Heating, Ventilation, and Air Conditioning (HVAC) Systems
Chapter Introduction
HVAC systems in recreational vehicles (RVs) are integrated assemblies that provide thermal comfort and indoor air quality, regardless of outdoor conditions. Unlike residential buildings, RVs present unique constraints: limited space, restricted electrical supply (12 V DC or 120 V AC), variable thermal loads depending on movement, and lightweight insulation. This chapter covers fundamental principles, components, diagnostic procedures, load calculations, and applicable Canadian regulatory requirements for the Red Seal exam.
Thermodynamic Principles Applied to RVs
Heat Transfer
Heat always moves from the warmer medium to the colder one, through three mechanisms:
Conduction: transfer through a solid (RV walls, ducts). Thermal resistance (R-value) measures opposition to this transfer. A material with R-10 transmits half the heat of an R-5 material for the same thickness.
Convection: transfer through fluid movement (warm air rising, coolant circulating). In an RV, natural convection is often insufficient; fans are used to force circulation.
Radiation: electromagnetic transfer (sun through windows, warm walls). Solar radiation can add 30 to 50% of thermal load on an RV parked in the sun.
Refrigeration Cycle
The vapor-compression cycle is the fundamental principle behind all RV air conditioners. It has four stages:
12.Compression: the compressor draws low-pressure refrigerant vapor (approximately 0.3 MPa) and compresses it to high pressure (approximately 1.8 MPa), raising its temperature to 70–90 °C.
13.Condensation: the hot vapor circulates through the condenser; outdoor air (or water) absorbs the heat, and the refrigerant condenses into a high-pressure liquid.
14.Expansion: the expansion device (capillary tube or thermostatic valve) suddenly reduces the pressure, causing partial evaporation and cooling to approximately 0–5 °C.
15.Evaporation: the cold refrigerant absorbs heat from the indoor air in the evaporator, fully vaporizes, then returns to the compressor.
Key points for the exam:
The refrigerant absorbs heat during evaporation and releases it during condensation.
The low-side pressure must be checked with a manifold gauge; pressure that is too low indicates insufficient charge or a blocked filter.
Subcooling (liquid below its saturation temperature) and superheat (vapor above its saturation temperature) are indicators of correct charge: typical subcooling of 5–8 °C, superheat of 5–10 °C.
Refrigerants Used in RVs
| Refrigerant | Type | Common Use | GWP (Global Warming Potential) | Saturation Pressure at 25 °C (MPa) |
|---|
| R-134a | HFC | RV air conditioners (2000–2020) | 1430 | 0.67 |
| R-410A | HFC (blend) | Recent air conditioners | 2088 | 1.65 |
| R-1234yf | HFO | New systems | 4 | 0.68 |
| R-22 | HCFC | Older systems (phased out) | 1810 | 1.04 |
Golden rule: Never mix refrigerants. A system designed for R-134a cannot accept R-1234yf without complete component replacement (seals, oil, expansion valve). Always check the manufacturer's label.
Heating Systems
Propane Furnaces
The propane furnace is the most common heating system in RVs. It operates independently of site electricity (except for the fan).
Main components:
Burner: mixes propane with primary air; combustion produces CO₂ and water vapor.
Heat exchanger: separates combustion gases from circulating air. A crack in the heat exchanger can introduce carbon monoxide (CO) into the living space — a deadly hazard.
Circulation fan: forces air through the heat exchanger and into the ducts.
Thermocouple or thermopile: generates an electric current (25–30 mV) to keep the gas valve open. If the flame goes out, the thermocouple cools and shuts off the gas.
Electronic controller: manages ignition, ventilation sequence, and safety features.
Typical operating sequence:
33.The thermostat calls for heat (closed contact).
34.The controller activates the combustion fan (pre-purge of 15–30 seconds).
35.The igniter (spark or hot surface) activates; the gas valve opens.
36.The flame is detected by the flame-sensing rod (flame rectification); otherwise, the controller locks out the system after 3 attempts.
37.The circulation fan starts after 30–60 seconds (fan-on delay).
38.When the temperature is reached, the gas shuts off; the fan continues for 60–90 seconds (fan-off delay) to remove residual heat.
