Heil furnace 8 flashes: High pressure switch closed when it should be open
Eight flashes means the high stage pressure switch was found closed when it should have been open. It is the high-fire counterpart of the two-flash code.
Applies to: International Comfort Products *9MPT and *9MPV two-stage gas furnaces using the Honeywell SV9541Q SmartValve, sold as Heil, Tempstar and Comfortmaker. Single-stage furnaces have no high pressure switch.
What it means
A two-stage furnace carries two pressure switches, one per firing rate, because high fire moves considerably more combustion air and needs its own proof of draft. Both must sit open at rest, and the control checks both before starting. This code means the high stage switch was found closed at that check. The reasoning is the same as for the low stage: a switch stuck closed proves a draft that does not exist. The practical difference is that a furnace may still manage low-fire operation, so the symptom can be reduced output rather than no heat at all.
Most common causes
- High stage pressure switch contacts stuck or welded closed.
- The sensing hose to the high stage switch pinched or kinked, trapping pressure.
- Water in the high stage sensing hose or its pressure tap.
- A short in the high stage switch wiring.
- The two pressure switches swapped during a previous repair — a specific and easily missed cause.
- The wrong switch fitted, with a setpoint that does not match the model.
How to diagnose it
- Confirm the furnace is genuinely two-stage. A single-stage model cannot produce this code.
- Note whether the furnace still produces some heat. Low-fire operation continuing while high fire faults is characteristic.
- Check whether the furnace was recently serviced, since swapped switches are a real cause and quick for a technician to check.
- Check the condensate drain for standing water, which puts moisture where these switches sense.
How to fix it
- Clear a blocked condensate drain if present — reasonable homeowner work.
- The pressure switches, hoses and wiring are technician work, for the same reason as every pressure switch fault: they prove safe venting.
- Ask the technician to confirm the two switches are fitted the right way round if the furnace has been worked on recently.
- Never jumper either switch. Both exist to stop the furnace firing into a flue that cannot carry the products away.
Notes
Because two-stage furnaces can often limp along on low fire, high-stage faults tend to be noticed when the weather turns rather than when they occur. The code is usually present for some time before the house stops keeping up.
Related
- 9 flashes — High pressure switch open when it should be closed
- 2 flashes — Low pressure switch closed when it should be open
All codes for this manufacturer
Heil, Tempstar and Comfortmaker 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 8 flashes means on every brand puts them side by side.