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Thread: Boilers, BDRs and TPI

  1. #11
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    So now I've been using Dan's development code, I can see much more clearly when Evohome is asking for the boiler to be on. This has, however, highlighted a couple of interesting things...

    Firstly, my Evohome was using 4 minutes minimum on time, and 6 cycles per hour. 4 minutes seems like a long time. What do others use? My boiler is a Potterton Suprima 100L 30kWh heat-only non-condensing, non-modulating dinosaur.

    Secondly, it seems that during the day when temperatures are fairly stable, Evohome is using the minimum it can to keep the house warm: 4 minutes on, 6 minutes off. However, during the 4 minute period the boiler shuts down because it's reached its flow temperature. So it will fire up for a minute or so, then shut down for a minute or two, then fire up for 45 seconds, then shut down for the remainder of the cycle. This seems particularly inefficient to me. Does it also suggest other problems with the system, such as undersized radiators (failing to take enough heat from the flow)?

    I will soon be replacing the boiler with a modulating, condensing one and attempting to set up OpenTherm too, so hopefully these problems will soon be behind me, but I'm certainly interested to hear people's opinions on whether this indicative of other issues.

  2. #12
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    Quote Originally Posted by dty View Post
    So now I've been using Dan's development code, I can see much more clearly when Evohome is asking for the boiler to be on. This has, however, highlighted a couple of interesting things...

    Firstly, my Evohome was using 4 minutes minimum on time, and 6 cycles per hour. 4 minutes seems like a long time. What do others use? My boiler is a Potterton Suprima 100L 30kWh heat-only non-condensing, non-modulating dinosaur.
    I use 6 cycles per hour and 1 minute minimum ontime - eg the default. I experimented with 3, 6, and 9 cycles per hour and with 1 and 2 minute minimum on times and for my system at least, my conclusion was that the default 6 cycles and one minute worked the best.

    Reducing the cycles to 3 per hour made the system far too slow to respond to small changes in heat demand in a timely fashion - say bumping up the temperature in a room half a degree or even one degree, when the boiler TPI duty cycle is already quite low. It could be 20-30 minutes before I'd notice any extra heat from the radiator. I also found it allowed the room temperature to fluctuate more than 6 cycles did when overall system demand was low. So I wouldn't recommend 3 cycles per hour unless someone had a boiler that really didn't like being cycled more often. (Oil fired ?)

    I found no advantage to more than 6 cycles per hour - in theory it would give a "smoother" control, but it has a side effect that it effectively makes your minimum on time percentage higher. EG if you use 6 cycles and 1 minute minimum on time, the minimum heat demand that the boiler can provide is 10% - or a 10:1 modulation ratio, in effect. If you were to use 12 cycles per hour you now have 5 minute cycles with a 1 minute minimum on time, now your minimum boiler output is 20% or a 5:1 modulation ratio - the smaller modulation ratio is not good when heat demand is minimal.
    Secondly, it seems that during the day when temperatures are fairly stable, Evohome is using the minimum it can to keep the house warm: 4 minutes on, 6 minutes off. However, during the 4 minute period the boiler shuts down because it's reached its flow temperature. So it will fire up for a minute or so, then shut down for a minute or two, then fire up for 45 seconds, then shut down for the remainder of the cycle. This seems particularly inefficient to me. Does it also suggest other problems with the system, such as undersized radiators (failing to take enough heat from the flow)?
    If you have a non-modulating boiler, like me, what you describe is normal. Under a low heat demand quite a bit (or all) of the BDR91 on time during a cycle may largely coincide with an off period for the boilers own flow thermostat. There's not much that can be done to avoid this, the boiler has to shut off when it reaches its set flow temperature, which will happen quickly if the radiators are hardly flowing and most of the flow is returning via the ABV.

    If you have a modern modulating boiler it's probably a sign that the current heat demand is lower than what your boiler can modulate down to - either an oversized boiler or insufficient modulation ratio. But keep in mind that in a large house with a lot of radiators and a boiler specced for them that it would be almost impossible not to have the boiler cycling off and on if say only one radiator is demanding a small amount of heat. The heat demand of a single radiator is usually below the minimum heat that a boiler can modulate down to.

    I think you'll find that your excessive minimum on time is not allowing evohome to reduce the boiler output low enough - effectively you're telling it that it has to run the boiler at at least a 40% duty cycle, or not at all. So it waits until a 40% heat demand builds up before it fires the boiler at all, but when it does so the boiler runs for long enough to hit its own flow temperature cutout. Change the minimum on time back to one minute and I suspect you won't see the boiler hitting its flow temperature limit under low load conditions nearly as often.

    On my system I usually see very low loads (one bedroom radiator on at night in winter) resulting in an average flow temperature of around 40-50 degrees even when the boiler flow temperature thermostat is set to 70, because evohome is only running the boiler for about 1 minute out of each 10 minute cycle to maintain that one room at 17 degrees.
    Last edited by DBMandrake; 4th May 2017 at 05:07 PM.

  3. #13
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    Same here; 6 cycles per hour and 1 minute minimum on time and I have a modulating boiler. I think I messed around with these settings last year and came to the same conclusion as DBMandrake.

    Dan

  4. #14
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    I use OT, so was surprised to see that I was even offered these options in the UI but they are set to 1min and 6 per hour. I suspect however with OT these parameters are ignored.

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