How does a marquee heater actually work?
A diesel burner sits just outside the marquee wall, alight for hours, and inside, forty guests in cocktail dresses never smell a trace of it. That is not a filter doing clever work. It is a much simpler trick: the flame and the air you breathe never share the same pipe.
An indirect-fired heater burns diesel inside a sealed combustion chamber, transfers that heat through a metal exchanger into a completely separate stream of ambient air, and ducts only that clean warm air into the marquee. The combustion gases leave through their own flue and never enter the tent at all. That physical separation, not the size of the flame, is what makes indirect heaters the trade default for enclosed structures. Sizing runs at roughly 25kW per 100 sqm of lined marquee, with UK hire fleets spanning from around 15kW units up to 220kW or more for the biggest single heater.
It looks, from outside a marquee on a cold evening, like a fairly blunt piece of kit: a metal box, a growl of a fan, a duct running under the groundsheet. What makes it safe to run for hours next to several hundred guests is not obvious from the outside, and it is worth understanding, because it is the same reason a direct-fired heater, a perfectly good bit of kit on a building site, has no place inside a closed tent.
Two paths through the same box, never touching
An indirect-fired heater has, functionally, two separate journeys happening inside one casing. The first is combustion: diesel is fired into a sealed combustion chamber, where it burns and produces exhaust gas, carbon monoxide, carbon dioxide and the rest of what any flame produces. That gas has exactly one way out, a flue that vents it straight up into open air, entirely separate from anything heading into the marquee.
The second journey is the air a fan draws in from outside, ordinary ambient air with nothing added to it. That air is pushed across the outside surface of a heat exchanger, a finned metal barrier with the hot combustion chamber on one side and the cool intake air on the other. Heat crosses that barrier the way it crosses a saucepan wall, by conduction through the metal, not by the two air streams mixing. What comes out the other side is warm, and it is still just air: nothing from the flame has hitched a ride. That air is what goes down the duct into the marquee. Nothing else does.
Why a direct-fired heater has no place inside a closed marquee
A direct-fired heater is not a worse machine, it is built for a different job. It blows its supply air straight through the flame itself, so whatever the burner produces travels with the warm air: carbon monoxide chief among the concerns, alongside carbon dioxide and other combustion by-products. That is a reasonable trade on a construction site or in a genuinely open structure, where fresh air is moving through regardless and nobody is sitting still in one spot for hours. It stops being reasonable the moment you close the sides, fill the space with seated guests, and run it for an evening: exactly the conditions a wedding marquee creates, and exactly why UK hire suppliers offer indirect units for that job and not direct-fired ones. HSE guidance on carbon monoxide risk from fuel-burning equipment makes the same point in more general terms: the hazard is about enclosed, poorly ventilated spaces, not about any particular type of burner being inherently unsafe.
Sizing runs on floor area, not guest count
The trade starting point is roughly 25kW of indirect heat per 100 sqm of lined marquee, which for a 170 sqm marquee (around 120 guests banquet-style) points to a single 50kW unit or two 25kW units on independent thermostats for different zones. Leave the marquee unlined and that figure moves up sharply: canvas alone holds almost no heat in, while a full ceiling lining acts like an extra layer of glazing, trapping warm air that would otherwise escape straight through the roof. UK hire fleets for events span a wide range to cover this, from around 15kW units suited to a small marquee up to 220kW or beyond for the largest single heater on a big structure, which converts to a little under three-quarters of a million BTU per hour for anyone used to seeing heat output in that unit instead.
The thermostat is doing more work than the burner
Left permanently alight, a heater sized for a hard frost would cook a mild October evening and burn diesel for no benefit. The fix is the same one a house boiler uses: a thermostat, sited inside the marquee rather than on the heater casing, reads the actual air temperature where guests are and switches the burner on and off to hold it. That is why a good supplier positions the room stat somewhere representative, not tucked by the entrance where it never feels the room the way the dance floor does.
The running cost is diesel, not just the hire fee
A workable rule of thumb across the hire range is around 0.1 litres of diesel an hour for every kW of output. A 25kW heater burns roughly 2 to 3 litres an hour; an 85kW unit around 8; the largest 220kW units used on big marquees around 20 to 22. Run a single 25kW heater across a 12-hour event day and that is 25 to 35 litres of diesel on top of the unit hire fee, which is exactly the same question worth asking about a generator: is fuel included, or billed on return.
What this means for your event
Practically: work out your marquee's floor area first, because that is what the 25kW-per-100-sqm rule actually runs on, not the guest count on its own. The marquee size calculator gets you there, and the heating & cooling hire guide has current guide prices and what to ask a supplier before booking, particularly whether lining is included and whether fuel is metered on return.
Marquee heater engineering, questions answered
What is actually different between a direct-fired and an indirect-fired heater?
A direct-fired heater blows air straight through the flame, so whatever comes off the burner, carbon monoxide, carbon dioxide and other combustion by-products, goes out with the warm air. An indirect-fired heater keeps the flame sealed inside its own combustion chamber and only pushes air across the outside of a heat exchanger, so the air reaching you has picked up heat but never touched the flame or the exhaust gas. The difference is not how hot either gets, it is whether the air path and the combustion path ever mix.
Why can't a direct-fired heater just run with the marquee sides rolled up for ventilation?
Rolled-up sides help, but they do not reliably guarantee enough air exchange for however many guests, however still the evening air is, for however many hours the heater runs. Direct-fired units are the right tool for genuinely open structures and outdoor work where fresh air keeps moving through regardless, and for construction sites and drying-out buildings where nobody is inside for hours at a stretch. A marquee full of seated guests for an evening is exactly the enclosed, occupied, hours-long scenario that indirect heaters exist for, which is why UK hire suppliers do not offer direct-fired units for that job.
Where does the exhaust go on an indirect heater if it is not blown into the tent?
Out through its own flue, entirely separate from the ducting that carries warm air into the marquee. The heater sits outside the structure with two things leaving it: a duct of clean warm air going in one direction into the tent, and a flue venting combustion gases straight up into open air in the other. They never share a pipe.
How does the heater know when to stop working once the marquee is warm enough?
A thermostat inside the marquee, not on the heater itself, reads the actual air temperature where guests are and cycles the burner on and off to hold it. Left to run flat out with no thermostat, a heater sized for a cold snap would overheat a mild evening and burn diesel for no benefit, so the room stat is doing the same job a house boiler's thermostat does, just for canvas instead of brick.
How much diesel does heating a marquee for a weekend actually cost?
Roughly 0.1 litres an hour for every kW of output is a workable rule of thumb across the hire range, so a 25kW unit burns around 2 to 3 litres an hour, an 85kW unit around 8, and the largest 220kW units used for big marquees around 20 to 22. Run a 25kW heater for a 12-hour event day and that is 25 to 35 litres of diesel on top of the hire fee, which is why it is worth asking a supplier whether fuel is included or billed on return, the same question you would ask about a generator.
Now work out your marquee size
Guest count and layout in, marquee class out, which is what heating actually scales from. Then build the full kit list for your event.