Introduction
If you’ve ever been on a call where the heat’s dead and the coil’s corroded, you know the stakes. When we change the coil and furnace in the house, we’re not just swapping parts—we’re restoring comfort, safety, and efficiency. This guide shows how pros plan, install, charge, and commission a full change-out the right way. We’ll cover safe isolation, compliant refrigerant work, proper venting, and combustion checks. If you’re a contractor who’s tired of callbacks, read on.
Quick Answer
Quick Answer: When we change the coil and furnace in the house, the pro workflow is: verify sizing and code compliance, safely isolate services, recover refrigerant, replace the evaporator coil, set and connect the furnace, evacuate and charge, then commission heat and airflow. Document, handover, and close with a clean proposal and invoice.
Table of Contents
Key Takeaways
- A clean change-out lives or dies on planning: correct sizing, matched equipment, and compliance saves hours onsite and avoids callbacks.
- Pressure test (around 20 bar or 300 psi), evacuate to 500 microns, and verify subcooling/superheat—don’t guess the charge.
- Confirm flue slope, condensate routing, gas pressures, static pressure, and temperature rise; fix issues before handover.
- Document everything: commissioning data, photos, and client sign-off. It protects you and builds trust.
Planning When We Change the Coil and Furnace in the House
When we change the coil and furnace in the house, planning is half the job.
Compliance and Licensing
- Refrigerant handling: F-Gas certified technicians (UK/EU) or equivalent credentials are required to recover and charge refrigerants.
- Gas work: Gas Safe registration is required in the UK for any gas appliance installation, testing, and commissioning.
- Electrical: Follow BS 7671 (18th Edition) and Part P for domestic electrical work.
- Building Regulations: Check Part L for efficiency, ventilation standards, and flue routing/condensate discharge requirements.
Sizing and Matching
- Heat load: Perform a heat loss/heat gain calculation. Oversizing the furnace can cause short cycling; undersizing leaves rooms cold.
- Coil match: Ensure the evaporator coil matches the outdoor unit and metering device. Confirm the AHRI match or manufacturer-approved pairing.
- Airflow: Target around 400 cfm per ton of cooling (roughly 680 m³/h per 3.5 kW) unless the manufacturer specifies otherwise.
- Nitrogen, vacuum pump capable of 500 microns, micron gauge, digital manifold, silver brazing rods, wet rags/heat-block, combustion analyser, manometer, leak detector, flaring/brazing kit, and a recovery machine and cylinder.
- Consumables: line driers, TXV (if not factory-fit), new filter/diffuser, sealants, mastic, hangers, unions, and isolation valves as required.
Proposal and Client Expectations
Set scope before ripping in. Outline what’s included: new coil, furnace, line drier, condensate components, flue work, thermostat verification, and commissioning data. Use Donizo to capture site photos and voice notes, then generate a branded proposal for e-signature the same day. This reduces back-and-forth and gets the green light fast.
Safe Isolation, Recovery, and Removal
Shut it down and make it safe first.
Isolation
- Power: Lock-off and tag the isolator. Verify dead with a tester.
- Gas: Close the isolation valve upstream of the furnace. Verify with a manometer and gas leak detector before disconnecting.
- Refrigerant: Connect recovery machine and cylinder; recover fully per environmental regulations.
Disconnection and Removal
- Remove the old coil: Protect the home with dust sheets. Disconnect the line set at the coil, remove the condensate line and pan sensors, and lift the coil out.
- Strip out the furnace: Disconnect flue, electrical, gas union, condensate (if condensing), and duct connections. Note platform heights and plenum transitions for a clean reinstall.
Common mistake: Cutting corners on recovery or skipping nitrogen purge during later brazing. It creates acid and scale, which kills compressors and TXVs.
Installing the New Evaporator Coil
A tidy coil install sets up the rest of the job.
Orientation and Drainage
- Correct airflow direction: Align as marked by the manufacturer.
- Pitch: Ensure the coil case and pan have a slight fall towards the drain. Install a proper condensate trap (around 75 mm water column) and route to an approved discharge.
- Pan switch: Fit a float switch or pan sensor to protect ceilings in loft installs.
Brazing and Line Set
- Nitrogen purge: Flow nitrogen at a low rate (roughly 2–5 L/min) while brazing to prevent oxidation.
- Heat management: Wrap TXV and service valves with wet rags. Use a 15 percent silver braze for strong joints.
- Filter drier: Install a liquid-line drier close to the indoor coil (or as specified). Replace any suspect line set; if reusing, flush thoroughly and pressure test.
Metering and Matching
- TXV vs fixed orifice: Match the metering device to the outdoor unit specification. If using TXV, target subcooling during charge. For fixed orifice, charge by superheat.
| Metering Device | Typical Charge Method | Typical Target |
|---|
| TXV | Subcooling | Around 8–12 K (as per OEM) |
| Fixed Orifice | Superheat | Around 10–15 K (as per OEM) |
Setting the Furnace, Vent, Gas, and Electrical
Get the box sitting right before you power anything.
Positioning and Ductwork
- Base: Level the furnace cabinet and seal transitions. Maintain at least 750 mm clear working space in front.
