Heat Exchanger Reference
Why this matters
Heat exchangers transfer heat between two fluids without mixing. Cracked furnace heat exchanger = CO leak into the home; failed evaporator coil = no cooling; failed boiler tube = no heat. The tech who knows heat exchanger types + failure modes diagnoses safely + correctly. This is the field card.
Heat exchanger types
Air-to-air: gas furnace combustion gases heat air stream. Refrigerant-to-air: AC evaporator/heat pump indoor coil (cools); outdoor condenser (rejects heat). Liquid-to-liquid: boiler hot water + system water; plate-frame OR shell-tube. Liquid-to-air: hydronic coil; hot water + room air. Refrigerant-to-liquid: chiller; water-cooled AC.
Gas furnace heat exchanger
Construction:
- Clamshell (atmospheric): stamped steel; older units; large surface area.
- Inshot tube + manifold: most modern; tubes carry combustion products.
- Stainless steel (condensing): handles low temperature acidic condensate; lifetime warranty common.
Function:
- Combustion gases heat metal walls
- Air flow on opposite side picks up heat
- Combustion products vent to outside
Cracked heat exchanger danger: CO leaks into supply air; serious health risk.
Inspection:
- Visual via inspection mirror + flashlight + bore scope
- Smoke test (with combustion analyzer)
- Pressure test (specialized)
- Cracks typically along weld seams OR thinned areas
Replacement: difficult + often not cost-effective; full furnace replace common when heat exchanger fails.
Evaporator coil (cooling)
Cold refrigerant absorbs heat from air.
Common types:
- A-frame (slant): two rows in inverted V shape
- N-coil: similar configuration
- Vertical: upflow OR downflow furnaces
- Cased OR uncased: pre-built box OR field-installed
Dimensions: typically 14" - 21" tall × 18" - 25" wide.
Fins: aluminum OR copper; tight (12 fins per inch typical).
Tubes: copper OR aluminum.
Failure modes:
- Refrigerant leak (formicary corrosion in some early all-aluminum coils)
- Bent fins (restrict airflow)
- Frost buildup (low refrigerant OR airflow)
- Dirty coil (poor heat transfer)
Condenser coil (cooling)
Hot refrigerant rejects heat to outdoor air.
Construction:
- Round OR rectangular outdoor cabinet
- Copper tubes wrap around
- Aluminum fins between tubes
- Fan blows air across
Fin damage: easily bent; restricts airflow; clean + comb periodically.
Cleaning: hose off dust + debris annually; chemical coil cleaner for buildup.
Common failures: leak at brazing joint, fin damage, scale buildup, fan motor failure.
Heat pump heat exchanger
Reversible cycle: indoor coil + outdoor coil swap roles.
Heating mode: outdoor coil = evaporator (absorbs heat from outside); indoor coil = condenser (releases heat indoors).
Cooling mode: opposite.
Reversing valve: switches refrigerant flow.
Defrost cycle: reverses temporarily to clear ice from outdoor coil; common winter.
Boiler heat exchanger
Cast iron: long-life; common older boilers.
Stainless steel (condensing): modern high-efficiency; resistant to condensate corrosion.
Aluminum: lighter; condensing boilers; sensitive to pH.
Fire-tube: smoke + flame in tubes; water around.
Water-tube: water in tubes; flame around.
Sealed combustion: outdoor air for combustion; sealed from house.
Common failures: sediment buildup, scale, corrosion (from improper water chemistry), cracked sections.
Hydronic coil (air handler)
Hot water OR steam heat passes through coil.
Air blows across: picks up heat.
Indoor air handler: same coil cabinet as cooling; sometimes shared.
Heat exchanger material: copper tubes + aluminum fins typical.
Water-heater heat exchanger
Direct-fired tank: burner heats tank water directly.
Tankless coil (built-in boiler heat exchanger): heats domestic water in passing.
Indirect-fired (tank with internal coil): boiler heats coil; coil heats tank water.
Plate heat exchanger
Stacked plates with channels: high surface area; compact.
Used for:
- Solar hot water exchange
- Boiler / DHW exchange
- Chiller / water-source heat pump
Common brands: Flat Plate, AIC, Bell + Gossett.
Sizing: per BTU/hr + temperature differential.
Shell + tube heat exchanger
Outer shell + internal tubes: industrial + commercial.
Used for: large hot water systems, chilled water systems.
Cleaning: tube brushes; pressure wash.
Common heat exchanger problems
Cracked furnace HX: CO leak; replace HX (rarely cost-effective) OR full furnace replace.
Frozen evaporator: low charge, dirty coil, bad fan; immediate shut down OR damage.
Dirty condenser: high head pressure; high amp; reduced cooling; over-pressure trip.
Boiler scale: reduces efficiency; flush + descale annually with hard water.
Aluminum coil corrosion: galvanic with dissimilar metal in water + acidic environment.
Hydronic coil leak: pinhole from pH OR oxygen + corrosion.
Inspection + maintenance
Annual (during tune-up):
- Visual inspect for cracks, corrosion, dirt
- Clean coils + fins
- Comb bent fins
- Pressure test if required
- Flush condensate drain
Combustion equipment specifically:
- Combustion analyzer measures CO + O2 (cracked HX reveals as elevated CO OR backdraft)
- Visual + flashlight + mirror inspection
- Some manufacturers require periodic HX inspection per warranty
Coil cleaning
Foaming cleaner (NuCalgon Evap-Pow'r, Frost King): spray + rinse.
Liquid alkaline cleaner: stronger; commercial use.
Rinse thoroughly: residue corrodes long-term.
Don't bend fins: combs straighten if bent.
Don't pressure-wash too high: damages fins.
Refrigerant leak repair (heat exchanger leak)
Find leak: electronic detector, UV dye, soap bubble.
Tiny pinhole: braze + seal; leak fix.
Crack: replace component (coil OR compressor).
Pre-fab kit: some coils replaceable as cassette.
Customer education
Annual maintenance = catch heat exchanger issues before failure.
CO detector required in homes with fuel-burning equipment.
Aging equipment: heat exchangers fail at ~ 15 - 25 year mark (lifespan).
Replace vs repair calculation: when HX cost > 50% of new equipment.
Truck stock for HX work
Coil cleaner spray, fin comb, flashlight + inspection mirror, borescope (specialty), pressure test kit (nitrogen + fittings), brazing supplies, refrigerant + recovery tank (for evac before HX work).
Common service-call scenarios
"AC not cooling": check evaporator coil (frozen? dirty?) + condenser coil (dirty? fan working?).
"Furnace short-cycling": check heat exchanger limit switch + airflow + venting.
"Burning smell from furnace": check heat exchanger for crack + combustion air + dust burning off (first start of season normal).
References
- ASHRAE Fundamentals + Refrigeration Handbooks
- AHRI 410 (heat exchanger standards)
- AHRI 1230 (heat pump performance)
- Manufacturer technical data
- Manuall internal: Combustion Fundamentals, Common Compressors