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Thread: Evohome - boiler on VERY frequently

  1. #31
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    Quote Originally Posted by garmcqui View Post
    Hi Top Brake,

    It wasn't just 3am though, I just happened to choose this time as an example. The same thing happened at from 10am last night (when I started recording), then 10.10, 10.20, 10.30 and every 10 minutes (approx) all the way through until 5am when the boiler came on for longer due to hot water demand.

    Regarding the double binding - since installing it, I have always had the "Boiler Demand" set to NONE - but Rameses above seemed to suggest that myself and Jonstatt should be able to see settings such as cycle rate, minimum on time etc, which we couldn't.

    Tonight I have done a factory reset on the Evohome controller, removed the batteries for 30+ mins, done a hard reset on each HR92 and cleared the binding on both BDR91 relays.

    I am now in the process of slowly rebinding them, being sure to leave plenty of time between each one so as not to confuse it.

    Thanks, and apologies if the above is confusing!

    Gareth

    I was careful to ensure I didn't have the double binding, but I did find that weird things were happening until I pulled the batteries from the controller and rebooted (requests for heat when all radiator zones were closed). There are still short requests for heat though (correctly when radiator zones are open)....not quite as short as you are noticing and I will be curious to see what you report back.

    Top Brake : You mention s and y-plan are supported for legacy installations. But I fancy many installations today of EVOHOME will be using this. I see no reason why you couldn't add the boiler controls for those configurations if only you would support the notion of controller firmware upgrades. There is a USB port there....I realise there is a time/cost investment to come up with a Windows/Mac firmware updater viable for home use, but I sense this continued unwavering determination never to release a firmware update for this product which I find disappointing.

  2. #32
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    Quote Originally Posted by top brake View Post
    No need to replace any valve, you can simply latch open the existing 2 port heating valve or remove it. Then re-use the existing heating relay as a boiler relay. When installing like this you need HR92 on the radiators. There is a small typo in the evohome manual at figure 4, should say V4043.

    I am not saying you need to make adjustments to the boiler, your boiler installer may need to change the settings for you now an evohome system is installed if previously no TRV were fitted. A modern high efficiency condensing boiler is a very sophisticated appliance with a modulating gas valve and clever control electronics (incidentally often made by Honeywell). The Honeywell product design and development team understands these capabilities and evohome was developed to work in harmony.
    Ok I see, but by latching the central existing heating zone valve open, doesn't this mean that even if there is a call for heat for DHW, the entire central heating circuit will be heated (apart from the radiators which are shut off, either by HR92's or other TRV's)?

    All radiators originally had manual TRV's except one, but now all radiators are either controlled by HR92's with a couple (utility room, 2nd bathroom) still having manual TRV's set only to the frost setting.


    So, how did the system do last night? Well, we had a 30sec demand for heat at 22.32, but then no demand until 23.58! Brilliant, I though. But then from that point onwards there was a demand for heat every 10 mins (for 7 hours), each lasting about a minute.

    So - is this normal behaviour? Should the Evohome be instructing the CH zone valve to open EVERY 10 mins?

  3. #33
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    If there's a room that needs to be kept at a certain temperature, a little heat will have to be added every 10 minutes. Unless that 1 minute of heat per 10 minutes is making the temperature rise too much. In that case, it should stop adding heat. But if the temperature remains exactly on setpoint (or maybe 0.2 above or below), then it will continue to ask for heat every 10 minutes, to maintain that temperature. That's normal.

    Normally, you would be able to change the 10 minute cycles to 20 minute cycles, and you would be able to change the minimum on time to 2 or 3 or 4 or 5 minutes, but you don't have those options available if I understand correctly? These options would probably decrease the boiler firing frequency a lot.
    Last edited by erik; 15th March 2015 at 12:27 PM.

  4. #34
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    Quote Originally Posted by erik View Post
    ..... and you would be able to change the minimum on time to 2 or 3 or 4 or 5 minutes, but you don't have those options available if I understand correctly? These options would probably decrease the boiler firing frequency a lot.
    I'd still love to know what that "min on time" is ? It seems fairly obvious that this should be the minimum boiler demand time - but still can't get an answer from Honeywell

  5. #35
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    It's the minimum time that the BDR91 will send an ON signal to the boiler. So let's say the BDR91 calculates that it needs to do 30 seconds on and 9:30 off to achieve a certain heat demand. If minimum on time is set to 1, it will go on for 1 minute instead of 30 seconds. Either that, or it will stay off untill the demand is at least 1 minute. Still unclear on that. In my system, I've never seen the BDR91 skipping a 10 minute cycle. So I guess the former is the case.

  6. #36
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    Quote Originally Posted by erik View Post
    It's the minimum time that the BDR91 will send an ON signal to the boiler. So let's say the BDR91 calculates that it needs to do 30 seconds on and 9:30 off to achieve a certain heat demand. If minimum on time is set to 1, it will go on for 1 minute instead of 30 seconds. Either that, or it will stay off untill the demand is at least 1 minute. Still unclear on that. In my system, I've never seen the BDR91 skipping a 10 minute cycle. So I guess the former is the case.
    yes - that's what I thought but obviously doesn't work as mine is set to 5 minutes but I still see 1 minute demands - looks like others do too.....what is the default value ?

  7. #37
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    I've just been thinking - on my (S-plan) system, when the BDR91 sends a boiler demand, the CH zone valve opens and this in turn switches on the boiler. However, as soon as the heat demand from the BDR91 ends, the zone valve closes immediately and boiler turns off. Now there must still be a lot of heat left in the system, but due to the zone valve being closed the heating circuit is isolated so it can't be circulated through the radiators, instead it's just pumped around the bypass circuit for 5 minutes until the pump overrun ends.

    This can't be the best way to do it, surely? Maybe I would be better doing what Top Brake suggests, latching the CH zone valve permanently open and configuring the BDR91 as a boiler relay instead? Hmmm...

    Quote Originally Posted by erik View Post
    In my system, I've never seen the BDR91 skipping a 10 minute cycle. So I guess the former is the case.
    Surely it must miss a 10-minute cycle if the room temperature is above what is being asked for?
    Last edited by garmcqui; 15th March 2015 at 05:52 PM.

  8. #38
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    The default is 1 minute. I'm not sure how it behaves in other setups... I do know for example that cycles/hour setting (default 6) does not apply when using an Opentherm relay. So other settings might be ignored by certain setups as well.

  9. #39
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    Now I could be wrong with this thought but understand that what switches the boiler on is actually the pump. In other words the thermostat triggers pump on and that then tells the boiler to fire up. Therefore if the boiler senses the water temperature is at maximum the boiler will switch off but the pump will continue. TRVS that are open still calling for heat will continue to get it as the pump is running, then when the water temperature has gone down the boiler fires up again.

    I am no expert but that seems to me to be what is happening for me. Indeed before Evohome I had a wireless thermostat and that triggered the pump and not the boiler.

  10. #40
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    pretty sure (on mine anyway) the thermostat/evohome controls the zone valves, which control the boiler, which in turn controls the pump.

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