Return grill near sealed combustion furnace

I inspected a new modular home, and the propane fueled sealed combustion furnace has an open return grill within 3 feet of the furnace. I know this is not allowed for furnaces that are NOT sealed combustion. The modular home installer says their systems have been engineered (by who, not sure, but this is a reputable company).

First off, where does the 10 foot rule come from, what agency or reference? How does this 10 foot rule apply to sealed combustion appliances?

As part of my follow up on this issue, I performed a Combustion Appliance Zone test on the furnace room (the CAZ is a relatively small area with a door). The return grill does depressurize the CAZ. However, when the CAZ door is open, there is no home pressure change. The system is balanced with the CAZ door open.

The worst case CAZ depressurization was measured at -38pa, CAZ door closed. The depressurization limit for a sealed combustion furnace is -50pa, so from the perspective of safe operation, it is safe. All other appliances in the home are electric.

I have done an internet search, and this question stumps me. I need to get a report out and make a decision on how to address this issue.

The 10 feet rule does Not apply to sealed combustion appliances.

You are over thinking this. A properly designed and operating 90+ furnace will not have any effect on indoor air.

I appreciate your comment. I do know there is a depressurization limit to a sealed combustion appliance; it’s -50pa. Most other combustion appliances are between -3pa to -15pa. I’m cognizant of this issue from my BPI training/certification as a Building Analyst. I have reported on three homes where the home was tight enough to actually exceed the -50pa limit. The first time was a surprise and an alert to the issue.

I am reporting on this installation as safe, but I did note that the area between the bottom of the CAZ door and the floor should not be restricted. Restricting the area between the door and the floor can make the CAZ depressurization more negative. Unfortunately, I did not test this potential condition with something stuffed under the door.

The issue of concern to me is CO gas entering the home, and the lawyer following a death. We do have CO induced deaths in our area periodically, every few years, and I don’t want one of my inspections to be one of those homes. Thus, my over thinking or paranoia/CYA.

If I were in charge, I’d seal off the grill and this issue would be gone. I’m reporting to the buyer, the manufacturer, and the modular home installer. I have to be fair and objective to all parties.

Then you would most likely be sealing off the return air supply. At the furnace in most manufacturered/modular homes the return air is usually supplied to the blower through slots in the furnace access door/grill.

But it is…

A negative pressure with the door closed is the result of a “Leaking” Sealed Furnace.

Good job finding this, but you need to learn what you found and what to do about it.

This is not an issue with the 10 foot rule, but it still is a significant issue. Furnace operation and safety devices are referenced to indoor pressures. As the HVAC is a closed loop system, no indoor referenced pressure measurements should be detected unless there is a problem.

If the furnace combustion intake is not connected to the exterior, the CAZ should be vented to provide air to the furnace without a pressure differential with reference to the interior.

There is a ducted supply and return air system servicing the rooms of the home. Sealing that grill on the return air plenum would not seal off the return air to the blower. It was not the sole source for return air to the blower.

The issue is that the furnace room was too small and was closed with a solid door. The return air taken from the furnace room with the CAZ door closed was greater than what was supplied through the space under the door. I put a piece of paper over the grill located in the return air plenum, thus sealing the grill, and my CAZ pressure became very acceptable with the CAZ door closed.

That eliminated the issue that David brought up, namely whether the source of combustion air was outside or inside the home. Good point, because a combustion air pipe could be improperly terminated or broken, another source of CAZ depressurization. I previously identified both the combustion air and flue pipes during my visual inspection of the building exterior.

Thanks for all replies. I got it figured out.

First question. You stated modular home. Was it a modular or manufactured home? There could be vastly different building requirements.