Installing a whole house water filter
A whole house filter goes on the main line after the meter or the well pressure tank, and before the water heater. That one position treats every tap in the house and keeps sediment out of the heater tank. Allow two to four hours, and fit a three-valve bypass at the same time, because without one every future cartridge change means shutting off the whole house.
The filter itself is the simple part. What decides whether the installation is good is the position on the line, the bypass, and the order the cartridges go in: the housings are identical, nothing stops you fitting the carbon block first, and doing so ruins it before its media is spent.
Where on the line it goes
This is the decision the instructions in the box cannot make for you, because it depends on your plumbing rather than on the product. Working from where the water enters:
| In order | Why there |
|---|---|
| Water meter, or well pressure tank | Everything downstream is yours to change. Nothing goes before this. |
| Main shut-off valve | You need this working before you start. Test it closes fully first. |
| Spin-down screen, on a well | Catches sand before it reaches a cartridge you have to buy. Flushable, so it costs nothing to run. |
| The filter, with its bypass | After any pressure tank, before the water heater. This is the spot. |
| Water heater | Filtered water reaches it, so sediment stops collecting in the tank. |
| Branch to outside taps | Optional, and worth teeing off BEFORE the filter. Garden water does not need cartridges spent on it. |
On a private well, the filter goes after the pressure tank. Fitting it before the tank puts the cartridge inside the pump's working cycle, where it sees the full pressure swing on every start, and a spin-down screen ahead of it will save you cartridges on any well that produces sand. The full treatment order for a well is on our well water page.
The bypass is the part worth planning
A three-valve bypass is two valves either side of the filter and one on a loop that carries water around it. Open the loop and close the pair, and the house has water while the filter has none. It costs three valves and half an hour on the day.
Without one, changing a cartridge means closing the main, draining down, and working against a house with no water. That is the difference between a job you do on a weekday evening and one you put off, and a filter whose cartridges are overdue is doing less than the plumbing it replaced.
Plan for the weight and the space
A three-stage 20 by 4.5 inch system is roughly 24 inches wide and 28 inches tall, and it needs clearance below the housings to drop the sumps off. Filled with water it is heavy, and it hangs off your pipework unless you mount it properly.
- Mount to a board, not to the pipe. Fix a plywood backing to the studs or the masonry and hang the bracket off that. The plumbing should carry water, not the unit.
- Leave clearance underneath. Allow the height of a sump plus room for your hands, or every cartridge change becomes a disassembly. Six to eight inches is comfortable.
- Put something under it. A drip tray or a floor drain nearby is cheap and turns a failed o-ring into a wet patch rather than a damaged floor.
What you need
- Pipe cutter, sized for your main
- Two adjustable wrenches, or the plastic housing spanner in the box
- PTFE tape for threaded joints
- Bucket and towels, because the line will not be empty
- Three ball valves and fittings, if you are adding a bypass
- Mounting board and screws, plus wall fixings rated for the weight
Nothing on that list is specialist except the housing spanner, and that ships in the box. The three ball valves are the only line item people skip, and they are the one worth keeping.
The sequence
- Close the main and open the lowest tap in the house to drain the line. Expect water in the pipe regardless.
- Mark and cut. Measure the unit against the run with the bracket held in place, and cut for the fittings you are using rather than for the unit itself.
- Fit the bypass first, then hang the filter off it. Doing it the other way round means working around the unit.
- Mind the direction. Each housing head has an arrow moulded into it. In on the arrow, out the other side.
- PTFE tape on threads, and stop when it is snug. Plastic housings crack when they are treated like brass, and the seal is the o-ring rather than the thread.
- Fit cartridges in order: sediment in the first position, carbon behind it. Wet the o-rings before seating the sumps.
- Open up slowly. Crack the main open and let the housings fill rather than hitting them with full pressure, then check every joint before you walk away.
- Flush before you drink it. New carbon sheds fines that colour the water grey for a while. Run the nearest cold tap until it is clear.
What goes wrong
- The shut-off will not close fully. Find this out before you cut, not after. An old gate valve that has not moved in a decade often will not seat, and that turns a filter fitting into an emergency.
- Cartridges fitted in the wrong order. Nothing physically prevents it and the system gives no warning. See which cartridge goes where.
- Overtightened housings. The sump seals on an o-ring, so more force does not make it tighter, it makes it cracked.
- No bypass. Cheap to add on the day, expensive to retrofit, and the reason cartridge changes get postponed.
- Outside taps left downstream. Garden watering through a filter is cartridge life spent on water nobody drinks.
Before you buy the system
Two things about the unit itself change the installation. The port size has to match your main, and 1 inch is the common fitting on three-stage systems, so a 3/4 inch line needs reducers and gives up a little more pressure. And the cartridge size decides the running cost: 20 by 4.5 inch is an industry standard, so the parts stay cheap and easy to source, while smaller or proprietary sizes tie you to one supplier.
Which system is worth fitting, scored on owner ratings, certification, cartridge count and warranty, is on best whole house water filter systems. What it costs to keep running is on the running cost guide.
Common questions
Where should a whole house water filter be installed?
On the main line where it enters the house, after the water meter or the well pressure tank, and before the water heater. That position treats every tap and stops sediment collecting in the heater tank. Tee off any garden or outside taps before the filter so they are not consuming cartridge life on water nobody drinks.
Do I need a plumber to install a whole house water filter?
It is a competent do-it-yourself job if you are confident cutting and joining your main line and your shut-off valve works. Call a plumber if the main is galvanised or lead, if the shut-off does not close fully, if the run needs soldering you would rather not do, or if local code requires licensed work on the supply. The filter is the easy part; cutting into the main is the part that goes wrong.
How much pressure does a whole house filter take?
A few pounds per square inch when the cartridges are new, and more as they load. Sediment is what costs you pressure, because the cartridge fills from the outside and restricts flow long before it stops catching anything. If your supply is already weak, a three-stage system on a 1 inch line is a bigger ask than a two-stage, and a 5 micron sediment cartridge is a gentler start than a 1 micron.
Do I need a bypass?
You want one. A three-valve bypass lets you isolate the filter and keep water running to the house while you change cartridges, which turns a whole-house shutdown into a ten minute job at the housing. It also means a failed housing or a leaking o-ring does not leave you without water while you wait for a part.
Which way round do the cartridges go?
Sediment first, carbon behind it, and the housings will not stop you doing it the other way. A carbon block in the first position loads from the outside with grit and is ruined long before its media is spent, which spends the most expensive cartridge in the system protecting the cheapest. Water flows in the direction of the arrow moulded into the head of each housing.
Related
- Best whole house water filter systems, scored and ranked
- Whole house replacement cartridges, and which position each one belongs in
- Well water systems, where the treatment order matters most
- How water filters work, the six mechanisms and what each one misses