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Thread: Boiler Pump/Fan cycling at random? Evohome at fault?

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  1. #1
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    Default Boiler Pump/Fan cycling at random? Evohome at fault?

    I have a full Evohome setup, controls UFH and individual room rads.

    During the night, the pump (or could be the fan?) activates for 30 odd seconds or so, then quickly dies off and repeats this every ??-20minutes or so. The boiler does not fire - or didn't on the 3-4 cycles of this.

    The system isn't calling for heat (BDR91s are unlit for hot water/ CH) but the pipes in the airing cupboard are warm from the system firing up earlier in the night (as per normal)

    My son's room is kept at 17 degrees, and last night the room temperature was reading 17 degrees throughout the hour I spent scratching my head trying to work out why the boiler was activating the pump(?) or fan(?).

    Sticking my head in the airing cupboard sounded like water was being circulated, but as I said the boiler wasn't firing. The pipes were slightly warm so the system had been on the last 30mins properly.

    It did this 3-4 times before I eventually gave up and put a pillow over my head.


    --> Now, could this be because the room temp was flicking above/below the room stat at 17 degrees? This causing the boiler to start the ''fire up'' sequence, then the temp would rise slightly, causing he boiler to start the ''shut down'' sequence....before the temp would flicker again and the boiler would fire back up....BUT...Evohome's controls preventing the boiler from cycling too often was ''regulating'' these on/offs?

    How can I prevent this? Any suggestions on what could be causing it? Due to the boiler being near the bedroom these unwarranted activations are really bugging me at night.

  2. #2
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    Quote Originally Posted by Andehh View Post
    I have a full Evohome setup, controls UFH and individual room rads.

    During the night, the pump (or could be the fan?) activates for 30 odd seconds or so, then quickly dies off and repeats this every ??-20minutes or so. The boiler does not fire - or didn't on the 3-4 cycles of this.

    The system isn't calling for heat (BDR91s are unlit for hot water/ CH) but the pipes in the airing cupboard are warm from the system firing up earlier in the night (as per normal)
    Some more details about your system would help - I assume you use S Plan ? Do you have two BDR91's or three ? EG, is the boiler fired by a 3rd BDR91 or is it fired using the limit switches on the zone valves ?

    Is your pump powered directly from the boiler with an internal pump overrun or is your pump externally connected perhaps with its own overrun timer ?

    If the BDR91's are not turning on and the boiler is not firing I can't really see how the Evohome would be the cause - because if the relays came on the boiler would fire too. If the relays are definitely coming on it must be something else.

    Could it be frost protection of the boiler kicking in and circulating the pump ? In some systems air temperature below 5 degrees in the boiler is used to trigger frost protection and the return flow temperature is used as a shut off - so when the boiler gets too cold it fires and keeps running until the return pipe reaches typically 30 degrees then shuts off again. This would all happen without the BDR91's being involved.
    My son's room is kept at 17 degrees, and last night the room temperature was reading 17 degrees throughout the hour I spent scratching my head trying to work out why the boiler was activating the pump(?) or fan(?).

    Sticking my head in the airing cupboard sounded like water was being circulated, but as I said the boiler wasn't firing. The pipes were slightly warm so the system had been on the last 30mins properly.

    It did this 3-4 times before I eventually gave up and put a pillow over my head.

    --> Now, could this be because the room temp was flicking above/below the room stat at 17 degrees? This causing the boiler to start the ''fire up'' sequence, then the temp would rise slightly, causing he boiler to start the ''shut down'' sequence....before the temp would flicker again and the boiler would fire back up....BUT...Evohome's controls preventing the boiler from cycling too often was ''regulating'' these on/offs?

    Are you sure that the BDR91's weren't coming on for a short period ? In case you aren't aware, the BDR91's use TPI (time proportional integral) to control the boiler.

    This means that the boiler will be operated at a reduced duty cycle when heat demand is low, this lowers the average flow temperature. The default cycle time is 10 minutes (6 firings per hour in the installer menu setup) and a minimum on time of 1 minute. So depending on demand it could run for anything between 1 and 10 minutes per 10 minute cycle. (Or not at all)

    This means that if there is a small heat demand (such as keeping a bedroom at 17 degrees at night) the boiler may in fact be firing for about 1 minute in every 10 minute period, and you may have just not noticed this. For some of the remainder of the 10 minute cycle your pump may be running in pump overrun. You would have to watch the relay for a full 10 minutes to confirm whether this was the case.

