Honeywell evohome and OpenTherm integration
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Originally posted by The EVOHOME Shop View PostI 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...
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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 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.
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Originally posted by Edinburgh2000 View PostI 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.
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.
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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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Originally posted by the crooner View PostWhat 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.
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.
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Originally posted by the crooner View PostNothing 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
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; 7 August 2017, 10:40 AM.
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Originally posted by DBMandrake View PostIf 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.
* 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.
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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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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)
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Originally posted by Dan_Robinson View PostThe higher the flow temperature the lesser the proportion of the flue gases are condensing on the heat exchanger.
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?
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Originally posted by HenGus View PostStarting 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.
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?
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Originally posted by The EVOHOME Shop View PostThe fact is that radiators will be 36% undersized if you swap from a SE boiler and the radiators were sized for this.
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
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