Garald Posted January 17, 2023 Share Posted January 17, 2023 Yet another heat pump installer visited today - he seemed serious, or at least both receptive and opinionated. (He seems to be an overt fan of overdimensioning things. I suppose that, since the heat pump we are considering has an inverter (meaning it works well even when it's working well beneath its full capabilities), this isn't terrible.) One thing he said is that he was not fond of (our architect's plan) to put wooden benches on top of some long, low radiators. Nice and comfortable, but he said it was not efficient. Now, technically, I don't even know how to start thinking about the efficiency of a radiator, as usually efficiency is defined in terms of energy that is converted into heat, and that's precisely what we want here. Is what is meant that the air in the room won't heat as quickly as it should? Not my place: Link to comment Share on other sites More sharing options...
SteamyTea Posted January 17, 2023 Share Posted January 17, 2023 I doubt it would impede the airflow much. You could always put a small fan behind the radiator to increase airflow. Talking of fans, not sure where you decided to place your ASHP in the end, but you could put it in that garden shed and fit a secondary fan to blow air into, or out of, the shed. That would overcome the airflow problem. May be a bit noisier, but a large fan and slow speed can still shift a lot of air. Link to comment Share on other sites More sharing options...
PeterW Posted January 17, 2023 Share Posted January 17, 2023 Cast iron column rads are pretty inefficient anyway so adding a bench to the top won’t make it any worse. They rely on more radiation / convection from the thick casting rather than with fins as per modern steel rads. Link to comment Share on other sites More sharing options...
Garald Posted January 17, 2023 Author Share Posted January 17, 2023 1 hour ago, PeterW said: Cast iron column rads are pretty inefficient anyway so adding a bench to the top won’t make it any worse. They rely on more radiation / convection from the thick casting rather than with fins as per modern steel rads. "Inefficient" here meaning "they heat by the same amount eventually, but they take their time to get going"? It's difficult for me to see where "lost wattage" would be going if not heat. Link to comment Share on other sites More sharing options...
PeterW Posted January 17, 2023 Share Posted January 17, 2023 6 minutes ago, Garald said: "Inefficient" here meaning "they heat by the same amount eventually, but they take their time to get going"? It's difficult for me to see where "lost wattage" would be going if not heat. Yes just that - need longer to respond and the heat output isn’t as good. Cast is very good at holding heat - transfer by convection requires the material to lose heat hence the use of thin steel means that newer rads “feel warm” quicker and tend to have higher surface area. Link to comment Share on other sites More sharing options...
SteamyTea Posted January 17, 2023 Share Posted January 17, 2023 3 minutes ago, Garald said: It's difficult for me to see where "lost wattage" would be going if not heat. Heat, is the old word for energy. Energy [J] and power [W] are not the same thing, why they have different names and units. The reason that power may be reduced is that the airflow past the radiators may be lower, this has the same effect as reducing the surface area. You can think of a radiator (which in this context is really a convector) as a wall. It has a hot side, a cold side, a surface area and a power transfer coefficient. So as soon as there is a higher temperature on the radiator surface, it heats the colder room air. This air rises and is replaced by the colder room air. If the temperature difference stays the same, then the air will flow past at a fixed rate because of the density differences. At 16°C it is 1.22 kg.m3, at 30°C is it 1.72 kg.m3. As the temperature difference reduces, the bouncy difference reduces so the flow rate reduces. As power transfer is proportional to temperature difference, less power is imparted into the air, this reduced the airflow even more, so less power. You get to the point where the room air and the radiator surface temperature are in equilibrium and there is no power transfer. So reducing the airflow is similar to putting a thick blanket over it. Link to comment Share on other sites More sharing options...
SteamyTea Posted January 17, 2023 Share Posted January 17, 2023 (edited) 31 minutes ago, PeterW said: Cast is very good at holding heat Not that good, the worse of the common radiator materials. Grey Cast Iron is 460 kJ.kg-1.K-1 Mild Steel is 510 kJ.kg-1.K-1 7068 Alloy is 1050 kJ.kg-1.K-1 The material thickness is more important, in the middle. Grey Cast Iron is 53 W.m-1.K-1 Mild Steel is 50 W.m-1.K-1 7068 Alloy is 190 W.m-1.K-1 Material density comes into it, not the most dense. Grey Cast Iron is 7150 kg.m-3 Mild Steel is 7850 kg.m-3 7068 Alloy is 2850 kg.m-3 With that information, the thermal effusivity [e] can be calculated. Edited January 17, 2023 by SteamyTea 1 Link to comment Share on other sites More sharing options...
SimonD Posted January 17, 2023 Share Posted January 17, 2023 8 hours ago, Garald said: One thing he said is that he was not fond of (our architect's plan) to put wooden benches on top of some long, low radiators. Nice and comfortable, but he said it was not efficient. Now, technically, I don't even know how to start thinking about the efficiency of a radiator, as usually efficiency is defined in terms of energy that is converted into heat, and that's precisely what we want here. Is what is meant that the air in the room won't heat as quickly as it should? I think that, as @SteamyTea has pointed out, the radiators effectiveness at heating a room is more down to convective currents that radiation. A wooden shelf above a radiator may not hamper that at all. In Sweden, it's a very common design element in houses where the window sill is extended (or cantilevered) out over the top of the radiators installed under windows. Provides some useful space, usually for house plants. Some of these are slotted which can be a nice effect. Link to comment Share on other sites More sharing options...
