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Thread: Opentherm with Evohome

  1. #1
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    Default Opentherm with Evohome

    After a few months off the case, I am back at looking at boiler replacement options with an opentherm connection. To date, I have considered ATAG but there are no installers in my immediate area and, latterly, Intergas. Valliant said that it wasn't a goer and I still await a response from Viessmann. Technical support teams tell me that their installers have been given some training on the opentherm capability but as there are so many opentherm devices on the market they have to led by the device manufacturer.

    One forum that I read recently suggests that Intergas boilers do not cope with Evohome multi-zoning. The advice is that it may be just as good to install a boiler with weather compensation and no opentherm. I know that Evohome does not use external weather compensation but I struggle to see how, for example, a variable boiler temperature (weather adjusted) would work with, say, optimum start on a HR92? Wouldn't the boiler be continually adjusting and the HR92 continuously learning?

    Another forum suggested that as most of the savings benefit has already been accrued by installing Evohome, the cost/benefit of opentherm doesn't stack up.

    I am also struggling to get my mind round how a system boiler works with opentherm? I have seen a reference in the past that opentherm only works with combo boilers.

    What I am looking for is a replacement system boiler for an Ideal FF280 in an 'S'plan configuration. The two BDRs control the 2 motorised valves.

    Am I wasting my time with this? I am really not keen to have my Evohome set up 'destroyed' whilst a well-meaning heating engineer learns on the job.

  2. #2
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    I think there might be a bit of confusion on the part of those that you were reading as to what weather compensation and opentherm control do - they both control the flow temperature, but for quite different reasons, and in theory both can work together or independently to optimise efficiency under different conditions.

    So first, weather compensation:

    All this does, is automate the process of turning your flow temperature down in warmer weather, and proportionally based on measured outside temperature.

    In the winter you obviously need a high flow temperature - if it's set too low it will either take far too long to get the house up to temperature, or it may not even reach the desired zone temperatures in some zones at all. If the flow temperature is too low the heat output from radiators may actually be less than the heat loss through the walls...so in cold weather you turn it up.

    The biggest drawback of leaving your flow temperature high all year round (which I suspect a lot of people that don't know better do) is that your boiler may not be in condensing mode very often as flow temps have to be below about 55 degrees on the return leg to gain condensing efficiency benefits. If you have your flow temperature set to 75 degrees and the system isn't balanced properly (pump too fast, radiators not balanced etc) to get a decent temperature drop on the return then the return temperature may be far to hot to condense and your shiny new condensing boiler is running at about 75% efficiency instead of 90%...

    So turning the flow temperature back a bit when the weather isn't that cold will make your boiler run in condensing mode most if not all of the time improving efficiency. Further gains come from less heat loss from the boiler casing, the flue, and less heat loss from the pipe work that is running unnecessarily hot to radiators that are largely closed off.

    From a comfort point of view, running an unnecessarily high flow temperature when heat loss through the walls is low will tend to cause temperature overshoots due to the radiators continuing to radiate a lot of heat for long after the valves are closed. This is particularly bad with manual TRV's - Evohome is smart enough to try to turn off the valve well in advance of reaching the set point if it detects the temperature is rising fast but there is still only so much it can do. If your flow temperature is optimal for the heat loss temperature regulation of the rooms will be better.

    So weather compensation helps improve efficiency and comfort during warmer weather but does nothing for you in the depths of winter which is exactly when you are using the most gas. If the weather is really cold outside weather compensation will crank your flow temperature up high and leave it there, regardless of whether your house is in the heat up phase, or is up to temperature and in the "maintenance" phase where it is just trying to maintain steady temperatures.

    To work well you also need weather compensation that has "slope" adjustment so that it can be adjusted to match the heat loss characteristics of your house, as the better your insulation is the more rapidly it needs to drop the flow temperature as outside temperatures rise. If you don't have slope adjustment or it isn't set properly the flow temperature it selects under given conditions will be too hot or too cold and you'll have to manually override it anyway. From what I've read some cheaper weather compensation controls don't provide a slope adjustment - if they don't have it, avoid!

