Tall, narrow radiators

[orion.osiris]

Anyone know if this format is as efficient as the conventional, rectangular one?

[A.Lee]

Define efficiency?

All radiators come with a BTU (or similar) rating, so they will put out that amount of heat, whether they are horizontal or vertical. Where that heat goes is different though. A radiator low down is likely to heat up a room more than one that touches the ceiling, as heat rises, it'll warm the room as it rises, one that is high up will warm the ceiling a lot more, and the room above it.

[Bill Wright]

[e-mail] wrote:

As it happens I am an expert in the field of heat aperture optimisation. Firstly I have to mention that almost all radiators are rectangular; it's just that their aspect ratio varies. Circular and polygonal radiators were briefly popular in the late 19th century, and were installed in large numbers in the Tsars' palaces of Russia. However, the annoyance this caused amongst the plumbers was the start of the discontent that eventually triggered the Revolution. Taking that as a warning, the European upper classes quickly went back to rectangular radiators, usually ones that were very easy to fit. They also instituted a tradition of providing tea with unlimited sugar for all members of the Plumbing, Heating, and Lavatory Engineers Union, amalgamated with the Association of Women Sewerage Operatives All heat should be provided at a dimensional aperture appropriate for the application. Convected heat is irrelevant here. The problem with these radiators is that the heat, whilst it is tall, is also narrow. This is what we experts call heat aspect ratio. You have to stand in exactly the right place to get the benefit. However, once in that place you can feel the warmth all the way from your ankles to your head, passing though your knees, knackers, belly, and chest. These radiators are OK for thin people, but the obese may find that their edges remain cold. However I understand that a vertical rotisserie is available from www\hdmi\lardarse.com. This item is splashproof, so suitable for wetrooms. The fat person stands on the turntable and is rotated about a vertical axis at approx 5rpm. These items are ideal for wetrooms where space precludes the installation of a normal width radiator. It should be noted that these radiators are useless for the warming of quadrupeds, unless the animals can be trained to stand erect. The heat from these radiators is quickly dissipated by draughts from open doors, since the shape of the draught is approximately the same as the shape of the heat. Hope this helps.

Bill

[harry]

They will have to be bigger than a horizontal one. Heat output is reduced because the air passing over the upper part of a tall radiator is already warmed by the lower part.

[Robin]

Can you point to published work which shows that please (as ISTM there are several other variables)?

[rbel]

When discussing this point with the heating engineers who revamped our system earlier this year they agreed with the above comment but said that we were unlikely to notice the difference in practice. In our case we were replacing an elderly longitudinal double panel rad in a modern hallway and wanted something with minimal depth. We chose a from the Stelrad Vistaline range and have been very pleased with it.

While the theoretical heat output is similar to that of the old rad the comfort level of the area is noticeably improved, so much so that we have been able to do away with the small rad in the cloakroom which opens off the hallway.

[alan]

With these it is advisable to put aluminium foil on the floor to catch the dripping fat.

[polygonum]

And a large person of middle eastern origin carrying an extremely sharp blade helps remove any bits that start to get browned...

[Andrew Gabriel]

Does that win over the chimney effect which gets a significantly higher draft running up taller (versus wider) radiators?

[Andrew Gabriel]

Precisely. The only definition I can think of which makes any sense is heat transfer into the room versus size of the radiator, in which case which dimension are you interested in being minimised (width, height, thickness)?

Working from most efficient (on this basis) to least...

Most efficient will be fanned units, if you count them as radiators - they can be made very many times smaller for the same heat output, and hidden in kickspaces.

Next will be regular finned utility radiators. When I looked at the datasheets 12 years back when I installed mine, for a given surface area of the front, a taller one has higher output than a wider one, which I presume is due to a longer chimney effect drawing a larger airflow over the panels/fins. Obviously, single panel, 1.5 panel, double panel, triple panel increase output for any given front surface area, but that depends how you feel about radiator thickness.

After these come radiators with integral side panels and top grills, which slightly reduce the output for any given size.

Unfinned radiators come next.

Finally, designer radiators are the least efficient of all, with the metalic finishes (e.g. chromed) being the worst of all, but this is often made up for by designer radiators being much bigger anyway, partly from a style point of view, but also because they would otherwise be completely useless.

When comparing data sheets, make sure you are comparing outputs at the same rated delt-T, and also note that these figures are almost never based on ideal condensing boiler operating conditions, which you should compensate for by oversizing if you want the best _system_ efficiency.

They're normally placed near the floor, but I've seen instructions that say they should be no closer to the finished floor level (e.g top of the carpet) than the horizontal distance from the wall to the front face, or that gap will reduce airflow through them.

