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# How to Follow Up on Unpaid HVAC Invoices
Start with the thing that determines everything else: most unpaid invoices are not refusals.
The invoice arrived during a busy week, got put down, and nobody thought about it again. There is no dispute, no cash-flow crisis and no intention to avoid paying. There is an invoice sitting in an inbox.
That matters because it decides the tone of the whole sequence. A first reminder written as though the customer is avoiding you damages a relationship that was never in trouble — and a company that never sends the first reminder at all is the one waiting ninety days.
The sequence
Six steps, from the day the invoice goes out to the day a person takes over.
| When | Channel | To | Purpose |
|---|---|---|---|
| Invoice sent | Customer | Deliver it, with a way to pay | |
| 3 days before due | Customer | Courtesy reminder | |
| Due date | Customer | Payment due today | |
| 3 days overdue | SMS | Customer | Short, because a fourth email is not read |
| 7 days overdue | Customer | Firmer, restating terms | |
| 14 days overdue | Task | Office | Automation stops. A person owns it. |
The last row is the most important one and the one most systems get wrong. Past two weeks, another automated email changes nothing. What is needed is a phone call, a payment plan, or a decision.
What each message says
1 — Invoice sent
Not a chase. The delivery, and it should make paying trivial.
Subject: Invoice INV-1043 from Summit Comfort Heating & Air
Hi John,
Thanks for having us out on Tuesday. Your invoice for the capacitor replacement at 123 Main Street is attached — $398.75, due 23 September.
You can pay online here: [link]
The full details of what Mike found and what he did are on the invoice. Any questions, just reply.
The work description is in there deliberately. It is the last moment a query costs nothing to resolve.
2 — Three days before due
The highest-value message in the sequence, and the one almost nobody sends.
Subject: Invoice INV-1043 due Friday
Hi John — a quick reminder that invoice INV-1043 for $398.75 is due on Friday 23 September.
Pay online: [link]
If it has already gone out, please ignore this.
It arrives before anything is wrong, so it costs nothing in goodwill, and it catches every invoice that was simply forgotten. That closing line matters — it gives an already-paying customer somewhere to put the message.
3 — Due date
Subject: Invoice INV-1043 — payment due today
Hi John, invoice INV-1043 for $398.75 is due today.
Pay online: [link]
If there is a problem with the invoice, reply and we will sort it out.
That last line is doing real work. It invites a dispute you would rather hear on day zero than on day sixty.
4 — Three days overdue, by SMS
Short, because an email is evidently not being read.
Summit Comfort: invoice INV-1043 for $398.75 was due on 23 Sep. Pay here: [link] — or call (555) 0134-8800 if there is a problem.
Under 160 characters, one link, one phone number.
5 — Seven days overdue
Firmer. Not hostile.
Subject: Invoice INV-1043 — now 7 days overdue
Hi John,
Invoice INV-1043 for $398.75 was due on 23 September and remains outstanding.
Pay online: [link]
If there is a problem with the invoice or the work, please call me directly on (555) 0134-8800 — we would rather fix it than chase it.
Our terms are Net 15 and balances beyond 30 days may carry a late charge.
Note what it does not do: threaten, guess at why, or apologise for asking.
6 — Fourteen days overdue: a person
The automation stops and a task lands on somebody's queue with the invoice attached.
Call. Do not email. The purpose is to find out which of four things is actually going on:
- They never received it — resend, and check the address
- They dispute something — resolve it, then reissue
- They cannot pay right now — agree a plan and put a date on it
- They are avoiding you — now you know, and the decision changes
Emails cannot distinguish between those four. A phone call does it in three minutes.
Aging, and why age beats size
Group every outstanding balance by how late it is:
| Bucket | What it means | What to do |
|---|---|---|
| Current | Not yet due | Nothing |
| 1–30 days | Just late | The sequence handles it |
| 31–60 days | Needs a call | A person, this week |
| 61–90 days | Escalate | Owner or manager call |
| 90+ days | A decision | Payment plan, collection, or write-off |
Work it oldest first. A $180 invoice at 120 days is a harder conversation than a $4,000 one at eight days, because collectability falls as money ages. Working the biggest balance first feels productive and leaves the hardest cases to get harder.
HVAC accounts receivable calculator →
The things that prevent the problem
Follow-up is the cure. These are cheaper.
Invoice the same day. Every day between finishing and sending is a day added to every payment, and it is the part entirely within your control.
Make it payable online. An invoice settled the moment it is read beats one requiring a cheque book.
State the terms on the invoice. "Net 15" means nothing if the customer never saw it. Put a real due date on the document, not a phrase.
Take payment on site for service work. The easiest invoice to collect is one settled before the technician leaves.
Ask for a deposit on installations. A changeout with nothing down is a large uncontrolled exposure.
Document the work properly. Most disputes are confusion, not refusal. An invoice that describes the fault and the fix does not generate the call.
Get the commercial details right first time. Purchase order number, the correct accounts payable address, the portal they insist on. Getting any of those wrong is a reliable way to wait sixty days for an invoice nobody was ever disputing.
What this will do to your numbers
Less than the internet will tell you, and more than doing nothing.
The honest position: there is no credible universal figure for how much a follow-up sequence improves collection, because it depends entirely on where you are starting from, what mix of work you do and what your terms are. A residential service company taking card payment at the door has almost nothing to gain here. A commercial contractor on Net 30 with no follow-up process has a great deal.
What you can measure is your own before and after: days sales outstanding, the share of the balance past 60 days, and the number of invoices that reach the fourteen-day escalation. Track those three against yourself, and ignore anyone quoting you a percentage.
When to stop
At some point an invoice stops being a receivable and becomes a decision. Payment plan, collection agency, small claims, or write it off and move on.
That decision depends on the amount, the customer and your own appetite, and no article can make it for you. What matters is that it is a decision — an aging report nobody acts on is a list, not a process, and the 90+ bucket quietly growing is the most common shape of that failure.
See the follow-up sequence running against a real ledger →
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