Other things being equal, is a large radiator at a lower temperature going to heat a room more efficiently than a small one at a higher temperature?
Advantages I can see:
Daniele
I suspect the dominant factor will be surface area for heat transfer - if you're thinking about "effectiveness" rather than "efficiency".
In terms of energy efficiency, it would probably be mostly about matching the flow and return temperatures of the whole system to the ideal operating characteristics of your boiler.
I think I would say yes, but not for the reasons you give.
The biggest factor is that lower temperature radiators allow your boiler to run more efficiently (assuming it's a condensing boiler), and there's quite a change in efficiency from running at the old high temperatures (which a condensing boiler can still do if required) to running at the low temperatures a modern condensing boiler much prefers.
Another factor is that large radiators for lower temperature use can be used at high temperatures too to pump lots of heat into a house, e.g. to heat it up from cold very quickly, without you having to turn on the heating a long way in advance, or worse leave it on when you're out.
Something else I observe is that large radiators operating at low temperatures generate almost no temperature overshoot when the thermostat cuts the heating off, whereas smaller hot radiators do, and if you want to avoid that you'd need a more expensive thermostat or controls which learn the amount of overshoot your radiators give in order to predict an earlier switchoff.
There are also some safety benefits with cooler radiators, relating to the young/old/infirm being less likely to burn themselves.
The only disadvantage I can think of is the physical size they take up which some people might object to (never bothered me), and they will be a little more expensive to buy in the first place. I have installed one heating system where the radiators run at about 45/40 (flow/return) most of the time, and I'm very pleased with it. That's actually much cooler than even most condensing systems are designed to run, but it makes the boiler just about as efficient as it possibly can be.
Equal to what though? Biggest practical difference is in matching the radiator temperature to the "ideal" output temperature for your particular boiler. If it's a recent condenser, this will be lower than traditionally. In that case, you may well need larger radiators than otherwise to achieve equivalent heating power.
In a well-insulated house though, you shouldn't need 100% duty cycle to run radiators to reach an acceptable temperature in the UK climate. In that case there's no problem with less powerful radiators running for more of the time (assuming that control gear is smart enough to handle issues of cycle time).
You can make theoretical comparisons based on pipe losses for hot, fast cycles vs. warm, slow cycles, but these get dwarfed by the boiler efficiency. So use an efficicent boiler, and keep this boiler happy by using its best choice of output temperature.
Surely the point here is efficiency of the condensing boiler (if there is one). You need to have the water returning to the boiler below about 50 Celcius so that the boiler can do maximum condensation and therefore be most efficient.
To do that you need oversized radiators and to feed them in at the top out at the diagonally opposite bottom corner.
see, for example:
The return must be from the bottom, but the feed doesn't matter very much. Certainly, with a pumped system, there's no requirement that it be at the top.
I'll be amazed if you can find a boiler which isn't at its most efficient with the lowest possible water temperature.
I might believe one that suffers from reliability problems if the water is too cold - but I'd be surprised if this is the case with a condensing one. They are *designed* to suffer condensation!
Andy
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