Small Package Unit Changeout Planning

Purpose

A rooftop changeout is decided weeks before the lift, on a roof, with a tape measure. The jobs that go badly are almost never bad installs; they are surveys that skipped one of four things: the curb and its duct openings, the electrical service, the gas supply, and the tenant's operating hours. Each is discoverable in a 90-minute visit, and each one discovered on lift day instead leaves a crane on the clock, a hole in a roof and a business with no cooling.

This procedure produces one artifact: a scope document specific enough that the crane, the electrician, the curb adapter and the downtime are all agreed before anything is ordered. It is written for a two-tech shop that does not own a crane and does not need to, because the lift belongs to a rigging contractor who needs measurements from you and no opinions.

Scope

Covers the pre-sale survey and scope document for replacing a single-package rooftop unit on an existing curb, in the sizes a residential-leaning shop takes on strip retail, small offices and restaurants.

Does not cover the install day sequence, owned by the system changeout day-of-install standard, or startup and charge verification, owned by the commissioning SOP. Does not cover the rigging plan, the pick or the crane setup, which belong to the rigging contractor. Does not cover structural adequacy of the roof, which is an engineer's determination.

Roles and responsibilities

Procedure

1. Capture what is there and why it is being replaced. Photograph the full nameplate and record model, serial, nominal tonnage, heat type and input, voltage and phase, minimum circuit ampacity, maximum overcurrent device, refrigerant type and approximate age, plus the failure that triggered the decision. Acceptance: a legible nameplate photo and every field transcribed, with the failure named. What wrong looks like: a proposal written from a tonnage somebody remembered, which is how a 7.5-ton unit arrives for a 10-ton opening. Stop rule: an unreadable nameplate means the data comes from the manufacturer by serial number before anything is quoted. Hazard: none physical, a records task done on a closed cabinet.

2. Measure the curb and the duct openings, not just the footprint. Outside curb dimensions, curb condition and gauge, and the supply and return opening sizes and positions dimensioned from one named corner, with which side the supply is on written down. Acceptance: a dimensioned sketch showing both openings located from the same named corner, plus a photo of the curb with the old unit off if possible. What wrong looks like: a footprint that matches perfectly and duct openings shifted four inches, which is discovered with a crane holding the new unit. Stop rule: openings that do not line up mean an adapter curb, which is quoted with its lead time, not fabricated on the roof. Hazard: measuring an open curb means an open hole in the roof deck, so it is guarded and never left unattended.

3. Compare the existing electrical service against the new unit's nameplate, not against the old one's. Record the existing conductor size and material, the termination temperature rating, the overcurrent device and the disconnect rating, then read the proposed unit's minimum circuit ampacity and maximum overcurrent device. Acceptance: conductor ampacity from NEC Table 310.16 at the equipment's termination rating, which is 75 C on most equipment lugs, at or above the new unit's minimum circuit ampacity, and the overcurrent device at or below the new maximum, with sizing for air conditioning equipment taken from NEC Article 440 in the edition your jurisdiction has adopted. What wrong looks like: assuming the same tonnage draws the same current, when a new unit with a different fan motor or added electric heat does not. Stop rule: any shortfall becomes an electrician's line in the proposal with its own price and lead time, before the sale, not a discovery on install day. Hazard: reading conductor sizes means opening a disconnect, so it is isolated and proved dead per the rooftop disconnect SOP first.

4. Verify the gas supply against the new input, not the old one. Measure the pipe size at the unit, trace and measure the developed length back to the meter, add up every other appliance on that run, and check the new unit's input against the sizing tables in the fuel gas code your jurisdiction has adopted. Acceptance: pipe size, developed length, total connected load and the table result written down, plus confirmation that a shutoff and a drip leg exist at the unit. What wrong looks like: a higher-input unit fitted to a line that was marginal for the old one, which starves it under full fire on the coldest day. Stop rule: an undersized run is a piping line in the proposal, priced before the sale. Hazard: this survey does not open, disturb or test the gas train, and any smell of gas at the unit stops the survey and routes to the gas leak response before anyone continues; leak testing belongs to install day.

5. Record weight, dimensions and access, then hand the lift over. Record both operating weights, compute the difference, photograph the crane setup area, the swing path and the overhead obstructions, and measure the distances and heights the rigging contractor will ask for. Acceptance: both weights, the computed difference and a dimensioned photo set, sent to the rigging contractor in writing. What wrong looks like: a shop deciding for itself that a roof will carry another eighty pounds. Stop rule: any weight increase is referred to the building owner and their engineer, and the shop states plainly that structural adequacy is not its determination. Hazard: none physical in the survey, though the swing path walk keeps you off parapet edges, which the fall protection SOP governs.

