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Thread: Honeywell evohome and OpenTherm integration

  1. #31
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    Quote Originally Posted by The EVOHOME Shop View Post
    As Hengus has said, the tests he did were performed for 3 weeks during April and his heating system isn't necessarily correctly designed or balanced for a condensing boiler. In my opinion there are various ways OpenTherm can be used to achieve heating control strategies and it is understanding this aspect that I am most interested in.

    At the end of the day the OpenTherm Protocol Specification does not require the 'master' to do every OpenTherm command and because of this, the boiler manufacturers should make their boilers capable of modifying the settings that the controls are not required to do. By Viessmann not allowing these settings available, they are in essence forcing people to buy Viessmann's own controls or an OpenTherm controller that is capable of this. In my opinion this is damaging to the OpenTherm brand as people will start seeing 'incompatibilities' with some OpenTherm products and this is no fault of the manufacturer that has produced them.



    OpenTherm doesn't require the system to be designed and correctly balanced for it. The heating system design specifies how the system should operate and condensing boilers require a 20 degree differential between flow and return. Most design for 75/55 but you can design the system at a lower temp (70/50 for instance) to aid condensing effect (you would need to increase radiator sizes to use lower temps). The fact is that the heating installer is the expert here and if they feel the new boiler is not capable of maintaining a designed temperature differential of 20 degrees because the radiators are undersized, they should advise that they are changed.

    I am using an OpenTherm monitor from here - http://otgw.tclcode.com/otmonitor.html
    Thanks Richard. I have no reason to doubt the knowledge/experience of my installer. He came with a TB recommendation. He looked carefully at the house size, radiators etc and calculated that I could get away with an 18kW system boiler. We went for a 24kW boiler because of the 250L cylinder re-heat requirement.

    Are you using an HGI 80?

  2. #32
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    Quote Originally Posted by HenGus View Post
    Thanks Richard. I have no reason to doubt the knowledge/experience of my installer. He came with a TB recommendation. He looked carefully at the house size, radiators etc and calculated that I could get away with an 18kW system boiler. We went for a 24kW boiler because of the 250L cylinder re-heat requirement.

    Are you using an HGI 80?
    No problem, I am sure your installer was good but I am just re-iterating that the system should be designed correctly to remain efficient no matter what demand is being thrown at the boiler.

    I am using the 80 to monitor zone temps, IDS software to monitor the Intergas boiler and OpenTherm monitor to look at thermostat commands... I am trying to give the best feedback to the customers that choose OpenTherm and it would be poor of me if I didn't understand everything that happens (hence I have setup the facility to do this and also train installers here on OpenTherm).

    top brake has asked for feedback and specific cases where there are issues with evohome and OpenTherm. I am trying to help differentiate between OpenTherm 'issues' and system design/boiler specific issues.

    You have to bear in mind that to be an OpenTherm thermostat, you do not have to have every OpenTherm control parameter the 'master' can have. This then creates 'issues' when the manufacturer of the boiler knows this and uses this to their 'benefit', knowing only their controls satisfy their requirements (because they have not allowed the installer/user to modify the settings).

    This manufacturer behaviour is not in the sprit that OpenTherm was developed and quite frankly upsets me. Any manufacturers I find doing this will be named and shamed and I will have a recommend list and an avoid list on The EVOHOME Shop based on 'compatibility'.
    Last edited by The EVOHOME Shop; 1st August 2017 at 01:06 PM.

  3. #33
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    It would be less hassle for Honeywell to update their software to include a CH maximum setting and more convenient for the customer.

  4. #34
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    Quote Originally Posted by The EVOHOME Shop View Post
    No problem, I am sure your installer was good but I am just re-iterating that the system should be designed correctly to remain efficient no matter what demand is being thrown at the boiler.

    I am using the 80 to monitor zone temps, IDS software to monitor the Intergas boiler and OpenTherm monitor to look at thermostat commands... I am trying to give the best feedback to the customers that choose OpenTherm and it would be poor of me if I didn't understand everything that happens (hence I have setup the facility to do this and also train installers here on OpenTherm).

    top brake has asked for feedback and specific cases where there are issues with evohome and OpenTherm. I am trying to help differentiate between OpenTherm 'issues' and system design/boiler specific issues.

