Real scam - carbon monoxide is a by product of the combustion process. 

Home improvement products, Real Estate, encyclopedias, it doesn't matter those are standard cold call numbers since the dawn of man or something.If you know them it's easier.  

Just decline the application? 

There are several products to buy in the store or you can buy a small extractor from any big box store.

Using comparable sold properties with similar condition to your end product, within a 1/2 mile (sometimes a little further), less than 6 months old, within 20% of square footage, not crossing major boundaries ( high ways, school districts, city lines, etc).

Generally the conventional, non FHA loans have been cheaper and preferred and FHA was a backup for those that could not otherwise qualify for those loan products.  

So I was wondering if there are any particular things you can do to make the product irresistible to buyers.

HELOCs are typically bank products, not mortgage company products, so they have a lot more leeway in how they underwrite them. i.e. they are not trying to meet some government organization's criteria, just their own.

How are you getting 5.125% fixed for 30 years on a commercial product?

Have a blessed and productive day!