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Thread: Honeywell evohome and OpenTherm integration

  1. #61
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    Quote Originally Posted by The EVOHOME Shop View Post
    I think I've started something here...


    What did I tell you?
    Kind Regards - Dan Robinson (Jennings Heating Ltd)

  2. #62
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    Fascinating reading! Roll on winter with my new boiler - though I'm hoping my rads won't be far off as the house is only 10 and the old boiler was a leaky condensing glowworm.

  3. #63
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    Quote Originally Posted by The EVOHOME Shop View Post
    I think I've started something here... The thread is about OpenTherm and evohome, but this is one small factor. There is a lot more importance in the heating design and its at this point you will discover the gaps in your knowledge.

    So, lets get to the basics...

    Each room in your house has a certain heat loss (based on heat loss calculations for that room) and the rooms heat loss is calculated based on say a -4 degrees C outside temperature. The radiator is then sized based on the heat loss of the room, but using the design characteristics that you want.

    So say I want a 70 degree C flow and 50 degree C return (ideal conditions for a condensing boiler). This gives you a Mean Water Temperature (MWT) of 60 degrees C. Say then you want the room to be 21 degrees C, this then gives you a Mean Water to Air Temperature (MW-AT) difference of 39 degrees C (60 - 21 = 39). As all radiators are tested for their output at a MW-AT of 50 degrees C you have to recalculate the radiator size. To give you some perspective, a MW-AT difference of 50 degrees C would mean an 81 degree C flow temp, 20 degree C differential (so a 61 degree C return temp which is above condensing temps) at a required 21 degree C in the room air temp.

    So if I give the example of a Stelrad Compact 600mm x 1000mm Double Panel Radiator (K2). It has an output of 1732 Watts at a MW-AT difference of 50 degrees C (most will state DT50*C in a radiator book). If it was now to be used with a system running with a MW-AT of 39 degrees C (70/50) the actual radiator output would now only be 1204 Watts, some 30.48% too small! You would now need to have a radiator some 30% bigger in output verses the old radiator, as its output in Watts was was originally sized for a MW-AT of 50 degrees C.

    If you were just to swap a condensing boiler into a house where the original radiators were sized for a boiler designed with a 82 flow and 71 return (MW-AT difference of 56 degrees at an air temp of 21 degree C), then the original radiators would actually be some 36.76% too small...

    Pump selection (including pump speed selection based on the index circuit) and system balancing must be done with caution, as flow rates to the radiators are directly calculated on the heat output of the radiator and this is expressed in kg/sec (or can also be expressed as litres/sec, as 1 kg of water is equivalent to 1 Litre of water). For example, a room requiring a 2000 Watt radiator would need a correctly sized radiator at 70/50 to obtain the correct output, but would also need a flow rate of 0.024 kg/sec in order to supply enough energy to that particular radiator to obtain its output.

    I can go on, but hopefully the above shows there is much more behind heating design than a thermostat controlling the room by relaying the information back to the boiler...
    Was it something I said? Thread has gone dead?

  4. #64
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    If you were just to swap a condensing boiler into a house where the original radiators were sized for a boiler designed with a 82 flow and 71 return (MW-AT difference of 56 degrees at an air temp of 21 degree C), then the original radiators would actually be some 36.76% too small...
    I guess your suggestion that it is pointless to install a condensing boiler without also fitting larger radiators throughout the house would kill off the current industry that is pressing householders to install modern efficient condensing boilers. All those cold calls telling us that the government requires us to install a modern efficient boiler would be less persuasive if they had to tell us that we would get 37% less heat output from our radiators.

    I remain sceptical about that though as surely we can achieve a lower CH return temperature by slowing the pump down (or throttling it) and maintaining a fairly high boiler outlet temperature and hence average radiator temperature? I am not convinced it is necessary to change all the radiators when upgrading the boiler.

  5. #65
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    Quote Originally Posted by Edinburgh2000 View Post
    I remain sceptical about that though as surely we can achieve a lower CH return temperature by slowing the pump down (or throttling it) and maintaining a fairly high boiler outlet temperature and hence average radiator temperature? I am not convinced it is necessary to change all the radiators when upgrading the boiler.
    If you slow the pump down you could achieve the 20 degree drop across a smaller radiator, yes, but a slower flow of water can only carry so much heat energy. There are physical limits on the lowest volume flow of water that can carry enough heat to say radiate 1000 watts from a radiator panel of any size. The amount of heat energy a flow of water can carry is a function of both the flow temperature and the flow volume. Restrict the flow volume too much and it simply can't do the job and the radiator won't heat up sufficiently.

    Which when you think about it, is exactly the mechanism a TRV uses to control the average surface temperature of the panel to regulate the room temperature - the restricted flow of water simply can't carry the full amount of heat that the panel is capable of dissipating so the temperature drops. To achieve the 20 degree drop from one side of the panel to the other and still put out the desired amount of heat into the room requires a bigger radiator, there's no way around the physics.

    Of course it's quite likely that many houses already have "oversized" radiators in most rooms that are sufficiently large for a condensing boiler. So you'd probably get away with it in many houses, however if the original radiators were "correctly" specced and not oversized for an old boiler then yes they would be too small for a condensing boiler to operate efficiently. And in many cases people just won't be getting the potential efficiency savings due to lack of temperature drop on the return.

  6. #66
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    Nothing you said.

    But what you said was interesting.

    All of my ramblings on this thread are related to my sons house which he has just bought. Its identical to our own, albeit built about 5 years earlier.

    In our own house, currently rented out, we installed a brand new Ideal combi boiler last year which was a majot improvement over the 25 year old potterton system boiler even with existing radiators.

