--- title: "Why One Room in an RV Stays Cold With the Furnace On" description: "A rear bedroom stayed cold for two winters while the furnace ran normally. Where ducted heat gets lost, and how to measure each register against the plenum." focus_keyword: "why one room in an rv stays cold with the furnace running" canonical: https://ocrv.me/blog/the-furnace-that-ran-and-never-warmed-the-bedroom/ kind: blogPost updated: 2026-07-29 source: OCRV Center --- # The Furnace That Ran and Never Warmed the Rear Bedroom > A furnace that lights, runs and satisfies its thermostat can still deliver almost nothing to one room. The fault is usually **the duct run, not the appliance**, and **a probe thermometer finds it**. ## Why does one room in an RV stay cold while the furnace runs? Because the duct serving that room is losing its air before it arrives. A separated floor boot, a crushed flex run or a blocked return will all starve one register while the rest of the unit heats normally. We measure every register against plenum output in our Yorba Linda bays before touching the furnace itself. - Compare each register temperature against the plenum reading - A separated floor boot dumps heat into the belly cavity - A closed door with no return grille chokes the whole run - Short cycling on the limit switch is a symptom, not a cause ### Key facts | Item | Value | | --- | --- | | Unit | 38 foot destination fifth wheel | | Plenum output measured | 128 degrees Fahrenheit | | Rear bedroom register | 79 degrees Fahrenheit | | Mechanical and electrical labor | $260 per hour | | Cause found | Floor boot separated from the duct in the belly | Two winters of a cold bedroom had produced two visits somewhere else, a replaced thermostat and a general sense that the furnace was undersized. The furnace was not undersized. It was a Suburban NT series unit doing exactly what it was designed to do, and it was heating the space under the floor extremely well. Ducted heating faults on a towable are almost never in the appliance. They are in the twenty odd feet of thin wall flex duct that runs through the belly cavity between the furnace plenum and the last register, held together by tape, staples and optimism. Here is how to find where the air goes, using a thermometer and about forty minutes. ## Four readings that ended the guessing The furnace ran for eight minutes to stabilize, then we read four registers with a probe thermometer inserted into the outlet rather than held above it. Plenum output at the furnace was 128 degrees. The living area register measured 121. The mid unit register measured 114. The rear bedroom register measured 79, in a coach where the ambient interior temperature at the time was 68. Eleven degrees above room temperature at the last register is not a heating system. It is a small volume of air that has been in an unheated space long enough to give up most of its energy. A drop of seven degrees from plenum to the nearest register is expected. A drop of forty nine degrees over one run means the air is either not arriving or is arriving mixed with cold air from somewhere it should not be touching. ## How the ducting is actually built A furnace sits in a cabinet or a bench base with a sheet metal plenum on its outlet. From there, thin wall flexible duct, typically four inch, runs through the floor cavity or along the underside of the floor inside the coroplast belly wrap. Each register has a sheet metal boot that passes through the floor deck, and the flex duct clamps or tapes onto the boot from below. Nothing about that assembly is welded, bolted or inspected after the unit leaves the factory. It is a taped joint at the bottom of a boot, in a cavity that vibrates for every mile the unit travels, in a space that is neither heated nor sealed against outside air. The single most common failure we find is a boot that has shaken free from its duct, and it is invisible from inside and from outside. - Thin wall four inch flex duct, taped or clamped at each boot - Boots pass through the floor deck and are sealed from below - The belly cavity is unheated and vented to outside air - Every joint lives on the underside of a vibrating floor ## The three ways a run loses its air First is separation. A boot pulls off the duct and the furnace heats the belly. The tell is a warm patch on the coroplast underneath, findable with a bare hand on a cold morning, and often a faint hot dust smell from the underside rather than from the register. Second is crushing. A run passes behind a cabinet base or through a tight floor pocket and something has stood on it, been screwed through it or simply pinched it during construction. Flex duct kinks at a small radius and stays kinked. That produces a register that blows warm air at low volume, which is different from the separation symptom where the volume is fine and the temperature is not. Third is return path. Warm air delivered to a closed room with no return grille