A heat pump is the cheapest heat on the market. It is also, in an RV, the one that stops working first — and it stops at a temperature most buyers would describe as merely chilly.
The number is around 40°F. Below that, output falls off a cliff. Somewhere in the low-to-mid 30s the unit spends more time defrosting itself than heating you. This is not a defect, not a brand problem, and not something a better thermostat fixes. It is a direct consequence of bolting a heat pump to a vehicle roof, and once you understand why, the correct way to specify one becomes obvious.
What a heat pump actually does
It doesn’t make heat. It moves it.
Heat energy exists in all air above absolute zero, so even at 40°F there is usable heat in the outside air — a heat pump extracts it, concentrates it with a compressor, and releases it indoors. That’s why it beats every combustion appliance on running cost: you’re paying to relocate energy rather than to create it.
The measure of how well it does this is coefficient of performance, or COP. A COP of 2 means two units of heat delivered for every one unit of electricity consumed. Resistance heating is fixed at 1.0 by definition. A heat pump at COP 2 halves your heating bill relative to a plug-in heater.
The catch is that COP is not a constant. It falls as the outside air gets colder, because there is less heat available and the compressor has to work harder against a bigger temperature difference. At around 40°F, a typical RV rooftop unit produces roughly 2 kW of heat per 1 kW of electricity; as the temperature drops, that ratio falls until the unit can no longer hold the setpoint.
At some point COP approaches 1, at which stage you are running an expensive, noisy, complicated resistance heater. Well before that point, the practical problems arrive.
Where RV rooftop units actually stop
The consensus among owners and manufacturers is consistent enough to treat as settled. Most RV rooftop heat pumps, including the common Coleman Mach units, stop delivering efficient heat below about 40°F, at which point they either switch to an auxiliary source or become ineffective.
Manufacturer control logic backs this up. The Dometic manual specifies that auxiliary heat operation is triggered when the outdoor sensor reads below 30°F, and where a furnace is fitted, the control selects furnace mode as the auxiliary source. In other words: the manufacturer has already conceded the point and built the handover into the firmware.
Between roughly 40°F and 30°F, you’re in the degraded band. The unit still runs. It still produces some heat. But it loses efficiency rapidly and enters frequent defrost cycles as the outdoor coil freezes, and each defrost cycle means it briefly reverses — blowing cool air into the cabin while it melts ice off the outdoor coil.
Owners report running units into the mid-30s with poor results and being unable to start them at all in the low 20s. That matches the physics.
Why the floor is structural, not a tuning problem
Here is the part that explains why an RV heat pump performs so much worse in the cold than the one in your house.
A residential heat pump has a separate outdoor unit. Its own fan. Its own coil, sized generously. It sits on a raised pad so meltwater from the defrost cycle drains away and doesn’t refreeze around the base.
A rooftop RV unit has none of that, and the reasons are physical:
The fans share a motor. In a rooftop package, the indoor and outdoor fan blades are on the same motor shaft, so there is no way to run the outdoor fan independently during defrost. Residential units use that trick constantly. A rooftop unit can’t.
There is nowhere for the meltwater to go. Defrost melt on a roof has no drain path and will refreeze before it can run off the side of the vehicle. In sustained cold, an RV heat pump can build ice on the roof rather than shed it.
The coil is tiny. Roof-mounted equipment is constrained to a 14-inch opening and a low profile for clearance and drag. Coil surface area is what determines how much heat you can extract from cold air, and there simply isn’t room for more of it.
Put those three together and it is not practical to design a rooftop unit that works well in colder weather. It’s a packaging constraint. No firmware update, no refrigerant charge, no service call changes it.
The comfort problem nobody warns you about
Even inside its working range, a heat pump feels different, and this generates a steady stream of “my heat pump is broken” complaints that are nothing of the sort.
A propane furnace delivers air at roughly 130–140°F. A heat pump delivers air at typically under 100°F, which feels cool coming out of the vents.
Body temperature is 98.6°F. Air arriving at 95°F feels cold against skin even though it is warming the room. The furnace produces a small volume of genuinely hot air; the heat pump produces a larger volume of mildly warm air. Both raise the interior temperature. Only one feels like heating.
The practical consequence is that heat pumps are poor at recovery. If you arrive at a cold rig and want it comfortable in twenty minutes, the furnace does that and the heat pump does not. Where you have both, the sensible pattern is furnace to bring the space up, heat pump to hold it there.
The off-grid problem: it’s the same 1,400W appliance
This is where the enthusiasm usually dies.
A rooftop heat pump in heating mode draws essentially the same power it draws in cooling mode — the compressor is the same compressor, running the same cycle in reverse. For a 13,500 BTU class unit, that’s somewhere around 1,300–1,500W.
At COP 2, 1,400W of input yields around 9,500 BTU per hour of heat. That’s genuinely efficient. It’s also still 1,400W, and 1,400W from a battery bank through an inverter is around 1,600W at the terminals.