Most radiator mounting brackets have two mounting positions for spacing the radaitor near or far from the wall. I would assume the far from wall setting would be marginally better, but never seen any figures.

There's also a technique one manufacturer is trying with double panel radiators, where the water first circulates through the front panel, and is then transferred to the rear panel. The idea here is to generate more radiative heat from the front (facing into the room) than from the rear (facing the wall). Part of the claim here is that with a cooler wall behind, less heat will be lost out through the wall.

[newshound]

That is just basic physics/engineering; do you have a problem with this?

There is however one (slight) advantage from vertical radiators in one special case. If you have an outside wall, its surface temperature is likely to be lower than the room in cold weather, especially if it is not very well insulated. When you are near it, you will *feel* cold. If you now cover this with a hot radiator, you will *feel* warmer.

[newshound]

Interesting point. It's going to be non-linear. Some of the vertical designs almost have internal "chimneys", but they are not very wide so wall friction won't help.

[Robin]

I don't have any problem with the *hypothesis* harry put forward. But the little I can recall of my basic physics education made me wonder about a couple of other, possibly countervailing, points:

a. is air passing out of a "horizontal" radiator at the temperature of the radiator? If not, air flowing from the top of the vertical radiator is more likely to have approached or reacxhed equilibirium with the radiator's surface temperature and would then deliver more energy for a given flow; b. is the flow of air into the sides of the radiator negligble? If not, the vertical radiator gains a greater height over which cold air can flow into the radiator to be warmed.

I'd rather hoped there would be evidence (empirical or modelling) on this.

I was also interested as to how radiator vendors managed to advertise vertical radiators with much the same output per sqaure meter as horizontal radiators if the former were so patently less effective at transferring energy.

[harry]

Because there is no energy change in a heat exchanger, you can't really describe it's characteristics as an efficiency.

In any given heat exchanger, all heat transfer is based on the three "Ts".

Time, The length of time heated and cooled fluids remain in juxaposition. The longer the better.

Temperature. The temperature difference between the heated and cooled fluids. The greater the temperature difference the better

Turbulence. How turbulent the flow of the heated and cooled fluids is. More turbulent is better.

The first two are linear. The third is harder to quantify. It changes as the velocity of the fluids changes.

In practice the only way to determine accurately how much heat will be transferred can only be found out experimentally, ie suck it and see.

The actual path that the fluids take through a heat exchanger can also vary considerably in operational circumstances. eg in how the pipework is connected in a four connection domestic.radiator. The optimum being diagonal unless there are internal baffle(s).

So there are no clear answers, only general principles.

[The Natural Philosopher]

airflow speed dominates heat transfer from a hot object to air.

But with slow moving air all that really matters is the surface area.

If you look at a vertical surface in air with a thermal camera, you will see the air rolling off it in all sorts of turbulent waves. It doesn't stick to the surface.

[rbel]

snipped as I do not know the answer.

I think much will depend on the design of the individual radiators.

From my personal experience of comparing one particular vertical rad with one traditional longitudinal rad of equivalent output, I can say that from our viewpoint the vertical rad produces an equivalent (in fact better) level of comfort, which is what it boils down to at the end of the day.

There are two factors which may influence this outcome in our circumstances - the old longitudinal rad was a double panel design with one panel facing on to the room and one to the wall, whereas the vertical is all in one plane with well spaced tubes, and the vertical's position is slightly more central to the room layout than its predecessor.

[SteveW]

I can't give you any figures, but I can quote experience. We had a properly sized radiator in the kitchen - I can't remember the width, but it fitted in a 1200mm gap between cupboards, so probably 1000mm. We needed the space for a washer and a dryer and the only other wall space available was a narrow space between two doorways. We fitted two 400mm rads one above the other and plumbed with chromed pipe and fittings. We have found them very effective.

SteveW

[Robin]

Agreed. It was the OP who mentioned efficiency; I wasn't going there :)

I couldn't even begin to determine if a domestic radiator will have laminar flow back/front/both/neither. I'm not even sure if I could have done so 40 years ago. But I've been pointed to Keith Moss in "Heat and Mass Transfer in Building Services Design" who looks at the effect of radiator height and concludes "a long, low radiator will therefore give lower convective output than a short tall radiator of the same area". I am not saying he is right in theory let alone in all practical circs. But I was glad to see I ain't the only one who doubts the answer is as simple as tall/thin needing to be bigger than long/low.

[harry]

As an example. If you have a long, low radiator with two bottom connections, the water tends to go straight across the bottom of the radiator unless there is an internal baffle.