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Old 03-18-2018, 11:42 AM   #1
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Water Filter Math

We (I) am looking at adding a water filter to our fresh water system. Looking at most of the inexpensive filter housings, they all are listed as just one or two gallons-per-minute while my pump is rated at 4.2 gpm. A filter with a gpm rating as high as the pump is a great deal more expensive ($125 versus about $30) and is quite a bit larger.

While the price and size aren’t deal-breakers, our last boat did have a cheap Whirlpool filter (two actually... one for sediment that looked like a spool of string... and the other a charcoal cartridge... and both available off-the-shelf at Lowe’s). I never noted whether it was a restriction point on the old boat or not, but I do know the math was about the same; a 1 gph filter and a 3.3 gph pump.

Would a 1 or 2 gph filter really be a restriction to (or on) a system with a 4+ gph pump?
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Old 03-18-2018, 12:19 PM   #2
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yes, they can be bad.

I have a non restrictive sediment filter coming into the boat and a 2 stage Whirlpool under the galley sink to a separate gooseneck spigot for drinking water and occasionally cooking.

Les than $150 for both and no issues with flow rates.
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Old 03-18-2018, 01:36 PM   #3
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Originally Posted by Tom.B View Post
We (I) am looking at adding a water filter to our fresh water system. Looking at most of the inexpensive filter housings, they all are listed as just one or two gallons-per-minute while my pump is rated at 4.2 gpm. A filter with a gpm rating as high as the pump is a great deal more expensive ($125 versus about $30) and is quite a bit larger.

While the price and size aren’t deal-breakers, our last boat did have a cheap Whirlpool filter (two actually... one for sediment that looked like a spool of string... and the other a charcoal cartridge... and both available off-the-shelf at Lowe’s). I never noted whether it was a restriction point on the old boat or not, but I do know the math was about the same; a 1 gph filter and a 3.3 gph pump.

Would a 1 or 2 gph filter really be a restriction to (or on) a system with a 4+ gph pump?
A 1 or 2 gallon water flow shouldn't be a problem as long as it's downstream of the pressure tank and pump pressure switch.

Below are two posts from other theads with the filter I use. It's a 7 GPM carbon block that cost about $100 between filter housing and cartridge.

http://www.trawlerforum.com/forums/a...8&d=1503147758

Trawler Forum - View Single Post - Low water pressure

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Old 03-19-2018, 01:42 AM   #4
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Usually ebay or Amazon are the best and cheapest places to find filters and housings. Don't buy a housing without seeing what a filter replacement costs. 10" x 2.5" is the most common filter size. On ebay a 1 micron cartridge is about $8. Less if you buy more. 4 cartridges for about $11. They should handle your pump flow fine.
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Old 03-19-2018, 06:07 AM   #5
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Thanks y’all. That was one on my concerns. But honestly, the big filter Ted used is good for around 50,000 gallons. That is several hundred tank refills and, by my estimate, 10 years of use. A bit of an exaggeration for sure, but the reality is that it won’t need changing for a while. Our old one was just a trip,to Lowe’s and we changed it annually. Which is more expensive? Once every year or two or once... like... never? :-)
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Old 03-19-2018, 06:29 AM   #6
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Some of these filters have clear plastic housings.

Just as clear plastic water lines promote growth of green slime ,

I would be cautious of using a filter with a clear housing.
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Old 03-19-2018, 07:38 AM   #7
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Thanks y’all. That was one on my concerns. But honestly, the big filter Ted used is good for around 50,000 gallons. That is several hundred tank refills and, by my estimate, 10 years of use. A bit of an exaggeration for sure, but the reality is that it won’t need changing for a while. Our old one was just a trip,to Lowe’s and we changed it annually. Which is more expensive? Once every year or two or once... like... never? :-)
IMO, the part that's missed is what the cartridge does. If all you want is to trap some sediment, the smaller size filter housing will work fine. My goal was to filter all the water, remove the sediment (if there was ever any), remove the chlorine (taste anyway), and be able to take a shower while someone is doing dishes or flushing the head. To remove chlorine and taste, you generally need a carbon block filter. These have very low flow rates. Going to the larger filter size gives me the quality of water I want, and the flow rate I desire. With smaller filters and the lower water pressures of a boat, you either get quality or quantity, not both.

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Old 03-19-2018, 07:45 AM   #8
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What are you trying to filter out? The carbon/particulate filters that are rated for ~2 gpm need that flow rate for the carbon and filter media to have the contact time to be fully effective. A higher flow rate may still work but not up to a 100% of its specifications. It’s like taking a Racor 500 series filter that’s rated for 60 gallons per hour and using it to filter 100 gallons per hour.

We filter all the water before it goes into the tanks. It takes awhile at 2 gpm but we’re not loading up the pump and no filters to change in the engine room or under the sink.
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Old 03-19-2018, 09:07 AM   #9
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What are you trying to filter out. .
For us, this is the question too. With an RO system no filtering of that water should be needed. We have never had dirty dock water to contend with so what goes into our tanks is always municipal water.

As Larry says, you can use a hose filter, and I only would worry about + 100 micron so the filter rate from a hose can be pretty fast. We drain our water tanks to the bilge annually, very little grit shows up.

Our water system has never plugged up or given problems so never seen need for after tank filters. The first house water pump was replaced after 10 years, it is now a running take out.

BTW, SS or plastic water tanks are a bonus in this regard. No internally generated stuff like AL or iron.
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Old 03-19-2018, 09:37 AM   #10
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Assuming you don't mind showering or washing dishes in unfiltered water, put a small canister carbon filter under the sink on the cold water line. You'll have clean water to drink with no material impact on pump output.
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Old 03-19-2018, 09:48 AM   #11
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I have a whole-boat filter for sediment and one carbon under each sink. The filters I put in under the sink are 1 micron which really slows the flow. You want to change filters regularly because the stuff you might catch in the filter will grow over time and add a taste.
How often do you open your taps “full?” Usually on a boat you are conserving water so max flow is rare...
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Old 03-19-2018, 11:24 AM   #12
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This question seems to frequently arise on TF. A comment in the past has been, "How many filter their incoming house water?" I can say when I've been on non city well water sediment traps are not uncommon.
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Old 03-19-2018, 11:55 AM   #13
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yeah, but a lot of marinas water systems back East are pretty suspect.

I know at my home marina, they break the lines all the time, or get freeze damage and I just dont know how well they flush them.

of couse, staying at nicer, more expensive marinas has its perks....
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Old 03-19-2018, 12:07 PM   #14
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yeah, but a lot of marinas water systems back East are pretty suspect.

I know at my home marina, they break the lines all the time, or get freeze damage and I just dont know how well they flush them.

of couse, staying at nicer, more expensive marinas has its perks....
Yup, like bad fuel, bad water needs to be addressed. Or avoided.
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Old 03-19-2018, 01:22 PM   #15
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Yea... And that it kinda part of it. We change marinas fairly often and are just being careful. Our current marina's water is pretty hard. We were thinking of a big charcoal filter just to improve the taste. Sediment doesn't seem like a huge issue right now. I peeked into the fiberglass water tank thru a small hole the other day and it looks clean. Not bad for 18 years old. I do plan on adding a couple of access ports later to be able to clean it better should the need arise.
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Old 03-21-2018, 03:05 PM   #16
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Is the use of Ultraviolet inline or intank common?
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Old 03-22-2018, 06:59 AM   #17
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"We change marinas fairly often and are just being careful."

In any new marina TASTE the water to decide if you want it in your tanks .
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