6. Check controls, condensate and the code items that do not carry over. Count the existing thermostat conductors against what the new unit needs, confirm the condensate route and trap suit the new unit's static per the rooftop condensate SOP, and establish whether the replacement must carry an economizer. Acceptance: conductor count recorded, condensate route noted, economizer question answered by the building department rather than assumed. What wrong looks like: a five-wire cable pulled twenty years ago against a new unit that wants seven, found at commissioning. Stop rule: whether a replacement requires an economizer, and above what capacity, is set by the energy code edition your jurisdiction has adopted and is a permit question, so ask before quoting a unit without one. Hazard: none physical, a survey and a phone call.

7. Agree the operating window and the downtime before you agree a date. Establish the business's hours, its closed day, its busiest hours, and how long it can go without conditioning. Acceptance: an agreed date and start time, a stated maximum downtime, and the tenant's own confirmation they can trade or close through it. What wrong looks like: a restaurant changeout starting at eleven on a Friday, the fastest way to lose an account you just sold. Stop rule: no agreed window means no lift date, because the crane is booked against the window. Hazard: none physical, a conversation held with the tenant and the account holder together so neither hears it second-hand.

8. Produce the scope document and get it signed before anything is ordered. One document naming the proposed unit, the curb strategy, the electrical scope, the gas scope, the controls scope, the rigging contractor, permit responsibility, the downtime window and every item's lead time. Acceptance: a signed scope on file and a lift date set behind the longest lead item. What wrong looks like: ordering the adapter curb on a verbal approval, which is unreturnable and built to your sketch. Stop rule: nothing is ordered until the scope is signed, including the unit. Hazard: none physical, an office task, and the one that protects every physical step on install day.

The record this produces

The scope document itself, plus the survey behind it: nameplate photos and transcription, the dimensioned curb and opening sketch, existing conductor size, termination rating, overcurrent device and disconnect rating against the new nameplate figures, gas pipe size, developed length and connected load, both operating weights and the difference, access photos with dimensions, thermostat conductor count, condensate notes, the permit and economizer answer with who gave it, and the agreed downtime window.

The survey file makes install day predictable and the next changeout at that site quick. It also settles arguments: when a crane invoice shows two extra hours, the dimensioned sketch and the access photos show whether the delay came from something the survey should have found.

Worked pass: 7.5-ton gas and electric package unit over a restaurant, failed compressor

Unit 22 years old, compressor grounded, customer has chosen replacement over repair. Survey booked for a Tuesday morning before opening.

Step 1: nameplate photographed and transcribed, existing minimum circuit ampacity 27.4 A, gas input recorded, R-22 charge noted. Step 2: curb outside dimensions 46 in by 88 in, sound, 14 gauge; supply opening 20 in by 30 in starting 8 in from the west end, return 24 in by 30 in; sketch dimensioned from the northwest corner and photographed. The proposed unit's footprint matches the curb but its supply opening sits 4 in further east, so an adapter curb goes into the scope with its lead time.

Step 3 FAILED. The existing feeder is #10 copper into a 40 A fused disconnect. The proposed unit's nameplate reads minimum circuit ampacity 41.2 A and maximum overcurrent device 60 A. At the 75 C column of NEC Table 310.16, #10 copper is 35 A, and 35 A is below the 41.2 A the new unit requires, so the existing conductors cannot carry it. Under the stop rule this did not become an install-day discovery: a new feeder and a 60 A disconnect went into the proposal as a priced electrician's line, with the panel checked for capacity at the same visit.

Step 4: gas pipe measured at the unit, developed length traced to the meter, connected load added, and the new unit's input checked against the adopted fuel gas code sizing table with the existing size passing. Shutoff and drip leg present. Nothing was opened. Step 5: old operating weight 705 lb, new 780 lb, so 780 minus 705 is 75 lb more, which is 75 divided by 705, or 10.6 percent. That went to the property manager for their engineer's sign-off, along with the access photos and dimensions to the rigging contractor.

Step 6: existing thermostat cable has five conductors against the new unit's seven, so a control cable pull went into the scope; the building department confirmed the economizer requirement for this capacity under the adopted energy code before a unit was quoted. Step 7: the restaurant closes Mondays, so a Monday 7 am start with a one-day maximum downtime was agreed with the owner and the property manager in the same conversation.

Step 8: the scope was signed with the adapter curb at three weeks and the electrician at two, so the lift was booked four weeks out and nothing was ordered until the signature was on file.

What the failure teaches: everything about this changeout looked like a like-for-like swap. Same tonnage, same footprint, same gas line, same curb. The conductor shortfall came from a nameplate difference of 13.8 A between two units of identical nominal capacity, and it would have surfaced at four in the afternoon on install day with the old unit already in a dumpster and a restaurant opening the next morning.

References

• NEC Table 310.16 read at the equipment's termination temperature rating, and NEC Article 440 for sizing air conditioning branch circuits from nameplate values, in the edition your jurisdiction has adopted
• The fuel gas code your jurisdiction has adopted, for pipe sizing against developed length and total connected load
• The energy code edition your jurisdiction has adopted, for whether a replacement unit must carry an economizer, confirmed with the building department rather than assumed
• See related: System Changeout Day-of-Install Standard; Rooftop Disconnect and Service Receptacle Verification; Rooftop Drain Pan and Condensate Routing