It can be from any number of types of heat source and it does not have to be central heat.There has to be good window screens on every window to allow the occupants to open the windows for air circulation without getting eaten alive by mosquitos.

We put some in the air conditioning duct work as well, I think that helped get rid of the smoke smell that would circulate through that system.

It may be you just need to operate the HVAC fan alone without heat/cool to circulate enough air to evaporate the normal excess.

If so, replace it.Otherwise you might Invest in a water circulator. 

Was the agent a standard residential agent that posted to the MLS and utilizing standard board of realtor contractors, or was this a large property, commercial listing with customized Investment memos created and circulated and a substantial marketing budget/expense?

Fed burn the dollar from circulation by increasing RRP (REPO activity), including not swapping the old debt with newer matured debt3.

The fact of the matter is, there's more money out there waiting to be lent out than there probably is in circulation.

Is water constantly circulating or does it work more like a hot water heater (ie water sits in pipe for some amount of time)? 

Someone recommended we get an air return grill put into the basement to suck the air out and circulate it through the house. 

There are what 60 Trillion US dollars in circulation world wide right now and with all the central banks, many of which are still doing QE look at japan.