Looking for information concerning possible mold problems

My girlfriend and I have been renting a house to a woman and her three children since mid-September. Prior to that, my girlfriend lived there for 6 years. It did seem to be a humid house, and she often ran a dehumidifier, but there was no visible mold. Before renting the house, I completely gutted and remodeled the bathroom. There was some mold in an isolated spot inside the wet wall, which is between the bathroom and kitchen. That was all removed as part of the renovation, and replaced with blue drywall. The entire house was re-plumbed with PVC and copper since the wet wall was open. Other pertinent facts about the house are: vented crawlspace with plastic on the ground (it's always remained dry), very large attic space with little to no ventilation (this has not been addressed yet), there is a ground level utility room off of the kitchen that houses the furnace, water heater, and washer dryer. It's a 1940s build.

After about two months in the house, the tenant told me that the house has a musty smell and this smell is on their clothes. I told her I would look into it and get a professional contractor there to deal with it if necessary. That was about 2 weeks ago, and I planned on getting over this Friday to check walls for moisture; house humidity; attic humidity; and look around for visible mold. I confirmed that appointment with her today and in replying, she stated that she loves the house but she and her kids have had pneumonia, ear infections, bronchitis, and generally bad allergies. She said she hopes we can figure it out because they love the house.

Obviously, I have an obligation to look into this, and I plan to do so. At this point I have a couple concerns. If indeed there is a mold problem and it's that severe, I doubt that there is anything I can do to alleviate it. My other concern is that it seems odd that the 4 current tenants are suffering very severe reactions to mold when no other occupant has ever suffered even slight allergies (my girlfriend had two roommates when she lived there). I guess I'm not sure how to handle this. I think the only thing I can do is contact a professional to go in and check things out, however, I'm sure that will cost a great deal. If they don't find anything, then the tenant can choose to break the lease. If a problem is found, it would simply be dealt with however necessary.

How would others handle this? Should I even bother looking into things myself, or just hire a company for peace of mind, if nothing else? This is a new situation for me, so I'm hoping someone here has some experience and/or knowledge that might prove helpful.

Follow your nose.
If it smalls moldy, there's mold someplace.
Common areas are, no vapor barrier on the ground if there's a crawlspace.
Lack of ventilation in crawl space.
Leaking pipes.
Condensation on pipes under the house.
Outside grade not running away from the foundation.
No gutters.
Outside of foundation never got waterproofed.
Mulch of flower beds made of landscape timber forming ponds again the foundation.
Dryer vent not running to the outside.
No bathroom vent.
Non vented propane heaters.
No soffit and roof vents.
Old balloon framing not fire sealed.
Sure it's mold and not an issue with heating system?
ECT.

Of everything you mentioned, I'd guess that lack of attic ventilation and water pooling up near the house are the most likely problems. The attic is huge and actually has windows on both gables, but has no ventilation. We recently got a new roof and gutters but the downspouts terminate about 4 feet from the house and water pools up around there on a couple corners. I would put longer runs off the downspout, but they'd be running through the yard. Perhaps I need to do some digging and put in a drainage system when it warms up (or maybe use those collection barrels?).

Would it make any sense to crack open the windows in the attic? If the water near the house is a problem, would that cause mold to grow underneath? I've never been in the crawlspace, but the plumber spent a considerable amount of time down there and said it was all dry.

What problems could the heating system cause? I can certainly get an HVAC company to check things out and clean the ducts if that's a potential problem.

New roof and new plumbing dosn't say there isn't a problem and maybe a new one. Wet dirt in the crawl space can be smelly even with out mold. Do you own inspection first and check everything.
Install a co2 detector and check for gas leak if you have gas. Confirm that the tennents use the range fan and bathroom fan and does the furnace have fire air brought from outside.

I would open the attic windows until you get around to venting that space. And yes, there are a variety of ailments that can come from dirty air ducts.

I went over there today. The humidity levels were in the high 30%s, so that seems normal. The attic was in the low 40%s. I checked the drywall around the plumbing with a moisture meter and didn't get any high readings. I did crack the attic windows. I had an HVAC company look into the furnace. It is old and he said the heater core (I may be mixing up part here?) pipes could be an issue soon. He saw what could be cracks on the pipes where they bend, but with the view from the camera he couldn't be sure. He said he didn't smell anything, and I have a CO2 detector about 10 feet away from it in the kitchen. Either way, it sounds like that will have to be replaced sooner than later, but with the A/C components included, it's obviously a small fortune for some people. It did occur to me that the floor drain in the utility room might be low on water since it doesn't see much water at all, so I poured a bucket full in there to make sure no methane is escaping. I cleaned the gutters and made sure the downspouts were at least 4 feet from the house, which is the best I can do until I put proper drainage in.

That's about all I can think to do short of having a mold inspection company come in and take a look. I don't mind to have the done, I just don't want to get into that if it's not necessary. I do know that there is batt insulation between the floor joists in the vented crawlspace, which seems like a bad idea. I can't say I'd be surprised if there's a moisture/mold issue there.

There is a primer valve that you can use. It dribbles water to the floor drain when you use water.
The only problem with a vented crawlspace is with older houses there are way to many holes in the floor, in new houses, every hole is kept as small as possible and then they are filled with a fireproof foam. If you have a smelly crawspace, you might look at a vent fan that is made for that.

if your crawl space is smelly, coat the dirt with lime.

you need to air it out.

I forgot to mention in my last post that the attic does have a large exhaust fan, but it does not work. I've attached a picture of the fan and one gable end, including a window. What's the consensus on these fans? It seems odd to have this large of an exhaust fan with no intake. Should I repair/replace this fan and crack the windows?

That might be good in the summer to keep the attic cool, in the winter yopu are more worried about heat loss from the top of the exterior walls that melt snow on ther roof causing ice dams and leaks.
The problem with cracking a window now is rain coming in.