Where’s the Best Place to Put a Radiator in a Room. Part 1: Such Things are Important

With the gulf stream having shifted south and now flowing happily somewhere high over Paris, the summer weather in the UK has been ‘unseasonably’ (literally) wet and miserable. People’s heating has been thermostatically popping back on all over the country which for July is unheard of. So with this as the background and in the context of a stagnating world economy, ever continuing civil strife in the middle east and the forthcoming Mayan end of the world I thought now would be as good a time as any to look into the all important question of where best to place a radiator in a room. Such things are important.

Not everyone on our planet needs heating to remain comfortable indoors. Generally the northern hemisphere is cooler than the south and sure, it varies over the year. Click this image to see a typical annual global air temperature animation:

Southern California, Florida, Italy, Spain all look comfortably appealing! You’ll need to heat or cool your house if the outside temperature is cooler or hotter (respectively) than you’d like the temperature to be inside. Which begs the question what temperature would you feel comfortable in? 20degC (68degF) is the simple answer though as we’ll find out in later parts to this blog you need to look at more than just air temperature to gauge comfort.

Let’s focus on room heating. To maintain a desired internal room temperature you have to add heat into that space. That heat will seep/leak/flow/pass* (*delete according to your analogy liking) to the cooler outside. The potential difference in outside vs. inside temperatures will be related to the ease by which the heat passes from hot to cold and also to the amount of heat you add. Leaving aside radiative and conductive heat transfer, convective heat transfer (air moving cold or hot about) is central to the effect that a radiator has on the temperature distribution in the room. Hot air rises. Actually, it should be ‘hotter air rises’. A radiator will add heat to the air which will increase its temperature, reduce its density relative to the colder air around it and rise upwards as a consequence. Using the power of CFD simulation using FloVENT let’s have a look to see what’s going on. Oh, o, nice warm air moving towards the ceiling. Buoyancy, friend to the swimmer, bane to those tasked with heating a space.

As hotter air rises so must colder air drop. What would cool the air in a room? An outside wall and especially a window. In these triple glazing days the ability of a window at letting cold into a room is not as severe as it once was but does serve a good demonstrative purpose. Let’s remove the radiator and introduce a window (gotta love the freedom virtual prototyping gives). The cold air drops straight to the floor and along it in a not so nice foot level draft. Socks and slippers time and a complete lack of lazing cats.

To get a feel of the interplay between these ying and yang thermal warriors let’s put the radiator back and check out the resulting hot and cold air flow patterns:

Wow, somewhat complex. Even the simplest of configurations can give rise to very involved interacting 3D airflow patterns. Just as well air in reality is see through otherwise we’d be walking round in a dazed state of wonderment all the time. The cold air is kind of trapped in the window recess by the hot air coming up from the radiator. Not all of it though, some leaks out the edges of the sill.

All well and good but let’s get back to what we’re trying to find out; the best place for a radiator. Next time we’ll discuss what we mean by best. Unless we get a handle on that our investigations will be meandering… at best!

12th July 2012, Ross-on-Wye

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About Robin Bornoff's blog

Views and insights into the concepts behind electronics cooling with a specific focus on the application of FloTHERM to the thermal simulation of electronic systems. Investigations into the application of FloVENT to HVAC simulation. Plus the odd foray into CFD, non-linear dynamic systems and cider making. Robin Bornoff's blog

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Commented on July 13, 2012 at 12:51 am
By Chris Hill

The “cold air entrapment” around the window is one of those nice, but non-intuitive, things which sometimes arise in the world of fluid flow. Our ancestors probably worked this out by trial-and-error, long before the advent of CFD.

Best place for the radiator? My guess would be under the floor, uniformly distributed, PROVIDED you can prevent any serious heat leakage into the ground below. Do I win?

Commented on July 18, 2012 at 12:43 pm
By Michael Higgin

My school physics teacher (a smart chap who worked out the Masquerade jewelled hare riddle but was beaten to the treasure itself by a whisker) had us do experiments on heat loss. The best colour to radiate is matt black. So why are radiators white? Because they are not radiators at all but convectors! Underfloor heating gives you uncluttered walls and toasty feet, but perhaps the centre of the room is the best place for a convection heater (though in the way of everything else) to set up a nice circular flow of even distance to each wall.?

[...] form the main part of the heating of my family’s house. And yes, some of them are placed under the windows, but not enough! Dave Roberts, FloTHERM/FloVENT QA [...]

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