    You have to bear in mind that to be an OpenTherm thermostat, you do not have to have every OpenTherm control parameter the 'master' can have. This then creates 'issues' when the manufacturer of the boiler knows this and uses this to their 'benefit', knowing only their controls satisfy their requirements (because they have not allowed the installer/user to modify the settings).

    This manufacturer behaviour is not in the sprit that OpenTherm was developed and quite frankly upsets me. Any manufacturers I find doing this will be named and shamed and I will have a recommend list and an avoid list on The EVOHOME Shop based on 'compatibility'.
    Thanks Richard. I know that you are doing your best to sell Evohome/Opentherm but it is hardly surprising that most installers run a mile when third-party smart controls are mentioned, Given the marginal savings benefit of Opentherm over a BDR and weather compensation, I just wonder how long it would take the average house owner to recover the cost of changing an established heating system into something that is ideally configured from an Evohome/Opentherm point-of-view?

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    Quote Originally Posted by blowlamp View Post
    It would be less hassle for Honeywell to update their software to include a CH maximum setting and more convenient for the customer.
    I am sure this is something they could consider, but like I said its not an OpenTherm requirement.

    Quote Originally Posted by HenGus View Post
    Thanks Richard. I know that you are doing your best to sell Evohome/Opentherm but it is hardly surprising that most installers run a mile when third-party smart controls are mentioned, Given the marginal savings benefit of Opentherm over a BDR and weather compensation, I just wonder how long it would take the average house owner to recover the cost of changing an established heating system into something that is ideally configured from an Evohome/Opentherm point-of-view?
    Hengus, I'm not trying - OpenTherm has the capability to be best plug and play solution. Plus OpenTherm isn't a compromise, where using weather comp on a boiler and a BDR91 as a boiler relay switching the boiler is (some zones may not get to temp unless the WC curve is set correctly).

    I am used to modifying cars and you can't just upgrade a turbo without remapping the engine - effectively adding a condensing boiler to a existing standard efficiency heating system where the rads have been sized for 82/71 will mean you will not get the best results (OpenTherm controlled or W/C controlled). Radiators will be generally undersized when swapping for a condensing boiler, due to an existing higher flow temp and higher return water temp (Mean Water Temperature (MWT) would be 76.5 deg C with 82/71 verses 60 deg C with 70/50).

    Does that make sense? You will have the same efficiency issue with a condensing boiler upgrade, irrespective of the control type (ON/OFF or OpenTherm or W/C).
    Last edited by The EVOHOME Shop; 1st August 2017 at 03:53 PM.

  6. #36
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    @Richard - what you are saying makes some sense; however, .....

    Myth 3

    "Condensing boilers need larger radiators."

    Although a marginal benefit of about 3% (in terms of heating capacity) can be achieved by coupling a condensing boiler with oversized radiators, it proves impractical and not that cost-efficient as one might expect. Thus, one shall not worry about the appropriateness of operating a condensing boiler with standard size radiators, all the more so when most of the current heating systems have already integrated large radiators.

    Source: Green Match/EST/Building Regs

  7. #37
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    Quote Originally Posted by The EVOHOME Shop View Post
    I am used to modifying cars and you can't just upgrade a turbo without remapping the engine - effectively adding a condensing boiler to a existing standard efficiency heating system where the rads have been sized for 82/71 will mean you will not get the best results (OpenTherm controlled or W/C controlled). Radiators will be generally undersized when swapping for a condensing boiler, due to an existing higher flow temp and higher return water temp (Mean Water Temperature (MWT) would be 76.5 deg C with 82/71 verses 60 deg C with 70/50).
    Hi Richard, would you mind explaining a little bit more? When you say 70/50 do you mean 70C flow and 50C return (measured at the boiler)? And I assume these measurements are taken while the house is already at the correct temperature, i.e. the heating system is "maintaining" temperature, not running flat out to heat up the house.

  8. #38
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    Quote Originally Posted by HenGus View Post
    @Richard - what you are saying makes some sense; however, .....

    Myth 3

    "Condensing boilers need larger radiators."

    Although a marginal benefit of about 3% (in terms of heating capacity) can be achieved by coupling a condensing boiler with oversized radiators, it proves impractical and not that cost-efficient as one might expect. Thus, one shall not worry about the appropriateness of operating a condensing boiler with standard size radiators, all the more so when most of the current heating systems have already integrated large radiators.

