Combustion Safety After a Partial Conversion
Why this matters
A partial conversion takes out the appliance that used to dominate the house's air and leaves the small ones behind. The heat pump replaces the furnace, a bigger range hood goes in over the new induction cooktop, the weatherization crew seals the attic, and the gas water heater in the basement now lives in a tighter house with stronger exhaust fans and no furnace warming its flue. Each change was correct on its own. Together they can pull a natural-draft water heater's flue gas back into the basement every time the dryer and the hood run at once, and nobody tests for it because nobody touched the water heater.
This card owns the house-as-a-system check: worst-case depressurization as a concept, the CO alarms that stay required, and the commissioning test that closes the job. The flue consequence of losing the furnace belongs to What Removing a Gas Appliance Does to the Ones Left; makeup air for the hood belongs to Kitchen Hood Makeup Air.
Before anything else on site
Wear a personal CO monitor from the moment you enter any house that still has a fuel-burning appliance. If it reads 35 ppm or more in living space (a common shop default taken from NIOSH's 35 ppm recommended exposure limit, which is an 8-hour average, so a short reading at that level is a signal rather than a dose), or any alarm in the house is sounding, get the occupants outside, open doors on the way out, and call the gas utility or 911 from outside. Do not start a depressurization test in a house that is already reading CO. The test deliberately creates the worst condition the house can produce, and you only run it once the house is clean at rest.
The terms you need
Combustion appliance zone (CAZ). The room or space containing a fuel-burning appliance, defined by the walls and doors that separate its air from the rest of the house. A water heater in an open basement has a big CAZ; one in a closet with a louvered door has a small one, and the door position matters.
Depressurization. Exhaust fans, clothes dryers and duct leakage on the return side remove air from the house. In a leaky house, outdoor air replaces it through cracks. In a tight house, the easiest path in may be down the flue of a natural-draft appliance, which reverses its draft. The pressure difference is measured in pascals (Pa) with a digital manometer, CAZ relative to outdoors. A pascal is tiny: natural draft itself is only a few pascals, which is why a few pascals of fan-induced suction can beat it.
Spillage. Flue gas escaping at the draft hood of a natural-draft appliance instead of going up the vent. Brief spillage at startup into a cold flue is common; spillage that does not clear means the appliance is losing the pressure fight.
Worst-case depressurization test. A structured attempt to find the most negative pressure the CAZ can reach under normal household operation, then fire each vented appliance under that condition and see whether it drafts. The procedure lives in consensus standards, most often the Building Performance Institute's BPI-1200 standard practice, which binds you only where a weatherization program, a utility program or a contract requires it, and in some state weatherization field guides. Those standards also set the pressure limit by venting type, and natural-draft appliances get the tightest limit, a small number of pascals. Use the table your program has adopted rather than a number from memory.
Why a partial conversion moves the numbers
Three things change, and they move in the same direction.
The house gets tighter. Conversions travel with air sealing, because a heat pump sized for a sealed house is smaller and cheaper to run. Less leakage means each cubic foot a fan exhausts produces more suction.
The exhaust gets stronger. An induction cooktop invites a bigger hood. A heat pump dryer, by contrast, is ventless and removes an exhaust device (see related: How to Install a Heat Pump Dryer and Close the Old Vent), but many conversions keep a vented electric dryer that still pulls air out of the house. Bath fans get upgraded during the same remodel.
The furnace's help disappears. A running furnace kept the shared flue hot and supplied a strong draft for the water heater to join. A ducted heat pump's air handler can also depressurize a basement CAZ through return-side duct leakage, and a new air handler with leaky return panels in the same room as the water heater is a repeat finding.
The test, as a concept
The order matters more than the instrument.
- Set the house to winter-at-rest. Exterior doors and windows closed, fireplace dampers in their normal state, every exhaust device off. Read the CAZ baseline pressure to outdoors. Baseline is the stack effect and wind with no fans.
- Add every exhaust device the house has. Range hood on high, bath fans, a vented dryer running. Read the CAZ again.
- Add the air handler and test the door. Turn on the heat pump's air handler, then check whether opening or closing the CAZ door makes the reading more negative, and leave it in the worse position. Interior doors to rooms with supply registers and no return can pressurize those rooms and depressurize the rest.
- Net out the baseline. Worst case minus baseline is the fan-induced depressurization, and that net figure is what the standard's limit applies to.
- Fire each vented appliance under worst case, smallest first. Watch for spillage at the draft hood with a smoke pencil or a mirror, for the time window your standard sets, and measure CO in the flue after the appliance has run long enough to stabilize.
