Foil Radiator Reflectors - are they of benifit?

[Richard Owen]

Not sure though about the 25% surface etc. But that's probably along the apples-and-pears lane: if you had a 1200mm (4ft ) radiator, then it's outside would probably be just as warm in a single panel version as it is in a double panel double fins version. The surface facing the wall is the same for both cases, so the radiation is the same.

Klaus

Aaaaah, but it isn't. For a given emitter output, the area of the panel facing the wall for a double panel, finned version, is less than half the area of the panel facing the wall on the single panel version. For a given temperature, the radiation will be the same for both emitters per unit area. It's just the fact there is less unit area on the double panel, double finned version that lowers the amount of radiation.

[martin W]

Martin,
are you aware that the temperatures stated in the Stelrad brochure are DIFFERENCE temps between the room to be heated and the radiator temp? Operating a radiator at 25°C in a 20°C room will yield just 5% of the rated output.

Klaus

 

Oh Sugar! I Missed that one!
 Do Stelrad do any radiators at around 2meter high x 3 meters wide? 

[martin W]

Richard, All, just to clarify I am NOT going to buy a heat pump (well for the forseable future). Whilst the rad sizing for heat pump is in a different thread and has spilled over to here, I just want to clarify that the rad sizing is an aside as I am replacing the radiators anyway. Maybe I should have started a different thread on how to optomise rad sizes for heat losses.

Anyway, I think the general feel from this thread is that some sort of backing on the wall (if it is external) 'may' or 'is' of benefit to reducing the heat lost to the wall behind the radiator. I suppose it also means just don't pay too much for the said deflecting / backing material behind the radiators.

It also seems like double paneled, double finned radiators are the best for emiting heat to the room and not the wall they are mounted on.

Just a question on sensible raidator sizing (bit off topic I know), if I can fit a 600mm high x 1800mm radiator (single panel Stelrad K1 type) which has more than enough heat output for the room size, is it of any benefit to sit a K2 double panel instead? I think I have read it is best to fit the bigest physical size rad in a room as it has more even head disapation than a smaller physically sized rad of the same heat output (BTU's). But what about the oversizing of radiators?

[KLD]

Martin
as long as all rooms have a similarly oversized radiator, you'll be fiiiine. You can simply run the system at a reduced temperature, to accommodate for the oversizing. Trouble starts if there is another room that has a "normal" radiator, which would require higher temperatures to achieve enough heat output to keep the room warm.
I seem to remember you said somewhere else your boiler is of the non-condensing type? And modulating only by a factor two or thereabouts? With your now better insulated home you might find that quite often the heat load is much lower then the output of your boiler, and it will start short cycling. But that's another thread altogether 

Klaus

[KLD]

For a given emitter output (...)

Yes, that is the 'pear'-spective in my last post 

K.

[martin W]

Klaus, I think I will have a cycling issue anyway as by my reconing my house heat loss / demand is now around 21,000 BTU's. The minium combi-boiler ( non codensing type) output is 38,000 BTU's. Oh and the solar and WBS will contribute to house heating (well solar might a bit). WBS output is 4KW so around 15000BTU's. Might have to open a few windows this winter .

[tony.]

martin w,

wont your boiler modulate down to the required load?

What make and model is your boiler?

tony

[martin W]

Hi Tony, its a Halstead Ace Combi Boiler, I believe it modulates, but the specs say minimum output to Central Heating is around 39,000 BTU's with max around 82,000 BTU's. I just assumed that when the heating temp is set on min it will output around 39,000BTU's. The burner will certainly cut in and out with the pump running, but I am unsure how to check for modulation. I am sure I have read it is a modulating type, but I can't put my hand on the data at the moment.

[KLD]

Martin,
Boiler flow temperature and output power are separate things. The boiler increases the water temp passing through by some amount, normally 20°C I believe. For a given flow rate (pump speed) this temp increase equals a certain amount of energy per time, i.e. power. Now, the radiators emit this energy into the house, and the water temp drop between inlet into the rads and outlet by the same amount (20°C). Whether it's 30/50°C or 50/70°C, the energy content of the water is the same. If your rads are small, they need higher temps to be able to emit enough energy to decrease the water temp by 20°C.
Your gas boiler is less choosy, the flames are several hundred °C, so whether the temp levels are arount 40 or 60°C doesn't really matter. Except for condensing boilers, for which the second heat exchanger (the condenser) want's to be below 55°C.

Under part load conditions, i.e. when the heat loss of the building is not at their design maximum, the radiators don't need to emit as much energy. Most often the flow rate (i.e. the pump speed) is fixed, so you can only lower the temp spread between flow and return. The boiler doesn't need to put as much energy in to heat the water by only say 10°C, so it burns less gas -- this is the modulation.

Your heating load over the heating season varies much more then just a factor of two. For the lower outputs the boiler will switch on and off more or less frequently, with increased losses.

I hope this makes sense!?

Klaus

[wookey]

I fitted radiator reflectors (foil-faced 2mm foam glued to walls) behind rads in last house. They may have made a small difference - there is no way to know. Theory sugests they might. I'm not sure I'd bother again (in fact I've jus taken 2nd rad out of this house - hope to get rid of them all eventually).