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Thread: Evohome hybrid system possible?

  1. #1
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    Default Evohome hybrid system possible?

    I have something like 20 radiators in total, once I include hallways and utility rooms, etc. More than Evohome easily supports and more than I would care to spend on smart TRVs anyway. I have a single heating zone currently.

    It strikes me not every room really needs a smart controller, some areas are just fine with traditional timer control and traditional TRVs. Does anyone know if the Honeywell system will allow this sort of hybrid setup? My concern is that the hub will only instruct the boiler to come on if at least one smart TRV is demanding heat. Or can the hub also be instructed to come on in timed zones regardless of the smart TRVs?

    I suppose in reality it's a bit unlikely I would want heat in the non-smart areas when no smart zones are 'on' though? So maybe I could set up smart zones in the rooms/areas that are more 'fickle' in terms of heating use? For instance the hallways could get heating whenever the heating is on and thus stick with traditional TRVs.

    Anyone have more knowledge on what the system allows, or tips, or stories of similar setups?

  2. #2
    Automated Home Ninja Dan_Robinson's Avatar
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    Your can have multiple rooms in a zone.

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    A zone can have multiple radiators in it. My lounge, for example has 2. So you won't necessarily have 20 zones just because you have 20 radiators.

    You can (and arguably should) bundle together rooms where you leave the doors open, too. For example, I have a "downstairs" zone which has 6 radiators in it because I simply never close the doors between those rooms so trying to heat them to different temperatures on different schedules would be pointless. This could also cut the number of zones you need. In this case, it's often a good idea to enable the "multi-room zone" setting for this zone, so that each radiator can perform independently as each room will likely have different thermal characteristics.

    As for "traditional" mechanical TRVs, yes, you can mix them. As you rightly point out, Evohome won't know anything about them, so won't know to turn the heating on if those rooms are cold. But beyond that, they work just like normal. If the heating's on and the room is cold, the TRV will open a little bit. There's an argument that says this might confuse Evohome's learning algorithms - heat being extracted from the system by radiators it's unaware of - but I've never noticed any real problems. I use mechanical TRVs in my downstairs toilet, and a couple of cupboards that have radiators in (don't ask!)

    Finally, no, you can't simply command the system to come on at certain times.

    Don't forget that you'll need some sort of bypass too. Either one radiator with no TRV (mechanical or Evohome), or else a bypass valve to prevent dead-heading your pump if all the TRVs close and the pump is still running (which can happen during the pump overrun period, for example).

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    There is nothing stopping you from having both a timer and/or the Evohome both triggering the boiler. There was a time when I was using a traditional timer to determine the house heating time and then using Evohome to just control the smart TRVs within that time slot. The down side with this is that its not then a smart TRV, in that it can't turn on the boiler. But you can have this setup running in tandem, so both Evohome and the timer can fire the boiler.

    The BDR91 can also be wired to allow the Evohome or a timer to fire the boiler.

  5. #5
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    Quote Originally Posted by dty View Post
    A zone can have multiple radiators in it. My lounge, for example has 2. So you won't necessarily have 20 zones just because you have 20 radiators.
    Yes I'm aware of that but I don't especially want to buy 20+ smart TRV units as they are pretty expensive, if I don't need smart functionality in all areas!

    Side question - do all controllers in the same zone have to be set to the same temperature?

    Don't forget that you'll need some sort of bypass too. Either one radiator with no TRV (mechanical or Evohome), or else a bypass valve to prevent dead-heading your pump if all the TRVs close and the pump is still running (which can happen during the pump overrun period, for example).
    Isn't a modern boiler smart enough to avoid heating when there is no demand? I mean my current system has TRVs on every radiator so in the summer if I turn my heating on, are you suggesting something could go wrong?[/quote]

    Quote Originally Posted by bruce_miranda View Post
    There is nothing stopping you from having both a timer and/or the Evohome both triggering the boiler. There was a time when I was using a traditional timer to determine the house heating time and then using Evohome to just control the smart TRVs within that time slot. The down side with this is that its not then a smart TRV, in that it can't turn on the boiler. But you can have this setup running in tandem, so both Evohome and the timer can fire the boiler.

    The BDR91 can also be wired to allow the Evohome or a timer to fire the boiler.
    Can you clarify? You seem to be suggesting a few different scenarios possible here...

    1)You can have both Evo and a regular timer box wired 'in parallel' both connected to the boiler controller, so if either asks the boiler to be on, it will be? e.g. I set my existing controller to say "come on 10pm-midnight" and then add the evo system, and this timeslot will always have heating regardless of what the smart-zones want?

