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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
    Site Sponsor The EVOHOME Shop's Avatar
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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.

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