Radiant temperature.  Elephant in the room?

In a cold 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/#what_is
“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?  Why are we fixated with air temperature?

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 than normal. They could choose to keep the hall at a medium temperature all the time, this may add to comfort by removing (or reducing) the cold surfaces, but heating 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 in a room with low radiant heat.  I soon 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 might 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 solid cold walls and no insulation.

However, my experience of such heaters is that they are usually sized far too 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…. you won’t find a salesman giving that pitch!

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. 1kW in, 1kW out.  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 impossible….  like asking a good chef for just one plain cheese sandwich!   And controlling it is another matter.  Turning radiation on and off is not generally comfortable.   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 ‘comfort’ because its subjective.  The net-effectiveness of installed heating systems is hard to evaluate.  The individual products are being developed, but on the installation front, there seems to be little incentive to drive forward the little incremental improvements.  There still seems to be a long way to go, and I’m guessing that in the future will be better at gauging and correcting any shortfall in radiant heat with older buildings.

i)          https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/128720/6923-how-much-energy-could-be-saved-by-making-small-cha.pdf

16 thoughts on “Radiant temperature.  Elephant in the room?”

  1. Just installed an infrared heating system in our local village hall. It uses six 2.4 kw units which are fixed to the ceiling. Today was the first real test of them as the hall starting temperature was around five degrees. Up until today I’ve been impressed but the experience was not so good this day as, although a certain quick warming direct to my body was welcome, the air was still chilled, leaving hands very cold. Any movement through this cold air meant instant cooling too. Obviously it’s pretty instant loss of heat when switched off too. The overall control is by a room thermostat which may seem a bit odd as it’s bodies that are heated and not the air but I think there’s some medium wave heating which does nudge up the stat a little but we’ve never yet reached the 18 degrees it’s set at. We’re not sure that this system is suitable for very cold days.

    1. Thanks for your thoughts. I guess at that low temperaure, you won’t exactly be comfortable, but turn them off, and you’ll be wortse! We can experience this outside on a cold day… I would rather be in the sun than the shade. yes, having an air thermostat seems a little strange, but there is no radiation-sensing thermostat, to my knowledge, on the market.

  2. Like so many things, ‘obvious’ when someone has explained it!
    I remember my physics (black-body radiation etc.), but never considered this comfort factor, though I experience it almost every day: living on my own, I use the kitchen table as desk/worktable, sitting with my back to the south-facing window. Evenings or overcast wet days like today, I need to wear a lightweight gilet to stop my back getting cold even though the temperature is a steady 19°C. But on sunny mornings, I sit in the sun and toast my back even if it’s frosty outside. The whole of the downstairs stays at a steady 19°C even though the CH thermostat is programmed to vary the temperature. I went away for half-term (grand-children) setting the CH to ‘Holiday’ programme (18°C 24/7). On my return, the boiler was Off, and the actual temperature 18.5°C! I don’t know how much it had been on while I was away, probably not much, but it ran much of that evening try to raise the temperature (of the whole ground floor (65sq m) ½°.
    Reason? Massive stone walls. Want to be cool in summer and warm in winter? Live in a old stone house with 3′ thick stone walls.

    1. I have the fortune to live in an old chateau with metre+ thick stone walls, heated with a (John Cantor assisted) GSHP via the floor and ceilings. The biggest problem with stone walls is that insulation is almost zero, I have managed to insulate the outside walls reasonably well, windows, floor and ceilings exceed modern standards. This leaves some massive internal walls + floor slab within the insulated “box” (I estimate approx 1MJ/degC). The result is an incredibly comfortable feeling in the house, especially winter. There are no draughts, cold spots though I only heat to 19ishC. My thoughts on this feeling of comfort was due to the thermal inertia, I heat at night on economy electricity and the temperature will only drop one degree during the day when zero outside. Certainly this is a big help in the summer when this mass stays cool during the day and can be allowed to cool overnight.
      We are having a unusually cool May and it feels chilly inside, though the temperature is over 18C. Last night I put the heat pump on: the room temp has barely changed (inertia) yet the sensation is much more comfortable which I can only put down to the increased floor (and ceiling) temperature.
      This adds more fuel to the radiant temperature argument, and is perhaps where the feeling of comfort comes from, rather than the inertia of the stone mass keeping the air temperature more constant.

  3. It strikes me that you do know something about heating, it’s just that you have never, until now realised that the jigsaw pieces of your knowledge have not been arranged in a manner to allow you to gain a better understanding. But they are now … 🙂
    I like your phase “distorted sense of comfort”; it suits the explanation to the user and it fits into the technical discussion.
    I think your experience has been mine by the seems of it – with ufh (I prefer hf =a heated floor) the body experiences “the lack of cold” as opposed to “gaining heat”. When i heated the concrete floor of my flat with a 70mm timber floor containing the heating pipes (and insulation) and I felt the floor, it felt barely warm, which was a concern. Then I touched the only visible part of the original concrete floor by the front door – “aagh!”, it was freezing. And that was my lightbulb moment.
    Sadly the topic is almost unheard of because, dare I say, that heating engineers (typically plumbers) are not that well educated. You only get intelligent understanding in the intelligent, and even the intelligent benefit from suitable education and training.
    Btw, the wall surfaces will not approach the air temperature of the room unless highly insulated. A 10 inch solid wall with plaster and -1C outside and 21C inside is around 15C if I remember rightly. So bearing in mind the affects of body heat lost by radiation to a cold surface, that is why those old houses are so wickedly cold.
    Thanks for an interestingly article.

