The following link provides a household fly guide sheet to help you identify your household flies. This article also includes suggestions for trapping in order to confirm the fly's identity.
https://extensiondata.missouri.edu/pub/pdf/agguides/pests/g07388.pdf.  It is important to confirm the ID of the flys to make sure the right control is being applied.  

Phorid fly larvae can live in damp, high organic matter soil, like potting mix. If you have house plants or brought plants inside for the winter, that might be a possible source. For additional info on Phorid Flies See: http://www.phorid.net. Phorid fly's can also be associated with fungus, leaking waste pipes which while a lower possibility, can potentially pose more of a health risk.

Fungus gnats can also be found around wet organic matter. For additional info see: http://ipm.ucanr.edu/PDF/PESTNOTES/pnfungusgnats.pdf. Yellow sticky traps can be set-up around any house plants. One article even suggested that even areas right outside your doors should be checked for wet organic matter since this may be an entry into the house. Additional controls can be identified if you confirm fungus gnats.

Drain flies or moth flies are another possibility. On closer inspection, these can look slightly fuzzy. The larvae of these flies can live in the slime found in bathrooms, kitchen sink drains and garbage disposals. They can also be found in drains that don't get regularly used much like an A.C. drain, an unused shower or basement utility sink. I suspect you had already poured boiling water down all your drains. Also see: https://entomology.ca.uky.edu/files/efpdf3/ef615.pdf

The Virginia Cooperative Extension (VCE) 2021 Pest Management Guide identifies the following control for Drain flies

There are a large number of Aerosol sprays and Bacterial foam products on the market. If you can identify the product your plumber may have already used, then try again with a different product. Please fully read the label and carefully follow the label directions.

Master Gardeners are not able to identify pest control companies, but here is a link that may help you select a company to meet your needs: http://npic.orst.edu/pest/selectpco.html

We hope this info helps you confirm the identity and control your house flies. Thanks for contacting the Loudoun County Help Desk.  

Brenda - we asked the VT Entomologist to confirm the species and are waiting to hear back if it is ID'd as a phorid fly.  

The following article from Penn State provide additional suggestions for how you might investigate further and possible direct a plumber if we are able to confirm as a phorid fly.

https://extension.psu.edu/phorid-flies

Hi, Can you send some in for ID?  Eric

Brenda, The VT Entomologist reviewed your photos but then asked if you could mail a sample with several fly's to:

Department of Entomology (MC0319)

Price Hall, Room 216A, Virginia Tech

170 Drillfield Drive

Blacksburg, VA 24061

The flys should be shipped in a small box or pill bottle so they do not get crushed.  

Thank you!

The samples will need to be submitted through the Loudoun Extension office, for contact information, see:  https://loudoun.ext.vt.edu/

Hello, 

I have specimens ready to ship, but based on the message posted above on 5/28, I am not sure whether I should send them directly to VA Tech or through your office. Please confirm whether I should mail, or drop off at your offices on Miller St. 

Thank you so much!

Brenda Harrington

Brenda - I tried to skip a step since I thought your photos were sufficient for the Insect Lab to quickly ID.  

The lab's feedback was is that you need to first drop your sample at the VT Extension Office for the horticulturist to review  using the Extension Office microscope.  Here is the address:  

The EO Map address is: https://goo.gl/maps/K4uxTq8eU5U6FaBQ7

Virginia Cooperative Extension Office 

750 Miller Drive S.E. Suite F-3
Leesburg, VA 20175

<personal data hidden>

I have alerted the extension office that you will be dropping your sample this week if possible on tues or wed between 9:00 am to 4:00 PM. 

Once they have had a chance to review they will notify you by email of the insect's ID.  If they are not able to confirm the fly they will send it to the VT insect lab.  

Again, my apology for the ID delay.   
- Becky

Brenda - the office is closed today but I have alerted them that you could be expected to drop your sample on Tue or Wed this week between 9:00 AM and 4:00 PM.  

-Becky 

I look forward to seeing them. 

Thanks Brenda - I have forwarded your previous control attempts to the Horticulturist who you met today.