    I've devoted a lot of time and some years of DIY experience plus over 20 years knowledge of actually living in the house to developing a plan for a complete overhaul of the heating and water for my sons house leading to him spending a large amount of money in the Evohome shop, a Megaflow 210litre cylinder, and all new radiators, meticulously calculated in the Stelrad Stars app, with an adjustment applied to allow us to operate at lower flow temperatures to take advantage of the condensing Baxi boiler currently installed.

    What really surprised me, and caused me to revisit the entire calculations, was just how small the new radiators are compared to the cast iron monsters installed 30 years ago.

    We bought mainly Delonghi radiators from Amazon, plus feature radiators for the hall lounge and bedroom 1.

    My next project when the house is occupied in the next week, is to set up flow and return temperature monitoring on the boiler, and hopefully by then the smart gas meter will be installed so we can understand just how much output the 210l cylinder requires, and exact;y how often we need to reheat it.


    Initial thoughts are that the heat up is quick, and it stays red hot for days. How it will cope with the rigours of family life will be interesting.

    My job over the next 2 days is to replace the downstairs radiators which means some concrete floor chasing.

    And then on to Evohome....

    So there is more to central heating than meets the eye and it is necessary to invest a bit of computer time in it, or find an installer you think will do an equally good or better job than yourself.

    Tony

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    Quote Originally Posted by the crooner View Post
    What really surprised me, and caused me to revisit the entire calculations, was just how small the new radiators are compared to the cast iron monsters installed 30 years ago.
    You're comparing the big old fashioned cast iron radiators with no convecting fins against modern flat panel radiator with convectors ? (possibly double convectors ?) If so yes, convecting fins make a massive difference to the ability of the radiator to put heat into the room via convection rather than just direct radiation...

    I changed the old single panel non convector in our bathroom (which always struggled to even reach 20 degrees in winter) for a compact type 21 double panel single convector which was slightly shorter and about the same width and the difference in heat output in the room is incredible... it can now get the room about 4 degrees hotter than the old panel in the same winter conditions with ease.

  8. #68
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    Quote Originally Posted by the crooner View Post
    Nothing you said.

    But what you said was interesting.

    All of my ramblings on this thread are related to my sons house which he has just bought. Its identical to our own, albeit built about 5 years earlier.

    In our own house, currently rented out, we installed a brand new Ideal combi boiler last year which was a majot improvement over the 25 year old potterton system boiler even with existing radiators.

    I've devoted a lot of time and some years of DIY experience plus over 20 years knowledge of actually living in the house to developing a plan for a complete overhaul of the heating and water for my sons house leading to him spending a large amount of money in the Evohome shop, a Megaflow 210litre cylinder, and all new radiators, meticulously calculated in the Stelrad Stars app, with an adjustment applied to allow us to operate at lower flow temperatures to take advantage of the condensing Baxi boiler currently installed.

    What really surprised me, and caused me to revisit the entire calculations, was just how small the new radiators are compared to the cast iron monsters installed 30 years ago.

    We bought mainly Delonghi radiators from Amazon, plus feature radiators for the hall lounge and bedroom 1.

    My next project when the house is occupied in the next week, is to set up flow and return temperature monitoring on the boiler, and hopefully by then the smart gas meter will be installed so we can understand just how much output the 210l cylinder requires, and exact;y how often we need to reheat it.


    Initial thoughts are that the heat up is quick, and it stays red hot for days. How it will cope with the rigours of family life will be interesting.

    My job over the next 2 days is to replace the downstairs radiators which means some concrete floor chasing.

    And then on to Evohome....

    So there is more to central heating than meets the eye and it is necessary to invest a bit of computer time in it, or find an installer you think will do an equally good or better job than yourself.

    Tony
    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.
    Last edited by HenGus; 7th August 2017 at 10:40 AM.

  9. #69
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    Quote Originally Posted by DBMandrake View Post
    If you slow the pump down you could achieve the 20 degree drop across a smaller radiator, yes, but a slower flow of water can only carry so much heat energy. There are physical limits on the lowest volume flow of water that can carry enough heat to say radiate 1000 watts from a radiator panel of any size. The amount of heat energy a flow of water can carry is a function of both the flow temperature and the flow volume. Restrict the flow volume too much and it simply can't do the job and the radiator won't heat up sufficiently.

    Which when you think about it, is exactly the mechanism a TRV uses to control the average surface temperature of the panel to regulate the room temperature - the restricted flow of water simply can't carry the full amount of heat that the panel is capable of dissipating so the temperature drops. To achieve the 20 degree drop from one side of the panel to the other and still put out the desired amount of heat into the room requires a bigger radiator, there's no way around the physics.
    Indeed. But I am suggesting having a temperature drop greater than 20 degrees across the radiator. The heat extracted from the CH flow at any individual radiator is a function of:
    * CH flow temperature
    * CH return temperature (from that radiator)
    * Flow rate through that radiator.

    So, if we are constraining the return temperature to be no higher than 50 deg C, then we have to increase the flow temperature and/or reduce the flow rate to get the heat output we need. I accept that, for some small radiators, that could require a very high flow temperature. My understanding, however, is that condensing boilers require only a low enough return temperature to allow the latent heat of condensation from the flue gases to be recovered (and hence high efficiency). I believe this is not inhibited by having a high outlet (CH flow) temperature.
    Last edited by Edinburgh2000; 7th August 2017 at 10:53 AM. Reason: Typo (word missing)

  10. #70
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    If a property has been comfortable for the past 15 years, then the radiators are of an adequate size.

    We lived for 25 years in the house and were never happy that it was properly heated. I suspect a lot of houses had poorly designed systems, have sludged up radiators, and money grabbing contractors undersized specified pipe sizes.

    I think a complete rethink is required if you really want to get the best from a new boiler

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