(reading time: 10 mins)
Could infrared be the renewable alternative to gas central heating for Britain’s ageing housing stock?
As my twenty five year old combi-boiler approaches the end of its life, I’ve been looking for a more renewable heating option. Infrared heaters are fairly cheap to buy, around £2.5k for my house. By contrast my lowest quote for air source heating was £11k, only £4.5k of which could be recovered with the government renewable heat incentive (RHI) payments.
So during the winter 2019/2020 I tested far infrared (FIR) panels in one room. However I was not convinced infrared heaters are better than other forms of electric heating, such as underfloor or electric radiators, for reasons I’m giving below.
How I tested far infrared (FIR) heating
I live in a 3 bed Victorian terraced house, double glazing throughout and a good level of insulation (over 300mm) in the attic. There is 100mm of insulation between the ground and first floor rooms. The gas heating system comprises a 25+ year old Worscester Bosch combi boiler, and 6 radiators, with a total capacity of approx 22500 BTU/6600 watts.
To decide on infrared wattage required I used this calculator from heating manufacturer Herschel. This seemed to be the most realistic calculator online. Other sites selling infrared heating seemed to estimate the wattage required suspiciously low, less than half the wattage of my current gas C.H. radiator (1100w) so I stuck with the Herschel calculator. This estimated between 840W and 1400W required for my back bedroom depending on the options chosen.
I bought two 400w, one 600w panel heaters and a wireless thermostat. The Herschel heaters include mounting brackets, and are a simple enough job for a competent D.I.Y’er to fix to the ceiling, supposedly the best location for infrared. I installed two of them in a back bedroom measuring 3.1m by 3.9m. This is sometimes a guest room, but mostly the computer and yoga room and I chose it because apart from the living room this is the room that is occupied most in waking hours.
In November 2019 my electrician installed a new circuit for the heaters into the attic, and I turned off the gas radiator in that one room. I quickly found the two 400w heaters were not enough, so swapped the 400w near to my desk for the 600w and stuck with that 1kw setup for the rest of the winter. Once a week, Thursday, about 9pm, I took a meter reading for both gas and electric.
With my gas central heating I’ve long been in the habit of switching it on and off manually, rather than using any sort of program, which has significantly reduced my bills. So I adopted the same practice with the infrared heaters; not switching them on until I got home and knew I would be spending some time in the room, often switching them off if I knew I’d be out of the room for a prolonged period.
Is infrared more efficient?
Infrared heating appeared to have a potentially similar annual cost to running my gas boiler. Now, before anyone feels compelled to scream in capitals in the comments section about the unit cost of electricity being nearly 4x that of gas, please read the numbers below to see my reasoning first!!
So how could the running cost of infrared be similar to gas? Is infrared heating significantly more efficient?
It’s worth defining what we mean by efficiency here. The efficiency of a heater is determined by how much heat energy is generated for the amount of fuel it consumes. Most standard forms of electric heating have a near 100% efficiency, meaning they turn nearly 100% of electrical energy into heat energy. This is the same whether it’s an infrared, convector heater, or an electric radiator.
However the main claim for infrared is that it can consume less energy because it warms objects in the room directly, i.e. the furniture and you. The air is warmed as a secondary benefit, whereas radiators or convectors warm the air first, and warming you is a secondary benefit.
55% of what we get from the sun is invisible infrared, similar to what a far infrared (FIR) panel produces. Imagine going from the shade into direct sunlight on a winter’s day. The temperature of the air around you is basically the same but you feel significantly warmer in direct sunlight than in shade.
So the idea of infrared heating is you can feel comfortable at a slightly lower air temperature, and so use less energy.
What puts infrared ‘in the shade’
In the four corners of the back bedroom are the door, the computer desk, a wardrobe and a futon sofa. There’s not much point in the heater warming the air inside the wardrobe, or the air behind the futon, as a radiator or convector would do. The infrared heater just warms the outer surface of the wardrobe and cushions of the futon. So in theory infrared heaters could use less energy to create a comfortable temperature for the occupants.
However I found this to be a distinct disadvantage of infrared. In winter 2019/20 I was mostly at the computer desk, with the 600W heater on the ceiling above and slightly behind me. The heater was close but frequently my legs felt cold, because they were effectively in shade under the desk! And this was significant, requiring me to turn the thermostat up, and so losing any advantage of the infrared.
Another disadvantage is that infrared heat only seems to travel so far. My ceilings are a fairly standard 2.5m high. But I’ve read descriptions of infrared heating that says their effectiveness is reduced if the object they’re warming is further than 2m away. In at least half the room the first object the infrared will hit is the floor, 2.5m away.
For this reason I should possibly have relocated the 600w heater so it would be directly above me seated at the desk, although I did move the desk slightly more into the centre of the room to be more in the heater’s path. So infrared heaters have to be carefully positioned to benefit occupants if they are to be save power compared to a radiator or convector.
Another claim is that infrared uses less power because it provides more or less instantaneous heat. This may be true if you’re close enough to the heater. Otherwise you need the rest of the room to warm up to feel the benefit, negating the speed of a heater getting up to temperature.
I also noticed that on cold days where the gas C.H. was turned off for the rest of the house, I felt cold from breathing in cold air from outside the room, again requiring the infrared thermostat to be turned up. This could be a particular problem if you have a purely infrared system in an older house where draughts are common. Perhaps the answer here is to combine radiators/convectors with carefully positioned infrared heaters.
Now to the back of the envelope calculations.
