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Thread: Evohome : valve position status on the HR92 vs evotouch

  1. #1
    Automated Home Sr Member
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    Default Evohome : Cycles per hour parameter - what is that ??

    During the last maintenance of my heating system, the technician came and changed the advanced settings -> system parameters -> cycles / hour from 6 to 3, he told it was better without being really able to say why.

    I have a standard ON / OFF thermostat, no modulation as far as I know, and in the past this parameter ws set to 6 cycles per hour ... but what is that pramameter really doing ?

    I am not sure this is linked, but my heating system is beginning to heat those cold days, and I have the felling like the rooms take more time to heat ... not sure if this is only a feeling or linked tp that parameter ?

    Many thanks !!
    Last edited by morpheus; 28th October 2019 at 06:53 PM.

  2. #2
    Automated Home Guru
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    Started wondering about that myself. When I installed the system I actually raised that number to the maximum it would allow because I do not have a boiler but only a hot water valve (city block heating). My thought was that the controller would be the spider in the web, hence the name controller, but gradually I found out that it is actually just a remote to the HR92s that run completely independent of each other. Most annoying part in this being that the controller does not report or likely is even aware of local overrides. IMO that is something of a design error or at least an oversight from a technician that doesn't have any children of his own.

  3. #3
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    The cycles per hour setting controls the length of the TPI cycle for the boiler relay. (And heating zone relays if you have any)

    12 times an hour means a 5 minute cycle, 6 times an hour (default) a 10 minute cycle and 3 times an hour is a 20 minute cycle.

    Unless you have an oil fired boiler I would not set it to 3 cycles per hour. That setting is only there to support oil fired boilers as far as I know, and there is no benefit for gas fired boilers.

    I've tried 3, 6 and 12 cycles per hour on my regular (non condensing) gas boiler and monitored the behaviour of the system, (temperature regulation of zones, response to changes in requested set points etc) and came to the conclusion that the default 6 cycles per hour works best.

    12 cycles per hour in theory might give more responsive control of the heat demand since a cycle completes in 5 minutes however there are several drawbacks. The major one is that there is still a minimum one minute on time.

    With a 10 minute cycle and one minute minimum on time the boiler will not fire at all if the heat demand is below 10%. With a 5 minute cycle and one minute minimum on time the boiler will not fire at all unless zones are calling for at least a 20% heat demand. This is a major problem in low heat demand scenarios such as one or two bedrooms ticking over with a very low demand at night.

    In my experience this high minimum heat demand threshold caused the bedrooms to fall well below their set points (more than 1C) before the boiler would come on to bring them up again, and the rooms then overshot the set point significantly as the HR92 has kept opening further and further while the boiler was not running. With the 10 minute cycle the rooms only fell about 0.5C below the set point before the boiler would come on and there was much less overshoot.

    5 minute cycles also doubles the switching of both the relay and the boiler itself for no real benefit.

    The 3 cycles per hour setting has a different problem - at 20 minutes the cycles are so long that any requested change in heat demand from the zones takes forever to take effect. This also allows rooms to drop well below their set point before they can get heat in low heat demand scenarios. It also tends to cause the flow temperature to cycle between maximum and very low, as the burn and off times are both twice as long, whereas the 10 minute cycles tend to average the flow temperature better for partial heat demands.

    So my opinion is stick with 6 cycles an hour for a regular gas boiler.

  4. #4
    Automated Home Sr Member
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    Thanks a lot ... my boiler is a gas with condensation, it was initially set to 6 cycles / hour and worked like that for the last 3 years, but a month ago the boiler technician set it to 3.
    What are exactly those cycles ? I mean the Evotouch I have is a ON/OFF ... is this the maximum number of ON the evotouch will send to the boiler
    6 cycles means It can sent 6 times an "ON" to the boiler, or 3 times (2x ON and 2x OFF) ?

    I guess the technician reduced from 6 to 3 in order to have the boiler working less, I mean have less starts and stops ... thinking it would be sufficient to get the same quality of comfort ?

  5. #5
    Automated Home Sr Member
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    Quote Originally Posted by DBMandrake View Post
    The cycles per hour setting controls the length of the TPI cycle for the boiler relay. (And heating zone relays if you have any)

    12 times an hour means a 5 minute cycle, 6 times an hour (default) a 10 minute cycle and 3 times an hour is a 20 minute cycle.

    Unless you have an oil fired boiler I would not set it to 3 cycles per hour. That setting is only there to support oil fired boilers as far as I know, and there is no benefit for gas fired boilers.
    That's interesting. I have a (fairly large) oil-fired boiler and have left the settings at default because of a lack of explanation of what they do and why they should be set to a particular value. Is there any such documentation?

  6. #6
    Automated Home Ninja
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    I have mine set at 3 cycles specifically for the reason so that boiler isn't cycling as much, and a minimum on time of at least 2 minutes. I have an older non condensing boiler and whilst it probably would be fine on 6 cycles I'd rather have it set to 3 cycles to reduce the amount of on/offs.

    We don't suffer from any issues with Zones overheating etc. and it's been on this setting from day 1.

    The only issue I have currently is a Zone not meeting the set point, however I think this is due to that particular zone and change in weather conditions. However it's been OK over the past couple of days so perhaps it's adapted.

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