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Thread: Evohome Opentherm - tearing my hair out!

  1. #51
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    The HGI80 is a starting point. But remember what I said about the Heat Demand message interpretation not being correct in Domoticz for it. This will cause you some more issues. It's particularly broken for OT!

    There are several things you need here. An eBUS monitor to log what the boiler is doing. An OT monitor to log what the boiler is being asked to do. And then the HGI80 which logs the Evohome chatter.

    Oh and did I tell you, you can't graph Heat demand and Temperatures in the same graph on Domoticz. So very quickly you are going to start looking for other graphing tools.

  2. #52
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    Quote Originally Posted by bruce_miranda View Post
    The HGI80 is a starting point. But remember what I said about the Heat Demand message interpretation not being correct in Domoticz for it. This will cause you some more issues. It's particularly broken for OT!

    There are several things you need here. An eBUS monitor to log what the boiler is doing. An OT monitor to log what the boiler is being asked to do. And then the HGI80 which logs the Evohome chatter.

    Oh and did I tell you, you can't graph Heat demand and Temperatures in the same graph on Domoticz. So very quickly you are going to start looking for other graphing tools.
    Ok great, off shopping I go! Just ordering the OT gateway on Nodo-Shop.

    BTW, and off this topic, does anyone know who runs this site (automatedhome), and is it just me or are cookies broken on it? As I have to log in on each visit?

  3. #53
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    Cookies working fine for me...Add that to the list of things that aren't working for you :-) While you are shopping, pick up an eBUS monitor too, you really need that more than the OT Gateway.

  4. #54
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    Quote Originally Posted by billytkid View Post
    I agree at position 15 there shouldn't be any heat demand, but I am seeing some. I've ordered an HGI80 to try to get to the bottom of this, this should arrive for the weekend.
    I posted this a while ago in another thread but as it is pertinent here I'll post it again. This is the relationship between HR92 valve pin position (0-100) and reported heat demand for the zone shown in the system installer menu: (0-100%)



    This is based on carefully taken measurements of several of my zones. (All zones gave identical results, but I checked to make sure that local learning of HR92's did not alter this in any way)

    In short, pin positions 0-30 result in no heat demand, 30-70 has a linear slope, and 70-100 has another steeper linear slope with the knee occuring at about 70%. As you can see from this, a pin position of 15 should not call for any heat.

    Apparently the heat demand transmitted by an HR92 over wireless comms that can be captured by Domoticz with an HGI80 is the heat demand figure (left axis) not the pin position, in other words the controller displays exactly what it receives, but as I don't have an HGI80 I have not confirmed this myself.
    @DBMandrake - re the calling for heat point. It's a good question, I'm monitoring the flow and return temps from the pipe directly into Domoticz so this could be an overshoot in theory, but take a look at the attached. The boiler is able to modulate to 45degrees when it wants.

    Also, here is another graph from today:
    Attachment 1270
    It does seem to again 'blast' for heat then stop, then do it again, rather than go slow and long. Or, could this be (looking at flow/return temps) the boiler waiting for F/R to balance? It sees happy when return stays at 40 degrees, but is odd it takes an hour to get to this point of being happy. But then, again, EvoHome keeps calling for heat even after it's hit the temp - shouldnt it know it was close enough and pull back?
    Now that you've confirmed you're going by actual measured flow and return temperatures not logging the opentherm requested flow temperature from the evohome that completely changes the situation.

    Looking at your flow and return temperature graph above it's pretty clear to me that your boiler is unable to modulate low enough to meet the target flow temperature in times of low demand from the radiators. This is what is causing the very spiky up and down oscillation of the flow temperature, and the actual called for flow temperature from the Evohome is likely to be near the bottom of each oscillating spike on the yellow line, or slightly above the green return temperature line.

