What does the letters H and SV on these signs in Dublin mean?

When i first got to Dublin, Ireland, i was a little puzzled by some letters on small signs around the city. There were Hs and S.V.s. I saw them in houses and commercial buildings and my only guess was that they meant "historical", but the buildings didn't seem that interesting in any way, so i asked my teacher. She didn't know at first but soon came up with the answer. Well, one answer.

The letter H like you see in the pictures means Fire Hydrant and the numbers that appear on some of them are related to the distance from the sign and some other measure.

With that informations was easier to search the web for the other one, S.V.. This one stands for "sluice valve", not sewer vent as i was wondering. Does the sewer needs to vent? I guess. If you had poo flowing trough you all the time you'd need to vent too. So it happens that the sewer needs venting, or needed, as this entry on a "heritage walk tour thing" shows.

I'm not really sure what sluice means at this point but it has something to do with the water supply, and i'm not sure if this has something to do with fires either.

Apparently the signs are used in the UK too, maybe they came to the Emerald Island trough them.

So there you have it. Maybe i'll muster the courage to ask about the sluice valve to a fireman some day.