CSA B149.1 requirements (Natural Gas and Propane Installation Code):
Rule 5.20: gas appliances in recreational vehicles must be installed in accordance with the manufacturer's specifications and CSA Z240.10.1 (RV installation requirements).
Rule 6.2: the vent must be installed to prevent the entry of precipitation and to ensure adequate draft.
Rule 8.2: gas piping must be copper type K or L, or black steel, with approved fittings; joints must be accessible for inspection.
Each appliance must have an individual shut-off valve (Rule 6.8) and a pressure regulator (11 inches of water column, or 2.7 kPa, for propane).
Consumption calculation: A typical RV has a 30,000 BTU/h (8.8 kW) furnace. Propane has a heating value of approximately 25,000 BTU/L (or 91,500 BTU/gal). For one hour of continuous operation: 30,000 ÷ 25,000 = 1.2 L/h. A 30 lb (13.6 kg) cylinder contains approximately 30 L of liquid propane, providing approximately 25 hours of continuous operation.
Electric Heating
Newer RVs often offer electric heating as an option (baseboard or convection heaters). Advantages: no combustion, no CO emissions, quiet operation. Disadvantages: requires 120 V AC hookup (30 A or 50 A), high consumption (1,500 W per element).
Electrical load calculation: An RV park with a 30 A, 120 V hookup provides 3,600 W (30 A × 120 V). If the electric water heater (1,500 W), refrigerator (400 W), and converter (300 W) operate simultaneously, 1,400 W remains for heating. The Canadian Electrical Code (CE Code), Chapter V, Rule 8-200, requires that RV circuits be protected by circuit breakers; exceeding 80% of the continuous rated capacity is prohibited.
Radiant Heating (Radiant Floor)
Some high-end RVs use electric resistance heating mats under the floor. Control is via a thermostat with a floor sensor. Advantage: uniform comfort, no dust circulation. Disadvantage: slow response (30–60 minutes to reach temperature).
Air Conditioning Systems
Roof-Mount Air Conditioners
The roof-mount air conditioner is the most common type in RVs. It is mounted on a 360 mm × 360 mm (14 in × 14 in) opening in the roof, with a foam sealing gasket.
Components:
Roof unit: compressor, condenser, condenser fan, expansion device.
Interior unit: evaporator, evaporator fan, air filter, controls.
Roof trim: frame, gasket, air deflector.
Typical capacities: 13,500 BTU/h (3.9 kW) for medium-sized RVs, 15,000 BTU/h (4.4 kW) for larger units. Capacity is measured in BTU/h (British Thermal Units per hour); 1 BTU = 1,055 J. To convert to watts: BTU/h ÷ 3.412 = W.
Example: 15,000 BTU/h ÷ 3.412 = 4,396 W ≈ 4.4 kW.
Electrical requirements: A 15,000 BTU/h air conditioner draws approximately 12–13 A at 120 V AC. It must be on a dedicated 20 A circuit (CE Code, Chapter V, Rule 8-110). Never connect an air conditioner to a circuit with other loads.
Heat Pumps
Roof-mount heat pumps operate like a reversible air conditioner: in heating mode, the cycle reverses (the 4-way valve changes the refrigerant direction). The evaporator becomes the condenser and vice versa.
Advantages: high efficiency (COP of 2.5 to 3.5, meaning 2.5 to 3.5 W of heat per watt of electricity consumed), no combustion.
Limitations: reduced efficiency below 5 °C; most RV heat pumps stop functioning below 0 °C. A backup heat source (electric or propane) is required.
Quick diagnostic: If the heat pump blows cold air in heating mode, check the 4-way valve (24 V AC supply, coil continuity) and the thermostat (correct position).
Portable and Window Air Conditioners
Less common in new RVs, but present in budget models. They are less efficient (poor sealing) and consume more energy for the same capacity. Their installation must comply with the manufacturer's ventilation requirements.
Ventilation and Air Quality
Ventilation Requirements
CSA Z240.10.1 (Installation Requirements for Recreational Vehicles) specifies minimum ventilation rates:
Natural ventilation: at least 4% of the floor area must consist of openable windows or skylights (Rule 5.2).
Mechanical ventilation: a range hood must exhaust at least 100 ft³/min (2.8 m³/min) to the outside (Rule 5.4).