Twelve hours of a cold night at 50% duty comes to roughly 9.6 kWh — which is more than a 700Ah 12V lithium bank holds. The arithmetic is essentially identical to running air conditioning on battery, and it fails for the same reason: the recharge, not the storage, is the binding constraint. In December, with short days and a low sun angle, your array will not refill that.
So be clear about what a rooftop heat pump is for:
It is a shore-power appliance that saves propane. It is not an off-grid heating system.
Used that way it’s excellent. In a campground with electricity included in the site fee, its marginal cost is zero and it will preserve your propane for cooking and hot water across an entire shoulder season. That is a real and worthwhile benefit. It just isn’t the benefit it gets sold as.
For anyone comparing running costs across the alternatives properly, the cost-per-BTU comparison puts the heat pump at the top of the table when it works, and off the table entirely when it doesn’t.
Cabins are a different machine class
If you’re heating a fixed structure rather than a vehicle, everything above stops applying — because you can install equipment that solves each of the constraints.
Cold-climate mini-splits are rated for continuous operation well below 0°F, with published capacity and COP data at those temperatures. They achieve it through variable-speed compressors, vapour injection, much larger outdoor coils, dedicated outdoor fans, base-pan heaters and elevated mounting with proper drainage. Every problem the rooftop unit has, this equipment addresses with hardware.
Three things to get right when specifying one:
Size at your design temperature, not the nameplate. A unit rated at 12,000 BTU is rated at 47°F outdoor. At 5°F it might deliver 7,000. The manufacturer publishes capacity tables at multiple outdoor temperatures — use them. A system sized on the nameplate figure will be badly undersized on the coldest night of the year, which is precisely the night it matters.
Specify the backup explicitly. Every heat pump installation should have a defined answer to “what happens below the design temperature, and what happens during a power cut?” A wood stove, a propane heater or a diesel air heater all serve. What doesn’t serve is finding out in January that there wasn’t a plan.
Consider DC-native units if you’re off-grid. A growing category of 48V DC mini-splits runs directly from a battery bank without an inverter, in a power class well below rooftop RV units. For a solar-powered cabin, this changes the calculation substantially — though the same low-temperature capacity questions apply and should be asked of the specification sheet.
What a heat pump is genuinely good at
Having spent this article on the limitations, the honest counterweight:
Shoulder season is its home ground. Between about 45°F and 65°F, nothing beats it. Cheap, quiet, no propane consumed, no combustion, no carbon monoxide risk and no moisture added to the interior — which, in a vehicle, is worth more than people realise.
It dehumidifies. Unvented propane appliances add roughly a gallon of water to your interior per gallon of fuel burned. A heat pump does the opposite. In a damp shoulder-season climate, that alone can justify it.
It’s dual-use hardware. One rooftop box, two seasons. If you were buying an air conditioner anyway, the heat pump variant typically costs a modest premium and adds a genuinely useful capability for two or three months of the year.
It has no fuel logistics. No bottles to fill, no tank to tap, no fuel to carry.
How to specify one honestly
Four questions, answered before you buy:
- What’s the coldest temperature you’ll realistically be in? If the answer is above 45°F, a heat pump alone is fine. Below 40°F, it is a supplement.
- Will you be on shore power? If not, it’s a supplement regardless of temperature.
- What’s the backup, and does the changeover happen automatically? Some RV thermostats hand over to the furnace without intervention; some don’t. Find out which you have.
- What have you done about the envelope? A heat pump in a well-sealed, well-insulated rig holds temperature comfortably at outdoor conditions where the same unit in a leaky one gives up. Insulation moves the floor down far more cheaply than better equipment does.
For full-timers, the heat pump’s value shows up in the utilities line rather than the purchase price — a shoulder season on electricity included in a site fee instead of on bottled propane is a meaningful monthly difference, and it’s one of the costs that rarely gets itemised properly before people commit to the lifestyle.
Frequently asked questions
At what temperature does an RV heat pump stop working? Useful output falls off around 40°F and becomes marginal in the mid-30s. Manufacturer controls commonly hand over to auxiliary heat below 30°F. The unit will often still run below that; it just won’t hold the setpoint.
Is a heat pump better than a propane furnace? On shore power in mild weather, yes — significantly cheaper and quieter. In cold weather or off-grid, no. Most rigs benefit from having both and using each in its range.
Why does my heat pump blow cool air? Two reasons. Supply air at under 100°F feels cool against skin even while warming the room, and defrost cycles briefly reverse the unit to melt ice off the outdoor coil. Both are normal.
Can I run a heat pump off solar and batteries? A rooftop unit, not realistically — roughly 1,400W of draw means about 9–10 kWh across a cold night, more than most banks hold and more than a winter array will replace. A small DC mini-split in a well-insulated cabin is a different proposition.
Will running a heat pump below 40°F damage it? Generally no, though sustained operation in freezing conditions can cause ice build-up on a roof-mounted unit. The more common outcome is simply that it fails to keep up and you switch to another heat source.
Do cabin heat pumps work in genuinely cold climates? Yes. Cold-climate mini-splits are rated for continuous operation well below 0°F. They are a different class of equipment from anything mounted on an RV roof, and they must be sized against capacity at your design temperature rather than the headline rating.