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Thread: Honeywell evohome and Vaillant ecotec plus 838

  1. #1
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    Default Honeywell evohome and Vaillant ecotec plus 838

    Hi all, I'm new to these forums but am looking to do lots with home automation so hoping I can learn from all the experts here I'm looking at installing the Honeywell evohome system at home and have the above-mentioned boiler which is less than a year old. Some people have told me that before getting the system installed I might want to check the wiring guide for my boiler to ensure it won't get "upset or confused" at constantly being turned on and off by the evohome. Does anyone have any knowledge or experience about this?

    The other thing which occurs to me from a simplistic view is that if the boiler is being turned on and off much more frequently, is that not going to put more stress on it thereby ageing it quicker? Or making it prone to break down? It might be totally nonsensical but just wanting to check.

    Any advice or guidance will be much appreciated!

    TIA

  2. #2
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    Should work without a problem. The entire point of evohome is that every room can satisfy it's own heating needs independently of each other. ALL the other single thermostat solutions including the so called smart ones like NEST etc, are smart about the single zone they are installed in. So you can still have cold rooms when the room with the stat is heated enough. Does this result in the boiler coming on more frequently....perhaps. But it's all about general comfort. And once the house is evenly heated, the boiler should turn on less often rather than more.

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    Quote Originally Posted by freddyq View Post
    The other thing which occurs to me from a simplistic view is that if the boiler is being turned on and off much more frequently, is that not going to put more stress on it thereby ageing it quicker? Or making it prone to break down? It might be totally nonsensical but just wanting to check.
    When you use a boiler relay with the Evohome it uses TPI - "Time Proportional integral" that cycles the boiler on and off in a regular cycle if there is only a small heat demand. By default the cycle period is 10 minutes so it will come on from some amount of time from 1 minute out of 10 to 10 minutes out of 10 depending on the demand.

    Most modern digital thermostats these days use TPI because it is more energy efficient and gives a much better control of the temperature as it is able to maintain a much more constant temperature compared to a conventional thermostat that runs the boiler constantly until the temperature is about 1 degree too high, then turns it of until the temperature is 1 degree too low. (Which can be a long time on some systems)

    An alternative if your boiler supports OpenTherm is to use the OpenTherm bridge instead of a boiler relay - now instead of cycling the boiler on and off the Evohome "asks" the boiler for a lower flow temperature when there is less heat demand, so the boiler runs constantly but at a reduced burn. OpenTherm is more efficient and gives better control than TPI but not many boilers support it or require add in modules to support it.

    Normal gas fired boilers should be fine with TPI. Mine is over 20 years old and it has been running with the Evohome with TPI for over a year without any problems.

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    Thanks both for your replies - very useful insight and I'm feeling more satisfied about taking the plunge

    @DBMandrake - you mention "When you use a boiler relay with the Evohome" - do you mean an additional relay installed to the boiler or the BDR91 relay used by evohome?

    Also one other thing I've recently been trying to understand relating to the bypass point, I got told by one person that I should have an automatic bypass installed if I'm getting the evohome but (and I guess this might be why they suggested it) I didn't mention that although I was asking about 8 TRVs (so two packs) I actually have 9 radiators and 2 towel rails. So the additional radiator which is a small one in the hallway and the towel rails will not be controlled. I get the impression that as a result, the need for a bypass is negated/reduced because even if the boiler was asked to fire up when all TRVs are closed (I understand the evohome does have a tendency to do this) there will be somewhere for the heat/hot water to go. If later I decide to control the towel rails and last radiator then I'd need to get a bypass installed. Is that all correct?

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    Are you saying you have no motorised zone valves?

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    I don't have anything installed yet - due to be installed on the 12th. I will be setting up 8 TRVs on radiators around the house so yes, I will have the motorised zone valves. I'm just trying to figure out exactly what DBMandrake meant.

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    Sorry, I meant before you even start putting in evohome. Do you currently have motorised zone valves to divert the boiler's hot water to either CH or HW circuits? The reason I ask is, if you do, you will need to figure out how to turn those on when you put evohome in. Putting the radiator controls in is fine and putting the boiler relay will fire the boiler up when the radiators want heat, but if you have a zone valve that's shut those radiators aren't going to get hot. This question is also related to your ABV question.

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    Quote Originally Posted by freddyq View Post
    Thanks both for your replies - very useful insight and I'm feeling more satisfied about taking the plunge

    @DBMandrake - you mention "When you use a boiler relay with the Evohome" - do you mean an additional relay installed to the boiler or the BDR91 relay used by evohome?
    By boiler relay I mean the supplied BDR91
    Also one other thing I've recently been trying to understand relating to the bypass point, I got told by one person that I should have an automatic bypass installed if I'm getting the evohome but (and I guess this might be why they suggested it) I didn't mention that although I was asking about 8 TRVs (so two packs) I actually have 9 radiators and 2 towel rails. So the additional radiator which is a small one in the hallway and the towel rails will not be controlled. I get the impression that as a result, the need for a bypass is negated/reduced because even if the boiler was asked to fire up when all TRVs are closed (I understand the evohome does have a tendency to do this) there will be somewhere for the heat/hot water to go. If later I decide to control the towel rails and last radiator then I'd need to get a bypass installed. Is that all correct?
    If you have at least one radiator that is open all the time (lock shield valves on both sides left open, no TRV) then technically no, you don't need an automatic bypass valve.

    However if you ever decide to fit HR92's (or even manual TRV's) on all remaining radiators in the future you'd need an automatic bypass valve. It's good to have one anyway because it regulates the maximum differential pressure across the radiators.

    Without one you'll find that your radiators are quiet when they're all heating up but if there is only one or two active they might hiss quite a lot because the pressure increases. An ABV helps stop that happening as it lets you set the maximum differential pressure across the radiators. When enough radiators are open it doesn't flow at all, so there is no effect under those circumstances, but when not many radiators are open and the pressure gets higher it acts as a pressure relief valve.

    It's part of the building regulations for all new installations now I believe.

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    Thanks @bruce_miranda - I have no idea to be honest. How do I find out? Is it a boiler thing or an installation thing?

    @DBMandrake - thanks I'm with you. Do you think I can get the evohome installed and setup now and then add the automatic bypass valve later? Or is there a reason why I should really get the ABV installed with or before the evohome?

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    Hang on, my bad, you have a combi boiler. So you need to check because many modern combi boilers have an ABV fitted inside them. Also you won't have any motorised zone valves too because you have combi boiler. The valves are all built into the boiler. Which does mean you can't have the evohome HW kit too. So just the boiler relay with the radiator controls should be fine. If all your radiators don't have the radiator controls, you'll need to work out how they demand for heat.

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