    If this was happening then I'd say this is probably normal and it is doing its job to maintain 17 degrees in your sons room. Was the bedroom radiator warm to the touch by any chance ?

    The evohome system doesn't work on a simplistic "if the temperature is below 17 degrees turn on the radiator and if its above turn it off". It's a fully proportional and adaptive system that will make small adjustments to both the boiler duty cycle and the radiator valve opening to achieve a constant steady set point by warming the radiator up just enough, rather than cycling the temperature above and below the set point by turning the radiator full on and off as a conventional thermostat would.

    Just because the temperature is 17 degrees or even slightly above does not mean that the radiator does not need to be warm to maintain that 17 degrees - if there is heat loss from the room then a small amount of heat must constantly be put into the room to maintain a steady 17 degrees, and that is exactly what it will try to do.
    How can I prevent this? Any suggestions on what could be causing it? Due to the boiler being near the bedroom these unwarranted activations are really bugging me at night.
    But are the activation's really unwarranted if heat is required to keep your sons bedroom at 17 degrees ? Do they stop if you drop the set point in your sons room a few degrees ? (try this next time they happen) If they do, then the system is behaving as designed to maintain that 17 degree set point.

  3. #3
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    Thank you very much for your detailed reply!

    It is a S plan, with 2 x BDR91s one for heating one for hot water. I believe these control the motorised valve that triggers the boiler - but I may be wrong on that!

    The system works as intended to be honest, it does a very good job.

    The Boiler has the system pump within it, and yes the radiator in Son's room was warm - and seemed to stay relatively warm for the entire duration of my nights ''investigation''.

    I think you are entirely right in the sense that I underestimated the Evohome system in how it manages a room temperature. I envisaged it sensing a room temp drop, turning on boiler, opening the rad valve and effectively ''nuking'' the rad in that one room to boost the temp and continue doing so until that room hit required temp & then it would shut off and let it overshoot & then slowly drop down.

    I didnt realise Evohome was quite so good with micromanaging the rad temperature to maintain an even/exact room temp. I did wonder why his room stayed firmly on 17 degree the entire time.


    edit to add; For the upstairs and downstairs hallway rads, and upstairs toilet rads, we keep these unregulated so that any demand for heat warms them up as well - just to keep a reasonable temp throughout the house on very cold nights. I presume this means that the boiler just adds a tiny bit more hot water into the system as it compensates for the fact that Son's radiator needs 'X amount of warmth', but the rest of the rads in the house are draining that 'X-1' warmth already? It therefore adds on +1 heat to compensate? If that makes any sense...
    Last edited by Andehh; 20th October 2016 at 02:40 PM.

  4. #4
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    Quote Originally Posted by Andehh View Post
    Thank you very much for your detailed reply!

    It is a S plan, with 2 x BDR91s one for heating one for hot water. I believe these control the motorised valve that triggers the boiler - but I may be wrong on that!

    The system works as intended to be honest, it does a very good job.

    The Boiler has the system pump within it, and yes the radiator in Son's room was warm - and seemed to stay relatively warm for the entire duration of my nights ''investigation''.

    I think you are entirely right in the sense that I underestimated the Evohome system in how it manages a room temperature. I envisaged it sensing a room temp drop, turning on boiler, opening the rad valve and effectively ''nuking'' the rad in that one room to boost the temp and continue doing so until that room hit required temp & then it would shut off and let it overshoot & then slowly drop down.

    I didnt realise Evohome was quite so good with micromanaging the rad temperature to maintain an even/exact room temp. I did wonder why his room stayed firmly on 17 degree the entire time.
    Yes it's very good at making small adjustments to the heat output of the radiators to find a balance where the radiator heat equals the heat loss, thus the room stays at a constant temperature without the normal large up and down temperature swings of a thermostat that turns the system right off and fully on again. Even conventional manual TRV's which in theory are a proportional controller tend to over-react and thus cause the radiator to cycle between too cold and too hot.

    It can't always achieve this depending on the conditions, sometimes the temperature may cycle up and down slightly, this can happen if your flow temperature is too high or you have very low heat loss in your house, (or the weather just isn't properly cold yet) but more often than not it will find a stable balance where the temperature doesn't fluctuate significantly.