Researching the viability of infrared, I found there’s a rule of thumb that you should expect infrared heaters to be on 1/3 of the time. The heating would be needed on average 6 hours a day for 6 months. The total capacity of the central heating radiators is 6.6kw.
So 6h x 182 days of the year x 6.6kw = 7207kwh.
Multiply that by 1/3 (0.33) = 2402kwh
Multiply that consumption by 15p per unit to get £360 per year.
That is slightly higher than my estimated £296 on gas for heating (8000kwh of gas x 3.7p). However electric heating can bring other savings beyond the unit cost.
Electric heating doesn’t need regular maintenance, so I can subtract £90 per year as I wouldn’t need an annual boiler service.
As I already have an electric shower, if I installed an inline electric water heater and electric hob (the kitchen is due a refresh), I could switch off mains gas completely and save myself £60 per year gas standing charge.
So potentially those two create £150 savings each year. And depending on the age of the boiler there’s also a reduction in repair costs of course.
Perhaps most importantly, as I mostly have the house to myself, it’s easier to switch off heaters in rooms that aren’t in use. Every hour I don’t use 1100w of electric heating in a room will certainly take 1/6th off the house’s total heating bill for that hour. I can of course switch off central heating radiators, split the house into zones or use thermostatic radiator valves (TRVs) to cut consumption.
But I doubt switching off one gas 1100w radiator will cut gas consumption by a sixth, because the boiler still has to fire up, and heat the water in the pipes etc, so the savings from an electric heater that is turned off are more significant and predictable than a gas C.H. radiator being turned off.
So despite the much higher unit cost of electricity compared to gas, taking into account these other factors, trialling infrared heating in one room seemed to be worth a go.
However my experience this winter showed that the ‘1/3 on’ assumption was too optimistic. By looking at the increased consumption on my weekly readings, and comparing to days when I didn’t use the infrared at all, I found the infrared heaters were actually on 50% of the time. That raises expected electric consumption for the whole house to 3300kwh – a fuel cost of £495 not £360 per year, and £199 per year more than I’m currently spending.
Can electric heating ever work in an old house?
Do any internet search and the standard answer you’ll get about electric heating is, don’t bother, it’s a waste of money because a unit of electricity costs three to four times as much as a unit of gas and modern gas boilers are 90% efficient. However this isn’t the whole story.
Older boilers like mine would have been about 80% efficient when new. At the last service I was told it’s running very well considering its age – difficulty in obtaining parts will be what finally kills it. So lets say its efficiency has now dropped to 70%. Even so, if I take 30% waste from the 8000kwh of gas I used each year, that still requires 5600kwh energy use to heat the whole house with a 100% efficient heating system, not 3300kwh. So factoring out inefficiency of the boiler, the gas heating system overall still appears to be wasting significant amounts of heat.
The big unknown for me when considering any form of electric heating is how much benefit does the house get from the heat in the pipes carrying hot water to the radiators? Does that 5600kwh vs 3300kwh difference mean I should nearly double the electric heating requirement in each room to compensate for lack of pipes warming the house? Or is the heat in those pipes mostly being wasted, even with good pipe lagging?
I’ve not been able to find a definitive answer to that question. But consumer association Which? here state it takes 4200kwh to heat an average uk home for a year with electricity.
Now compare that to the average UK domestic consumption of 8000-17000kwh for gas, the majority of which is for heating. If highly efficient electric heating needs 4200kwh on average, a gas consumption 2-4x higher than electric does suggest gas central heating systems as a whole may waste a lot of energy.
If anyone knows of research that answers the question of how much heat generated by a gas boiler actually makes it into the living space, please let me know!
So am I still considering replacing the gas boiler with infrared heaters? In short no.
Due to the issues I’ve outlined above, I don’t believe far infrared (FIR) will cost less to run than other forms of electric heating. They might make useful supplemental heaters however. I’ll probably keep one positioned directly over my desk, and another directly over the bed in the front bedroom, as the top of the bed is less than 2m from the ceiling.
Another downside of infrared heat is that it easily escapes through glass and through uninsulated floors. This is borne out by the fact that Herschel’s calculator shows a significant increase in heat requirement for this back bedroom to 1400w if I had an uninsulated floor. That’s more than the existing 1100w gas C.H. radiator, which was perfectly adequate before I insulated the floor of the back room.
I insulated the suspended timber floors upstairs because it was a few hours work. However downstairs would require the complication and expense of fitting PIR insulation under the floor boards. Without floor insulation I would likely need to increase the downstairs infrared capacity significantly above the 2 x 1500w radiators I have now – pushing my heating requirement, and bills, way beyond the 6600w total for the house I allowed for above.
However electric heating is perhaps not a complete non-starter. Electric oil filled radiators are cheaper to buy than infrared, would be less affected by uninsulated downstairs floors, and I could do much of the installation work myself. Although fuel costs will be higher than gas, it still has the potential annual saving of £150+ outlined above, which could largely offset this cost.
Anyone considering a new gas boiler should also bear in mind it’s unlikely gas will get cheaper than it is now, because government needs to discourage consumption. By contrast the lower generation cost of renewables may lead to electricity cost reductions in future.
The UK especially needs to find an alternative to heating our homes with fossil Methane gas. The latest research shows oil and gas production are releasing more methane than previously thought – methane is a far more potent global warming gas than carbon.
I’ll be taking all this into account when it comes time to replace my heating system. Unfortunately, it seems infrared is not the silver bullet for problem of heating Britain’s homes renewably.