Radiant temperature.  Elephant in the room?

In an un-heated exhibition room at the Centre for Alternative Technology many years ago, there was one of those good-for-your-posture kneeling chairs.  Surrounding this chair was a curved hardboard sheet covered with Aluminium foil. You were invited to sit, surrounded by a shiny foil surface. The effect was quite dramatic, you could instantly feel the warmth reflected back from your body.

This chimed well with claims from various underfloor heating manufacturers that for the same level of comfort, you can have a slightly lower air temperature. This is due to the radiation emitted from the floor

The theory is that every surface emits radiant heat, and this depends on the surface’s absolute temperature (e.g., no radiation at -273°C). Things absorb it too, so radiation bounces back and forwards between objects.  E.g., between you and a wall.

I experienced this when I first installed some underfloor heating in a hall in Scotland. Having finished the floor (with wooden covering) in 1 day, I slept there and assumed my thermometer was incorrect because I was comfortable in a 16°C room.  It was only later that I realised that the floor’s radiation had distorted my sense of comfort.  Not that the floor felt particularly warm, but it wasn’t cold.  This was my first experience of underfloor heating.

I also came across this slide about human body cooling at an energy exhibition a long time back.  I find this one hard to believe but if it is even half true, it is showing how dramatic the effect of radiation is on comfort. Cold stone floors in very hot countries certainly seem to help at midday.

‘Radiant temperature’ is a thing, and is documented in CIBSE and ASHRAE.. There is plenty to read on the topic to support the notion.  Indeed, to quote https://www.hse.gov.uk/temperature/thermal/factors.htm
“Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment.

Why therefore is the topic almost unheard-of in general heating speak?   And why are the standard ‘comfort’ conditions suggested to be 21°C air temperature etc. with no mention of surrounding radiation?

This particular topic came up for me while advising on the heating at a village hall. The hall is infrequently used and un-insulated, so if they choose a quick ‘blast’ of heat from cold, the cold walls and cold floors would have a negative effect on comfort.  Occupants would radiate themselves, but get little back from the surroundings.  To compensate for the lack of radiation, and to restore comfort, the air temperature may need to be set warmer. They could choose to keep the hall at a medium temperature all the time, this may add to comfort by removing the cold surfaces, but it is also a continuous loss of heat to the outside.   I started to feel that I knew nothing about heating since I had no idea how to work out how much ‘discomfort’ could be caused by cold walls’, and how much warmer the air temperature needs to be to achieve comfort.  I then quickly realised that nobody else knew either.

I got around to thinking that in constantly-heated houses, it isn’t really a problem.  The walls and surfaces can get up to around 20°C, and these all emit radiation, so there is no ‘shortfall’ of radiation.

For intermittently heated buildings, and those with stone walls and stone floors in particular, then there could at times be a considerable lack of radiation and this could lead to discomfort.

There is of course a well-established, simple and effective way of addressing this ‘shortfall’, and that is to fit infrared heaters overhead.  Indeed, they are used frequently in churches, which must be the hardest of all buildings to heat, being infrequently used and with no insulation.

However, my experience of such heaters is that they are usually sized generously by installers wanting ‘its’s nice and toasty warm’ feedback from occupants.   But many such community buildings are on a limited budget, so maybe the design brief should be – low running cost with acceptable warmth.

Any direct electric heating does however seem inefficient, and when comparing heat pumps with any direct electric heating, it is common to assume that any electric heater has a COP of only 1. i.e., it’s 100% efficient. But this is not the whole story. If a small amount of electric radiant heat can mean we can drop the air temperature say 1°C, then our heat pump could have a reduced input of 10%. (i)  So, using some radiant heat only at times of occupation, or before the wall surfaces have warmed, could be a net benefit.

The more I think about it, maybe every old intermittently-heated building should have at least a small amount of radiant heat input, but finding an installer to fit a small infra-red heater might be hard….  like asking a good chef for just one plain cheese sandwich!   And controlling it is another matter.  Radiant globe sensors do exist, but you won’t easily find them.  Hopefully, as time goes on, controls will get better.

I was pondering the development and fine-tuning of heating systems.  For cars, fuel economy is so easy to measure, so there is a natural evolution towards better efficiency, but for heating, it’s extremely difficult to measure net-effectiveness of installed heating systems.  The individual products are being developed, but on the installation front, there seems to be little incentive to drive forward little incremental improvements.  There still seems to be a long way to go.


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