older/newer GCH radiator efficiency?

That would be true if all water out of the rad were at room temp. It isn't, so the rad's energy input = flow rate x (inlet temp rise - outlet temp rise) x SHC of water.

A rad is 100% efficient because there is nowhere for any waste energy to go. Thus energy input & energy output are actually the same thing for a rad.

NT

That's conservation of energy.

Efficiency involves the concept of usefulness - as in the traditional definition "the ratio of the useful work performed by a machine to the total energy expended". On that it's entirely reasonable to consider how a radiator in a CH system affects the ratio of the energy released where it's wanted to the total energy expended (i.e. including all the energy released where it's not - e.g. in the boiler, pump, and pipes under suspended floors).

Looking at it another way, I'd consider a radiator which took water in at 70 degrees and passed it out at 65 degrees to be less efficient than one which passes it out at 55 degrees. The second done is doing a much better job at putting the energy where it's wanted.

Hence the scare quotes.

And as was previously explained, of not much real relevance to a radiator.

Fair enough with a boiler, but not the topic under discussion.

A radiator does not need to extract the maximum energy from the primary water on each pass. Since it gets to have multiple subsequent attempts, and any heat left in the water is either lost into the fabric of the house, or deducted from that that the boiler needs to replace. Neither of which are bad outcomes usually.

With a modern boiler, there is merit to the the total heating load of the rads presenting enough load to the boiler, for a significant part of the heating season, to keep return temperatures low enough for maximum condensing heat recovery. However this is not really a new consideration since even with old non condensing fixed output boilers, inadequate load could result in short cycling, which also lowered fuel efficiency.

Radiator balancing houldn't be hit and miss...

Phil Addison

FWIW, I was in a similar situation a few years back. I had a new combi fitted, and I was persuaded to keep the original rads by the plumber - something along the lines of 'they just work, they don't make them like they used to'. He power flushed the system and it did, indeed, work. However:

• There was no way the system could get the house up to a reasonable temperature, even after I'd put in insulation and draft proofed. The house is/was double glazed, but as a Victorian terrace, difficult to apply thorough insulation.
• It would take some hours to heat up. They were almost all long, tall (but low profile) rads. I think the plumber's reasoning was that the system must have been designed properly in the first place, and the new boiler certainly wouldn't do any harm.
• The system was noisy and still pretty sludged up. The magnetic filter would have needed endless cleaning had I been bothered, and when I drained the system to fix a leak the water was filthy.
• The enamel/paint was peeling on most of them - not a major concern, but it didn't do the decor any favours.

So I've ended up replacing most of them with new finned radiators, overspeccing by about 20%. I mains water flushed, and reverse flushed, for pretty much a whole day until it ran clear. I could redesign the system to fit decor and comfort - some rooms now have 2 smaller rads for example. I've also routed pipes away from the floor and behind skirting - much neater. This didn't need new rads of course.

The main issue with resiting was finding decent wall - some are on stud partition. And routing the pipes was sometimes a challenge.

Newer designs won't be more efficient in an energy consumption sense, but will heat up and, depending on the design, pump out heat more quickly. And if appearance matters, they might look better. Very pleased now with the way it works.

If I had my time again I'd have done it all at the outset. But obviously milages vary . . .

Is that efficiency or is it effectivness?

It's theta_R, thermal resistance.

Paul

It's a fair cop that I conflated the 2. My mitigation is that I was trying to stop the post growing even longer. Slightly longer 2nd attempt:

The second radiator is more effective (assuming it's a good thing to have at least the option of warming the room faster) and (very probably) also more efficient in that the CH system delivers heat to the room more efficiently.

Nothing is ever 100% efficient. A rad will make a tiny amount of noise due to the water circulating. That noise is unwanted use of energy.

Tried that. The system is highly complex ie not just a pair of pipes that goes from one rad to the next. There are rads on long spurs, with different lenghts/bends for the pipes, spurs with rads connected overlapping and rad acroos the main pair. It makes the system very sensitive to adjustments. Less than 1/8 turn of a valve can completely stop (or start) the flow through that rad or any of the others. Then you have the settling time after an adjustment if you're traying to get an even temp drop across all rads.

Thermal imaging camera allows you to see how much flow is (or isn't!) going through a rad by the size of the rising hot water column from the inlet. This also responds very quickly to any changes.

water in

Effectiveness, but the real world gets in the way.

A rad designed to output x BTU with temps of 80 C flow and 70 C return and 20 C room temp will have have an associated water flow rate.

If you want 80 C in and 55 C out, you have to reduce the amount of water flowing through the rad to give it a chance to transfer the heat to the room. Which will be also be at a slower rate than the

The amount of heat transferred to the room is only related to the surface area of the rad (fixed) and the temperature difference between the rad and room. A rad running at a lower average temp than the design is going to be undersized, fullstop. No amount of messing about with flow/return temps either by boiler settings or water flow rate are are going to affect that.

If the new boiler ever arrives here I'm planning on putting fans under the radiators in some rooms until redecoration allows new (and sometimes relocated) radiators. I couldn't find evidence of how to quantify the effect but a trial with 3 x 100mm PC case fans (under-run at 9V) seemed to have some benefit.

If the noise doen't escape from the room it'll end up as heat. You can tell that happens as a noisy CH system at night makes one all hot and bathered ;-)

I don't see how that invalidates the procedure, surely it should work for any layout (well, at least a domestic layout with one boiler and one pump?

The procedure boils down to opening all the hand and balance valves wide-open, then progressively turning down the 'too-hot' balance valves. You don't necessarily want the same temperature drop across each, that's only true if all the rads are the exact perfect thermal size for the room, but you do want them all to be capable of reaching full temperature right across each radiator - no cool outlets. But note that in some cases this will be impossible due to bad design of the pipework - or maybe sludge. I had a case where the largest rad in the house was furthest from the boiler but was fed by a long length of 10mm tube. Calcs proved there was no way the pump could generate enough head to feed that rad, and the answer was to re-pipe it in 15mm. I had a LOT of trouble convincing the plumbing contractor to do that!

Indeed, the lag can be a pain.

I haven't had the chance to use an IR camera, and can see it would be helpful to see the immediate effect of a small turn of the valve, but I emphasise that you don't want full flow in all the rads. Any oversized rads need less than full flow, and any close to the boiler need throttling back to increase their flow resistance so as not to bypass (steal) the flow from the furthest. A common misconception is that increasing flow-resistance will make a radiator cooler. Not necessarily: for a rad already getting as much heat as it can dissipate, closing its valve a bit just makes the pump increase its pressure so that rads further away get more flow.

Phil Addison

no, it's an explanation of radiator efficiency.

Yep. And it's 100% for all rads as already explained

It's completely reasonable. But it's not radiator efficiency, it's something else.

You'd be wrong then.

NT

probably, though I don't know what 'second one' you refer to

and (very probably)

no.

no. You are still not grasping what radiator efficiency is.

NT

incorrect. Another example is an electric convector heater.

Sometimes. Gravity flow systems make no water noise when kept below boiling point. Very well specced & designed systems in mild weather make no detectable noise. Systems that do produce noise, as most do, can expend at most millwatts on noise versus kilowatts on heat. That's in the region of a million to one, making no detectable difference to even a nonideal system. I question the blame of that laying with the radiators rather than the pump, but even if you decided it's all the rads' fault you'd still get a figure of 99.9999%.

NT

Ah, reality. I hope you didn't copy Steptoe's heating system :) Though IIRC that was single pipe.

NT