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Thread: New Evohome firing boiler 24/7

  1. #1
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    Default New Evohome firing boiler 24/7

    Hello I am new to Evohome and to this forum.

    New Evohome install on old Vaillant thermocompact last week by local plumbing company. CH&HW relays and the rad valve heads. Specific plumber had installed Evohome on new builds in a previous job.
    After I got home in the evening I set up Wi-fi which triggered firmware update after which it rebooted.
    That night it had a comms error with the relays and the house was cold in morning.
    Plumber returned and rebound hot water relay.
    System has then worked mostly as expected but I had a suspicion it cycles boiler all night.
    Last night I confirmed this two hours after last heat demand. System status shows 0% demand but boiler doing minimal one minute cycles with pump overrun.
    Tested again this morning using the off setting and checking 0% demand and the same.

    Any systematic troubleshooting advice from the experts I have observed posting here would be appreciated. I have pinged an email to Honeywell but I am sure it might take a while to get a response. I can do Evohome settings stuff but if itís wiring I will call in installer to do. Many thanks!

    Edit:
    I forgot to say the house being cold was a boiler pump problem that was fixed. I also can now add that last night I checked and the relay green lights are NOT illuminated when the boiler is cycling so could this mean itís not an Evohome problem?
    Last edited by SMM; 8th March 2018 at 05:50 PM. Reason: Add extra detail

  2. #2
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    Someone on another forum had a similar problem. This was the advice offered which solved the problem:

    Ďturn the boiler thermostat up, the cylinder is probably set for 60c and the boiler is set at 50c so the cylinder will keep calling for heat, turn the boiler stat up until the display reads 75-80c.í

  3. #3
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    Quote Originally Posted by SMM View Post
    Edit:
    I forgot to say the house being cold was a boiler pump problem that was fixed. I also can now add that last night I checked and the relay green lights are NOT illuminated when the boiler is cycling so could this mean it’s not an Evohome problem?
    So you have an S-Plan or Y-Plan system using two BDR91 relays ?

    Usually in that situation the two relays are wired to control the heating and hot water zone valves respectively and it is the limit stop switches on the zone valves (orange wires) that are both wired to fire the boiler if there is either a heating or hot water demand.

    So if both BDR91's green and red lights are off and the boiler is still firing the problem is not with Evohome.

    It could be the wiring from BDR91's to zone valves is wrong, from the zone valves to the boiler is wrong, or there is a problem with one of the zone valves.

    I would start by double checking that both zone valves are mechanically closed at the time when the heating and hot water should be off but the boiler still seems to be running.

    On a two port valve (S-Plan) you can usually tell by pushing the manual filling lever - if it is hard to push and you can hear it moving the valve was closed, but if the lever feels limp and very easy to push it was already open.

    Establishing whether the zone valves are in fact closed when the boiler is still firing when it shouldn't will help narrow down the problem, but my bet is you have a straight forward wiring mistake.

    To help diagnose the wiring you can also press the button on each BDR91 to temporarily override it into the on or off position - the green light will come on or go off. You can then check whether the respective zone valve is physically opening and closing as it should.
    Last edited by DBMandrake; 8th March 2018 at 09:58 PM.

  4. #4
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    DBMandrake thank you! I greatly appreciate you taking the time to contribute.

    Once I noted the lack of lights on the BDR91s (x2) I did wonder if a wiring accident was involved. I am not a plumber, but on visually checking I have two Danfoss HPA2 valves on the flow side, one on the CH and one on the HW cylinder coil. What I had not mentioned was that we recently bought the house that had been unoccupied for some time and I had had a power flush performed due to symptoms of significant sludge in some rads and a ~15yo system, immediately prior to the Evohome installation (same day). On checking the valves both had been left in manualÖ perhaps from that flush operation? I have now placed both into auto and note that the CH valve is now resistive to placing into manual (correct) whereas the HW valves is easily pushed with zero resistance (incorrect), implying as I understand it that it is open and it may be guilty of firing the boiler? I also note that the old timer-only controller remains wired in (but with both CH and HW firmly set to off).

  5. #5
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    Yes if the hot water zone valve is remaining open then that is probably why the boiler is still running. It would probably lead to your hot water being very hot as well - have you noticed problems with the hot water being too hot ?

    If you have the Evohome hot water kit - which consists of a BDR91 and the CS92A battery powered temperature sensor, and either an insertion probe for the cylinder (non vented cylinder) or strap on sensor (vented cylinder) then the old hot water timer should have been removed or disconnected.

    Evohome does the scheduling for the hot water as well as the temperature control. (Although if the cylinder already has a safety over temperature cut-out, that should be kept in circuit, but set a few degrees higher than the hot water temperature configured in Evohome to ensure that it's Evohome that controls the temperature)

    If the hot water zone valve is still open when the hot water BDR91 is off then it does indeed look like a wiring problem between that BDR91 and the hot water zone valve. I would get them back to fix the wiring and ensure the old hot water timer is disconnected.

    When wired correctly you should be able to open and close the hot water zone valve by pushing the button on the appropriate BDR91. Once that works correctly and that zone valve follows the green light the system should work correctly.

    Is you cylinder vented or unvented ? And what do you have your boiler flow temperature set at ?
    Last edited by DBMandrake; 9th March 2018 at 07:43 AM.

  6. #6
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    DBMandrake, very many thanks again for your further reply which gave me confidence to push the installers firmly to revisit the wiring this morning when they were thinking it might be a motorised valve problem. I pushed and they called in their electrician today to double check things and I then got a (slightly sheepish) voicemail when at work to say that the wiring was indeed the fault and everything was now in order (and the old time clock was now gone at my request too). I have already gone to listen to the silence from the boiler this evening and it made me smile!

    FYI re my system itís Vaillant thermocompact from 2003, S-plan, 2x 2-way motorised valves, 2xBDR91, open vented cylinder with strap on Evohome sensor, 14 rads (12 Evohome, 1 TRV, 1 towel rail with open valves assume itís intended to be a bypass though I wish I felt allowed to turn it down!), no cylinder safety cut out, I am mindful of need for boiler flow (65) to be above cylinder target temp but I keep that low (49) anyway due to toddler. And yes the HW had been overshooting a lot.

    Unrelated to the original topic, re my one remaining TRV: can one use an HR92 in a bathroom or does humidity etc. preclude this? Honeywell have a very unhelpful FAQ that quotes the operating parameters but doesnít say yes or no! (One of my zones, hall and landing, is formed of two rads so I have one zone slot free)

  7. #7
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    Glad you got everything sorted!

    The HR92 is fine in a bathroom, I've used one on my bathroom radiator which doubles as a towel rail for a couple of years without problems.

    The only thing I would say is avoid pressing the button or turning the knob with wet hands as while they will cope with humidity they are not a water tight design.

    If you want to have HR92's on all radiators then you would need to check that there is an automatic bypass valve installed. If there isn't, one radiator or towel rail needs to remain open to provide a bypass for the boiler.

    On my system I have HR92's on all radiators (and no separate towel rail) but I have an automatic bypass valve installed in the boiler closet.
    Last edited by DBMandrake; 10th March 2018 at 11:02 AM.

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