Oil filled radiators: can the heat output per kW vary between models?

[Harnser]

There's a number of reviewers (Screwfix, Toolstation) who say the cheaper (£35) 2kW oil filled radiators give out less heat than their more expensive 1.5kW one. By more expensive they're comparing to the 3x the price Dimplex, the 2x the price Winterwarm ones.

This got me curious because I can't see why performance would change.

It's a heater. Now it's been a long time since school but I recall that electricity can be converted to light or heat. Unless the heating element is lighting up inside the cheap radiators then the energy's got to be released as heat.

A particular design could heat up faster, or transfer the heat more efficiently to the air. But for all of them, that 2kW draw should be converted into the same amount of heat. Even with a crap element, the heat will have to go out into the room.

Over to you. Are these reviewers nuts or have I overlooked a fundamental?

 

[Rannoch]

I would agree. I have a Delonghi Dragon I use if I run out of wood at my man cave. I also have a cheaper oil radiator from ebay. The Dragon puts out more heat on a low setting than the cheaper rad on full

 

[Werewolf]

Think of it in terms of computer cooling and you might start to see how it can be different.

A difference in the design of the radiator fins can have a huge effect of output, a cheap one might have a much lower surface area which means that it's not as good at letting the heat "out" so it hits the working temperature and the element cuts out but because it's not radiating the heat into the room as well it's effectively got a lower output than a lower wattage one with better design that is constantly putting heat out and has the element running more.
There might also be a difference in the fluid used inside it, as that could make a big difference if it's one that transfers the heat better (think thermal paste vs pad, or a good thermal past vs a cheap one).

If you've ever compared old wall radiators to modern ones you'll notice that the newer ones tend to be far thinner and usually have a bunch of fins (like a heatsink) whilst older ones were often just a stamped bit of heavyish metal where you basically had it alternating between what was effectively about a one inch pipe and an inch of thick flat metal.

Basically they're set up to only hit a certain temperature on the radiator (something like 70c), so the better it can dump that heat into the room the more time the element will be on, thus the more it'll output in a given time period.
A 2kw design with a poor design might spend half it's time waiting for enough heat to have been transferred into the air for it to turn the element on again, whilst a better design with a 1.5k element might be on all time (so the 2kw might only really be doing 1kw of heating under some conditions, whilst the 1.5kw might do that all the time).

In terms of price with the brand name ones you'll probably also be paying for the brand name, and probably a bit of a better build quality.

 

[jaybee]

Do the maths assuming the more expensive ones are more efficient. Depending on usage, check the cost in electric and make a decision. I would not have thought there was a lot in it.

 

[Werewolf]

Do the maths assuming the more expensive ones are more efficient. Depending on usage, check the cost in electric and make a decision. I would not have thought there was a lot in it.

Aye

They're probably no real difference in cost to run to get the room up to temperature in the long run, but the difference is likely in how fast they'll get the room warmed up.

When we swapped our old radiators (70's style), to the modern "finned" versions the house heated up far faster but the cost to run didn't change much at all.

 

[b0rn2sk8]

Aye

They're probably no real difference in cost to run to get the room up to temperature in the long run, but the difference is likely in how fast they'll get the room warmed up.

100% this, the cost to heat the space is determined by the heat loss of the room and as you say the 'quicker' the heater can get the heat out, the quicker the room will get up to temp.

 

[LuckyBenski]

Do the maths assuming the more expensive ones are more efficient. Depending on usage, check the cost in electric and make a decision. I would not have thought there was a lot in it.

There's no such thing as electrical/energy efficiency with heaters. Any other device - a TV, dishwasher, light bulb - efficiency is defined as how much energy is used in the function (flashy pictures, sounds, spraying water around), versus how much is wasted as heat.

In a heater the efficiency is 100% aside from tiny amounts of noise and any LEDs it might have.

The real factor is purely how quick it releases that heat energy into the environment. Think of watts as "lumps of coal per second". If you put 10 lumps of coal on your fire while it's roaring, you will get all the coal energy quite fast. If they burn slow and low, it might take twice as long - but just as much energy went into the room.

The reality is, whatever a label says on the product, it's quite possible they draw different amounts of power even if both labelled same wattage.

Heat output per kW is in fact, kW. It's a question of speed and as mentioned above that's dependent on the radiator shape, not the wattage.

 

[Harnser]

Basically they're set up to only hit a certain temperature on the radiator (something like 70c), so the better it can dump that heat into the room the more time the element will be on, thus the more it'll output in a given time period.
A 2kw design with a poor design might spend half it's time waiting for enough heat to have been transferred into the air for it to turn the element on again, whilst a better design with a 1.5k element might be on all time (so the 2kw might only really be doing 1kw of heating under some conditions, whilst the 1.5kw might do that all the time).

Thanks all. I'm quoting this as it got to the heart of the question I had.

My conclusion: since I don't care how quickly the room gets hot and as a 1.5kW heater should be enough for my room, it's a no brainer to buy the cheap £35 2kW heater and save £60.