    When the overall system demand is high (many radiators requiring heat) it will tend to run the boiler at a high duty cycle (and thus high average flow temperatures in the pipes) and adjust the valves on the radiators of rooms that don't need much heat right down so they are only just flowing, to find the right balance, however when the total system demand is low due to only one or two radiators needing heat (like bedrooms at night time set to a low temperature) it instead opens the valve on that radiator fairly wide but cycles the boiler on infrequently so you get a very low average flow temperature. This is the most efficient way to get your boiler to produce only a small amount of heat for a single radiator, and also gives very fine control of temperature as the radiator panel is only warm.
    edit to add; For the upstairs and downstairs hallway rads, and upstairs toilet rads, we keep these unregulated so that any demand for heat warms them up as well - just to keep a reasonable temp throughout the house on very cold nights. I presume this means that the boiler just adds a tiny bit more hot water into the system as it compensates for the fact that Son's radiator needs 'X amount of warmth', but the rest of the rads in the house are draining that 'X-1' warmth already? It therefore adds on +1 heat to compensate? If that makes any sense...
    If you have some uncontrolled radiators that piggy back on any calls for heat they will also warm up a small amount.

    They will act as a "vampire" load that the Evohome is not aware of (since there are no HR92's on them) which means the boiler duty cycle would have to be greater to achieve the same radiator surface temperature in your sons room due to the additional radiators taking their share of the boilers heat.

    This will be compensated for automatically though - as the Evohome will just keep slowly increasing the boilers duty cycle until the bedroom can maintain the target temperature - it doesn't care what the specific duty cycle is, it only knows to increase or decrease it, so it will adapt to the vampire loads.

    If you use OpenTherm instead of a BDR91 it doesn't even have to adapt - it just asks the boiler for a specific flow temperature and it's the job of the boiler to deliver this, no matter whether there is one radiator or 4 drawing heat! So the same specific flow temperature would be correct for the bedroom regardless of how many other uncontrolled radiators are piggybacking on the heat demand.
    Last edited by DBMandrake; 21st October 2016 at 12:14 AM.

  5. #5
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    Quote Originally Posted by DBMandrake View Post
    If you use OpenTherm instead of a BDR91 it doesn't even have to adapt - it just asks the boiler for a specific flow temperature and it's the job of the boiler to deliver this, no matter whether there is one radiator or 4 drawing heat! So the same specific flow temperature would be correct for the bedroom regardless of how many other uncontrolled radiators are piggybacking on the heat demand.
    This is the system I have, deliberately. I kept the two towel rails in the bathrooms without any TRVs. We leave the bathroom windows open a lot. We have the HRxx on every other radiator. With the Opentherm the boiler is not bothered about the two towel rails. But that keeps the towels nice and dry whenever there is any other heat demand. The only downside is that the bathrooms cannot demand their own heat, but we have never ever found the bathrooms to be too cold. So this configuration works for us. I also didn't like ugly (there, I said it!) white plastic controllers on our shiny stainless steel towel rails.

  6. #6
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    I've got a problem:

    Well, I solved my repetitive communication faults that occurred every day at 17:19 (Fault) and then 20:39 (Restored), it appears to be because I had bound one boiler BDR also under Boiler Relay setup, having already used the guided configuration to add HW control where it asked me to bind the CS92, then HW BDR then finally boiler BDR. System appeared to work normally until last night!

    OK, so I did the full reset routine, factory reset, then batteries out, then reset all HR92 and cleared the BDR91 bindings, then started again and only adding the BDR's via the guided installation using HW, this added the CS92, then HW BDR then finally HTG BDR, all worked OK. But at 23:00 last night the HTG BDR continued to cycle on/off every 10-mins, coincident with the cycle setting of 6-cycles / hour, maybe.

    At 23:00 the schedule sets all the zones to 15C and just prior to this all zones were at their set temperatures of about 20C and until then all was working as expected. But, the cycling of the BDR kept going for (10-mins then on for about 1-min) for an hour or so that I let it, then I decided to see if the Quick Action function would stop it, yes it did using the Economy function. Having confirmed that by waiting more than 10-mins, I then switched it off (resumed the schedule) and everything was back to normal.

    So this morning thinking the HTG BDR was not being controlled correctly, I enabled Boiler Control and bound the HTG BDR to that function, previously it said disabled. But the communication errors had returned (at least I can now replicate that condition), so I have now disabled that again and normal operation resumed and no communication errors.

    I just called Honeywell and explained the problem with BDR cycling after the schedule has reduced all rooms to a night-time 15C and they weren't that fussed, they said the system was working normally and it might take a couple of days for the system to normalise, but if I'm getting a lot of inefficiency by the system doing this, I should call them back and they will open a call-log and refer the problem to the second-line support, so that's what I'm going to do.
    Last edited by g6ejd; 25th October 2016 at 03:20 PM.

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