    At the point where some demand starts at about 13:40 you can see flow temperature spikes up quickly from 28 to 48 degrees and then drops back down to about 32 degrees again, then next time it goes up to 50C and back down to 40C. These oscillations in flow temperature are not a result of the Evohome requested flow temperature jumping up and down - it will be either constant or gradually increasing once every few minutes. (as HR92's gradually open)

    So what will be happening is at 13:40 one or more HR92's just start to open - there will be a very minimal water flow through the valves as they will only be slightly open. The Evohome is probably calling for about 35C initially, however the heat load on the boiler is so low (due to so few radiators being open, and those being open only slightly) that at a minimum kW burn the temperature quickly and uncontrollably climbs to 48C.

    When trying to modulate to an OpenTherm requested flow temperature the boiler will allow a certain amount of "overshoot" due to low load, say 10C. This is probably adjustable as one of the boiler configuration parameters. If it exceeds the maximum allowed overshoot amount it has no choice but to shut down the burner. The temperature now quickly falls below the requested temperature from the Evohome, (dropping to 32C here) and it then comes on again when it is too low.

    If the boiler can't modulate it's heat output low enough for the low demand conditions then it has no alternative but to cycle on and off like this trying to approximate the requested temperature without overshooting too much. What else can it do ?

    If either the load is increased (radiator valves open up more and/or more radiators come online) or the requested flow temperature increases or a bit of both, the amount of heat required to reach an equilibrium will increase beyond the minimum modulation level of the boiler and the temperature will become stable as the boiler can adjust it's output suitably without having to cycle on and off.

    You say the boiler is able to modulate to 45C when it wants to - yes it can, when it has a higher load from the radiators. Not when the load is very low. All boilers have a minimum load that they can supply while staying within their modulation range, so all boilers will suffer from this cycling effect at very low load levels like a proportional control system trying to put a very small amount of heat into the house in warm weather.

    So unfortunately I think you've probably been over analysing the situation and chasing your tail about nothing. There is nothing wrong other than the boiler being unable to modulate low enough, and there is little you can do about it.

    However this in itself is not really a problem because total gas use is still going to be very low. You see the temperature shoot up to 48C and then drop down to 32C again and assume gas is being wasted, but the heat output and gas burnt to do that was very low, precisely because there is so little load on the boiler in those conditions. Flow temperature alone is not an indicator of how much gas the boiler is using or how efficiently it is running.
    Last edited by DBMandrake; 9th May 2018 at 11:07 AM.

  5. #55
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    FWiW, I do not have the smart monitoring kit that some of you guys have; however, I would suggest that a simple meter optical reader provides a lot of useful information about gas usage and, more importantly, how best to run Evohome with or without Opentherm. When my house was fully up to temperature - on a cold day - I have seen boiler flow temperatures in the low 50s/high 40sC. Usage monitoring has also shown that if the house is occupied for most of the day, it is much more efficient just to leave the house to run at the set zone temperatures. I appreciate that heat loss is different for all homes; however, I have concluded that if I am going out for less than 3 hours, there is little to be gained by going for the Economy setting.

    I have also concluded that for my boiler and Opentherm, the % boiler demand often bears no relationship whatsoever to the zone demanding the maximum % heat. I am guessing that this is down to inherent Evohome/HR92 delays. All that said, I have given up bothering. I just let the boiler; Evohome and Opentherm do their thing without worrying if I am squeezing all the kWhs worth of savings out of the system.

  6. #56
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    Quote Originally Posted by DBMandrake View Post
    I posted this a while ago in another thread but as it is pertinent here I'll post it again. This is the relationship between HR92 valve pin position (0-100) and reported heat demand for the zone shown in the system installer menu: (0-100%)



    This is based on carefully taken measurements of several of my zones. (All zones gave identical results, but I checked to make sure that local learning of HR92's did not alter this in any way)

    In short, pin positions 0-30 result in no heat demand, 30-70 has a linear slope, and 70-100 has another steeper linear slope with the knee occuring at about 70%. As you can see from this, a pin position of 15 should not call for any heat.

    Apparently the heat demand transmitted by an HR92 over wireless comms that can be captured by Domoticz with an HGI80 is the heat demand figure (left axis) not the pin position, in other words the controller displays exactly what it receives, but as I don't have an HGI80 I have not confirmed this myself.

