If your LED grow room is too cold in winter, the obvious answer is usually to add a heater.
But before you spend electricity on something that only produces heat, there’s another option worth considering: add a 315W CDM grow light alongside your LEDs.
Modern LED grow lights are incredibly efficient, which is one of the main reasons they’ve become the standard for indoor growing. The downside is that they produce far less radiant heat around the canopy than older HID lighting systems.
During summer, that’s a massive advantage. During a cold UK winter, it can leave an LED grow room struggling to maintain temperature.
A 315W CDM gives you some of that useful radiant warmth back while simultaneously adding more plant light, a broader spectrum and extra intensity across the canopy.
So if you’re already considering adding a few hundred watts of heating to your grow room, CDM gives you another option: use that electricity to generate useful light and heat at the same time.
Why Does My LED Grow Tent Get So Cold in Winter?
This is becoming increasingly common as growers move away from HPS and other traditional HID grow lights.
Modern LEDs convert more of their electrical input into usable plant light and produce considerably less direct radiant heat around the canopy.
For most of the year, that makes environmental control easier.
In winter, however, it can create a heat gap.
Cold intake air enters the grow room, extraction continually removes warm air and an unheated garage, loft, spare room or outbuilding can draw even more heat away from the tent.
The result is a setup where the lights are running but the growing environment is still colder than you want it to be.
That’s why growers searching for how to heat a grow tent with LED lights, how to increase heat in a grow room or how to keep a grow tent warm in winter often end up looking at tube heaters, oil-filled radiators and electric fan heaters.
Those all have their place, but if you mainly need additional warmth during the lights-on period, adding supplementary lighting can make a lot of sense.
How to Increase Heat in a Grow Tent Without Just Adding a Heater
If your LED grow tent is only a few degrees below where you want it, you don’t necessarily need to install a large heater.
A 315W CDM grow light produces noticeably more radiant warmth around the growing area than a typical modern LED fixture while still turning that electrical input into useful horticultural light.
That distinction is important.
A 315W electric heater uses approximately 315W to generate heat.
A 315W CDM also uses approximately 315W, but you’re getting radiant heat plus an additional high-intensity grow light.
CDM isn’t magically cheaper to run watt-for-watt. The advantage is that the additional electrical load is doing two useful jobs instead of one.
If your room needs more warmth and your plants can make use of additional light intensity, that can make CDM a much more attractive winter addition.
Why Use CDM with LED Grow Lights?
CDM – also commonly referred to as CMH or Ceramic Metal Halide – produces a broad, natural-looking spectrum that makes it particularly interesting as a supplementary grow light.
The Omega 315W 3000K CDM Grow Light Kit produces high-intensity full-spectrum light at around 1.9µmol/W, while its spectral output also includes UVA and UVB wavelengths.
Modern LEDs still offer greater outright electrical efficiency, which is why we’d keep LED as the main lighting source in most modern grow rooms.
But that isn’t really the point of adding CDM.
LED and CDM complement each other.
Your LEDs provide efficient primary lighting, while the CDM adds extra intensity, a different spectral profile and useful radiant heat during the part of the year when that warmth is actually beneficial.
That’s why CDM remains such a useful option for supplemental grow lighting in 2026.
The Big Benefit: Quality, Aroma & Essential Oil Production
If all you needed was heat, you could simply buy a heater.
The reason CDM remains interesting is what else it brings to the grow room.
Its broad-spectrum output and additional UV content have made CDM particularly popular with growers focused on crop quality, aroma, flavour and essential oil production.
That’s one of the reasons experienced growers continued using CDM as supplementary lighting even after converting their main grow rooms to LED.
You retain the efficiency and intensity of your existing LED setup, then introduce another type of light to broaden the overall spectral output hitting the canopy.
For flowering and fruiting plants where the goal is to push quality as well as overall light intensity, CDM still has plenty to offer.
Running a 1.2m x 2.4m Grow Tent? This Setup Makes Perfect Sense
A 1.2m x 2.4m grow tent is probably one of the clearest examples of where supplementary CDM works.
A common lighting setup in a tent this size is two LED fixtures positioned side-by-side across the 2.4m length.
That naturally leaves a central position between the two fixtures.
Put a 315W Omega CDM right down the middle.
LED → 315W CDM → LED
You’re now adding extra intensity through the centre of the canopy, broadening the overall spectrum created by your lighting system and introducing a useful source of radiant warmth directly into the growing area.
For winter, it’s a very tidy way to upgrade an existing two-light setup.
You don’t need to replace your LEDs and you don’t need to go back to running a large HID system as your main lighting.
You’re simply adding another 315W of useful horticultural lighting into the space between the fixtures you already own.
Why the Omega 315W 3000K CDM Works So Well Alongside LEDs
The Omega 315W 3000K CDM Grow Light Kit is particularly well suited to supplementary lighting alongside LEDs.
Its 3000K spectrum has a warmer spectral balance that works particularly well with flowering and fruiting plants, while the CDM technology adds a different spectral profile to the output of a typical modern LED fixture.
That makes it a strong winter addition if you’re trying to achieve three things at once:
- Increase overall light intensity
- Broaden the spectrum reaching the canopy
- Add useful radiant heat to the grow room
Rather than replacing your existing LED lighting, the Omega 315W CDM can simply be positioned alongside it as a supplementary source of light and warmth.
Radiant Heat Becomes an Advantage in Winter
For years, growers were trying to get heat out of their grow rooms.
That was one of the biggest advantages when LED technology started replacing traditional HID lighting.
Winter changes the calculation.