Bathroom ventilation: an exhaust fan of at least 50 ft³/min (1.4 m³/min) is required if the bathroom has no openable window (Rule 5.6).
Air change calculation: For an RV measuring 8 m × 2.5 m × 2.5 m (volume = 50 m³), an air change rate of 0.35 air changes per hour (ASHRAE 62.2 recommendation for living spaces) requires a flow rate of 50 × 0.35 = 17.5 m³/h ≈ 10 ft³/min.
Carbon Monoxide and Propane Detectors
The Canadian Electrical Code, Chapter V, and CSA Z240.10.1 require:
CO detector: installed in the sleeping area, within 3 m of each bedroom door (Rule 6.3 of CSA Z240.10.1). It must comply with CSA 6.19-01.
Propane detector: installed at floor level (propane is heavier than air), near the kitchen and furnace. It must comply with CSA 6.19-01.
Functional test: Press the test button; the alarm must sound within 5 seconds. Detectors have a lifespan of 5 to 7 years; check the manufacturing date.
Humidity and Condensation
Condensation on windows is a common problem in RVs. Causes: humidity generated by breathing (0.4 L/h per person), cooking, showers; insufficient insulation; inadequate ventilation.
Solutions:
Open skylights and windows during cooking and showers.
Use an electric dehumidifier (capacity of 10–20 L/day).
Ensure furnace vents are not obstructed (warm air carries more moisture).
Dew point calculation: If indoor air is at 22 °C with 60% relative humidity, the dew point is approximately 14 °C. If the window temperature is 10 °C, condensation will form. The approximate formula: Td = T - (100 - RH)/5, where Td is the dew point in °C, T is the air temperature, and RH is the relative humidity in %. Example: 22 - (100 - 60)/5 = 22 - 8 = 14 °C.
Diagnostics and Troubleshooting
Diagnostic Tools
Manifold gauge set: for measuring low and high pressures in the refrigeration circuit.
Infrared thermometer: for checking line temperatures (superheat, subcooling).
Clamp meter: for measuring compressor current (excessive current indicates a worn compressor or dirty condenser).
Electronic leak detector: for locating refrigerant leaks.
Combustion analyzer: for checking CO₂ and CO in furnace exhaust gases.
Systematic Diagnostic Procedure
94.Visual inspection: look for oil leaks, loose connections, blocked filters, obstructed vents.
95.Electrical check: measure supply voltage (120 V AC ± 10%), check fuses and circuit breakers, test start capacitors (a faulty capacitor prevents the compressor from starting).
96.Pressure check: connect the manifold, measure pressures at rest (equalization) and during operation.
97.Interpretation:
| Symptom | Low Pressure | High Pressure | Probable Cause |
|---|
| Insufficient cooling | Low | Low | Insufficient charge, leak |
| Insufficient cooling | Low | High | Blocked expansion valve, clogged filter |
| Insufficient cooling | High | High | Dirty condenser, faulty fan |
| Insufficient cooling | High | Low | Worn compressor (internal valves) |
| Evaporator freezing | Low | Normal | Low refrigerant, reduced airflow |
Refrigerant Recovery Procedure
CSA B52 (Refrigeration Systems Installation Code) and Environment Canada's Halocarbon Regulations require:
101.Recover all refrigerant before opening the circuit (Rule 5.4 of CSA B52).
102.Use a certified recovery unit (compliant with CSA C743).
103.Weigh the recovered refrigerant; the quantity must match the system's nameplate charge.
104.Never release refrigerant into the atmosphere — fines can reach $1,000,000 for individuals and $500,000 for businesses (Canadian Environmental Protection Act).
Propane Furnace Troubleshooting
Symptom: furnace will not ignite
107.Check propane supply: open the cylinder valve, check the regulator (outlet pressure 11 inches of water column).
108.Check the thermostat: contact closed (measure continuity).
109.Check the thermocouple: generates 25–30 mV; if less than 10 mV, replace.
110.Check the igniter: spark present? Hot surface glowing?
111.Check the controller: LED blinking? Error codes (see manufacturer's manual).
Symptom: blowing cold air
113.Check that gas is reaching the burner (smell of gas? — if yes, shut off immediately and ventilate).
