Heating expert! Furnace room positive pressure

I know negative pressure in a furnace room can be a combustion/exhaust safety issue. What about positive pressure?

I have a small furnace closet, the furnace and ducting fills the closet. There is no significant leakage to equalize the pressure between the inside/outside of the furnace closet. The furnace is a newer unit, York 95% efficient, direct vented with make up air supplied to a sealed cabinet. The cabinet is the component of closed combustion, not the combustion chamber.

Ducting: The filter access is well sealed. There is no return air register in the furnace closet. But there is an open supply register that supplies air handler pressurized warm air to the furnace closet. This air from the air handler will pressurize the furnace closet.

I did not measure the pressure with a monometer, because I didn’t have it with me. This was a home inspection. I wouldn’t be surprised if there will be +30 pa or more of pressure in that room with the closet door closed and the air handler running.

There is no opening in the wall to the exterior to equalize the furnace room pressure to the ambient atmosphere pressure.

Is this a safety issue? Does it effect the operation of the furnace?

I know on the negative side, depressurization of -50 pa is the limit of this furnace. If the air handler pressurizes this room above +50 pa, is that a safety issue?

Flex pipe on the right of the image exhausts air from the air handler, supplied to a location near the floor. Cabinet on the left is open, with the exhaust vent visible.

Here’s an image of the furnace with the cover removed. Combustion air on the left, vent fan and flue on the right. Burners visible under the combustion air termination:

My report referred this to a heating contractor for follow up. I’m just curious about this condition relative to furnace operation and safety.

There should be no supply air register to the furnace closet.

Also there should never be a return register in a furnace utility closet. Same issue as the return being located within 10 feet of the furnace.

Post the HVAC system model please.
High efficiency Condensing furnace. Sealed combustion direct vent. No need for make up air delivery unless another appliance, water heater, requires combustion air.
Move on.

Absolutely makes sense, which prompted my question. But where does this criteria come from? I understand that supply air is allowed when the furnace is located in an open basement or open room, with a register in the supply plenum. And yes, no return within 10 feet.

York, Model TG9S080B12MP11A, 2013 mfg. date, propane fueled, 80,000 BTU input

YORK TG9S SERIES Installation manual.
Go to page 22.
Sorry for the edit. I was called away.
*There is no return air register in the furnace closet.
A: Likely, Direct vent and seal combustion.
*The filter access is well sealed.
B: The filter access should have a pivoting door or or slide cover. The filter access should be sealed in a way to easily access, assess and change the filter if it is dirty.
*There is no opening in the wall to the exterior to equalize the furnace room pressure to the ambient atmosphere pressure.
C: Direct vent and seal combustion. High efficiency gas furnace.
*Safety issue?
D: I can not tell you. I was not there. Not enough information. Plus, I am not an expert.
*Flex pipe on the right of the image exhausts air from the air handler, supplied to a location near the floor. Cabinet on the left is open, with the exhaust vent visible.
E: Are you sure the flex pipe is not left over from the last HVAC system?
F: It is either an Air Handler or Furnace. If there is ‘a plenum’ above the furnace, it is likely the sheet metal cabinet, ‘a plenum’ for the Heat Pump evaporator coil.
Question: Is there Heat Pump Compressor is outside?

There is an AC condenser on the building exterior and an evaporator coil in the supply plenum above the furnace. Coil was inaccessible. No heat pump function in the exterior AC condenser cabinet.

The flex pipe was connected to the supply plenum; I checked. It also had air blowing out of it when the air handler was running. The furnace was not original equipment based on building age/furnace age.

The filter was accessed from the lower furnace cabinet access panel, inside the furnace cabinet, not at the typically located return duct, exterior of the furnace cabinet. That location was well sealed from the furnace closet room conditions.

I’m going to pursue this issue next time I talk to an HVAC associate of mine on the issue of supply air from the supply plenum, and furnace closet pressurization issues. This is part of a large condo complex, and he probably has seen some of these installations and knows of the furnace closet conditions.

I appreciate the comments and any additional input.

As an edit; I’ve come across this issue of furnace closet pressurization/depressurization on other inspections. One time I had my manometer with me and measured -35 pa of depressurization in the furnace closet. That was a sealed or closed combustion situation, where the depressurization limit of the furnace was -50pa. Still, -35pa is a lot!

Yes, air will blow out of the flex duct if it is connected to the supply plenum.
Likely previous flex duct from a duct mounted bypass humidifier.
Just my 2 cents.