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Thread: Honeywell evohome and OpenTherm integration

  1. #71
    Automated Home Ninja Dan_Robinson's Avatar
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    Lots of misunderstanding of efficiency, efficacy and cost effective upgrades.

    Because a house is comfortable on an SE boiler does not mean it will be comfortable on an HE boiler running at optimal efficiency. The higher the flow temperature the lesser the proportion of the flue gases are condensing on the heat exchanger. Much better to have the flow temperature limited to say 60 or 55 degrees (dew point varies by gas source and humidity). You get more latent heat out of the POC's then.

    Vokera produced a very good document on this, but sadly I am not at liberty to publish it as it is part of their training courses and the "contact" requested I keep it to myself.

    Yes original rads (cleaned internally and roughly balanced) will keep you comfortable, but the boiler will have to run at their design temp - 70 to 80 degrees. However, what is missing from the comments here (that I have seen) is that DESIGN temperatures are -1 in the UK, which we hardly get in the south/midlands for more than a few days, and even then, it is designed from cold. So there is a warm up factor. With smart controls, you do not have this warm up time to the fullest extent if you have set them up properly.

    So yes. On existing rads, you can keep a house perfectly comfortable at much reduced flow temps.

    For example.... I have a Swiss customer in North London is a drafty town house on an Old style Intergas (then Atmos). It is running Hybrid Weather comp and TPi controls. It has been tweaked a few times over the years, but the boiler is essentially only running at 75 degrees flow temperature when it is -3 outside and during a hot water recharge (granted he has a mahoooosive cylinder with a crappy old gravity annuls so it can be going for a while). Never have I had a complaint in the 9 years since we installed the system.

    In fact .... he had a problem with bacterial film growing in the header tank due to hte lack of decent high temperatures in the heating system.
    Kind Regards - Dan Robinson (Jennings Heating Ltd)

  2. #72
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    Quote Originally Posted by Dan_Robinson View Post
    The higher the flow temperature the lesser the proportion of the flue gases are condensing on the heat exchanger.
    Dan, Is it the flow temperature that is critical here, or the return temperature? Are condensing boilers built with counter-flow gas passages, meaning that the gas passes in the opposite direction to the water? If so, the gases on exit are passing over the water that is entering the boiler from the return leg.

    My question really is whether it is feasible to set up a boiler with high outlet temperatures but to constrain the flow rate such that the water loses enough heat through the radiators to return to the boiler at a low enough temperature to ensure condensation of the flue gases?

  3. #73
    Automated Home Ninja Dan_Robinson's Avatar
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    They're both critical.

    Different heat exchangers have different ways of handling things. Some are counter flow, some aren't, some have retardation or the flue gases, some don't.

    HEX surface area is also key.

  4. #74
    Site Sponsor The EVOHOME Shop's Avatar
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    Quote Originally Posted by HenGus View Post
    Starting from scratch or refurbishing an old property are different from the average man in the street that just wants a replacement boiler. If a property has been comfortable for the past 15 years, then the radiators are of an adequate size. I agree that there are savings to be made by getting a new boiler into condensing mode; however, if the graphs that I have looked at are to be believed, then the efficiencies are somewhat over-stated.

    For example:

    97% efficiency with a return temperature of 15C

    92% efficiency with a return temperature of 40C

    88.5% efficiency with a return temperature of 50C

    87% efficiency with a return temperature of 54C

    86% efficiency with a return temperature of 60 to 85C

    Not large enough, imho, to make the case for replacing perfectly sound radiators. Moreover, if a 24kW boiler is modulated down to 4kWs, then it must be saving money.

    I will get my tin hat.
    There is much more to the efficient running of a condensing boiler than this and whoever has stated those figures is probably a sales guy at a major boiler manufacturer who has been tasked with increasing sales numbers...

    I don't see the point in doing anything if it is going to be half a job. A direct boiler swap in an emergency is a stop gap and should be done with view to improve the overall system at a later date and to a heating design that ensures maximum efficiency of the boiler. The fact is that radiators will be 36% undersized if you swap from a SE boiler and the radiators were sized for this. Running the boiler hotter will just mean you will lose efficiency and it won't condense. Some boilers rely on this condensate to keep the heat exchanger clean which can then impact servicing and wear & tear.