114.Check the flame: blue and stable? A yellow flame indicates incomplete combustion (lack of primary air).
115.Check the circulation fan: is it running at the correct speed?
Symptom: smell of gas
117.Immediately shut off the gas supply.
118.Ventilate the RV (open all windows and skylights).
119.Do not operate any electrical switches (risk of spark).
120.Locate the leak with a soap solution (bubbles indicate the leak).
121.Repair or replace the faulty component; purge the line before relighting.
Thermal Load Calculations
Simplified Method for RVs
The thermal load of an RV is estimated by the sum of heat gains:
Q_total = Q_walls + Q_windows + Q_infiltration + Q_occupants + Q_equipment
Q_walls = U × A × ΔT, where U = 1/R (thermal conductance in W/m²·K), A = surface area in m², ΔT = indoor/outdoor temperature difference in K.
Q_windows = U_window × A_window × ΔT + solar gain (300–500 W/m² for a south-facing window).
Q_infiltration = 0.34 × V × N × ΔT, where V = interior volume in m³, N = air change rate in air changes per hour.
Q_occupants = 100 W per person (sensible heat) + 40 W per person (latent heat).
Q_equipment = sum of appliance power ratings (refrigerator, lighting, etc.).
Complete example:
RV measuring 8 m × 2.5 m × 2.5 m, walls R-5 (U = 0.2 W/m²·K), total wall area = 2 × (8 × 2.5) + 2 × (2.5 × 2.5) = 40 + 12.5 = 52.5 m². Windows: 3 m², U = 2.8 W/m²·K. ΔT = 30 °C (outdoor 35 °C, indoor 22 °C). Volume = 50 m³, N = 0.5 air changes/h. 2 occupants. Equipment: 500 W.
Q_walls = 0.2 × 52.5 × 30 = 315 W
Q_windows = 2.8 × 3 × 30 + 400 × 3 = 252 + 1,200 = 1,452 W
Q_infiltration = 0.34 × 50 × 0.5 × 30 = 255 W
Q_occupants = 2 × 100 = 200 W
Q_equipment = 500 W
Q_total = 315 + 1,452 + 255 + 200 + 500 = 2,722 W ≈ 9,290 BTU/h
A 13,500 BTU/h (3,956 W) air conditioner is therefore sufficient with a 45% safety margin.
Unit Conversions
| Unit | Equivalent |
|---|
| 1 BTU/h | 0.293 W |
| 1 W | 3.412 BTU/h |
| 1 ton of refrigeration | 12,000 BTU/h = 3.517 kW |
| 1 kPa | 0.145 psi |
| 1 inch of water column | 0.249 kPa |
| 1 L of propane | 25,000 BTU (heating value) |
Canadian Regulatory Requirements
Canadian Electrical Code, Chapter V
The CE Code, Chapter V, applies to electrical installations in RVs. Key rules:
Rule 8-200: branch circuits must be protected against overcurrent; continuous load must not exceed 80% of the circuit breaker's rated capacity.
Rule 8-110: each fixed appliance (air conditioner, furnace, water heater) must have an individual branch circuit.
Rule 8-300: conductors must be sized for the load; a 15 A air conditioner requires a minimum 14 AWG copper conductor and a 20 A circuit breaker.
Rule 8-400: receptacles must be of the locking type (NEMA TT-30R for 30 A, NEMA 14-50R for 50 A).
Rule 8-500: wiring must be protected against mechanical damage; use conduits or armored cables in accessible areas.
CSA B149.1 (Natural Gas and Propane)
Rule 5.20: installation of appliances in RVs — compliance with CSA Z240.10.1.
Rule 6.2: vents — minimum diameter, outdoor termination, 300 mm clearance from windows and air intakes.
Rule 6.8: shut-off valves — accessible, near each appliance.
Rule 8.2: piping — copper type K or L, or black steel; test pressure of 500 kPa (5 bar) for 30 minutes without pressure drop.
CSA Z240.10.1 (RV Installation)
Rule 4.2: appliances must be CSA or ULC certified.
Rule 5.2: natural ventilation — 4% of floor area in openings.
Rule 6.3: CO and propane detectors — location and operation.
Rule 7.1: heating ducts must be metal or non-combustible material, with a minimum clearance of 25 mm from combustible materials.