If your LED room is already struggling to maintain temperature, the warmth produced by a CDM lamp stops being wasted heat and starts becoming part of your environmental control strategy.
CDM produces more direct radiant warmth around the growing area than LED, helping warm the canopy and surrounding environment during the lights-on period.
This can be particularly useful in colder garages, lofts, spare rooms and outbuildings where incoming air temperatures can drop significantly through autumn and winter.
Instead of powering a heater and receiving nothing but heat, you’re adding warmth while increasing the amount of horticultural light available to the plants.
Our Take: Why We Still Rate CDM in 2026
We’ve been around CDM lighting for years, and this is a good example of an older technology finding a slightly different role rather than becoming obsolete.
We wouldn’t remove a good modern LED and replace it with CDM as the main lighting source.
But as a winter supplementary light, we still really rate it.
The big difference compared with simply putting another heater in the room is that you’re getting something horticulturally useful from the extra electrical load.
You can feel the radiant warmth coming from a CDM, but at the same time you’re putting another serious light source over the canopy.
In something like a 1.2m x 2.4m tent with two LEDs already installed, placing one CDM between them is probably our favourite way of using this technology now.
It fills the centre of the lighting footprint, introduces a different spectrum and gives you that extra bit of winter heat without having to return to a full HID lighting setup.
Other Ways to Keep an LED Grow Tent Warm in Winter
Adding CDM isn’t the only thing you should consider if your grow room is struggling with cold temperatures.
Before adding more electrical heating, make sure you’re not unnecessarily losing the heat you already have.
- Run your lights overnight – use the lighting during the coldest part of the day and schedule the dark period during warmer daytime hours.
- Don't over-extract – removing more air than necessary also removes valuable heat. Maintain enough extraction for temperature, humidity and fresh-air control without running excessively hard.
- Insulate cold floors – this is particularly important if the tent is positioned directly on concrete.
- Use warmer intake air – drawing air from a conditioned indoor room can make temperature management easier than pulling freezing air directly from outside.
- Maintain good circulation – move warm air evenly around the growing area to reduce colder pockets.
- Monitor minimum temperatures – check temperatures around canopy height during both lights-on and lights-off periods.
Getting those basics right can significantly reduce the amount of additional heating required.
When Should You Use a Heater Instead?
CDM isn't a replacement for every grow room heater.
The biggest limitation is simple: the CDM can only provide useful heat while your plants are supposed to be receiving light.
If your room is already warm enough during the lights-on period but temperatures drop sharply when the lights switch off, you’ll still need another form of thermostatically controlled heating or a warmer surrounding environment.
You also need to consider the amount of light already reaching your canopy.
If your plants are already receiving as much light intensity as they can productively use, adding another 315W purely for warmth isn’t the right solution.
CDM works best when your LED grow room is too cold during lights-on and there is still room to increase overall lighting intensity.
FAQ: Heating a Grow Tent with LED Lights
How can I make my LED grow tent warmer?
Start by reducing unnecessary heat loss. Run your lights during colder overnight hours, insulate cold floors, avoid excessive extraction and, where possible, draw intake air from a warmer surrounding room. If you still need additional lights-on heat, adding a 315W CDM grow light alongside your LEDs gives you radiant warmth and additional horticultural light at the same time.
Do LED grow lights heat a grow tent?
Yes. LED grow lights still produce heat, but modern fixtures generate less direct radiant heat around the canopy than traditional HID lighting. This is one reason LED grow rooms can become more difficult to keep warm during a UK winter.
Can I use CDM and LED grow lights together?
Yes. In fact, supplementary lighting is one of our favourite modern uses for CDM. Your LEDs remain the efficient primary lighting source while the CDM adds extra intensity, a broader spectral profile, UV wavelengths and useful radiant heat.
Is CDM good for a cold grow room?
It can be an excellent option when a grow room needs additional warmth during the lights-on period. A 315W CDM produces useful radiant heat while simultaneously adding full-spectrum horticultural lighting.
What's the best way to add heat to a 1.2m x 2.4m LED grow tent?
If you're already running two LED fixtures and the room is slightly too cold during lights-on, adding a 315W CDM between the two LEDs can be a very effective option.
It adds extra central canopy intensity, broadens the overall spectrum and introduces useful radiant warmth into the growing area.
Is CDM cheaper to run than a heater?
Not watt-for-watt. A 315W electrical load is still approximately a 315W electrical load.
The advantage of using CDM is that the electricity produces useful grow light as well as heat, whereas a conventional electric heater only provides heat.
Is CDM the same as CMH?
The terms CDM, CMH and Ceramic Metal Halide are commonly used to describe the same general type of ceramic-discharge horticultural lighting technology.
Can a CDM replace a grow room heater?
Sometimes, but not always. If your room only needs a modest temperature increase during the lights-on period, the radiant heat from a CDM may be enough to help bring temperatures back into range.
If your main temperature problem occurs during the dark period, you'll still need another form of heating because the CDM shouldn't be running when the plants are meant to be in darkness.
Use the Winter Cold to Your Advantage
For most of the year, growers are trying to remove heat from their grow rooms.
Winter changes the calculation.
If your LED grow tent is running too cold and you're already considering adding a heater, it's worth looking at what an additional 315W of CDM could do instead.
You get useful radiant warmth to help keep the environment dialled in, additional light intensity, a broader spectrum and the quality benefits that have kept growers interested in CDM long after LEDs became the dominant lighting technology.
For a 1.2m x 2.4m setup running two LEDs in particular, placing a CDM between the two fixtures is one of the simplest ways to add more light and useful winter warmth at the same time.