    I find it weird that people have bought evohome to increase comfort or improve efficiency, yet don't want their heating system optimal?

  5. #75
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    Quote Originally Posted by The EVOHOME Shop View Post
    The fact is that radiators will be 36% undersized if you swap from a SE boiler and the radiators were sized for this.
    My recent experience suggests that this is not the whole truth for several reasons:

    Modern windows are far more thermally efficient

    Folk will have upgraded the scant provision of loft insulation

    Cavity wall insulation may have been retrofitted

    Extensions and alterations may have taken place

    Rooms may have been repurposed and target temperatures may be incorrect

    And most importantly the system may not have been designed in the first place.

    So just upping radiator sizes by 36% willy nilly may be exactly the wrong thing to do.

    A more considered, holistic approach is required. Then tune your properly functioning system with Evohome for best results.

    Tony

  6. #76
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    Quote Originally Posted by the crooner View Post
    My recent experience suggests that this is not the whole truth for several reasons:

    Modern windows are far more thermally efficient

    Folk will have upgraded the scant provision of loft insulation

    Cavity wall insulation may have been retrofitted

    Extensions and alterations may have taken place

    Rooms may have been repurposed and target temperatures may be incorrect

    And most importantly the system may not have been designed in the first place.

    So just upping radiator sizes by 36% willy nilly may be exactly the wrong thing to do.

    A more considered, holistic approach is required. Then tune your properly functioning system with Evohome for best results.

    Tony
    I didn't suggest in my previous post that 'other' factors had changed - I was comparing like for like and if you do the calculations, my suggestion is based on simple physics used for heating design and is irrefutable.

    Room heat loss is based on the sum of its factors - if things have changed to prevent heat loss then these factors are then taken into consideration.

  7. #77
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    As I have a new boiler coming and am considering OpenTherm (Richard’s firm has already delivered the boiler - its in my garage all ready!) last night I read through the whole of this topic and reread a more recent section that I had started. I am not an expert by any means but three things come out of what I read and the advices:

    1. A return flow of about 50C is optimum for a modern condensing boiler.

    2. Radiators have changed little in the past 20 to 30 years but with the latest boilers some may find their existing radiators undersized putting a new boiler in. But amongst the experts helping in the forum the jury is out as to whether undersize is an issue. To my mind I think it could be as logic says if you have an adequate set up with an old boiler where the return flow is higher than say 50C and then you install something that is more efficient with lower temperatures then something better at the distribution end is needed. But it seems from what is said in the forum the difference is not massive. Perhaps it would take a long time to recover the capital cost of changing all the radiators thus making the issue more a theoretical one for the average user.

    3. Size. Size matters! But what is meant by size? Reading the postings it is easy for a layman to think if he needs a bigger radiator it is bigger in height and width and then the concern because it means alterations to pipework which may or may not be feasible and if it is it may be unsightly. But in fact by size what is meant is single panel, single panel convector, double panel, double panel convector etc.and for the layman reading this if your existing radiators are metric in size (height and width) then the equivalent if upgrading to a double panel etc. is available.

    As well as researching boilers, and I admit with a bias toward Intergas, I also measured every room and worked out what the house may need to heat it and what each room needs. I checked the calculations on a number of sites and all more or less come back with similar figures for BTUs and kWs. I found that but for two rooms my existing radiators (23 years old) are fine. My existing boiler (an old Baxi) has a flow of 65C and return of 55C at the boiler. That is not too far away from recommendations to achieve 70/ 50.

    I will therefore keep my radiators and see what happens first, other than change one that does need to be changed.

    I think also I may initially just connect the existing BDR91 relay to the new boiler thermostat connection and let it run and see how it goes before removing it and replacing with the OpenTherm Bridge. Fortunately, provided I can identify the right wire that feeds from my airing cupboard to the boiler downstairs I can leave the BDR91 in the airing cupboard and do a simple swap with the Bridge at a later date. I have an abundance of wires in the box in the airing cupboard (of similar colours!) my existing set up being boiler plus separate DMW.