has nowhere to go, so the whole system pressurizes and the furnace short cycles on its limit switch. The bedroom door being shut at night is enough to do this on some floor plans, and the owner experiences it as a furnace that runs constantly without heating. ## What was under the floor The rear section of coroplast came down. The bedroom boot had separated cleanly from its duct, and the duct end was lying about three inches away with the original foil tape still wrapped around it in a stiff ring. The tape had not torn. It had lost adhesion to the boot and slid off intact, which is what usually happens rather than a dramatic tear. Eleven feet of that same run was resting directly on the frame crossmember with no support, sagging into a shallow U between two members. A sagging run collects condensate at the low point in humid weather and it also loses heat to whatever it is touching. Neither of those two conditions was going to fix itself, and both had been there since the unit was new. > **note:** Foil tape on a vibrating duct joint is a five year part, not a permanent one. When a belly is open for any reason, spend the twenty minutes to check every boot joint that is reachable. It is the cheapest inspection available on a towable. ## What the repair involved The duct was trimmed back to clean material, reattached to the boot with a metal band clamp rather than tape alone, and then taped over the clamp as a secondary seal. The sagging span was supported with two nylon strap hangers off the crossmembers so it runs with a gentle downhill grade to the register rather than a low spot in the middle. The belly wrap went back with new fasteners and a taped seam. Retest after reassembly: plenum 129 degrees, rear bedroom register 112. That is a normal figure for the last register on a thirty eight foot run and it changed the room from unusable in winter to comfortable. Total time in the bay was under three hours including the belly work, at the mechanical and electrical rate. ## The test an owner can run in forty minutes You need a probe thermometer, the kind sold for cooking, and a notebook. Run the furnace with all registers open for ten minutes so temperatures stabilize. Then read each register in turn by pushing the probe an inch or two into the outlet, and read the one closest to the furnace last so you have a fresh reference. Write every number down with the register location. Any register more than about twenty five degrees below its nearest neighbour is the one to investigate. Then feel the belly wrap underneath that run with the back of your hand on a cold morning. A warm patch on the coroplast is a separated joint, and you have just located a repair to within a couple of feet without removing anything. 1. Run the furnace ten minutes with every register open 2. Read each register with a probe pushed into the outlet 3. Write down location and temperature for all of them 4. Flag any register far below its nearest neighbour 5. Feel the belly wrap under that run for a warm patch ## Questions and answers ### How much temperature drop between RV registers is normal? A few degrees per run is expected, and the register nearest the furnace should read close to plenum output. A drop of more than about twenty five degrees compared with the neighbouring register means that branch is losing air or heat. Forty degrees or more almost always means a separated joint under the floor. ### What does a warm patch on the belly wrap mean? It means heated air is escaping into the cavity instead of reaching a register. A boot that has separated from its duct is the usual cause, and foil tape losing adhesion is the usual mechanism. Feeling the coroplast with the back of your hand on a cold morning locates it within a couple of feet. ### Can closing a bedroom door stop an RV furnace from heating? On some floor plans yes. Warm air delivered into a sealed room with no return grille pressurizes the space, airflow through the whole system falls, and the furnace begins short cycling on its limit switch. Owners read that as a furnace running constantly without producing heat, which is exactly what is happening. ### Is foil tape enough to hold a duct to its floor boot? Not for the life of the unit. Tape on a joint that vibrates every mile eventually loses adhesion and slides off intact rather than tearing. A metal band clamp with tape over it as a secondary seal is the repair that lasts, and it costs a few dollars more than tape alone. ### Does a sagging duct run under the floor cause problems? Two of them. The low point collects condensate in humid weather, and duct resting directly on a steel crossmember bleeds heat into the frame. Supporting the run with strap hangers so it grades gently toward the register solves both, and it is worth doing any time the belly is already open. --- Source: [OCRV Center](https://ocrv.me/blog/the-furnace-that-ran-and-never-warmed-the-bedroom/). Last verified 2026-07-29.