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Thread: Evohome firmware 02.00.19.31 Beta Trial - Exclusive for Automated Home Members

  1. #541
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    Hi Kevin

    I've recently installed Evohome with Opentherm control on a S Plan setup. I've spotted the opening and closing of the CH valve periodically when under low load, why would it being doing this? I thought the purpose of Opentherm was to keep radiators at a low temp, in my case I've temporarily latched open the CH valve as my radiators weren't getting warm during this low temp cycling and thus the room wasn't getting fully up to temp.

    Dan
    Quote Originally Posted by kevinsmart View Post
    What boiler do you have? EvoHome does employ on/off control with OpenTherm under low load situations, but for me it is always at lower temperatures, typically under 40C.

    Have you connected an OpenTherm gateway to monitor the flow temperature request? It could be that what you are observing is the boiler firing with its startup fan rpm at say 40% before modulating down, but the requested flow temperature is soon overshot by 5C resulting in the boiler turning off, starting its anti-cycle.

    You may be able to tweak some of your boiler parameters to improve e.g. if you have Intergas, but fundamentally your boiler probably can’t modulate down low enough to satisfy the kW output needed, so you end up in this on/off situation with the boiler firing at higher output for short intervals.

  2. #542
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    Quote Originally Posted by Mid112 View Post
    I have Intergas xclusive 30 - it can modulate by design to 25C by design If I remeber correctly with 30C set by default.
    You're not understanding what was meant by "can't modulate low enough". It's not referring to the minimum flow temperature, but the minimum power output.

    If you have a 25kW boiler and a modulation ratio of say 5 to 1 that means the boiler can only modulate its heat output down to 5kW, eg 1/5th of the maximum output.

    If the combined heat demand from the radiators is less than the minimum power output of the boiler (which can very easily happen in warm weather, with a low requested flow temperature) then even at minimum burn the flow temperature will keep going up and can't be controlled. When it goes more than a few degrees above the requested flow temperature the boiler will actually shut the burner off for a while, and this will cause the burner to cycle on and off even though OpenTherm is requesting a constant low flow temperature.
    Last edited by DBMandrake; 30th September 2020 at 08:31 PM.

  3. #543
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    Quote Originally Posted by daniel@paynefamily.co.uk View Post
    Hi Kevin
    I've recently installed Evohome with Opentherm control on a S Plan setup. I've spotted the opening and closing of the CH valve periodically when under low load, why would it being doing this? I thought the purpose of Opentherm was to keep radiators at a low temp, in my case I've temporarily latched open the CH valve as my radiators weren't getting warm during this low temp cycling and thus the room wasn't getting fully up to temp.
    Dan
    In an S-Plan system with a heating zone valve the zone valve will always cycle on and off using TPI when there is a partial load. This is true whether you have OpenTherm or whether like me you have a boiler relay. It's the way the system is designed. It won't stop the rooms getting up to temperature as it will always adapt and find an equilibrium. Maybe you're being impatient and not giving the system long enough to adapt, because if the rooms are persistently below their set points it will gradually increase the heat demand to find a new equilibrium.

    Personally I don't like the heating zone valve cycling open and closed with every TPI cycle because it tends to provoke my ancient boiler into kettling. So I've made a tweak to the wiring logic in my system so that if the pump is running and the hot water zone valve is closed, the heating zone valve stays open continuously, ignoring the TPI cycling that would normally be sent to it. (But it does still do TPI cycling if hot water reheat is being performed to prevent the hot water reheat causing the room temperatures to overshoot)

    I find this an improvement on the default behaviour of the system.
    Last edited by DBMandrake; 30th September 2020 at 08:33 PM.

  4. #544
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    Quote Originally Posted by daniel@paynefamily.co.uk View Post
    Hi Kevin

    I've recently installed Evohome with Opentherm control on a S Plan setup. I've spotted the opening and closing of the CH valve periodically when under low load, why would it being doing this? I thought the purpose of Opentherm was to keep radiators at a low temp, in my case I've temporarily latched open the CH valve as my radiators weren't getting warm during this low temp cycling and thus the room wasn't getting fully up to temp.

    Dan
    Hi Dan,

    With EvoHome and HR92 on all radiators you really should keep the CH valve open. I have a Normally Open valve fitted which closes when DHW is being heated.

    I guess what is happening is that when EvoHome is switching to its low load on/off control, the CH valve is closing in conjunction with a 10C demand (off), and reopening when a higher flow temp is sent (on).

