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Thread: Honeywell evohome and OpenTherm integration

  1. #51
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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. 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.
    Thanks again. Another question: if each radiator is a different size, how can they all give 20 degrees drop? Wouldn't a bigger radiator drop a lot more than a smaller one?

  2. #52
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    Quote Originally Posted by rotor View Post
    Thanks again. Another question: if each radiator is a different size, how can they all give 20 degrees drop? Wouldn't a bigger radiator drop a lot more than a smaller one?
    That's what balancing is all about.

  3. #53
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    A quick google search shows that even installers cannot agree on the required temperature drop across radiators. Historically, it was 20F which equates to 11C but many rounded it down to 10C. Some installers are adamant that it should now be 20C and others think that anything between 10C and 20C is just fine - and these people are the professionals.

    I know that Richard is keen to maximise efficiency; however, given that the condensing/non-condensing saving is stated to be in the order of 3%, why would any homeowner want to go to the expense of having new radiators installed? Given that the 3% saving is on kWhs/year consumed, my saving would be 10000 x 3% or 300kWhs which equates to an annual saving of 8.93 per year on my present tariff. What am I missing?

  4. #54
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    Quote Originally Posted by HenGus View Post
    A quick google search shows that even installers cannot agree on the required temperature drop across radiators. Historically, it was 20F which equates to 11C but many rounded it down to 10C. Some installers are adamant that it should now be 20C and others think that anything between 10C and 20C is just fine - and these people are the professionals.
    More to the point, you might not be able to practically increase the size of your radiators for physical/aesthetic reasons. Many of mine are type 11 or type 21, so I can at least update them to type 22. However, many of them are also imperial sizes, so the physical/aesthetic issues still exist.

    Quote Originally Posted by HenGus View Post
    I know that Richard is keen to maximise efficiency; however, given that the condensing/non-condensing saving is stated to be in the order of 3%, why would any homeowner want to go to the expense of having new radiators installed? Given that the 3% saving is on kWhs/year consumed, my saving would be 10000 x 3% or 300kWhs which equates to an annual saving of 8.93 per year on my present tariff. What am I missing?
    I have just finished the first full year in our new house. My usage for the year was approximately 65,000kWh. So in my case I'd be looking to save about 33 since I'm only paying 1.69p/kWh. Even on your tariff it would mean 58. As you say, hardly worth it for the cost savings.

    I will be looking to upgrade my radiators, but mostly from a comfort perspective... some of my rooms heat up very slowly at the moment.

  5. #55
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    Quote Originally Posted by HenGus View Post
    A quick google search shows that even installers cannot agree on the required temperature drop across radiators. Historically, it was 20F which equates to 11C but many rounded it down to 10C. Some installers are adamant that it should now be 20C and others think that anything between 10C and 20C is just fine - and these people are the professionals.
    Achieving a drop of 11 degrees is difficult on very small radiators, (such as mini hallway types) let alone 20 degrees, so I doubt whether a house with a wide variety of radiator sizes can achieve 20 degrees on the smallest radiators in practice. You have to restrict the flow to the small radiator so much that it can hardly heat up while it is achieving a large temperature drop.
    I know that Richard is keen to maximise efficiency; however, given that the condensing/non-condensing saving is stated to be in the order of 3%, why would any homeowner want to go to the expense of having new radiators installed? Given that the 3% saving is on kWhs/year consumed, my saving would be 10000 x 3% or 300kWhs which equates to an annual saving of 8.93 per year on my present tariff. What am I missing?
    Where did you get a figure of 3% for efficiency increase for a condensing boiler ? If it was only 3% it wouldn't be worth the hassle - it would be lost amongst the noise of daily/weekly/monthly variations in the weather...

    A quick google suggests "at least" 10-12% increase in efficiency when condensing, which seems more likely. Also just compare the efficiency of older non-condensing boilers with a modern condensing boiler - 70-75% is typical for a non-condensing boiler such as mine, over 90% is typical of a condensing boiler in condensing mode - eg about a 25% improvement in efficiency. Granted, some of that will be improvements in heat exchanger/burner design etc but a lot of it must come from the actual condensing process.

    So 3% doesn't sound right to me at all, unless it is an average figure that takes into account what percentage of time the boiler is likely to be in condensing mode in a "typical" system, if so, that's not the right figure to use in this context. We want to know the efficiency increase when it is condensing, vs when it is not for the same boiler and system.
    Last edited by DBMandrake; 3rd August 2017 at 11:02 AM.

  6. #56
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    I think that in days of old balancing was more important to ensure each radiator got its fair share of water.