    Source: Green Match/EST/Building Regs
    If radiators are sized for a higher temperature output boiler (standard efficiency boiler) they will be undersized when the new boiler requires a lower flow temperature (condensing boiler) - this is a fact not a myth.

    Quote Originally Posted by rotor View Post
    Hi Richard, would you mind explaining a little bit more? When you say 70/50 do you mean 70C flow and 50C return (measured at the boiler)? And I assume these measurements are taken while the house is already at the correct temperature, i.e. the heating system is "maintaining" temperature, not running flat out to heat up the house.
    Yes, no problem.

    Modern condensing boilers are designed to have a flow and return differential of 20 deg C. Condensing boilers require a return water temperature of below 55 degrees C to be at their most efficient. So designing a heating system to operate at 70 deg C flow and 50 deg C return will guarantee efficient operation.

    When measuring the differential, this is done at the radiators - ideally 70 in and 50 out. The obvious downside of this is where the existing system was designed as an 82/71 system, the radiator sizing would have been specified much smaller than if they had been sized for a condensing boiler at 70/50. So if everything was to remain the same as before with a 70 degree flow temp, this will give less heat output to the room. If you increase the temperature of the flow, you will bring the boiler out of condensing mode.

    My advice is if you replace the boiler and it was an old standard efficiency boiler, the radiators should be looked at and correctly sized for efficient operation of the new boiler.

  9. #39
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    Quote Originally Posted by The EVOHOME Shop View Post
    Modern condensing boilers are designed to have a flow and return differential of 20 deg C. Condensing boilers require a return water temperature of below 55 degrees C to be at their most efficient. So designing a heating system to operate at 70 deg C flow and 50 deg C return will guarantee efficient operation.

    When measuring the differential, this is done at the radiators - ideally 70 in and 50 out.
    I've often wondered how much an automatic bypass valve interacts with a condensing boiler and spoils its ability to condense, especially if pump speeds, radiator sizing and ABV pressure setting are not optimised.

    It seems to me that on most systems with a fixed speed pump and an ABV, if the pump has sufficient delivery for a large number of radiators (say 10+) that when only one or two radiators are flowing in a zoned system, that there must be significant flow through the ABV that raises the return flow temperature into the boiler considerably (since it is blending hot water directly out of the boiler that hasn't passed through any radiators) and thus prevents efficient condensing despite the radiators themselves achieving their designed 20 degree drop. Is this the case ?

    Does this mean a variable speed pump is preferred for best efficiency with a condensing boiler in a zoned system where there may be times when one or two zones have high output but other zones are scheduled off ? (Which in a fixed speed system would result in high flow temperature to meet the demand from those zones, but also high return temperature due to the high ABV flow)

    I wonder if many condensing systems with fixed speed pumps and ABV's have the ABV set too low - I've often seen people say on here theirs is set as low as 0.2 bar which seems quite low to me, and at least on my system that results in significant flow through the ABV even with many radiators open which is not really necessary. (I have mine set to about 0.4)

    Just pondering...
    Last edited by DBMandrake; 2nd August 2017 at 10:33 AM.

  10. #40
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    Quote Originally Posted by DBMandrake View Post
    Does this mean a variable speed pump is preferred for best efficiency with a condensing boiler in a zoned system where there may be times when one or two zones have high output but other zones are scheduled off ? (Which in a fixed speed system would result in high flow temperature to meet the demand from those zones, but also high return temperature due to the high ABV flow)
    I had this conversation with my heating guy yesterday. I really need an ABV because I have HR92s on every rad and am concerned about the situation where the demand is removed (say when the overnight setback kicks in), but the pump overrun is on. He said to just take the HR92 off one of the towel radiators. I mumbled about wasting heat - especially when the bathroom is already warm or during the middle of the day when warm towels aren't required, and he said:

    "No one has shown the industry how to commission an abv with a modulating pump yet."

    As I understand it the two end up fighting each other. With only low demand and one or two valves open, the resistance is quite high and the pump ramps up its output pressure. This can cause the ABV to open which reduces the resistance. So the pump ramps down the pressure which makes the ABV close again. And repeat. Obviously, setting the ABV quite high would prevent this, but in my system I need high pressure to get hot water from the boiler at one end of the house to the study at the other end via the 1st floor and 15mm pipes.

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