    How does weather compensation work with Optimal start ? Although I don't have weather compensation, theory says it should work just fine. The reason being, if you have properly adjusted weather compensation it will tend to adjust the flow temperature so that warm up times remain about the same - so optimal start actually has less work to do as outside temperatures change.

    For example with a fixed flow temperature as weather gets colder it takes longer and longer for the rooms to warm up, optimal start gradually adapts to this on a day by day basis and brings your rooms on earlier and earlier, however if you use correctly adjusted weather compensation the colder weather would increase the flow temperature which would bring the warm up time back to about what it was before so optimal start doesn't have to adapt! Of course when the weather gets really cold and weather compensation has turned your flow temperature up to maximum if the weather continues to get colder warm up times would increase and optimal start would need to optimise further.

    So I don't have any concerns about mixing optimal start and weather compensation - in theory they should work together very well as long as your weather compensation is adjusted to match the house properly.

    OK, so OpenTherm control:

    This is the Evohome being able to request a specific flow temperature from your modulating boiler, and the boiler will modulate its burner output (rather than cycling it on to full burn and off repeatedly like old boilers) to try to hold that temperature regardless of the load presented by the radiators. When there is a high demand from one or more zones, the Evohome will ask for maximum flow temperature from the boiler. This is either a manually set temperature on the boiler, or the flow temperature chosen by the weather compensation.

    For example one or more zones being at least 1.5 to 2 degrees below the set point will cause the Evohome to request maximum flow temp. This provides the fastest possible warm up time. However once all active zones are within their +/- 1.5 degree proportional zone it will start to request a lower and lower flow temperature. Eventually it will find the lowest flow temperature that can meet the current demands of the most demanding zone in the house.

    The big advantage of this is that it allows the flow temperature to be dropped even during winter. It might be that you need 70-75 degrees flow temperature in the middle of winter to get your house warmed up in a reasonable time, but it's unlikely that it needs to stay that high to maintain those room temperatures once warm. You might be surprised how low the flow temperature can go and still maintain status quo even in winter. If it can drop the flow temp to 55 degrees for most of the day even in the middle of the winter that's potentially quite a big efficiency savings. Weather compensation can't do this for you as it doesn't know when the house is up to temperature and has a lower demand - all it knows is the outside temperature.

    Another benefit is that it allows the Evohome to more precisely regulate the temperatures in the rooms and the likelihood of temperature overshoots reduces considerably when the flow temperature is no higher than necessary.

    So what does the Evohome do if you only have a BDR91 not OpenTherm ? It does TPI (Time proportional Integral) cycling of the boiler, like any other TPI thermostat - this is a "poor mans" attempt to modulate the boiler flow temperature. It does this by turning the boiler on and off in (by default) 10 minute cycles. For example on for 3 minutes off for 7 minutes for 30%.

    This does work, but it can't be as accurate or efficient as using OpenTherm. It's not as accurate because OpenTherm lets it ask for a specific temperature, and the boiler will do its best to achieve that by modulating its burner, and largely independent of how many radiators you have open etc.

    With TPI all it knows is that increasing the duty cycle will increase average flow temperature and reducing it will reduce average flow temperature, but it has no idea what the flow temperature might be, because it depends on so many variables that it doesn't know, like whether the radiators are currently hot or cold, how much they're flowing, what the kW rating of the boiler is, what the manually set flow temperature is, what the outside temperatures are and so on. It's a bit of guess work but it is better than all or nothing control.

    It's also slow to respond to small set point changes - for example say you change your living room from 20 to 20.5 degrees. With TPI the heat output of the boiler is only averaged over at least a 10 minute period, If the boiler is currently running 2 minutes per 10 minutes and would need to increase to 3 minutes per 10 minutes to warm the room that much you may have to wait a full 10 minute cycle to get to the bit of the cycle where it turns on again so it can stay on a bit longer. With OpenTherm it could immediately command an increase from 50 to 55 degrees and the boiler would oblige as quickly as possible.