    When I put in my system, I didn't spend too long fussing over balancing, I just made sure the lockshield valve allowed adequate flow through the radiator, whilst being sure it restricted it enough for the sake of directing water to the other radiators in the system.

    In use, I feel the balancing is done in real-time by the TRVs, as they constantly modulate the circulation.
    Last edited by blowlamp; 3rd August 2017 at 05:51 PM.

  7. #57
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    Quote Originally Posted by blowlamp View Post
    I think that in days of old balancing was more important to ensure each radiator got its fair share of water.

    When I put in my system, I didn't spend too long fussing over balancing, I just made sure the lockshield valve allowed adequate flow through the radiator, whilst being sure it restricted it enough for the sake of directing water to the other radiators in the system.

    In use, I feel the balancing is done real-time by the TVRs as they constantly modulate the circulation.
    My installer shares this view, but I'm not convinced. This overlooks the early morning heat up where it's conceivable that the majority of radiators will be calling for heat (especially during the winter). Without a reasonably well balanced system, some radiators will get hot and some won't. I guess Evohome will eventually compensate by starting the more distant radiators earlier in the optimum start window. Maybe.

  8. #58
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    Quote Originally Posted by dty View Post
    My installer shares this view, but I'm not convinced. This overlooks the early morning heat up where it's conceivable that the majority of radiators will be calling for heat (especially during the winter). Without a reasonably well balanced system, some radiators will get hot and some won't. I guess Evohome will eventually compensate by starting the more distant radiators earlier in the optimum start window. Maybe.
    Which is why I put this:

    "When I put in my system, I didn't spend too long fussing over balancing, I just made sure the lockshield valve allowed adequate flow through the radiator, whilst being sure it restricted it enough for the sake of directing water to the other radiators in the system."

  9. #59
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    Quote Originally Posted by DBMandrake View Post
    Achieving a drop of 11 degrees is difficult on very small radiators, (such as mini hallway types) let alone 20 degrees, so I doubt whether a house with a wide variety of radiator sizes can achieve 20 degrees on the smallest radiators in practice. You have to restrict the flow to the small radiator so much that it can hardly heat up while it is achieving a large temperature drop.

    Where did you get a figure of 3% for efficiency increase for a condensing boiler ? If it was only 3% it wouldn't be worth the hassle - it would be lost amongst the noise of daily/weekly/monthly variations in the weather...

    A quick google suggests "at least" 10-12% increase in efficiency when condensing, which seems more likely. Also just compare the efficiency of older non-condensing boilers with a modern condensing boiler - 70-75% is typical for a non-condensing boiler such as mine, over 90% is typical of a condensing boiler in condensing mode - eg about a 25% improvement in efficiency. Granted, some of that will be improvements in heat exchanger/burner design etc but a lot of it must come from the actual condensing process.

    So 3% doesn't sound right to me at all, unless it is an average figure that takes into account what percentage of time the boiler is likely to be in condensing mode in a "typical" system, if so, that's not the right figure to use in this context. We want to know the efficiency increase when it is condensing, vs when it is not for the same boiler and system.
    Efficiency doesn't appear to fall off a cliff if the boiler isn't condensing. I have been looking at a number of condensing boiler efficiency graphs this morning which show, for example, a condensing boiler with a return temperature of 55C with a boiler efficiency of 87% which reduces to 85.5% with a return temperature of 70C. A boiler efficiency of 99% is only achieved when the flow return temperature is 10C. The extra savings come from the fact, as far as I can see, that the heat exchanger is bigger; there is usually some form of modulation and no pilot flame.

    Going from the theoretical to the practical. Looking back over the month of July 2016, my old boiler consumed an average of 10.74kWh per day. Last month, my daily average was 6.8kWhs with HW selected on for a similar period of time. Given that the boiler slowly cranks up to 70C when cylinder heating, it would only be in condensing mode for some of the time. The efficiency saving is high at 36%: I suspect that a lot of this is down to the fact that there is no pilot flame burning in the boiler for 24 hours a day.

    Edit:

    The graph in this article is typical of what a google search reveals:

    https://vintagegreenhome.wordpress.com/2012/10/17/are-you-getting-the-maximum-energy-savings-from-your-condensing-boiler-fall-is-a-great-time-to-check-your-settings-to-find-out/
    55C = 131F

    70C = 158F
    Last edited by HenGus; 3rd August 2017 at 03:45 PM.

  10. #60
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    Quote Originally Posted by rotor View Post
    Thanks again. Another question: if each radiator is a different size, how can they all give 20 degrees drop? Wouldn't a bigger radiator drop a lot more than a smaller one?
    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...
    Last edited by The EVOHOME Shop; 4th August 2017 at 10:13 AM.

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