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Thread: Evohome firmware Beta Trial - Exclusive for Automated Home Members

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  1. #11
    Automated Home Legend
    Join Date
    Sep 2014


    Quote Originally Posted by jb80 View Post
    I am interested to know whether the "Adaptive Load Scaling" modulation feature increases wear and tear on the boiler because of the frequent stop/start behaviour. I was thinking about this whilst watching this Vaillant video about their modulating thermostat which I gather is varying the power rather than switching the boiler on/off. My thoughts are whether the reduced energy costs using "Adaptive Load Scaling" modulation is offset by the increased wear and tear on the boiler. As a home owner I'd prefer to spend a few extra quid on energy to avoid replacing/servicing the boiler sooner than planned. Thoughts?
    Adaptive load scaling does not do what you apparently think it does. It does not cause additional "stop/start" cycles in any significant way.

    TPI (Time proportional integral) has been an integral feature (heh) of Evohome from day one, if you use BDR91 relays instead of an OpenTherm bridge. The default TPI settings are 6 cycles per hour and a minimum on time of one minute.

    In this configuration your boiler's control input will be switched on and off once every 10 minutes if there is a partial heat demand. (Partial being any heat demand other than 0% or 100%) All that changes with different heat demands is when in that 10 minute cycle it switches on - a higher demand means it switches on earlier in the cycle. (And always switches off at the end of each cycle)

    Adaptive load scaling simply changes the calculation of the heat demand being sent to the relays. With it turned off the heat demand is always equal to the highest heat demand from an individual zone. With it switched on, the heat demands from some zones are "scaled down" to a lower value based (we think) on previously measured characteristics of the zone and the outside temperature. There is also some additive contribution from zones where multiple zones with high heat demand cause more boiler demand than a single zone.

    For example my bathroom zone (which heats up and overshoots quite easily) might be calling for 65% heat by itself but the final contribution to the boiler relay might only be 30% from this.

    So the vast majority of the time load scaling won't cause any additional on/off cycles of the boiler, it will only affect the timing of them slightly within the 10 minute periods.

    The one exception might be if a schedule change causes one zone to come on to 100% - previously this would force the boiler relay to 100% (therefore staying on continously) until the temperature of that zone was with 1.5 degrees of the set point where it would go back to proportional switching. With load scaling some zones may not always be able to call for 100% from the boiler therefore the boiler will switch on/off during that warm up time when it would previously be on continously.

    But this would be a very tiny fraction of the day. In short load scaling depending on your schedules will have little to no effect on the total number of times the boiler cycles on and off per day.

    I'd also make the observation that TPI controls for boilers are now ubiquitous - you won't find a non-Opentherm (or equivalent) thermostat today that doesn't use TPI, so it stands to reason that boilers have to be designed to cope with TPI controls when that's basically all that you can buy for switched control. The days of a mechanical thermostat which only switches on and off once every hour or two with a 2 degree hysteresis are long gone...
    Last edited by DBMandrake; 11th May 2020 at 09:31 AM.

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