  8. #78
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    Quote Originally Posted by G4RHL View Post
    As I have a new boiler coming and am considering OpenTherm (Richardís firm has already delivered the boiler - its in my garage all ready!) last night I read through the whole of this topic and reread a more recent section that I had started. I am not an expert by any means but three things come out of what I read and the advices:

    1. A return flow of about 50C is optimum for a modern condensing boiler.

    2. Radiators have changed little in the past 20 to 30 years but with the latest boilers some may find their existing radiators undersized putting a new boiler in. But amongst the experts helping in the forum the jury is out as to whether undersize is an issue. To my mind I think it could be as logic says if you have an adequate set up with an old boiler where the return flow is higher than say 50C and then you install something that is more efficient with lower temperatures then something better at the distribution end is needed. But it seems from what is said in the forum the difference is not massive. Perhaps it would take a long time to recover the capital cost of changing all the radiators thus making the issue more a theoretical one for the average user.

    3. Size. Size matters! But what is meant by size? Reading the postings it is easy for a layman to think if he needs a bigger radiator it is bigger in height and width and then the concern because it means alterations to pipework which may or may not be feasible and if it is it may be unsightly. But in fact by size what is meant is single panel, single panel convector, double panel, double panel convector etc.and for the layman reading this if your existing radiators are metric in size (height and width) then the equivalent if upgrading to a double panel etc. is available.

    As well as researching boilers, and I admit with a bias toward Intergas, I also measured every room and worked out what the house may need to heat it and what each room needs. I checked the calculations on a number of sites and all more or less come back with similar figures for BTUs and kWs. I found that but for two rooms my existing radiators (23 years old) are fine. My existing boiler (an old Baxi) has a flow of 65C and return of 55C at the boiler. That is not too far away from recommendations to achieve 70/ 50.

    I will therefore keep my radiators and see what happens first, other than change one that does need to be changed.

    I think also I may initially just connect the existing BDR91 relay to the new boiler thermostat connection and let it run and see how it goes before removing it and replacing with the OpenTherm Bridge. Fortunately, provided I can identify the right wire that feeds from my airing cupboard to the boiler downstairs I can leave the BDR91 in the airing cupboard and do a simple swap with the Bridge at a later date. I have an abundance of wires in the box in the airing cupboard (of similar colours!) my existing set up being boiler plus separate DMW.
    As you know, it was a couple of years since I last had a home with Evohome; however, I did install at Atag boiler with Opentherm. In my view, boiler sizing is important: big is not always best with a condensing boiler. What is important is the modulating ratio. My experience over a couple of Winter seasons showed that it is difficult to get the return floe to <56C if zoning has closed off most of the heating system. The flow from the boiler has nowhere to go other than through the radiator that is open and the ABV (if you have one). Unless the boiler can get down to a low output by modulating then you will end up with high flow return temperatures (ie; no condensing) and the boiler cycling ON and OFF. Opentherm control will smooth out some of these bumps in the road but it cannot overcome the physics of the boiler; ie, it can only reduce the boiler to minimum output and that's it. That said, the problem in sizing the boiler is to some extent determined by how quickly want their homes to heat up, and HW demand.

    FWiW, even with the existing radiators in my old 5 bed home my installer calculated that I could sensibly get away with a 18kW boiler. After much head scratching, I compromised on 24kWs.

    PS. In my new build home with a WB 24k condensing boiler, I have gone for Tado rather than Evohome. It just works.

  9. #79
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    Moving from single to double (or even triple) convector rads also needs pipe alterations, because the radiator centre shifts.
    Also traditional boiler sizing is also quite outdated with a zoned system like Evohome. How often are all of your zones calling for heat at the same time? Because if they are, you aren't using Evohome for what it was designed.
    My house was sized for a 38Kw boiler which I have derated to 18Kw. It's amazing what you can get away with if you plan your house heating strategy well. And if you have a storage DHW then if you heat the HW after most of the zones are satisfied it takes away the need to have a massive boiler even for DHW. Because again the way boiler sizing is done, is they add the heating coil after all the rooms are added up.
    Last edited by bruce_miranda; 5th February 2021 at 10:54 AM.

  10. #80
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    Thanks. I suspect for CH I will be able to have the boiler running quite low intend initially too set the maximum for CH at 19.3kW (one of the options in the manual, the others are 25.7, 12.8, 9.0 and 6.4). Some experimenting to be done.

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