    It's also not as good from an efficiency point of view. If the Evohome commands 50 degrees through OpenTherm, the boiler will probably run constantly but with a low burn to maintain that 50 degrees. This is good for efficiency. With TPI if you had your manual flow temperature control set to 70 for example, and the Evohome wanted an average flow temperature of 50, it would cycle the boiler on and off every few minutes to try to get that average. However the boiler itself is still trying to reach the set 70 degrees (which it never can, because it keeps getting interrupted) so every time it comes on it goes flat out to try to reach 70 degrees, fails to get there, is turned off, then next time it goes flat out again for a few minutes. So the boiler ends up going flat out in short bursts instead of running continuously at a reduced burn, all to reach the same average flow temperature. This constant igniting, running at full burn for a short time then stopping again is not as efficient as running continuously at a reduced burn.

    Hopefully that gives some insight into Weather compensation vs OpenTherm control.
    Last edited by DBMandrake; 12th October 2016 at 04:25 PM.

  3. #3
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    I'm about to start first winter with a viessman system boiler (wb100), opentherm and full evohome on a large system (~20 rads). Last winter the system was installed, but not installed right - spent months debugging, I know more about it than my heating engineers now (who have since said they will never touch opentherm again.. not to put you off, but I think it scared them).
    I am now confident its right, so going to see what it can do. so if you can hang on a few months, I can give you some more detailed feedback!
    It hasn't got properly cold yet, what I am seeing so far in the last week since its started to get a bit chilly (and we passed the "national heating on" day )is an echo of what DBM has said. when the system is cold, rooms asking for a lot of heat (2 degrees or more out), OT tells the boiler to give it max output. When the setpoint is nearly reached it drops down boiler output. It will in fact drop down as far as "burner off, keep the pump on", when there is still some useful heat in the circulating water and perhaps only one zone calling for a tiny bit of heat, it keeps the water moving to squeeze out the last bit of energy e.g at the end of the evening heating period.
    I've also switched to an alpha2 pump in PP3 mode, noticing a substantial difference when in that "just one zone needs a bit of heat" mode - the pump backs right off and the flow noise level is close to zero.

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    Quote Originally Posted by sandyman View Post
    I'm about to start first winter with a viessman system boiler (wb100), opentherm and full evohome on a large system (~20 rads). Last winter the system was installed, but not installed right - spent months debugging, I know more about it than my heating engineers now (who have since said they will never touch opentherm again.. not to put you off, but I think it scared them).
    I am now confident its right, so going to see what it can do.
    I'm interested to know what scared them so much and why they had so much difficulty getting it working ? Was it compatibility issues between the two units ? In principle as long as the Boiler supports OpenTherm it should just be a matter of replacing the BDR91 with the OpenTherm bridge, binding that new device and making sure it's correctly wired...

    Maybe some first hand info of what problems they ran into and what the resolution was would help others contemplating it...

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    Ideal, ATAG, Viessmann or Ravenheat will all be great with evohome and OT Bridge. If you looking for an installer clued up on this looked for a Honeywell Connected Specialist or an ecotechnician www.ecotechnicians.co.uk.

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    Quote Originally Posted by top brake View Post
    Ideal, ATAG, Viessmann or Ravenheat will all be great with evohome and OT Bridge. If you looking for an installer clued up on this looked for a Honeywell Connected Specialist or an ecotechnician www.ecotechnicians.co.uk.
    Thanks all. Sadly, the southwest Midlands are not well served by either connected specialists or ecotechnicians. I may give the guys in Bromsgrove a ring.

