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I've recently come across low surface temperature radiators (E2 technology) as a form of heating. Unlike a traditional water radiator they operate at 30 - 50 C instead of 75 C. Small fans pull air across thin radiator fins to convect the heat around the room rather than use pure radiation. The heat source can be from a heat pump, whereas traditional radiators use boilers.

This brand brochure says it delivers 3.5kW in fan boost mode at 50C, and 1.98kW at 30C. This model uses 14 1W fans.

Here's a typical model from a different company

E2 technology radiator

There are claims that they are over 12% more efficient than traditional radiators. Is this reasonable?

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    Just a note, but traditional radiators can also operate at something like 40 C without extra fans. Don't know the specific English term but it gets used for instance in houses which get renovated but don't allow floor heating for instance. The radiators are usually a bit bigger and provided the house is well insulated there's no problem at all getting it warm. Don't know about efficiency numbers, but the claim is it's better than operating at 95 C. – stijn Nov 10 '19 at 19:14
  • Yes, but the issue is that they need to be about twice the size and you need the wall space to accommodate them. – Graham Chiu Nov 10 '19 at 19:22
  • You mean twice the surface? I assume that's technically sound but in reality I don't think that is a general rule because there are other factors. E.g. especially in the renovation case I mentioned. If I look around here there are many of those cases where the amount of energy needed for the house gets reduced drastically because of insulation and windows so the radiators sometimes don't even need to be resized but can just run at lower temperature. – stijn Nov 11 '19 at 8:30
  • Normal "radiators" are misnamed anyway. They mostly deliver heat by convection. Try holding your hand in front of one that's hot, then a similar distance above – Chris H Nov 11 '19 at 19:58
  • The other option with existing radiators and low temperature water is to run them for much longer. With an air source heat pump they're effectively running full time in cold weather. – Chris H Nov 11 '19 at 20:02
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The key is this part, from page two of the brochure:

The ULOW-E2 provides enhanced convection via in-built fans. This enables heat sources to operate at their most energy efficient settings.

The "heat source" is a boiler which provides hot water. Here's an efficiency curve for a boiler relative to the return water temperature (source):

enter image description here

You can see that higher efficiencies occur when the water in the boiler starts at a lower temperature, in "condensing mode." Condensing boilers are designed and programmed to operate in this region:

Condensing boilers [...] achieve high efficiency (typically greater than 90% on the higher heating value) by condensing water vapour in the exhaust gases and so recovering its latent heat of vaporisation, which would otherwise have been wasted. This condensed vapour leaves the system in liquid form, via a drain.

Once in the condensing region, you can continue improving efficiency by further lowering inlet water temperature.

Thus, two factors result in making the total system more efficient with a low temperature radiator:

  1. The boiler temperature can be set lower, since the radiator works at a lower temperature. This will inherently lower the temperature of the water returning to the boiler.
  2. The radiator is able to extract more heat from the water than a traditional radiator, further lowering the temperature of the return water.

Obviously, the boiler temperature must be sufficient to provide heating for the house. But with a system like this you could also implement a hot water reset strategy to vary the boiler setpoint based on the expected need for heating.

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  • So the energy used to drive the fans is much less than the extra energy extracted from the water? – Graham Chiu Nov 10 '19 at 18:54
  • That's the assumption, and it seems safe even without power consumption data for the units. The largest one produces up to 3500 W of heat with a rated electrical input of 12 W, for a ratio of 1:0.003. – LShaver♦ Nov 10 '19 at 20:28
  • The electricity used to power the fans will be charged at a higher rate than the gas used to power the boiler (typical UK installation), but may still cost less than its saved by the higher efficiency. It will also be partially from renewable sources. – Chris H Nov 11 '19 at 20:00
  • @ChrisH The UK seems to be special in that people have access to low cost (non-renewable) natural gas which is cheaper than electricity. In my own case I'd power the fans from batteries charged by my PV panels. – Graham Chiu Nov 12 '19 at 23:50
  • @GrahamChiu that may be the case; certainly gas is the main heating fuel for most people. We don't have much sunlight at the times of year we need heating, so unless you really need loads that are suited to batteries (and have an excess of solar power), running dedicated cabling to power the fans off batteries would seem rather a lot of effort and materials for little benefit. That's assuming a grid tie solar setup – Chris H Nov 13 '19 at 6:50

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