    Now that you've confirmed you're going by actual measured flow and return temperatures not logging the opentherm requested flow temperature from the evohome that completely changes the situation.

    Looking at your flow and return temperature graph above it's pretty clear to me that your boiler is unable to modulate low enough to meet the target flow temperature in times of low demand from the radiators. This is what is causing the very spiky up and down oscillation of the flow temperature, and the actual called for flow temperature from the Evohome is likely to be near the bottom of each oscillating spike on the yellow line, or slightly above the green return temperature line.

    At the point where some demand starts at about 13:40 you can see flow temperature spikes up quickly from 28 to 48 degrees and then drops back down to about 32 degrees again, then next time it goes up to 50C and back down to 40C. These oscillations in flow temperature are not a result of the Evohome requested flow temperature jumping up and down - it will be either constant or gradually increasing once every few minutes. (as HR92's gradually open)

    So what will be happening is at 13:40 one or more HR92's just start to open - there will be a very minimal water flow through the valves as they will only be slightly open. The Evohome is probably calling for about 35C initially, however the heat load on the boiler is so low (due to so few radiators being open, and those being open only slightly) that at a minimum kW burn the temperature quickly and uncontrollably climbs to 48C.

    When trying to modulate to an OpenTherm requested flow temperature the boiler will allow a certain amount of "overshoot" due to low load, say 10C. This is probably adjustable as one of the boiler configuration parameters. If it exceeds the maximum allowed overshoot amount it has no choice but to shut down the burner. The temperature now quickly falls below the requested temperature from the Evohome, (dropping to 32C here) and it then comes on again when it is too low.

    If the boiler can't modulate it's heat output low enough for the low demand conditions then it has no alternative but to cycle on and off like this trying to approximate the requested temperature without overshooting too much. What else can it do ?

    If either the load is increased (radiator valves open up more and/or more radiators come online) or the requested flow temperature increases or a bit of both, the amount of heat required to reach an equilibrium will increase beyond the minimum modulation level of the boiler and the temperature will become stable as the boiler can adjust it's output suitably without having to cycle on and off.

    You say the boiler is able to modulate to 45C when it wants to - yes it can, when it has a higher load from the radiators. Not when the load is very low. All boilers have a minimum load that they can supply while staying within their modulation range, so all boilers will suffer from this cycling effect at very low load levels like a proportional control system trying to put a very small amount of heat into the house in warm weather.

    So unfortunately I think you've probably been over analysing the situation and chasing your tail about nothing. There is nothing wrong other than the boiler being unable to modulate low enough, and there is little you can do about it.

    However this in itself is not really a problem because total gas use is still going to be very low. You see the temperature shoot up to 48C and then drop down to 32C again and assume gas is being wasted, but the heat output and gas burnt to do that was very low, precisely because there is so little load on the boiler in those conditions. Flow temperature alone is not an indicator of how much gas the boiler is using or how efficiently it is running.
    Hiya,

    thanks so much for the big reply, and yes this all does make sense.

    the only thing that still confuses me is that the boiler will be called for heat, but the only rads that are calling (ie on/near setpoint) are still stone cold. Which means the above is happening as you said, but just making my bathroom very hot (the rad in there is the loop back!

    I've got the HGI80 now, but have another post as install on a Pi is troublesome.

    Link to post on Domoticz Forum - http://www.domoticz.com/forum/viewto...p?f=34&t=23415

    Thanks again
    Last edited by billytkid; 11th May 2018 at 11:36 AM.

  7. #57
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    Quote Originally Posted by billytkid View Post

    I've got the HGI80 now, but have another post as install on a Pi is troublesome.
    If you do a search you should find a pre-baked image file that I posted, which you can use as a starting point. It's old now, and will need both Os and Application updates, but it should be enough to get you going.

  8. #58
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    Start with this. http://www.automatedhome.co.uk/vbull...Domoticz+image

    It has everything you need.

