what trades are in demand even when construction is low?

when construction is low, all the trades slow down, unless you are in service, like hvac, plumbing or electrical service. being a plumbers helper on a service truck would keep you busy. it would suck, but you could get into it without experience.

even with hvac, you can start right away. the HVAC school is for service technicians ( the guy who works on broken stuff). if you are a mechanic ( the guy installing HVAC) then you can start with no experience. as an HVAC mechanic (again, not a service tech) you can go from 10 bucks an hour to 25 bucks an hour in about 4 years if you are good at it and you get your license.

EDIT: no, you dont need an associated to do hvac. the hvac mechanics install duckwork and heating/cooling units. you can get hired on as a helper even without a high school diploma (although its preferred) depending on the company. this pays between 8-11 an hour and as much as 25 in as little as 2 years. although 4 or 5 years is more common. its almost always 40 hrs a week and it hurts for the first few weeks. but you get used to it. thats the same with any trade.

There is always room in the market for competent tradesman. HVAC is one of the higher paid services but is often seasonal. Either when it is really hot or realy cold do people call for help.

My service and repair business has suffered with the economy and is not recession proof.

When people don't have money they let things go.

I would look at the latest in HVAC and separate yourself from the crowd. Market the indoor air quality (IAQ) angle and duct testing and repair

Maintenance and repairs.