    2)I think you're suggesting I can have Evo set up but not wired into the boiler at all? I have actually considered that sort of idea - so the valves would open and close but whether there is hot water available is not controlled by the Evo hub but another system?

  6. #6
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    Quote Originally Posted by MrBoy View Post
    do all controllers in the same zone have to be set to the same temperature?
    Yes. Each zone has its own schedule of what the temperature should be and when. That schedule applies to all the actuators (not controllers) in the zone.

    Quote Originally Posted by MrBoy View Post
    Isn't a modern boiler smart enough to avoid heating when there is no demand? I mean my current system has TRVs on every radiator so in the summer if I turn my heating on, are you suggesting something could go wrong?
    That depends on what you mean by modern. And smart. And demand.

    Imagine this scenario... the house is heating up in the morning and all the zones have shut down apart from one. That zone just arrives at temperature. The HR92 (radiator sensor/actuator) will tell the controller and will shut itself down. The controller will tell the boiler to stop. The boiler will stop. But the pump won't. It will continue for some amount of time determined by your boiler settings. This is the pump overrun. It's designed to protect the boiler from thermal shock. However, if all your TRVs are shut, the pump is now trying to move water around a closed circuit which will cause it to burn out pretty quickly unless it's designed to allow that which very few residential pumps are, IMO.

    This is the same for any TRV system - Evohome or "traditional". There always has to be a way for the water to move if the pump is on and all the valves are closed.

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    Surely even with all TRVs shut the water has the whole house's heating circuit to run around? I would guesstimate in my house that's several hundred feet... either way, if it isn't a problem with regular TRVs would suggest it wouldn't with smart ones either - the potential issue is addressed?

  8. #8
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    No. Do you think your feed is connected to your return? Do you think your live is connected to your neutral so the electrons have somewhere to go?

    It is an issue with regular TRVs. Many modern boilers have a bypass built in. But a normal trick is to leave one radiator (typically a towel radiator) without a thermostatic valve of any kind.

  9. #9
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    Quote Originally Posted by MrBoy View Post
    Surely even with all TRVs shut the water has the whole house's heating circuit to run around? I would guesstimate in my house that's several hundred feet...
    Uhhh, no. Where is the water going to run to if all the valves are closed ? Modern homes are configured with all radiators in parallel off a main trunk branch, something like this:



    If all the radiator valves are closed the pump is trying to pump into a dead end, and unless it's a modulating pump that knows to slow right down and stop, it will be damaged. An automatic bypass valve (not shown above) connects between the output of the pump and the return line to the boiler but does not open and pass any flow unless the differential pressure exceeds what you set the valve to - which would usually only happen if less than one or two radiators were on.


    either way, if it isn't a problem with regular TRVs would suggest it wouldn't with smart ones either - the potential issue is addressed?
    Not necessarily - "most" houses tend to have a bypass radiator - this is often either the bathroom, or the hallway where the wall thermostat is mounted, or both.

    In fact in most traditional systems there is deliberately no TRV on the radiator in the room that the wall thermostat is in - this is because if you were to turn that TRV down the wall thermostat would never be satisfied and the boiler would keep running, so that radiator is left open and the room temperature controlled by the thermostat that controls the boiler.

    So there is always a radiator open to allow some flow through the pump - because of this even though guidelines still recommend an automatic bypass valve, it's possible (on the cheap) to leave it out if there is a bypass radiator, and many installations have a bypass radiator but no automatic bypass valve.

    On an Evohome system every zone has the ability to call for heat from the boiler not just that single zone, so its perfectly possible (and I would say desirable) to eliminate the bypass radiator and have ALL radiators in the house controlled. As soon as you do that you no longer have a bypass radiator and MUST have an automatic bypass valve in the system, which you may or may not already have. The only exception would be a system with an integrated variable speed pump that is specifically designed to be able to cope without a bypass, but that is not most systems.
    Last edited by DBMandrake; 18th September 2017 at 01:17 PM.

  10. #10
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    Water doesnt flow, unless it has a path. Hence why the flow pipe is cold if the radiator is shut off. So unless your boiler has an internal by pass, you should have an ABV. Non TRV Towel rails are OK but they too can be shut off (think kids in the bathroom).

    in terms of Evohome without boiler control, its absolutely possible. And if you want a smart timer, just do something similar. I had the boiler triggered by a Wemo, which gave me all the benefits of IFTTT etc, but a few zones were controlled by Evohome. So you become responsible for making sure the boiler is fired up, but then the advantage is you dont need smart TRVs on every rad. It also avoids the boiler from firing when you don't want e.g. a single room has dropped 0.5C below set point

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