CSA B52 (Refrigeration Systems)
Rule 4.3: systems containing more than 50 g of refrigerant must be equipped with overpressure protection devices.
Rule 5.4: mandatory refrigerant recovery before any service work.
Rule 6.2: refrigerant lines must be protected against mechanical damage and corrosion.
Pitfalls to Avoid
165.Confusing saturation pressures: R-134a and R-410A have very different pressures at the same temperature. A manifold calibrated for R-134a can be used for R-410A, but readings will be inaccurate if the scale is not appropriate. Always check the manifold label.
166.Forgetting to purge the gas line: After working on propane piping, purge with nitrogen or gas before relighting. An air pocket can prevent ignition and cause repeated attempts that lock out the controller.
167.Neglecting the total electrical load: An RV with a 30 A hookup cannot simultaneously run the air conditioner (13 A), water heater (12 A), and refrigerator (3 A) — total 28 A, but the 80% rule limits to 24 A. The main breaker will trip.
168.Confusing superheat and subcooling: Superheat is measured at the evaporator outlet (vapor), subcooling at the condenser outlet (liquid). High superheat (over 15 °C) indicates low refrigerant; low subcooling (under 3 °C) indicates an overcharged system.
169.Ignoring error codes: Modern furnace and air conditioner controllers have LED indicators that blink in code patterns. Always consult the manufacturer's manual before replacing components.
170.Using an inappropriate leak detector: Electronic detectors for R-134a do not always detect R-1234yf. Use a universal detector or one specific to the system's refrigerant.
171.Forgetting the gas line pressure test: After any repair, perform a pressure test at 500 kPa for 30 minutes. A pressure drop indicates a leak — do not proceed with ignition.
172.Not checking the start capacitor: A compressor that hums without starting is often due to a faulty capacitor. Measure the capacitance with a multimeter (it must be within the rated range ± 10%).
173.Confusing thermostat types: A propane furnace thermostat operates on 12 V DC; an air conditioner thermostat operates on 24 V AC. Do not interchange them.
174.Underestimating the importance of sealing: A faulty roof gasket around the air conditioner causes water infiltration and cooling loss. Always inspect the gasket during maintenance.
Summary
The vapor-compression refrigeration cycle is the foundation of all RV air conditioners: compression, condensation, expansion, evaporation.
Refrigerants R-134a, R-410A, and R-1234yf have different pressures and characteristics; never mix them.
The propane furnace uses a thermocouple (25–30 mV) for flame safety; a cracked heat exchanger is a deadly hazard (CO).
Minimum ventilation is 4% of floor area in openings (CSA Z240.10.1, Rule 5.2).
CO and propane detectors are mandatory in sleeping areas and near combustion appliances.
Thermal load calculations use the formula Q_total = Q_walls + Q_windows + Q_infiltration + Q_occupants + Q_equipment; an air conditioner should have a 20–30% margin.
CE Code, Chapter V, Rule 8-200, limits continuous load to 80% of the circuit breaker capacity.
Refrigerant recovery is mandatory (CSA B52, Rule 5.4); penalties for release are severe.
Systematic diagnostics (visual, electrical, pressures) prevent unnecessary component replacements.
Unit conversions (BTU/h ↔ W, kPa ↔ psi, inches of water column ↔ kPa) are essential for exam calculations.
Exam Tips
Memorize the saturation pressures of common refrigerants at 25 °C: R-134a ≈ 0.67 MPa, R-410A ≈ 1.65 MPa, R-22 ≈ 1.04 MPa.
Learn the furnace operating sequence (pre-purge, ignition, flame detection, fan delay) — this is a frequent question.
Practice unit conversions without a calculator: 1 BTU/h ≈ 0.293 W, 1 ton = 12,000 BTU/h.
Know the key rule numbers: CE Code 8-200 (80%), CSA B149.1 5.20 (RVs), CSA Z240.10.1 5.2 (ventilation), CSA B52 5.4 (recovery).
For diagnostic questions, first identify the symptom (low/high pressure, temperature), then the probable cause — use the diagnostic table in this chapter.
Never confuse 12 V DC circuits (lighting, furnace thermostats) with 120 V AC circuits (air conditioner, receptacles) — wire colors and voltages are different.