- Return and supply: Confirm correct filter rack size and low-resistance filter. High pressure drop causes noise and poor heat rise.
Flue and Condensate
- Flue slope: For condensing appliances, maintain a continuous fall or rise of roughly 6 mm per 300 mm in the direction specified by the manufacturer.
- Termination: Follow clearances from openings and property boundaries. Support flue sections at recommended intervals.
- Condensate: Use a trap and route to an approved drain; protect against freezing in lofts or unheated spaces.
Gas and Electrical
- Gas-tight: Reconnect with a new gasket/compound where required. Tightness test and leak-check with a calibrated detector.
- Pressures: Verify inlet and burner pressure with a manometer per the data plate.
- Wiring: Land supply to the correct terminals, earth properly, and follow the control wiring schematic. Confirm the thermostat heat stages and fan profiles.
Charging, Commissioning, and Verifying Heat
This is where callbacks are made or avoided.
Pressure Test and Evacuation
- Pressure test with nitrogen around 20 bar (300 psi). Soap every joint.
- Evacuate to 500 microns with the core tools removed. Perform a standing vacuum test for at least 10 minutes; microns should stabilise.
- Weigh in the factory charge, adjust for line set length per OEM tables (for example, add 15–30 g per metre beyond the factory allowance).
- TXV systems: Dial in subcooling to the manufacturer’s target (often around 8–12 K).
- Fixed orifice: Adjust charge to reach the target superheat (often around 10–15 K) at given indoor/outdoor conditions.
- Record suction/discharge pressures, line temps, subcooling/superheat, and ambient conditions.
Airflow and Static Pressure
- External static: Measure total external static pressure; many furnaces target less than 125 Pa to 200 Pa (0.5–0.8 inch water column) depending on model.
- Fan speed: Set blower taps to hit design airflow and the furnace’s nameplate temperature rise.
Combustion and Heat Verification
- Temperature rise: Confirm within the furnace nameplate range (commonly 25–35 C). Adjust airflow if out of range.
- Combustion analyser: Verify CO, O2, CO2, and flue gas temperature as per OEM limits. Correct with gas valve adjustments only within manufacturer parameters.
Pro tip: When we change the coil and furnace in the house, photograph gauges, microns, manometer readings, and analyser results. Attach them to your proposal record in Donizo for bulletproof documentation.
Handover, Pricing, and Paperwork
Tie the bow on the job the client can see—and the records you’ll need later.
Client Walkthrough
- Show filter access, thermostat modes, and how to read any fault codes.
- Explain maintenance intervals, including annual servicing and filter changes.
Documentation
- Leave commissioning sheets and warranty information.
- Store load calcs, AHRI match, photos, and readings. Use Donizo to keep a clean project record that converts the accepted proposal into an invoice in one click.
Pricing and Proposals
- Scope clarity prevents “while you’re here” scope creep. Use Donizo Voice to Proposal to capture detail onsite, generate a branded PDF, and secure e-signature before you start. Fewer surprises, faster approvals, and easier payment tracking.
When we change the coil and furnace in the house, a tight workflow—from proposal to commissioning—keeps labour predictable and margins healthy.
FAQ
Can I change a coil and furnace in the house myself?
Legally and safely, this is a licensed professional’s job. Refrigerant recovery/charging requires F-Gas certification, and gas appliance work requires Gas Safe registration in the UK. Incorrect flueing, charging, or combustion setup can be dangerous. Hire a qualified contractor with documented commissioning.
How long does it take to change the coil and furnace in the house?
A straightforward domestic change-out often runs 1 full day for an experienced two-person team, assuming no major duct or flue rework. Add time for new line sets, complex flue routes, or electrical upgrades. Plan a return visit if ambient conditions aren’t suitable for final charge verification.
Do I have to replace the coil when replacing the furnace?
If the home has an existing cooling system, the indoor coil should be evaluated for compatibility with the outdoor unit and refrigerant. Mismatched or old coils can bottleneck performance and cause refrigerant issues. Many contractors replace the coil with the furnace to ensure a matched, warrantable system.
What commissioning readings should I provide to the client?
Record and share: pressure test value, final vacuum in microns, subcooling/superheat, external static pressure, temperature rise, gas inlet/burner pressures, and combustion analyser results. When we change the coil and furnace in the house, these readings prove the system is safe and set correctly.
What’s the best way to avoid callbacks after a change-out?
Follow a checklist: pressure test, evacuate to 500 microns, verify charge by subcooling/superheat, confirm static pressure and temperature rise, and complete a combustion analysis. Photograph your instruments and file everything. Use Donizo to attach data to the proposal so any future tech sees the history.
Conclusion
When we change the coil and furnace in the house, success comes from process: correct sizing, code compliance, pressure testing around 20 bar, evacuating to 500 microns, and verifying subcooling, static pressure, and temperature rise. Close the loop= with clear documentation and a confident handover. Want to save hours on admin? Capture site details, generate a branded proposal, get e-signature, and convert to an invoice with Donizo. Get organised, get paid, and move on to the next install.