- Restore the house and fire again. This is the resume step. Turn every fan off, return doors to normal, fire each appliance once more and confirm it drafts and its flue CO is acceptable under normal conditions before you leave. A house left with the hood on high and the CAZ door shut is a house you just configured to backdraft.
Hazard at steps 5 and 6. You are deliberately making an appliance spill. Keep the personal monitor on, keep the test short, and if spillage continues past the window or room CO climbs, shut that appliance off at its own gas valve, turn the fans off, and open the CAZ to outdoors before you do anything else. Never run a depressurization test with occupants sleeping or with children in the basement.
CO alarms do not leave with the furnace
The International Residential Code, as adopted by your jurisdiction, requires CO alarms in dwelling units that have a fuel-fired appliance or an attached garage with an opening into the dwelling (R315 in the 2021 edition), outside each sleeping area and in a bedroom that contains a fuel-burning appliance. A partial conversion leaves at least one of those triggers in place in most houses: the water heater, the range, the fireplace, or the garage. A fully electric house with an attached garage still meets the code's trigger. State law layers on top and sometimes reaches further: California's statute counts a fireplace and an attached garage, and several states require alarms at sale or rental regardless of any permit. When your work needs a permit, the code's alteration provisions can require alarms be added in the existing house. See related: Smoke and CO Alarm Requirements Your Permit Triggers, which owns that trigger in full.
What changes on a conversion is placement. If the furnace was the only appliance on the second floor and the water heater sits in a basement below a bedroom, recheck that the required locations still hold, and check the alarms' replace-by date while you are looking. Alarms listed to UL 2034 carry an end-of-life date, typically several years from manufacture.
Worked example: the water heater that passed alone
A two-story house, 1,900 sq ft, has its gas furnace replaced with a ducted heat pump. The air handler goes in the same basement utility room as a 38,000 Btu/h natural-draft water heater, relined after the furnace left. The same job moves a vented electric dryer into the utility room and adds the customer's new 600 cfm range hood upstairs. The crew air-sealed the attic the week before.
The tech commissions it this way. Readings are illustrative of the method.
CAZ PRESSURE LOG basement utility room
house state CAZ to outdoors
at rest, fans off -0.6 Pa
+ hood high, dryer, 2 bath fans -3.9 Pa
+ air handler on -4.8 Pa
+ utility door closed -6.1 Pa <- worst
net fan-induced -6.1 minus -0.6 = -5.5 Pa
water heater at worst case: spilled, did not clear
water heater, house restored: drafted, flue CO ok
The net of 5.5 Pa exceeds the natural-draft limit in the program's adopted table, and the spillage confirms it physically. The water heater is fine on its own; it fails only when the house is run the way the customer will run it on a winter evening.
The fixes, cheapest first, and what each changes. Seal the air handler's return panels and the return plenum inside the CAZ, which removes the air handler's contribution. Undercut or louver the utility room door, or add a transfer grille, which removes the door's 1.3 Pa. Provide makeup air for the hood, which is the largest single contributor here. Retested after the first two, the reading would need to come in under the limit with the hood still running; if it does not, the honest answer is the one the conversion is already heading toward: move the water heater off natural draft, either to a power-vent or direct-vent unit or to a heat pump water heater, which removes the combustion appliance from the problem entirely.
What would change the call
A sealed-combustion appliance. Direct-vent and power-vent appliances tolerate far more depressurization than natural draft, because a fan or a sealed system moves the flue gas. The same 5.5 Pa would likely pass a direct-vent water heater, so the test result depends on what is left, not just on the house.
A leaky house. An older house with no air sealing may never reach worst case at all. The test still belongs on the commissioning list, because the air sealing usually comes later.
Vent-free appliances. An unvented gas log set or room heater exhausts into the room by design. A depressurization test does not apply to it in the same way, and some states and localities restrict them. Flag one to the customer and to the CO alarm check rather than testing around it.
Where you stop
Interpreting a failed test, redesigning ventilation, or deciding that a tight house now needs mechanical ventilation under the IRC's whole-house ventilation provisions as adopted is work for an energy auditor or HVAC designer certified under the program's standard. Walk in with your pressure log, the appliance inputs and venting types, and the fan list with their rated flows.
References
- BPI-1200 Standard Practice for Basic Analysis of Buildings, Building Performance Institute (where your program adopts it)
- International Residential Code, Section R315 Carbon Monoxide Alarms, 2021 edition as adopted; UL 2034 for single- and multiple-station CO alarms
- NIOSH recommended exposure limit for carbon monoxide
- See related: What Removing a Gas Appliance Does to the Ones Left; Kitchen Hood Makeup Air; Smoke and CO Alarm Requirements Your Permit Triggers