Seal water heater pipes

here is the info I found

https://homeguides.sfgate.com/seal-around-exterior-furnace-vents-33122.html

What are you trying to block by sealing this? If it’s for heat loss it would be something foam - like the spray foam insulation. If it’s for mice it would be mesh.

Looking at the picture, I would use a foam sealant.

Either steel wool or foam sealant can work. Personally, I'd use the foam sealant.

I would stuff it with steel wool for filler then add a collar of some type like a metal flange for appearance.

Zach,

You probably have some local building code that speaks to your situation. Generally speaking, you would be required to use a fire block caulk or fire block foam sealant. You could put in some copper mesh, but I don't think you really need to worry about mice in this situation. What's above your ceiling? Can you seal it from the top side as well? If so, foam from the topside down and then trim any excess with a utility knife. Because the ceiling looks like regular drywall, you can backfill with all purpose joint compound, tape, mud, and finish to match the ceiling like you would any other type of hole in drywall. It would be hard to cut a decorative flange around the different pipes.

Easiest would be using fire block foam sealant. I would first stuff aluminum foil to fill any gaps around the flue and pipes. Then use fire block expandable foam. Once it's expanded and hard trim it level with the ceiling. You can leave it or paint it. It needs to be fire block to prevent flames or smoke from migrating up through it. Kind of surprised it passed code when it was installed in the first place.

We use a foam spray sealer.

What are you trying to seal it from? I would think the spray foam might work the best, but agree with Seth in that you may want to speak with the building department in your area. They may have rules around how this needs to be done.

While I agree that spray foam is probably the easiest fix you need to check with your city to see if there are any codes in place that would dictate what you needed to do to keep things up to code.

Since they are water pipes, I would avoid the steel wool because any moisture from condensation would cause the steel wool to rust.

Check with the local codes to see what would be allowed and work best in your situation.

I would check local codes before I did anything.

I have aluminum caps on my water heater which looks just like yours. You can buy them at Home Depot or Lowes. If you cannot find one with holes to match. get a solid cap and drill holes to your dimensions

Block the lot in with lathes fitted to the wall and Hardboard, MDF or Plywood to cover.

you can wrapped them up https://www.amazon.com/Frost-King-Foil-Backed-Fiberglass/dp/B000BQSMCG/ref=sr_1_9?keywords=foam%2Bheater%2Bwraps&qid=1637852104&qsid=131-9223515-6136256&s=home-garden&sr=1-9&sres=B002G51BZU%2CB00427COLA%2CB07WCQZG5T%2CB01MY7TAWQ%2CB085DFC3YF%2CB07WJG9B5P%2CB0002YWMEG%2CB000BQSMCG%2CB00004TUCV%2CB0088AHK3E%2CB07GGFC3BR%2CB000IKOU9O%2CB08JYQCHFN%2CB003110YN8%2CB003P9W0RE%2CB000FPDJJE%2CB0756VY391%2CB00EUKHACW%2CB000DZHD0M%2CB00IS94BWS&th=1

Zach, you mentioned "I have a gas water heater and am worried that even fire block foam would be hazardous". So, you really should call the local building inspector and relay to them exactly what you said here.

There may well be specific codes to your area that you need to follow in this case. And the inspector is the person who will know. You should be able to call your town hall or city hall and they can put you in touch with the inspectors office.