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    it was a new boiler as well. I have some posts on here in other threads going into the gory detail. the main points where
    - they didn't actually install the OT bridge (they "forgot") they put in a boiler demand BDR91 instead and we/I didn't realise for weeks. much shouting when realised we were troubleshooting the wrong problem...
    - the viessman WB100 boiler needs a hot water kit input (triggered by the DHW BDR91) to set the output to maximum when you need DHW. without it, the opentherm call for heat from the evohome when DHW is the only demand, was for a very low temp, so my DHW would never warm up as it was firing the boiler only up to 50 odd degress. this took ages to figure out, I was helped out a lot by some CH engineers who know viessman backwards on diynot forum. There were some suggestions from honeywell (I think topbrake was involved under his real identity) regarding adding zone valves to put the system into W plan mode (flow goes to either CH OR DHW, but not both), which we did implement, but the killer fix was enabling the dhw kit on the viessman.

    because I made them come back so many times to actually get it right, at no cost to me, I am now in their "damn he was a difficult customer" books, they are somewhat traumatised by the experience, so I think they don't want to touch OT any more . And they ARE honeywell connected specialists.

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    Quote Originally Posted by sandyman View Post
    it was a new boiler as well. I have some posts on here in other threads going into the gory detail. the main points where
    - they didn't actually install the OT bridge (they "forgot") they put in a boiler demand BDR91 instead and we/I didn't realise for weeks. much shouting when realised we were troubleshooting the wrong problem...
    - the viessman WB100 boiler needs a hot water kit input (triggered by the DHW BDR91) to set the output to maximum when you need DHW. without it, the opentherm call for heat from the evohome when DHW is the only demand, was for a very low temp, so my DHW would never warm up as it was firing the boiler only up to 50 odd degress. this took ages to figure out, I was helped out a lot by some CH engineers who know viessman backwards on diynot forum. There were some suggestions from honeywell (I think topbrake was involved under his real identity) regarding adding zone valves to put the system into W plan mode (flow goes to either CH OR DHW, but not both), which we did implement, but the killer fix was enabling the dhw kit on the viessman.

    because I made them come back so many times to actually get it right, at no cost to me, I am now in their "damn he was a difficult customer" books, they are somewhat traumatised by the experience, so I think they don't want to touch OT any more . And they ARE honeywell connected specialists.
    And therein lies my concern. Installers of brands such as ATAG, Intergas, and Viessmann tend to be one man businesses. I have a local Intergas installer who gets excellent reviews for his boiler installations but there is no mention of smart controls on his website. Equally, local HW Connected Specialists are not listed as boiler manufacturers' recommended installers which has warranty implications.

    To return to my earlier - rather poorly phrased - question, what additional savings does Opentherm offer over a new condensing boiler with Evohome zoning?

  9. #9
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    @DBMandrake

    Thanks for your comprehensive and helpful explanation. More food for thought.

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    Quote Originally Posted by HenGus View Post
    And therein lies my concern. Installers of brands such as ATAG, Intergas, and Viessmann tend to be one man businesses. I have a local Intergas installer who gets excellent reviews for his boiler installations but there is no mention of smart controls on his website. Equally, local HW Connected Specialists are not listed as boiler manufacturers' recommended installers which has warranty implications.

    To return to my earlier - rather poorly phrased - question, what additional savings does Opentherm offer over a new condensing boiler with Evohome zoning?
    best I can say is subjectively so far (having run the same system , full evohome , new condensing boiler, with both control methods) is that so far it subjectively "smoother" with opentherm. I will know more once it gets properly cold as my OT was only fully sorted around april.
    Objectively, I have "loop" monitoring on my gas meter (don't have a smart one yet) and I'll gladly share my consumption data with you in the spring (of course, it may well vary more due to the severity or otherwise of the winter, time spent away from home etc, than anything due to the two control methods ).

    as DBMandrake said, you can get the system installed with one control type (if that helps you with your choice of boiler and installer) then swap it to the other if/when you feel like it - fairly simple change, wiring goes to and from the same place just different terminals on the boiler and a rebind in evohome.

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