I am being told that vertical radiators are not as efficient for the same volumetric flow as horizontal radiators.

[jon]

What is this nonsense, heat is dissipated by radiation, convection or conduction so how come efficiency becomes a problem.

[Fredxx]

For conventional central heating radiators most heat-loss is through convection. Air is a poor conductor.

It seems there is less overall air flow for a tall radiator than long for a radiator with the same area and construction.

So hardly nonsense.

[Andrew]

Try telling that to the female who has decided that "we need" vertical designer rads.

[jon]

So where does the heat loss go other than in the room..?

[Theo]

Hotter water returning to the boiler. Less gas gets burnt, but the room is colder.

(plus a bit into the wall, depending on its insulation)

Theo

[polygonum_on_google]

Likely true. But, in many cases, so what?

Let's assume a horizontal radiator is 100% efficient.

But how much of the time does that efficiency matter? I suspect a large proportion of radiators are throttled by a TRV. Further, if the radiator can cope on the coldest days, it probably has plenty of capacity in hand for most of the rest of the year.

Dropping efficiency to, I suggest, 90% would only really affect that small period during which the absolute maximum output is required.

[T i m]

Because efficiency is a measure of 'how well' not 'how' a system works.

With a tall / narrow rad, most of the air rising up / around the rad will already have been warmed by the lower sections and so only the lower sections will be working 'efficiently' (hot rad > cold air V hot rad > already warm air (= lower temperature gradient)).

With a wide / low rad, much more of the height / length of the rad will be transferring the heat from the rad > air > room and so it will (for the same area) be able to transfer the heat to the room efficiently / faster.

It is this way because the air is moving over the rad by natural convection and that means vertically. Make the rads 'fan assisted' and, (depending on the design) make far less difference re the orientation.

Cheers, T i m

[T i m]

<snip>

Fancy allowing anyone without the understanding of the physics to determine such things (irrespective of their gender).

Cheers, T i m

[Clive Arthur]

It is true (though 'effective' would be a better word) because of the reasons given by T i m and others. Look at the manufacturer's figures for power output and you'll see.

As a result, a vertical rad will be heavier and (usually much) more expensive. The upsides are that they take up less width, and when you come in from the cold you can stand against one.

[Martin Brown]

The physical geometry of the "radiator" affects how well it transfers heat to its environment. Convection is the most important by far.

Shiny metallic "radiators" like chrome towel rails in the bathroom hardly radiate any heat at all and once you put a towel onto them they don't do very much to warm up the air in the bathroom either.

Hotter water leaves the radiator exit port than would be the case for a radiator with a more appropriate geometry.

The critical thing is how much area is at each temperature and the airflow over it. The amount of heat that the "radiator" which actually loses most of its heat to the room by convection can transfer to the air. There is an optimum height, width and wall separation for maximum heat transfer. Insulating the wall behind a radiator improves it a bit if the wall is an outside one. You can see any radiators mounted on outside walls with thermal imaging. It matters less on an interior wall.

If you choose three radiators of 1m^2 with very different geometries.

Height x Width 0.5 x 2 1 x 1 2 x 0.5

The first will work best of all since it has a stripe of 2m length that is at inlet temperature and releases warm air low down in the room.

The second is a bit more practical and would still work well enough. Most are typically around a 1:2 ratio of height to length.

The last one has a much smaller stripe at the highest temperature and mostly delivers hot air to the ceiling (where it stays). They are a designer fad that serves no useful purpose (except to sell more replacement radiators when they prove to be unsatisfactory).

If you happen to live on the ceiling then they might be useful to you.

[Martin Brown]

More accurately you will have to stand against one to get warm!

[T i m]

<snip>

Yes, *once* the room is up to temperature?

Agreed, once it has brought the room up to temperature. ;-)

And how long it takes to get the room up to temperature. ;-)

My first experience of the reliance on 'central heating' was as a kid and when going back to my mates house after playing in the snow.

At our house we had pretty crude coal fired free standing boiler, gravity fed rad system and so also had electric fan heaters to 'top up' as / when / where required.

If you came in cold you could flick one on and sit right in front of it and be warmed up pretty quickly.

So, mate turns the central heating on and we have to stand next to the radiators waiting for some heat to get in them to be able to thaw our fingers out again. ;-(

Cheers, T i m

[Andrew]

But cannot sit on them :-(

[Robin]

As I think I said the last time this came up, my physics is nowhere near good enough to decide "by inspection" whether a vertical or horizontal radiator is better at transferring heat to a room. And (like some others) I saw too many /known/ unknowns - e.g laminar flow? temperature of air exiting top? chimney effect? - quite apart from the possible unknown^2

Ah yes:

[The Natural Philosopher]

Very easily. How 'efficient' is a radiator in a polystyrene box?

What the OP means is not heat in to heat out, but peak heat flow.

a vertical radiator will be transferring heat from its top to already hot air, so less 'efficient' in terms of peak heatflow into the room, in the same way that I have 2KW heaters the size of a cornflake packet because they are fan blown

With 'radiators' airflow is everything. There is not much 'radiation' going on at 60C...

[The Natural Philosopher]

The heat loss is imply LESS overall

[The Natural Philosopher]

wrong understanding of 'efficient'

how about a rewrite of the OP

'vertical radiators are less *effective* at heating a room than horizontal ones'

[The Natural Philosopher]

and UFH is better than either!

[charles]

So, you choose a slightly larger one

[Dave Plowman (News]

People do tend to use efficiency meaning how well it works, rather than the scientific meaning. And it could well be a horizontal rad of the same size as a vertical one gets heat into the room better.