  4. I am sure warm, draught free feet make a huge difference to how you feel. I’ve discovered tucking my feet inside a cloth smalldog basket is fabulous! That drops room temperature at least a degree!

    1. I have installed hydronic radiant ceiling heating using capillary mats supplied by beka. It works brilliantly. We laid the mats on the ceiling plasterboard and then when it was all pressure tested we poured a thin layer of self levelling compound over the mats. My only concern, which may be unnecessary, is mice eating the pipes. It’s a lot of pipe!

      1. Nick, if you read this reply, I’d be really interested in hearing about your experiences. I’ve seen the same system from Beka and it looks to tick all the boxes for a retrofit into a UK 1950s house. That being said, it’s hard to get Beka to supply it and there’s no-one in the UK that’s familiar with the technology that I can find! If you’re up for a chat, https://larkinweb.co.uk/contact/

    2. Hello Amanda,
      I have it!
      (The first floor of my house has ancient wooden floors which were not suitable to underfloor and I hate radiators, so I looked for another solution.)
      It works fantastically well. Similar to a underfloor heating (actually it is exactly that upside down) there is a sense of comfort without any feeling of direct heat.
      If you can’t have underfloor heat and don’t want radiators then it is a great solution.
      It might seem counter intuitive since warm air rises, but if the house is well insulated then ultimately if you put a given amount of energy in a well insulated box it warms up.
      Physically it is very straightforward: fix galvanised diffuser plates to the ceiling joists (suspended rails etc.) these have channels to clip the water pipe in, which is done exactly like for underfloor. Below this plasterboard panels are fixed in the normal way.
      Couple of points to be aware:
      1) You need extra insulation above the ceiling. This is because the ceiling is actually at 30C not 20C so you need to double the insulation to get the same heat loss.
      2) Care on cost and detail of the system. Initially a plumber proposed a Rehau system where the pipes are encased within special plasterboard panels which click together and make a connection for the water pipes also. Apart from being very expensive this to me (an engineer) is madness, you are left with dozens of push fit water connections in each ceiling, which are bound to fail at some point.
      I was prepared to invent my own system before the new plumber found the steel diffuser plates commercially.

      1. Looks like you have found a good solution Steven.. well done.
        The important thing is (as you say), the room needs to be well insulated. I’m told that hot head and cold feet are a bad combination, and this would be the experience in an old house. That said.. a small bit of radiant would help. The restaurant at CAT (Centre for Alt Tech) has pipes and plates in the ceiling. It all looks steel to me and big-ish bore. I don’t think anyone had done any testing on it there.
        I am often mentioning radiant heat, but many seem not to get it, even after explaining that when the heat of the sun is felt from a sudden break in the clouds, the air is no warmer. Its all down to radiation.

        1. I’m really interested in finding out more on how effective this is. I’m a physicist by degree and control engineer by profession. But I’m struggling with this one.
          My challenge is getting our 1950s house with solid concrete floors (no ufh possibilities there) ready for a heat pump. Using heatpunk the required radiators for 40C flow are bigger than many of our rooms allow (given doors, windows furniture etc)
          What flow temperatures would be suitable for ‘ceiling radiators’, do they have to be plastered into place. How do you calculate the heat emitted, are there any designs out there?
          I’m not interested in having the ‘average’ installed performance of an scop of 2.5 target is 4, and we already have std other insulation 300mm in loft, CW, good double glazing very low ACH, etc. The house isn’t thermally efficient being long and thin detached.
          So this concept looks worth more studying.

          1. Are you sure about your leaving your floor? un-insulated solid floors are a big problem. digging down,insulating (say4″), then pipes in screed is a BIG upheaval and cost, but you may be pleased for years to come.

            I don’t know a great deal about the heat output of ceiling radiators. I know the Centre for Alternative Technology have them (industrial ones) in their restaurant, but they run off hot biomass boiler. I have heard stories of radiators on the ceiling. The radiation element should be much the same, but the convection will be very low (zero?), so you will need a lot of them, and I would think it could be impractical. Weight and safety being a problem. One product that I have used (about page 64 of by book) is capillary matting. This is about 5 or 6mm pipes, and is pinned, then plastered to the ceiling. This should give the lowest working temperatures. apparently, on a wall it can emit 80w/sqm with water in the region of only 30C.
            All that said, your uninsulated solid floor is far from ideal. hot head and cold feet is not a good combination for comfort. I tend to feel that a little bit of radiation should always help, but too much of it in a poorly insulated house may not provide good comfort. (just realised I have already explained this in a previous reply to someone)

          2. Replying to John below.
            When we bought the house 12yrs ago and refurbished it before moving in, then we were recommended against digging out to insulate by our surveyor.
            In hindsight he was one of the olde-school “houses need ventilation not insulation” types, (I’ve learned since that these dinosaurs exist).
            So many tradesmen then and now said “it’s OK you always get insulation” and indicated 2 or 20mm. My excel thermal model for the house easily showed that 100mm was needed to not be heating the garden as well as the house.
            So now living here we don’t want the disruption
            Your book has arrived now, thanks for writing it.

    1. This is why UFH and Heat Pumps are so effective together because the heat pump is most efficient when always on, even at a low background temperature, which heats the thermal fabric of the building and radiates it back into the room from all directions, not just up through the floor.

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