Armstrong furnace 10 flashes: Watchguard — pressure switch opened five times during one heat demand
Ten flashes means the pressure switch opened five times within a single heat demand and the control locked out. That count is documented, and it points at a marginal draft rather than an absent one.
Applies to: Like the rest of this control’s codes, this one is common to Allied Air Enterprises’ A80UH1E and 80G1UHE gas furnaces — rebadged as Armstrong Air, AirEase, Concord, and Ducane.
What it means
Allied Air specifies the threshold exactly: five openings during one heat demand. That precision changes the diagnosis. A switch that never closes is a blockage; a switch that closes and drops out five times in one cycle is a draft that is marginal — present, but not reliably enough to hold. Conditions that produce that pattern come and go rather than being constantly present: wind at the termination, a condensate trap filling during the run, ice forming progressively, or an inducer weakening as it heats. Those are harder to find than a blocked pipe, because the furnace often works when a technician arrives.
Most common causes
- A partly restricted vent or intake, where draft is marginal rather than absent.
- A condensate drain that backs up during a cycle and clears between them.
- Wind at the vent termination disturbing the draft intermittently.
- An inducer weakening and unable to hold its output steadily.
- A pressure switch operating at the edge of its setpoint.
- A sensing hose partly obstructed, so pressure fluctuates.
- Ice forming progressively at the exhaust termination during a long run.
How to diagnose it
- Note that this is a repeated fault within one cycle, which points at an intermittent condition rather than a blockage.
- Check the outside vent termination for partial restriction, and note the weather — wind and freezing conditions both fit.
- Check the condensate drain for slow drainage rather than complete blockage.
- Note how long the furnace runs before faulting, since a consistent interval suggests something accumulating.
- Recall the stored history. Nine-flash codes preceding this one confirm the pattern.
How to fix it
- Clear the vent termination and condensate drain if either is partly restricted. Both are within homeowner reach.
- The pressure switch, its hose and the inducer are technician work.
- Have the vent system assessed properly if the restriction is not obvious. Marginal draft is harder to find than absent draft and produces exactly this code.
- Record when it happens — time of day, weather, how long the furnace had run. That information is something only the householder can supply and it materially helps.
Notes
A manufacturer publishing the exact trip count is genuinely useful. It tells you the furnace tolerated four interruptions before stopping, which means the draft is present most of the time — and that rules out the simple explanations.
Related
- 9 flashes — Pressure switch failed to close, or opened during a heat demand
- Error code recall — The last ten faults, retained even through a power cut
All codes for this manufacturer
Armstrong Air, AirEase, Concord and Ducane furnace codes — every code, with what differs between models.
Not sure this is your brand?
The same count means something else on other furnaces. What 10 flashes means on every brand puts them side by side.
What the fix costs
Intermittent faults bill differently from constant ones. The parts are the same as any pressure-switch code — $150–400 for the switch, $300–900 if the inducer is the weak link, per the furnace repair cost guide — but a draft that holds four times and drops on the fifth often tests fine during the visit, and a switch replaced on spec against an intermittent fault is the classic wasted spend. Expect the diagnostic time ($75–150, usually credited) to be the real product here, and treat a first-visit part swap without evidence with mild suspicion.
Do it yourself or call someone?
The homeowner's unique contribution to an intermittent fault is data. Note when the lockouts happen: during high wind, in hard freezes, late in long heating runs. Wind at the termination, progressive icing, and a condensate trap that fills as the furnace runs are exactly the conditions Allied's five-openings threshold is built to catch, and a pattern narrows the search enormously. Checking the outdoor termination for ice or obstruction is safe. Everything else — hoses, trap, inducer, and especially any temptation to bypass the switch — is technician territory.
The same pattern on another control
A pressure switch that cycles rather than fails outright is counted distinctly on the ICP SmartValve too: 6+2 flashes includes a cycling pressure switch among the abnormal events that add up to its soft lockout — on the same five-events-per-heat-demand arithmetic Allied uses here. The cross-brand list is on pressure switch codes.
What this looks like
From the outside, five pressure-switch openings in one demand usually shows up as the furnace starts and stops in short bursts before Watchguard finally locks it out — that walkthrough covers what to check in order.