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.