  9. #59
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    Thanks guys, however as I've got a live system I don't really want to go back to the start and reinstall from a backup.
    Also from the info Honeywell have given me I don't think I need the HGI80 anyway.

    However there is good news and info, Honeywell do trap far more info that I'd been led to be believe, including flow/return temps on the boiler.
    Now that I've had a full explanation I'm happy to surrender to the EvoHome gods and let it run it's own way, end details from Honeywell below:


    "I have had a look at the data from Mr Morrish’s Evo with OpenTherm modulating boiler and on/off kitchen zone
    I believe the questions were:
    Why was the boiler firing when zones were not calling for heat
    Why was the Kitchen valve closed when the boiler was firing
    Why was the kitchen above set point why the boiler is producing water at 50-60°C when the boiler can modulate to 30°C.
    I will try to answer these questions but I don’t think I will be able to fully answer them with the data we can gather through the portal. I can however suggest what may be happening and also some of the more general control challenges that will influence control operation.

    I have captured a series of screen shots from the portal, the first shows the Kitchen zone and includes Room temp, Setpoint, Outside temperature and the Boiler flow and return temperatures (measured at the boiler not to this particular to this zone - radiator size and flow rate will affect what each zone actually receives). This is for part of one particular day but it does seem to be typical of days either side of it.

    You will see that the Kitchen (slide 2) is above setpoint and will not be calling for heat, it should therefore not be cycling the zone valve and unless the valve is leaking the radiator will not be warm – I fully accept that this is not when the customer was commenting on the operation and will not be what they were observing but the pictures taken of the Evo do at least show similar temperatures. I would assume there are other forms of heat in the Kitchen, the oven, hob, kettle, South facing window or even just people as they give off a significant amount of heat compared a single zone at low load.
    I have then looked at the rest of the system and captured the zones that will at times be calling for heat at the same time. The heat demand causing the boiler demand are coming from five other zones. I am looking over a 6 hour period and the zone demands will either be very low or cycling between low and off at times during the period depending on when it is observed.

    This will be why the boiler is firing but it will perhaps not look logical, the reality is that the control is trying to maintain temperatures in each of these zones, all of which are losing heat. Even though they show on the display to be at temperature were we were to stop heating the temperature would fall and have to be recovered at some point later once the temperature had undershot the set temperature. Putting heat in large lumps is rarely a good idea for efficiency or control. Rooms are suppressing difficult to control accurately because of the thermal lag in heating systems, from air temperature into the sensor and all the design parameters for radiator sizing that may or may not have been correctly applied. It is quite typical for there to be a 20min lag from the control firing the boiler to the heat being registered in the room. To cope with the variability of room and heating design we have developed a Fuzzy logic control in Evo that learns each rooms characteristics, this is a constant process throughout the year but the initial big learning curve after installation can take about three weeks of operation (we can only learn when we are actually controlling temperature so it can take longer in the Spring or Autumn)

    The point about the boiler firing when the Kitchen valve is closed is very reasonable. It is obvious that it should be open when heat is available. What has been observed is a compromise caused by couple of factors. The boiler with the OpenTherm bridge is a fully modulating system, and the by far the best way to run a high efficiency premix condensing boiler but the boiler massively oversized during warmer weather and particularly when running a zones heating system.
    I don’t know the size of this boiler but Combi boilers are often 30kW, system boiler perhaps 20kW, they can modulate their output down to perhaps 7 or 8kW at minimum but this is also much higher than a whole house needs in warm weather, add zoning to this and a single radiator might only deliver 2kW at maximum output then modulate this down to 10% which is the minimum heat demand that we consider passing to the boiler."
    The attachment is very interesting on the level of detail Honeywell trap. - Sadly the forum limits the file size, so here is a link : https://www.scribd.com/document/3792...oney-Well-Info

  10. #60
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    Anyone else having trouble downloading that scribd document ? I can view it in the browser but downloading it as a file so I can get a better look at it just sits on a spinner... Have tried different browsers, disabling ad blockers, logging in to scribd etc...

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