Why such different results?

[Spooked]

Hi all.
I'm refitting a small bathroom.
Trying to calculate rad size, I get wildly different results from 580 - 1374 BTUs!!
All info entered the same.
2.4 x 1.95 x 1.7
1.1 m2 window. Double glazed plastic.
One outside wall
Heated room below
50mm insulated pitched above
I know it's always best to go higher but which is right?
Cheers

 

[Justin Passing]

For a bathroom - yes always go bigger, and use a TRV. A tall towel rail feels warmer than a low rad...
I find it easy to think in terms of 1kW electric fan heaters. 1kW is 3412 BTU/hr. So 580 is tiny. a small hairdryer!

The calculators vary a lot. Try whatever you have on a different room and see wha\tit comes up with, and work out wthat the rad gives.
The variables like water temps, delta T temperature increase, air changes yada yada will be different, but if they all use the same set of numbers it'll balance easier.
HTH

 

[dilalio]

Towel warmer is not a radiator.

 

[Madrab]

Unless the bathroom is really small then don't count on a towel rail to heat it up unless it's quite tall and black/anthracite or white and you don't warm/dry towels on at least half of it. Yours isn't the largest but you still want to have the largest the wall can take for it to have any chance.

 

[ericmark]

I messed up, failed to take into account the extractor fan which LABC insisted was fitted, air was replaced from the hall, which was not one of the warmest rooms in the house, and I had not considered how much air was drawn in from the hall.

The wet room had both a tall towel rail, which yes could hardly be used as a towel rail in a wet room, and under floor heating, which was useless. The only way to get wet room warm was to get the hall warm, the wall thermostat was in the hall, and not TRV in the hall, so set the lock shield to get the hall radiator warm, and the whole house got cold as the whole heating turned off.

The cure was to fit a TRV to the hall radiator set slightly lower to the wall thermostat, so the hall heated up quick until the last few degrees, when the TRV turned the radiator down, so giving the rest of the house time to also warm up. Curing, the hall heating also cured the wet room heating.

Why the books say don't fit a TRV in the room with the wall thermostat, I don't know, but to be fair it also says don't fit in a room with an outside door, and the stairs have a chimney effect. So the hall is not a good place to put the thermostat to start with.

But the point is with a small room with an extractor fan, it is where the air is replaced from which is important, could have had no heating in wet room, as long as the hall was warm.

 

[Spooked]

Thanks all, crazy.
dilalio, towel warmer? Wine?
Yes, I've seen that anthracite seems to be higher BTUs, stainless steel also seems to give out more heat.
I'm quite fancying one of those open bar rads with the flow and return on one side, so you can just slip your towel over.
80mm centres seems to be the most common (in case of future replacement).
ericmark, you appear to have had a nightmare.

 

[dilalio]

dilalio, towel warmer? Wine?

A towel rail is the same thing!

 

[Spooked]

So a towel rail isn't a radiator?
I thought they were called radiators because they 'radiate' heat.
I've just had a quick scan, some places call them towel rail radiators.
I'm not sure the shape defines the description. https://www.stelrad.com/radiators/range/classic-towel-rail/

 

[dilalio]

Long topic.
Look it up.
Even radiators don't solely radiate heat.
Towel rads primary function is to warm towels.
So... Not a room's primary heat source, hence... These exist...

Google Search

 

[Spooked]

Yeah.
I've taken a good look.
They are, without doubt, radiators.
My towel rail is the primary/only source of heat in my bathroom, and will be again when it's redone.
Same with my downstairs bathroom.
I'm curious.
What other source of heat are you using and, if you put in a big enough towel rail that will provide enough heat, why bother with a second source of heat?
It doesn't even matter, surely, if a radiator is the primary heat source or not, it's still a radiator.
I could have a small radiator in my living room, but my main heat source could be a log burner.
That doesn't change the description of the small radiator, it's just what they are called I guess.

 

[Madrab]

Radiators are simply units that emit heat. The issue is the term 'radiate' which isn't an ideal term as different types of radiator heat the the space in different ways. It's not really about how much heat it outputs rather than how efficiently it heats the space.

Standard K2 /T22 convecting rads heat the pace on a ratio of 90% convection - 10% radiation - currently the most efficient way to heat larger spaces using typical radiators
Old style column rads heat the space on a ratio of - 40% convection > 60% radiation
Tube style designer rads heat the space on a ratio of 20% convection > 80% radiation
IR projecting radiant heaters heat the space by 90% radiation > 10% convection

When it comes to small spaces like smaller bathrooms, then towel rads can heat the space but need to be vastly oversized to compensate for the fact that it primarily uses radiation to heat the space. It needs to be close to other items in the room that it can heat and that then heats other items close to it and so on and so forth, it will eventually warm the space but can take a very long time to do it, or will struggle in the cold months. Also given the fact that it will invariably have heat absorbing towels draped over it, so larger means more of the rad exposed to actually heat the space. So all in all they are very poor at space heating.

 

[dilalio]

Rads as we know them, mainly "convect" heat. Circulating warm air from floor to ceiling and creating convection currents.

Radiant heat is infrared and warms the body in a different process.

Most washrooms I do nowdays have UFH for primary heat source and towel rails to keep towels warm.

Failing that, the addition of a convector/radiator or... As is the usual case, in a standard 3-bed semi bathroom... A big ass towel rail... It's common but not the ideal!

So... In a nutshell there's what should be and then what is!

 

[Spooked]

Well, all the above said, it's no different to anything else in this crazy world.
Perhaps we are making our bathrooms too comfortable.
I'm from the 'Jack Frost' on the windows, boil the kettle days.