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Thread: Confused about Evohome and new boiler

  1. #1
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    Default Confused about Evohome and new boiler

    I have had an Evohome kit for a few years and have had no problems with it until we recently had our boiler replaced from a Grant Oil Combi to a new Worcester Greenstar Heatslave II Oil Combi and now it seems like both the boiler and evohome have a mind of their own.

    I am noticing that often the BDR91 relay will show a green light and the boiler demand light will be on but the boiler is not firing or sending any heat to the radiators, this can go on for up to an hour or more and causes the room temperature to drop a few degrees below the set temperature. When the boiler does eventually fire the BDR91 sometimes clicks off for a few seconds then back on which seems to make it worse because the boiler then stops again for a long period. The rooms all heat up fine in the mornings and evening it just seems to be a problem maintaining the level.

    I have also seen the green relay light come on for a few minutes then go off when the heating should be off i.e all rooms have set points a couple of degrees below room temp.

    We have had our installer back and he has thoroughly tested the boiler but doesn’t believe it is at fault. He advised us there was low current from the relay and we have since replaced the relay with a new BDR91 unit but this doesn’t seem to have made any difference.

    I have factory reset the controller and this has the default 6 cycle rate and 1 min on time.

    I appreciate the description is somewhat non specific but wondered if anyone can advise if any of this seems normal and if not whether this is likely to be the evohome or the boiler causing the issue?

    Chris

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    Automated Home Legend paulockenden's Avatar
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    “Low current from the really” makes me suspicious that the installer doesn’t have a clue what they are doing.

    It's a relay. A switch. It doesn’t provide power.

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    I have to agree it does seem a bit odd. What I don't understand is, the relay is just a switch so when it is on "green light" the boiler should be sending heat to the rads, if the boiler demand light is on and there is no heat going to the rads for up to a hour at a time then surely this is the boiler at fault?

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    Quote Originally Posted by bluecat View Post
    I have to agree it does seem a bit odd. What I don't understand is, the relay is just a switch so when it is on "green light" the boiler should be sending heat to the rads, if the boiler demand light is on and there is no heat going to the rads for up to a hour at a time then surely this is the boiler at fault?
    It sounds as though it may not be wired correctly and/or there is fault with boiler for the relay connects the circuit, the power goes to the boiler, it fires up. I wonder if the installer is wanting to find an excuse for his failure?

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    Automated Home Legend paulockenden's Avatar
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    What's the temperature of the flow/return pipes? If the water is hot then the boiler won't fire.

    Could be something like an airlock preventing the water flow.

    P.

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    I don't know much about oil-fired boilers, but I do know you're not meant to turn them on and off rapidly. Could it be that you need to play with the cycle time and minimum on time in Evohome? Is it trying to modulate the heat too rapidly and the boiler is ignoring it to protect itself? Check the installer manual for the boiler - it might tell you what the maximum number of hourly cycles is.

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    Quote Originally Posted by dty View Post
    I don't know much about oil-fired boilers, but I do know you're not meant to turn them on and off rapidly. Could it be that you need to play with the cycle time and minimum on time in Evohome? Is it trying to modulate the heat too rapidly and the boiler is ignoring it to protect itself? Check the installer manual for the boiler - it might tell you what the maximum number of hourly cycles is.
    I thought maybe this had something to do with it, I contacted Worcester directly to ask this question but they told me to contact Honeywell which perhaps I need to do. I can understand the modulation "TPI" where there is a large water tank but don't really see it as necessry for a combi boiler where the tank is very small and heats up quite quicky. I have looked around this site and elsewhere but couldn't find any recommendation for these settings on an oil combi? It would also explain why the heating comes on OK morning and evening but then gets confused keeping the temperature contant.

    @paulockenden flow/return pipes are stone cold so don't think that is the issue

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    There are lots of variables. You need to try to find a way to eliminate them one by one.
    EG could you (or your fitter) temporarily disconnect the Evohome relay, and connect a simpler one? If the problem persists you know Evohome is not at fault.

    Having said that, here is another avenue to check/eliminate -
    The situation you describe sounds like it could be caused by unbalanced radiators. As I understand it, all central heating systems need to be balanced for 2 reasons -
    1. To make sure hot water can reach ALL the rads without being "hogged" by some at the expense of others.
    2. To ensure water travels SLOWLY enough around the circuit to shed its heat sufficiently.

    If HOT water is returned to the boiler, the boiler will detect this and shut down to protect itself. The classic behaviour is (1) the system comes on from cold - typically in the morning or evening, (2) the house heats up quickly (3) the boiler then shuts down (4) room temperatures then drop significantly before boiler starts up again.

    The way to diagnose this is to check temperature of the return pipe into the boiler.
    You say you have done this - but your answer does not sound right -
    "flow/return pipes are stone cold so don't think that is the issue"
    When the system is running, the flow pipe should be hot, and the return pipe should be noticeably cooler, but will not be stone cold.

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    Automated Home Legend paulockenden's Avatar
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    Quote Originally Posted by bluecat View Post
    @paulockenden flow/return pipes are stone cold so don't think that is the issue
    So perhaps the pump is bu&&ered? Or maybe there's an airlock that's preventing circulation? Or a zone valve stuck shut somewhere.

    First thing I'd check is whether the pump is running.

    P.

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    Thanks for all the advice, I contacted Honeywell as I suspected the cycle rate or minimum on time might be having something to do with it and confusing the boiler and they told me

    "Please be advised that for an oil boiler you may set the Cycle rate 3 and Minimum on time 4" (Mine was set to default cycle rate 6 and minimum on 1)

    Since making this change for the last 2 weeks the rooms have all been keeping up to temperature so it looks like this has made a difference. I still notice the relay activate sometimes after the heating has gone off but I assume this is somehow by design.

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