Garald Posted January 17, 2023 Author Share Posted January 17, 2023 8 hours ago, SteamyTea said: Talking of fans, not sure where you decided to place your ASHP in the end, but you could put it in that garden shed and fit a secondary fan to blow air into, or out of, the shed. That would overcome the airflow problem. May be a bit noisier, but a large fan and slow speed can still shift a lot of air. I should be getting permission from the coop to place the external unit on the courtyard later this month. The harder part is to find a heat pump installer who doesn't ghost us! We may have found our man - we shall see. Link to comment Share on other sites More sharing options...
Garald Posted January 17, 2023 Author Share Posted January 17, 2023 The existing radiators were used by previous owners (at high temperature) to heat the room back when it was not insulated. If the contractor's tables give us the right figures for their power, then our on-the-spot calculations of their power in low temperature (see below) show that they are brutally overdimensioned (by a factor of more than 3 when it's -2C outside). So, I shouldn't really be concerned, should I? TL;DR: benches on radiators muffle their power, but do not waste energy. Link to comment Share on other sites More sharing options...
Garald Posted January 17, 2023 Author Share Posted January 17, 2023 Link to comment Share on other sites More sharing options...
SteamyTea Posted January 17, 2023 Share Posted January 17, 2023 55 minutes ago, Garald said: Reminds me of A Beautiful Mind. What you been drawing on the windows, Feynman Diagrams? Link to comment Share on other sites More sharing options...
Garald Posted January 17, 2023 Author Share Posted January 17, 2023 Just now, SteamyTea said: Reminds me of A Beautiful Mind. What you been drawing on the windows, Feynman Diagrams? Hah! I'm a mathematician, not a physicist... Link to comment Share on other sites More sharing options...
SteamyTea Posted January 17, 2023 Share Posted January 17, 2023 Just now, Garald said: Hah! I'm a mathematician, not a physicist... It shows mate. Had a debate with @Adsibob that mathematics is not science as he claimed. Link to comment Share on other sites More sharing options...
Garald Posted January 17, 2023 Author Share Posted January 17, 2023 1 minute ago, SteamyTea said: It shows mate. :(. Too little physical intuition? The physics I took in Uni mostly taught me that I should learn more about Fourier analysis and Hilbert spaces - an invaluable life lesson, but not strictly speaking one in physics. 1 minute ago, SteamyTea said: Had a debate with @Adsibob that mathematics is not science as he claimed. I'd say it's not a science in English (neither are the social sciences), because of the strong Baconian associations of the word. It's Wissenschaft, however, though so are many other things. In French, Spanish,... - it's an issue of definition. I'm happy to let maths be their own thing. Link to comment Share on other sites More sharing options...
SteamyTea Posted January 17, 2023 Share Posted January 17, 2023 5 minutes ago, Garald said: happy to let maths be their own thing. Yes. It is a good language, much clearer rules than spoken languages. Link to comment Share on other sites More sharing options...
Garald Posted January 17, 2023 Author Share Posted January 17, 2023 2 minutes ago, SteamyTea said: Yes. It is a good language, much clearer rules than spoken languages. Kant would make stronger claims (and Plato much stronger ones). Link to comment Share on other sites More sharing options...
saveasteading Posted January 17, 2023 Share Posted January 17, 2023 7 hours ago, SimonD said: A wooden shelf above a radiator may not hamper that at all. I don't agree. I think the flow of air upwards from an efficient radiator can be quite strong, and of course draws in more air from below. Any blunt obstruction close to the rad is going to interfere with that air flow. For a big lump of a radiator it may not matter so much. I have no evidence other than observation, in hydraulics labs and of fire experiments. Hot air rises. Obstructions limit flow. Link to comment Share on other sites More sharing options...
SteamyTea Posted January 17, 2023 Share Posted January 17, 2023 Just now, Garald said: Kant would make stronger claims (and Plato much stronger ones). I had to study Philosophy of Science. Only remember Popper, Kuhn, Lakatos and Feyerabend. Was interesting and gave me an insight into thinking, which was quite helpful when doing my teacher training. Link to comment Share on other sites More sharing options...
saveasteading Posted January 17, 2023 Share Posted January 17, 2023 29 minutes ago, Garald said: I'm happy to let maths be their own thing. Maths is a tool. Numbers don't exist. That is until a certain level, where maths becomes philosophy, but the mathematicians cannot necessarily understand the practical use of sums or quantities. I showed a construction drawing to a PhD mathematician, and he was a bit boggled by the concept of scale. Link to comment Share on other sites More sharing options...
SteamyTea Posted January 17, 2023 Share Posted January 17, 2023 2 hours ago, saveasteading said: I showed a construction drawing to a PhD mathematician, and he was a bit boggled by the concept of scale. What would have happened if you had shown it to a hairdresser, or a builder. Link to comment Share on other sites More sharing options...
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