Results 1 to 10 of 780

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

Threaded View

Previous Post Previous Post   Next Post Next Post
  1. #10
    Automated Home Legend
    Join Date
    Sep 2014


    Quote Originally Posted by kevinsmart View Post
    With older firmware, and OpenTherm, the boiler still used to fire if the aggregated demand was much less than 10%.

    With the newer firmware it only seems to send a flow temperature above 10C to fire the boiler if the demand is at least 10%.
    Oh, that's disapointing to hear.

    One of the things that attracted me to OpenTherm if I ever ended up with a new boiler is I hate the 10% minimum heat demand threshold the Evohome system has when using a boiler relay.

    It leads to significant temperature cycling under low loads such as one or two bedroom radiators on at night. The room has to fall significantly below the set point before a 10% heat demand is reached, that then leads to an overshoot, then another undershoot etc...

    I've always said that the minimum ontime TPI algorithm in the Evohome is wrong - yes there should be a minimum ontime for the boiler of say 1 minute, as its not good or efficient to short cycle boilers, however the mistake is in not sending any heat demand at all until there is a demand of 10% - that can be a significant dip below set points in low heat conditions, (and especially with the new load scaling algorithm) enough to trigger oscillations as the control algorithm in the HR92's are opening their valves progressively and wondering why nothing happens for such a long time...

    Currently it works like this with a relay:

    Heat Demand - Relay on time percentage:

    0% - 0%
    1% to 9% - 0%
    10% - 10%
    11% - 11%

    etc. Everything below 10% is ignored. Here's how it should work:

    0% - 0%
    1% to 9% - 10%
    10% - 10%
    11% - 11%

    This way minimum ontime for the boiler is respected but even a small heat demand is enough to trigger the boiler and avoid cycling of temperatures and overshoots in low head demand conditions.
    Last edited by DBMandrake; 20th September 2020 at 12:46 PM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts