Old Electric Furnace Adding Air Conditioning Step by Step Guide
Old Electric Furnace 3400 Adding Air Conditioning
✅ Parts List for Adding A/C to a Coleman 3400‑820 Furnace
(All parts below are compatible with the 3400‑820 and match items on your product page.)
A. Required Indoor Parts (Furnace Side)
1. A/C Control Box
- 3400‑5841 — Coleman A/C control box
- Includes: 24V transformer, fan relay, terminal strip
- This is the heart of the conversion.
2. Blower Upgrade (Required for A/C Airflow)
The 3400 heat‑only blower is too weak for cooling. Use one of these:
- S1‑3500‑6901/A — 4‑speed blower assembly, 1–3 ton (fits 3400)
If you ever need 4–5 ton:
- 3500‑7901/A — 4‑speed 3/4HP blower (fits 3400 cabinet)
3. Low‑Voltage Controls
- 3300‑3861 — 24V 40VA transformer (spare or replacement)
- 025‑38747‑000 — 4‑wire heat/cool thermostat
- 18/5 thermostat cable
- 18/2 low‑voltage cable to outdoor unit (Y + C)
4. Indoor Coil & Drain
Pick based on tonnage:
- S1‑S1831 — 1.5–3 ton mobile home A‑coil
- S1‑6324‑5101 — 18" x 19.5" drip pan
- 3240‑3051 — condensate trap
5. Misc Indoor Electrical
- Wire nuts, spade terminals
- Label tape
- Furnace service switch (if missing)
B. Outdoor Parts (Condenser Side)
1. Outdoor Condenser
- 2–3 ton straight cool (brand doesn’t matter)
2. Electrical
- 3400‑3261 — Coleman/Nordyne disconnect assembly
- Outdoor whip
- Proper breaker (per condenser MCA/MOP)
- Copper wire sized to condenser nameplate
3. Refrigerant Circuit
- Line set (suction + liquid) sized to tonnage
- Condensate tubing
- Optional: float switch
✅ Text Wiring Diagram (Technician‑Friendly)
Below is a clean, accurate wiring layout using the 3400‑5841 control box.
1. Transformer (Inside 3400‑5841)
Primary (240V):
- L1 → Transformer primary
- L2 → Transformer primary
Secondary (24V):
- R = 24V hot
- C = 24V common
2. Thermostat Wiring
|
Thermostat Terminal |
Connects To (3400‑5841) |
Function |
|
R |
R |
24V power |
|
C |
C |
Common |
|
G |
G |
Indoor blower (cool) |
|
Y |
Y |
Outdoor condenser |
|
W |
Furnace heat circuit |
Heat elements |
3. Outdoor Condenser Wiring
- Y (control box) → One side of outdoor contactor coil
- C (control box) → Other side of outdoor contactor coil
When stat calls for cooling:
- R → Y energizes condenser contactor
- R → G energizes blower relay
4. Blower Motor Wiring (Critical for A/C)
Neutral/Common:
- White → Neutral block
Heat Speed Tap:
- Medium/low → Furnace heat relay (existing 3400 wiring)
Cool Speed Tap:
- High → Fan relay output from 3400‑5841
Only one speed tap is energized at a time.
5. Line Voltage Overview
Furnace
- 240V supply → Furnace lugs
- Feeds: heat elements, blower motor, transformer/control box
Condenser
- 240V supply → Breaker → Outdoor disconnect → Condenser
- Low‑voltage Y/C only energizes contactor coil
Internal Components of the 3400‑5841 A/C Control Box
(This is the exact internal architecture used in the 3400‑series electric furnaces when adding A/C.)
1. 24‑Volt Transformer (Step‑Down)
- Primary: 240V
- Secondary: 24V
- Output: ~40VA
- Function: Supplies low‑voltage power for thermostat, Y/G/W circuits, and outdoor contactor coil.
This transformer is the same spec as 3300‑3861.
2. Fan Relay (Blower Cool‑Speed Relay)
- Coil: 24V
- Contacts: SPST (normally open)
- Function:
- Energizes cool/high‑speed blower tap when thermostat calls for G or Y.
- Isolates heat‑speed circuit from cool‑speed circuit.
This is the relay that makes the 3400 blower A/C‑capable.
3. Terminal Strip (Low‑Voltage Distribution Block)
Labeled terminals include:
- R – 24V hot
- C – 24V common
- G – Blower relay coil
- Y – Outdoor condenser contactor
- W – Heat call (passes through to furnace heat circuit)
This is the connection point for thermostat and outdoor unit.
4. High‑Voltage Feed Block
- L1/L2 feed from furnace
- Supplies transformer primary
- Supplies fan relay line voltage output to blower motor cool‑speed tap
5. Blower Cool‑Speed Output Lead
- Typically a blue or black lead depending on model
- Connects to blower motor high‑speed tap
- Energized only when relay closes
6. Internal Wiring Harness
- Pre‑terminated spade connectors
- Color‑coded low‑voltage harness
- High‑temp insulated high‑voltage leads
- Connects transformer, relay, and terminal strip together
📦 Summary Table — Internal Parts of 3400‑5841
|
Component |
Function |
Notes |
|
24V Transformer |
Powers thermostat & condenser contactor |
Same spec as 3300‑3861 |
|
Fan Relay (24V coil) |
Switches blower to cool speed |
SPST, normally open |
|
Terminal Strip |
R, C, G, Y, W distribution |
For stat + outdoor unit |
|
High‑Voltage Feed Block |
Supplies transformer & relay |
Ties into furnace L1/L2 |
|
Cool‑Speed Output Lead |
Sends 240V to blower high‑speed |
Only energized in cooling |
|
Internal Harness |
Pre‑wired connections |
Color‑coded |
┌──────────────────────────────────────────┐
│ 3400‑5841 CONTROL BOX │
└──────────────────────────────────────────┘
LINE VOLTAGE (FROM FURNACE)
┌───────────────┐
│ L1 L2 │ 240V SUPPLY
└─────┬─────┬────┘
│ │
│ │
│ └───────────────────────────────┐
│ │
▼ ▼
┌──────────────────────────────┐
│ TRANSFORMER (24V) │
│ Primary: 240V (L1/L2) │
│ Secondary: R / C │
└──────────┬───────────┬──────┘
│ │
│ │
▼ ▼
(R) (C)
│ │
│ │
│ ┌───────────────┐
│ │ TERMINAL STRIP│
│ │ R C G Y W │
│ └───────────────┘
│ │ │ │ │ │
│ │ │ │ │ └───→ W → Furnace heat circuit
│ │ │ │ └──────→ Y → Outdoor contactor
│ │ │ └─────────→ G → Fan relay coil
│ │ └────────────→ C → Common to stat & condenser
│ └───────────────→ R → 24V hot to stat
Darin DeVries
HVAC Part Store LLC
Coleman HVAC Parts
Technical Service Advisor
Author, entrepreneur, e-Commerce