Joep, what an old man, it seems to me that you have only just begun to live, you do not have a single gray hair, you have barely come out of tahuda puberty (I could not say that for Jo). 
Iām not counting on power yet, I have to take a test first, Kamil is a good help to me, he doesnāt fall for any "siphoning"
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Haha,
Tone, we canāt do much about age and gray hair. It is what it is. Btw, I saw a gray bearded man running a compressor yesterday. I wonder who that beard belong to? 
I have to ask you again - if youāre cooling off 90% of 10 kW to condence your medium - what kind of heat need does your floor heating have? A normal house would maybe need 2-4 kW continius heating during cold winter months. You would need a pretty good flow rate and heat need to cool down 9 kW continius, with the floor heating alone.
If I remeber right you have a storage tank connected to your boiler. With itās stratification you should have āunlimitedā supply of cooling ability, if the cooling heatex was fed from the tankās cool bottom and the water returned back to about the middle of it when heated up.
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Hi Tone,
From your projected below 10% conversion to mechanical? energy numbers you are now into best-case current Thermal-Electronic-Generator cells range.
3-4 different technologies on these.
Thier conversion efficiencies and durabilityās are slowly evolving upwards.
So far as brainiac-divorce driven . . . never ending; never good-enough, Alt-Energy fixations should be rated right up with alcohol abuse, street drugs and gambling addictions and abuse.
Steve Unruh.
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Jo, hello, I know you know that I didnāt mean you about gray beards and hair, but about late puberty, or as my daughter tells me, middle-aged crises, maybe ā¦
I have to admit that you are right when you state the power of underfloor heating, it probably doesnāt exceed 5 kW, but I also have radiators designed for low temperature. Yes, water is easy to play with, but I find it interesting to get some electricity from this game, you wrote nicely somewhere above "The beauty of this system is that it can work 24 hours a day, 7 days a week and is not dependent from the occasional heat of the flue gases ", the power produced will depend on the current heat consumed and on the heat difference. I must also admit to you that at the moment time does not allow me to realize, but it is good to have clear details of the project in my head, and maybe someone else will take up the challenge, ā¦
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LOL! So true!
It shouldnāt be funny I guess (since I resemble that remark), but thanks for the laugh 
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Excellent diagram, @Tone. Truly self explanatory. Make me thinking since morning. 
So few new things arise.
Turbine work is almost isentropic, ns ā„ 95%. With your engine cartainly impossible. If you will get 85%, you will be our hero.
Cold side is OK. 13Ā°C increase for 9 kW represent 10 l/min flow. Nothing strange for big enough floor. In winter 
HEX volumes could be quite big. For such power flows and low mass flow, you will need good chunk of copper. Low temp HEX tend to be pretty inefficient. Are you ready for it?
Steam drum and deareator are missing in your schema. Donno how in low temp organic cycle, but normal Rankine doesnāt work without them. Droplets in turbine and bubles in compressor are killers of performance and engines as well.
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@Tone , Iāve find a document on internet which might be of your interest. Unfortunately, it is pdf document, which is not authorized for upload. Finally I found working link to the publisher
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You could save it to Google Docs, make the file shareable with a link and he could download it that way.
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Kamil, thank you, lots of interesting reading for long evenings. Are you going to do something similar?
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I donāt think so. At least not soon. Have no usage or utility for such mechanism. Maybe one day, if I design our new house, I will consider such thing. True experience would be welcome.
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Hmm, obviously Iāll have to plow the wasteland, but first thereās a tractor, ā¦ I donāt have a real solution for the pump yet, maybe I could use a high pressure fuel pump from Mitsubishi GDI, for example, ā¦ but sealing is a problem. They tested different media in the document, somehow I think R-245fa is the most suitable, probably butane is also close to it, most of them are flammable substances, so we should not allow leaks.
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Arenāt fuels pumps little bit powerfull for your purpose? Normal range is in hundreds of bar. What about some oil pumps used in hydraulics?
In your designed temp range R-245fa is most promisinsg, providing double efficiency against propane. Also flames are not in concern. But the price really is 
For me, flamable gases are forbidden. I do water, electricity, simple constructions, welding, distillation, many other things. But I keep myself hundred miles away from gas.
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I donāt want to try to talk you out of a project that you find fun, but the time and money you will invest here vs efficiency/energy return is going to be low. Iāve been down this rabbit-hole with thermal solar + heat engine.
Organic Rankine Cycle engines are just steam engines for low temperature heat sources. Hobby steam engines are low efficiency and thatās using high temperatures on the hot side. Hotter hot sides improve efficiency and increase theoretical efficiency limits, thatās just thermodynamics (give Carnot efficiency a āwikiā).
Hobby ORC, at those lower temps, is going to be very low efficiency. I was going to suggest looking at car/truck pumps and A/C equipment. Itās cheap and available, runs off low voltage DC. ORC engines are just just ACs run in reverse. Seeing that you already have that thought. And yesā¦ Iād use propane/butane. Itāll be easy on the seals and low pressure which is good if you run higher temperatures and the seals get a little soft. Iād double check the max temp on your pump oil as well.
Quick edit/addition: Consider counter-flow heat exchangers to minimize the temperature difference between your heat source and your ORC system hot side. Itās more complexity but every degree counts when the hot-cold differential is so low. Youāll want a very effective cold sink as well for the same reason and as low temp as you can get.
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Tone,
Maybe enough power to keep some LED nightlights on, or trickle charge a battery bank. Donāt waste too much time, but the fun can be in trying out something new! Put some more feathers in your headdress, so to speak! 
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Carnot can be a bummer.
Ideally you would reverse your system such that you generated power off the top of whatever was available and used the āwasteā heat for domestic uses.
I spent a few years on Rankine cycle systems, āhigh efficiency steamā, before leaning into gasification. Hereās a few piston operated valve uniflow steam engines I built (400F, 400psi max). Turns out steam is not simpler:)
https://youtu.be/MR1qV843awI
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