  5. #545
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    Quote Originally Posted by DBMandrake View Post
    In an S-Plan system with a heating zone valve the zone valve will always cycle on and off using TPI when there is a partial load. This is true whether you have OpenTherm or whether like me you have a boiler relay. It's the way the system is designed. It won't stop the rooms getting up to temperature as it will always adapt and find an equilibrium. Maybe you're being impatient and not giving the system long enough to adapt, because if the rooms are persistently below their set points it will gradually increase the heat demand to find a new equilibrium.

    Personally I don't like the heating zone valve cycling open and closed with every TPI cycle because it tends to provoke my ancient boiler into kettling. So I've made a tweak to the wiring logic in my system so that if the pump is running and the hot water zone valve is closed, the heating zone valve stays open continuously, ignoring the TPI cycling that would normally be sent to it. (But it does still do TPI cycling if hot water reheat is being performed to prevent the hot water reheat causing the room temperatures to overshoot)

    I find this an improvement on the default behaviour of the system.
    Thanks. The added benefit of latching open the CH valve is our bathroom towel rail now regularly warms up our towels which is great. Given the unvented cylinder typically reheats when required in 10-15 mins I hoping we don't suffer any temp overshoots at the radiators whilst the boiler temporarily increases its temp to heat the water.

    Do the radiator valves also learn how many turns are required to allow a minimal flow of heat into the radiator, say a 9% heat request? Just wondering how they know given the variety of different TRV valves on the market.
    Last edited by daniel@paynefamily.co.uk; 30th September 2020 at 08:44 PM.

  6. #546
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    Quote Originally Posted by kevinsmart View Post
    Hi Dan,

    With EvoHome and HR92 on all radiators you really should keep the CH valve open. I have a Normally Open valve fitted which closes when DHW is being heated.

    I guess what is happening is that when EvoHome is switching to its low load on/off control, the CH valve is closing in conjunction with a 10C demand (off), and reopening when a higher flow temp is sent (on).
    Thanks. I have looked into this, but at this stage I'm trying to avoid any physical plumbing changes. Might fit the normally open valve if I start to suffer too many radiator temp overshoots whilst the cylinder is reheating.

  7. #547
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    Talking of TPI, I assume this TPI in action from 18:45 onwards?



    TIA

  8. #548
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    Quote Originally Posted by Somebody View Post
    Talking of TPI, I assume this TPI in action from 18:45 onwards?



    TIA
    Yes, it appears to be, every 10mins or so. A plot of the flow temperature request might show short 10C (off) requests.

  9. #549
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    Quote Originally Posted by Somebody View Post
    Talking of TPI, I assume this TPI in action from 18:45 onwards?



    TIA
    If you use a boiler relay, yes, if you use OpenTherm, no. TPI only applies to relays.

    Edit: On thinking it through a bit more if you use OpenTherm plus a heating zone valve, the zone valve will be performing TPI, and this will indirectly affect the flow temperature by cycling the heat demand the radiators put on the boiler, so even though the boiler is trying to reach and maintain the OpenTherm target the actual temperature will modulate up and down a bit every 10 minutes as the zone valve opens and closes.

    However if you have OpenTherm and no heating zone valve you wouldn't see rapid 10 minute "wiggles" like this.
    Last edited by DBMandrake; 1st October 2020 at 09:57 AM.

  10. #550
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    Quote Originally Posted by DBMandrake View Post
    If you use a boiler relay, yes, if you use OpenTherm, no. TPI only applies to relays.

    Edit: On thinking it through a bit more if you use OpenTherm plus a heating zone valve, the zone valve will be performing TPI, and this will indirectly affect the flow temperature by cycling the heat demand the radiators put on the boiler, so even though the boiler is trying to reach and maintain the OpenTherm target the actual temperature will modulate up and down a bit every 10 minutes as the zone valve opens and closes.

    However if you have OpenTherm and no heating zone valve you wouldn't see rapid 10 minute "wiggles" like this.
    Not quite. You will see this with OpenTherm and no heating valve when the low load algorithm comes into play. (I seem to keep repeating myself on this).

    The algorithm sends a >10C flow temperature request (on) followed by a 10C request (off).

    Typically the boiler in these low load situations will reach 5C above the flow temperature request and cut ignition. It will start to anti-cycle but the 10C off request followed by another flow temp request can interrupt this and result in further heating if the flow temp has dropped.

    The OpenTherm Monitor plots clearly show this behaviour.

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