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Thread: Hr92 92 on all radiators bar plinth heater with opentherm

  1. #1
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    Default Hr92 on all radiators bar plinth heater with opentherm

    Just wanting some advise,my system comprises of a Vaillant 831 ecotec plus combi boiler,vr33 opentherm bridge controlled by evohome with honeywell opentherm bridge.My radiators all have the hr92 fitted apart from the bathroom towel rad and smiths plinth heater in the kitchen,this is run off the heating not an electric one.What I would like to do is fit a hr92 to the bathroom towel rad so it can be set to dry the towels during the summer without running the rest of the heating.My boiler has an abv in the boiler itself,I have read about people fitting a separate one usually as far away from the boiler,this would not be possible for me due to the logistics of lifting the floor.My question is would it be ok to fit the new hr92 and leave everything as is,or would it be better to remove the hr92 from the hall,I know this would then heat up if the system was on as this would then become the bypass rad.Phew,that's it I think,would welcome any help from those in the know.
    Last edited by garymtitley; 19th August 2019 at 08:20 PM.

  2. #2
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    Positioning the ABV "as far away from the boiler as possible" is overrated IMHO. My ABV is only just past the (external) pump, so a total loop length from the boiler of under 2 metres or about a metre a side. Like you fitting it any further away would be infeasable as it would put it after the junction that splits the upstairs and downstairs feeds, so it would be either under the floor or in the ceiling rather than in the boiler closet!

    Its primary purpose is to regulate the maximum differential pressure across the main header pipes supplying the radiator network despite changes in the number of radiators that are open, and to provide a minimum flow rate to the boiler and pump (so the pump isn't trying to pump into a dead end) even if all radiators are closed - which on an all HR92 Evohome system can happen under certain circumstances for a period of time, such as during system shutdown (heating off mode) where the HR92's will will fully close before the boiler relay turns off and (likely) before the pump overrun ends.

    The ABV loop has a minor secondary function to dissipate a small amount of heat to help remove heat from the boiler heat exchanger under conditions where the radiators have all just closed and the boiler shut off but the pump overrun is still active.

    However the amount of heat dissipated is very small unless the ABV loop length is very long and it shouldn't really be needed with todays modern low thermal mass heat exchangers. So whether you make your loop 2 metres long or 4 metres makes very little difference.

    Also keep in mind that when you only need a small amount of heat to one or two radiators (say nighttime bedrooms) the longer your ABV loop is the more wasted heat there would be from the loop as the ABV would be flowing a lot in those conditions with little flow going beyond it to the radiators.

    Personally I think being able to control all radiators in the house is a major benefit of Evohome, so I would be inclined to get an ABV fitted where convienient near your boiler and put HR92's on all radiators, which is what I did. Like you I still want to use the bathroom radiator in summer to dry and pre-warm towels and also prevent the bathroom fogging up, despite not wanting radiators on in any other room. With all radiators controlled I can do this, and I even have the bathroom radiator scheduled to stay on for a while after we leave for work to finish drying towels while other radiators in the house go off.

    Before fitting an ABV make sure your system is not configured with an automatic pressure sensing variable speed pump - as an ABV does not necessarily play nice with a variable speed pump system as the pump will see the easy flow once the ABV opens as a sign that radiators have opened and that the pump speed should increase, which will cause the ABV to open even more and the flow to increase and the pump speed to increase further - which is exactly what you don't want.
    Last edited by DBMandrake; 19th August 2019 at 11:38 PM.

  3. #3
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    Thanks for the detailed reply,very informative and understood.My boiler has a two speed pump that runs on speed one at a certain kw setting and on speed two over a certain kw setting,can’t remember what these are without looking in the manual,but I’m pretty sure it currently running on speed two,therefore it’s fixed at whatever kw output happens to be selected.The distance from the boiler to an external abv would be a maximum of about 600mm-800mmroughly due to the boiler being installed in what was the airing cupboard.If this would suffice,I could get my heating engineer to fit one for me.The plinth heater could be switched off until its needed,I know you can fit these with an hr92 but it’s a bit of a faff and expensive because it’d then need a circular Honeywell wireless stat to communicate with the hr92,I think.

