for that sensor to something long eg. 59 minutes - temperature is automatically sent from the sensor when change is greater than resolution, polling is only used as a sort of health check, but if your polling is relatively quick then Cortex can effectively get readings higher than set resolution).

    To be honest I'm not advocating the above but it might be interesting to experiment with - when no one is actually using the room.

    You can as you say also increase the minimum ON time. If you can guess this right and conditions are consistent then you might achieve a similar end result. But you'd really have to have consistent conditions so not sure how practicable.

  3. #3
    Automated Home Guru jpdw's Avatar
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    Hi Karam,

    Thanks as usual for the excellent and detailed reply ....

    Quote Originally Posted by Karam View Post
    ...you could change the resolution of the temperature sensor which is serving it (you would also have to change the polling period for that sensor to something long eg. 59 minutes - temperature is automatically sent from the sensor when change is greater than resolution, polling is only used as a sort of health check, but if your polling is relatively quick then Cortex can effectively get readings higher than set resolution).
    I'd not even realised that would do the same things. Yes effectively I'm finding that the accuracy of the control system is too good compared with the thermal losses that I seem to be getting with such a wide difference between inside & out.

    Quote Originally Posted by Karam View Post
    To be honest I'm not advocating the above but it might be interesting to experiment with - when no one is actually using the room.
    This isn't really something I want to do either -- but I want to find a short-term (this winter) way to reduce cycling / improve efficiency while I solve the root cause of the problem - which is that the heating system (not the control system) needs to be running 18h of 24h to provide enough watts of heat to compensate for the losses.

    Your comment about the external temperature input made me have another look at the Supplementary Heat Source features ... In our case, as the heating seems to manage just fine maintaining ~17C when it's 5-10C outside, so the "use if less than <X outside" feature might be another way round - A test in the last hour shows our 2kw elec heat achieves in 20 minutes what the heating takes 4 hours to do. With a 13A relay and my spare DRB.....

    Still.. this evening's proved to me that what I really need to do is solve the thermal issues rather than trying too many wierd workarounds.
    Last edited by jpdw; 11th January 2009 at 02:30 AM.
    Jon

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