The most important question... what are you wanting to filter out of the water?

You calculate the required SFR (service flow rate) for the appliances and fixtures in the home, or get a plumber to do it, and get a filter assembly that can meet that requirement. In some instances even the popular 20" Big Blue filter housing won't, but you can put two or more in parallel.

If you're intent on installing a whole house filter then install pressure gauges before and after so you can monitor the pressure drop and replace the element(s) as soon as indicated.

Thanks, justalurker.
I was looking for something with automatic backwash. Would you recommend the pressure gauges even for that?
I think our SFR will be quite high as we have 2 kitchen sinks, 2 dishwashers, 2 washing machines, 6 showers, a whirlpool, etc. and when the house is full of 20+ people a good bit of those are in use.
I've read some of your posts regarding carbon filters but I'm still leaning in that direction. I will wait until our water is finally turned on (hopefully by Fri.) to test.

No gauges are necessary for backwashing filters but with the scant info you offered it is hard to tell what you intend.

THe SFR of a backwashing filter will depend on correct tank size and media volume of that filter. You'll need to know the sustained and peak SFR you need to size for.

If your intent is to remove chlorine or similar anti-bacterial additives think long and hard before you do.

By eliminating anti-bacterials at the service entrance the entire plumbing system, all appliances, and fixtures in your home will be unprotected. Might be more sensible, and less expensive, to filter POU.

I really hate repeating myself... the most important question... what are you wanting to filter out of the water? When you post what you want to filter then useful discussion can begin.

#1- Copepods
#2- Organic compounds
#3- Anything else testing shows is a potential problem

So I called a company that sells whole house filters and comes highly recommended on GW for their helpful customer service. I was told that the first step in figuring out gpm is to find out how many gpm my pipe is carrying in the first place. The way to do this, I was told, is to go to a faucet close the beginning of the line and time how long it takes to fill a 5 gallon bucket.
So I turned on the faucet that is on the 1.25 in supply pipe and got 15 gpm. But then I had someone turn on the washing machine supply while I ran my faucet. I got 15 gpm while he got about half that. So now I'm getting 22.5 gpm?
So clearly I need more info.
Here's what I know: The city has a 16 in main. My main is 2 in which then splits into a 2 in pipe for the fire sprinklers and a 1.25 supply to the house. The pressure gauge on the sprinkler pipe reads somewhere between 55 and 60 psi.
Does any of this help me figure out how to size an automatic backflow carbon filter?

AY, I got a little lost in the words. In your last message, you ended with, "how to size an automatic backflow carbon filter." Carbon filters are not intended to take out particles, like rust, dirt, or foreign material, they are intended to take out things that taste bad, like chlorine, etc. Further, they are not backwashable, since they give up their chemical internals to accomplish their mission. If you put in a carbon filter when you needed a particulate filter you will be changing them out often, since the particles will foul (means plug) the filter media.

What is the condition you are trying to correct? Let's start with that.

PPP, PE, PMP

pugparents- Thanks for helping. I don't understand why you think they're not backwashable as I've found numerous websites selling auto backwash carbon filters.
NYC has a seasonal influx of copepods that grow in the upstate reservoirs, filling our drinking water with many of the 30-50 micron crustaceans. They are all killed by the chlorine treatment, but I still do not want to be drinking that.
While exploring this issue, I discovered that NYC (along with many of our country's water systems) can be full of all sorts of contaminants, both organic and inorganic, which are known to be unhealthy. NYC is not so good at reporting water violations or general conditions to the EPA or the public in a timely manner. So I thought that a carbon filter was the way to go.
justalurker has voiced concerns about removing chlorine, but after looking into it further, I'm more concerned about the chemicals in the untreated water.

Now, I'm getting the picture, although there are still a few blurry parts: When you say, "I'm more concerned about the chemicals in the untreated water." What do you mean by "untreated"? Untreated by NYC? You shouldn't be getting any of that. Untreated by you: we are not there yet, as your system is not designed. I suspect the "backwashable" system you found was a multi-stage system. The first stage is almost always a roughing filter, that takes out the big chunks, like rust particles and your up-state critters. Usually this is a 20-30 micron polypropylene filter, but to be backwashable it cannot be a "depth" filter, but rather a surface filter--the particulate must be trapped on the surface of the filter media, otherwise how would it be backwashed off? This type (surface screen at about 30 micron) may be all you need to segregate the carcasses from what you want to drink. The next stage usually removes dissolved iron--I doubt you need this, but you could install a 20 micron depth filter to get out all the rust particles and dirt and stuff that gets loose in old municiple water systems, and the final stage is the carbon filter that removes chlorine and other similar chemicals, which is usually unnessary--more on this later. You could taylor the stages to suit your needs.

To one of your original questions, how much water flow can you get through a 1 1/4" service line? Probably 40 gpm. Using Hazen-Williams (google that!), the pressure drop would be about 50 feet of loss (that's about 22 psig, which is a good bit) per 100 feet of line and the velocity in the pipe would be about 10 ft/second--don't want to go much above that. But a better way to size the filter is to find out what simultaneous demand could you reasonable expect? Just add up all the users that could be using water at the same time, and size the filter for that. For the critters you describe, a 20-25 micron whole house filter should be all you need. I don't recommend the big blue filter housings, because I have seen them develop leaks and actually split apart at the joint where the two halves are fused together. The clear polycarbonate single molded housings or the full stainless steel housing with brass head are better choices. You can get either at w.w.grainger. The advantage of the clear plastic is you can watch the carcasses collect on the spiral-wound polypropylene filter media. ;-)

What the Lurker was warning you about is that if you remove chlorine with a carbon filter, than anyone with "soiled" hands who touches the nozzle end of a faucet could transmit bad critters to that faucet, and you will never know and the critters will multiply and that spigot will be come the fountain of yuck. I have a friend who contaminated his whole well through such a transfer of bacteria. Bad day.

Hope this helps.

P.