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    Both my bathroom radiator and en suite radiators have HR92s fitted. Each has its own schedule and if I need more heat, such as when we have guests and more towels to dry, I just turn it up. Never had a problem doing this and it’s a simple solution. In a way it is what Evohome is designed to do. My bypass is my downstairs we radiator. It is tiny but does the job fine.

  5. #5
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    Quote Originally Posted by DBMandrake View Post
    The ABV loop has a minor secondary function to dissipate a small amount of heat to help remove heat from the boiler heat exchanger under conditions where the radiators have all just closed and the boiler shut off but the pump overrun is still active.
    Not sure I agree that this is a minor secondary function. I see it as one of the main functions, if not THE main function.

    Over-run is there for a reason. When the flame goes off if the water stops moving then it'll either boil (adding gasses to the heating loop) or else heat to a degree where scaling becomes a problem. It's really important to remove the heat from the heat exchanger before this happens, and an ABV on a nice long loop is a good way to do it.

    If you have a spare room where it won't look too unsightly then an easy way to fit one without lifting floorboards is to fit it underneath a radiator, between the flow and return legs.

    P.

  6. #6
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    Quote Originally Posted by garymtitley View Post
    Just wanting some advise,my system comprises of a Vaillant 831 ecotec plus combi boiler,vr33 opentherm bridge controlled by evohome with honeywell opentherm bridge.My radiators all have the hr92 fitted apart from the bathroom towel rad and smiths plinth heater in the kitchen,this is run off the heating not an electric one.What I would like to do is fit a hr92 to the bathroom towel rad so it can be set to dry the towels during the summer without running the rest of the heating.My boiler has an abv in the boiler itself,I have read about people fitting a separate one usually as far away from the boiler,this would not be possible for me due to the logistics of lifting the floor.My question is would it be ok to fit the new hr92 and leave everything as is,or would it be better to remove the hr92 from the hall,I know this would then heat up if the system was on as this would then become the bypass rad.Phew,that's it I think,would welcome any help from those in the know.


    As you have a plinth heater then you don't need an ABV, because the plinth heater circuit is alway open due to its lack of any control valve.
    Last edited by blowlamp; 21st August 2019 at 11:26 AM.

  7. #7
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    That's great,thank you.Will get this done over the weekend.The boiler does have it's own abv built in as well.

  8. #8
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    I have hr92 on all rads and on my kitchen plinth heater , the plinth heater fan only operates when approx 45 deg detected in the plinth heater, therefore it comes on only when hr92 opens and calls evohome controller for heat. I have a abv fitted also. I put worscester greenstar cdi combi pump on a fixed speed pump curve from the boiller controls, this allows the abv to open if all hr92 are closed

  9. #9
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    [QUOTE=garymtitley;40669]Thanks for the detailed reply,very informative and understood.My boiler has a two speed pump that runs on speed one at a certain kw setting and on speed two over a certain kw setting,can’t remember what these are without looking in the manual,but I’m pretty sure it currently running on speed two,therefore it’s fixed at whatever kw output happens to be selected.The distance from the boiler to an external abv would be a maximum of about 600mm-800mmroughly due to the boiler being installed in what was the airing cupboard.If this would suffice,I could get my heating engineer to fit one for me.The plinth heater could be switched off until its needed,I know you can fit these with an hr92 but it’s a bit of a faff and expensive because it’d then need a circular Honeywell wireless stat to communicate with the hr92,I

    Hr92 on plinth heater talks to controller, the schedule can be adjusted at controller or by the phone app. No need for circular controller . Only hassle is ensuring you can reach the hr92 to change batteries .

  10. #10
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    Got the Hr92 fitted on the towel rad a couple of days ago and it’s working perfectly well,we can now dry our towels without running anything else apart from the plinth heater.Decided to leave the smiths plinth heater as is for now,so this just comes on when any of the others are calling for heat.My vaillant 831 has its own bypass built in so should be ok.I might in the future look at putting a Hr92 on the plinth heater and would then probably get an external Abv fitted just to be sure,will see how things go.

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